revision of the dinosaur stegoceras lambe - BioOne

Journal of Vertebrate Paleontology 23(1):181–207, March 2003 q 2003 by the Society of Vertebrate Paleontology

REVISION OF THE DINOSAUR STEGOCERAS LAMBE (ORNITHISCHIA, PACHYCEPHALOSAURIDAE) ROBERT M. SULLIVAN Section of Paleontology and Geology, The State Museum of Pennsylvania, 300 North Street, Harrisburg, Pennsylvania 17120-0024, [email protected]

ABSTRACT—Stegoceras (sensu lato) has been the recipient of a number of species. Most have been placed into synonymy with S. validum, some have been transferred to other genera (i.e., Gravitholus, Ornatotholus), whereas others have been interpreted as being sexual dimorphs. A parsimonious phylogenetic analysis using 49 characters, which include 9 new cranial characters based on pachycephalosaur frontoparietal domes, now permits a revision of the genus Stegoceras. This analysis concludes that Stegoceras validum (sensu stricto) is a primitive, incipiently-domed pachycephalosaur that is characterized by a well-developed squamosal shelf and open supratemporal fossae. It is the sister taxon to the fully-domed Pachycephalosaurinae. The taxon Ornatotholus browni has a parietal and displays features identical to S. validum and is therefore a subjective junior synonym. Specimens consisting of flat, paired frontals are immature individuals of S. validum. The holotype of ‘‘Stegoceras’’ lambei, and all morphotypes of this species, are unique in the construction of the posterior squamosal region and are placed in a new genus, Colepiocephale. Colepiocephale lambei is known solely from the Foremost Formation and is the oldest diagnostic pachycephalosaur from North America. An incomplete, and indeterminate, skull from the upper part of the Milk River Formation (upper Santonian) has the distinction of being the oldest known North American pachycephalosaur. ‘‘S.’’ sternbergi lacks a posterior squamosal shelf and has squamosals directed laterally and is placed in a new genus, Hanssuesia, as H. sternbergi. Although the taxon Gravitholus albertae displays some characters (i.e., retention of the posterior squamosals separated by a distinct median parietal) and is similar to H. sternbergi in some respects, the holotype is too incomplete for any definitive diagnosis and thus it is considered a nomen dubium. Sphaerotholus is a subjective junior of Prenocephale and the species Sphaerotholus buchholtzae is a subjective junior synonym of Prenocephale edmontonensis. The taxa Prenocephale brevis, P. edmontonensis, and P. goodwini, form a monophyletic clade with monotypic Asian taxa Prenocephale prenes and Tylocephale gilmorei as an unresolved sister group. They are united by the possession of a distinct row of nodes on the squamosal and parietals. Tylocephale, if valid, is interpreted as the sister taxon to the Prenocephale clade. Stenotholus kohleri is formally recognized as a junior synonym of Stygimoloch spinifer. Based on the presence of hypertrophied nodes Stygimoloch spinifier and Pachycephalosaurus wyomingensis are united in the clade Pachycephalosaurini new taxon. Small-to-medium size Late Cretaceous pachycephalosaurs were rather diverse, especially during the late Campanian (Judithian), and many species coexisted during Campanian and Maastrictian times. Ancestry and directionality of dispersal (in part) of the North American and Asian taxa remains uncertain and certainly antedates Campanian time.

INTRODUCTION Pachycephalosaurian dinosaurs, known primarily for their unusually thickened crania, are perhaps the most enigmatic and poorly understood dinosaurs. Numerous taxa have been named, and attempts to assess them in a phylogenetic framework have been a challenge to all who have studied them. Most known specimens consist of isolated frontoparietal domes, or parts thereof. Only a few specimens have postcranial material associated with cranial elements, so characterization of these dinosaurs largely rests on the morphology of the skull, primarily on the frontoparietal region. Of all the pachycephalosaurs named, the North American genus Stegoceras has been the recipient of most species. Early workers placed many of the small and medium-sized North American pachycephalosaurs in the genus Stegoceras (sensu lato), a practice that has largely continued to this day. Consequently, the genus has become (perhaps unintentionally) a ‘‘wastebasket taxon’’ for any small-to-medium sized North American pachycephalosaurs. Pachycephalosaur cranial elements have been interpreted as being highly variable, and consequently, have been rendered useless in assessing taxa at both the generic and specific levels. Morphometric studies concerning some pachycephalosaur crania have been attempted by Chapman et al. (1981) and Goodwin (1990). However, these

studies were conducted outside the bounds of a rigorous taxonomic framework where it was assumed (a priori) that most of the morphological variation expressed among the small-to-medium sized pachycephalosaurs was within a single taxon (Stegoceras validum). The conclusions regarding sexual dimorphism for these dinosaurs, reached by these workers, are now considered to be dubious at best in light of the work presented below. For many years the number of pachycephalosaur specimens, like those of many other dinosaurs, were not plentiful. However, in recent years, a plethora of specimens have been amassed, especially by the Royal Tyrrell Museum of Palaeontology. This fortunate circumstance has resulted in an unusually large sample that not only permits the recognition of consistent morphotypes, but now allows for a revision of the non-crown group pachycephalosaurs. Moreover, most of the newly collected specimens have excellent stratigraphic documentation, making precise biostratigraphic correlations and interpretations possible. In the late 1980s, I began a study of Stegoceras (sensu lato), and other pachycephalosaurs, in the belief that it should be possible to make some ‘‘taxonomic sense’’ of the array of morphologies exhibited by the numerous specimens known. Initially, I was frustrated dealing with the material, primarily because of the apparent morphologic variability among the fron-

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toparietal domes. Moreover, most of the specimens were largely incomplete, making taxonomic assessments even more challenging. I temporarily tabled the project in the early 1990s and, only recently, returned to this study spurred on largely by the discovery of two incomplete skulls from the Upper Cretaceous Kirtland Formation of New Mexico. I began my re-evaluation of these enigmatic dinosaurs in 1999 and have been able to identify consistent morphologies and character states among the array of specimens referred to ‘‘Stegoceras.’’ The first part of this investigation was recently published (Sullivan, 2000) which serves as a foundation for the results of this study presented below. MATERIALS AND METHODS I have studied all the pachycephalosaur specimens in the collections of the Royal Tyrrell Museum of Palaeontology, Canadian Museum of Nature, Museum of the Rockies, Royal Ontario Museum, and University of Alberta. In the past, while conducting other studies, I have had the opportunity to examine other pachycephalosaur specimens in the collections of the American Museum of Natural History, Carnegie Museum of Natural History, Denver Museum of Nature and Science, Los Angeles County Museum of Natural History, New Mexico Museum of Natural History and Science, and United States National Museum (Museum of Natural History). Also, I was fortunate enough to examine the holotypes of Prenocephale prenes and Homalocephale calathocercos in the Great Russian Dinosaur Exhibition that recently toured the United States. A list of key material studied during the course of this study appears in Appendix 1. Because most of the specimens in this study are largely represented by incomplete skull or frontoparietal material, I am only able to offer diagnoses using cranial autapomorphies (contra the claim of Williamson and Carr, 2003) as the predominant means for differentiating taxa. A list of new cranial characters identified in this study is provided in the phylogenetic systematic section below. I have built upon the recent work of Sereno (2000), reassessed his pachycephalosaur cranial characters, and added to his taxon-data matrix to produce the phylogenetic hypothesis of pachycephalosaur relationships presented here in Appendix 2. In my previous contribution (Sullivan 2000a), I focused on the taxon Prenocephale, resurrected the species ‘‘Stegoceras breve (5brevis),’’ and transferred ‘‘S.’’ breve and ‘‘S.’’ edmontonese to Prenocephale (P. brevis and P. edmontonensis, respectively) because of the distinct linear row of nodes that are largely on the squamosals, a character and taxonomic assignment that are reassessed below. The purpose of this paper is to conclude my taxonomic study of the non-crown group (i.e., non-Stygimoloch 1 Pachycephalosaurus) North American pachycephalosaurs previously assigned to Stegoceras (sensu lato), and to comment on the phylogenetic relationships and paleobiogeography of the Pachycephalosauria. Institutional Abbreviations AMNH, American Museum of Natural History, New York; CM, Carnegie Museum of Natural History, Pittsburgh; CMN, Canadian Museum of Nature (formerly the Geological Survey of Canada [GSC] and National Museum of Canada [NMC]), Ottawa; DMNS, Denver Museum of Nature and Science (formerly the Denver Museum of Natural History), Denver; LACM, Los Angeles County Museum of Natural History, Los Angeles; MOR, Museum of the Rockies, Bozeman; MPM, Milwaukee Public Museum, Milwaukee; NMMNH, New Mexico Museum of Natural History and Science, Albuquerque; PAL, Institute of Paleobiology, Warsaw; ROM, Royal Ontario Museum, Toronto; SMP, The State Museum of Pennsylvania, Harrisburg; TMP, Royal Tyrrell Museum of Palaeontology (includes specimens previously in the col-

lections of the Provincial Museum of Alberta [PMA], Edmonton), Drumheller; UALVP, University of Alberta, Laboratory of Vertebrate Paleontology, Edmonton; UCMP, University of California Museum of Paleontology, Berkeley; USNM, United States National Museum, Washington, D.C.; and YPM, Peabody Museum of Natural History, Yale University, New Haven. TAXONOMIC HISTORY OF THE PACHYCEPHALOSAURIDAE The first pachycephalosaur, Stegoceras validum, previously ‘‘S. validus,’’ was named by Lawrence Lambe in 1902, based on three frontoparietals (syntypes), that were originally interpreted by him as ‘‘prenasals.’’ Hatcher, in the classic, and posthumous monograph on the Ceratopsia (Hatcher et al., 1907), designated CMN 515 as the type (lectotype) and correctly identified the elements as pertaining to the frontals, parietals and occipital region of the skull. A few years later Lambe (1918) designated another species, Stegoceras brevis, from among the material that he had studied earlier, and noted that 12 specimens of these dinosaurs were then known, although their phylogenetic position was still less than certain. Gilmore (1924) reported on the first complete skull, and partial postcranial skeleton of Stegoceras validum (UALVP-2), but believed, based on tooth morphology, that Stegoceras validum is a junior synonym of the now well-known theropod dinosaur Troodon formosus. Brown and Schlaikjer (1943) were the first to review the dome-headed dinosaurs as troodontids, established the genus Pachycephalosaurus for the species T. wyomingensis (as P. wyomingensis [Gilmore, 1931]) and established two new species, P. grangeri and P. reinheimeri. They restricted the genus Troodon to include T. formosus and ‘‘T.’’ validus [sic] and named ‘‘T.’’ sternbergi, and ‘‘T.’’ edmontonensis as new species of ‘‘Troodon.’’ This error in synonymy, first proposed by Gilmore (1924) and subsequently adopted by Brown and Schlaikjer (1943), was later corrected by Sternberg (1945). In their discussion of Stegoceras validum (sensu lato), Brown and Schlaikjer (1943) assumed, a priori, that they were dealing with a single species and that morphological variation within this taxon was due to differences in individuals, ontogenetic variation and/or gender. In a subsequent and important paper, Sternberg (1945) established the family Pachycephalosauridae, reviewed all previously named taxa, and correctly recognized that Troodon is a theropod dinosaur. Sternberg (1945) also resurrected Stegoceras as a valid genus and named a new species, S. lambei. Russell (1948) reaffirmed Sternberg’s assessment that Troodon is indeed a theropod dinosaur. Galton (1971) named the first putative pachycephalosaur from outside North America. Dubbed Yaverlandia bitholus, it is an enigmatic partial ‘‘skull cap’’ from the Lower Cretaceous (Barremian) Wealden Marls of the Isle of Wight. This taxon lacks shared-derived features in common with pachycephalosaurs. Specifically, Yaverlandia is characterized by: (1) individually domed frontals; (2) frontals that do not participate with the edge of the orbits; and (3) putative supratemporal fenestrae that are associated with the posterior edge of the frontals. Therefore, it has been removed from the Pachycephalosauria (Sullivan, 2000a). The first putative pachycephalosaur from Asia, was described by Bohlin (1953). The specimen consisted of a fragment of the cranium, which Bolin dubbed ‘‘Troodon’’ (Stegoceras) bexelli. The whereabouts of the holotype is unknown, and it is presumed missing. Based on geographic parsimony, it is likely referable to Prenocephale, or some other Asian taxon, rather than to Stegoceras (sensu stricto). However, because of its incomplete nature, the taxon remains a nomen dubium as stated by Galton and Sues (1983).

SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS Twenty years later the first unequivocal, pachycephalosaurs from Asia, were described (i.e., Homalocephale calathocercos, Tylocephale gilmorei and Prenocephale prenes) by Maryan´ska and Osmo´lska (1974). Later, additional taxa were added to the Asian pachycephalosaurs with the descriptions of Wannanosaurus yansiensis (Hou, 1977) and Goyocephale lattimorei (Perle et al., 1982). Dong (1978) named Micropachycephalosaurus hontuyanensis, based on a fragmentary specimen (dentary fragment and incomplete postcrania), that Sereno (2000) considered a nomen dubium because the taxon lacks any pachycephalosaur characters, or autapomorphies, an assessment that is maintained here. Wall and Galton (1979) established the species Stegoceras browni that was later placed in a new genus Ornatotholus (Galton and Sues, 1983). In the same paper, Wall and Galton established a new genus and species, Gravitholus albertae, based on a strange frontoparietal dome from the Oldman Formation of Alberta. The taxon Majungatholus atopus was described and named by Sues and Taquet (1979), based on rather incomplete skull material from the Late Cretaceous of Madagascar, and placed in the Pachycephalosauria. The recovery of additional, and more complete, specimens have recently revealed that Majungatholus is an abelisaurid theropod (Sampson et al., 1998). Galton and Sues (1983) named the new taxon Stygimoloch spinifer based on a cluster of hypertrophied nodes attached to a left squamosal, from the Upper Cretaceous Lance Formation of Montana. Later, Giffin et al. (1987) named another taxon from the Hell Creek Formation, Stenotholus kohleri, which was subsequently determined to be a junior synonym of Stygimoloch spinifer by Gabriel and Berghaus (1988), but, contrary to Goodwin et al. (1998), this has not been formally synonymized. The abstract of Gabriel and Berghaus (1988), cited by Goodwin et al. (1998), does not constitute a formal synonymy as clearly stated in Article 9.9 of the International Code of Zoological Nomenclature (1999). Therefore, Stenotholus kohleri is technically valid. Goodwin (1990) did equate the two taxa and suggested that S. kohleri is a subjective junior synonym of Stygimoloch spinifer. Following Gabriel and Berghaus (1988), and Goodwin (1990), I formally synonymize this taxon below. Galton and Sues (1983) synonymized the species Pachycephalosaurus grangeri and P. reinheimeri with P. wyomingensis, based on the presence of non-hypertrophied, nodosed spines adorning the posterior region of the cranium. In addition they symonymized Tylosteus (Leidy, 1872) with Pachycephalosaurus. Williamson (1999) and Williamson and Sealey (1999) reported on and briefly described an incomplete skull (NMMNH P-27403) from the Kirtland Formation of New Mexico that they interpreted as being close to the Asian genus Prenocephale. In a subsequent abstract Williamson and Carr (2001) dismissed the occurrence of Prenocephale in North America and recently they established a new genus Sphaerotholus and two species S. goodwini and S. buchholtzae (Williamson and Carr, 2003). For reasons given below, Sphaerotholus goodwini is a subjective junior synonym of Prenocephale and the species S. buchholtzae is a subjective junior synonym of P. edmontonensis. Sullivan (2000a) concluded that some specimens from North America share features in common with the Asian taxon P. prenes and that these features preclude reference to other (North American) pachycephalosaurs. He transferred the taxa ‘‘Stegoceras’’ breve and ‘‘Troodon (5Stegoceras)’’ edmontonense to the Asian genus Prenocephale, based on retention of discrete nodes (aligned in a row) on the squamosal, pronounced doming of the frontoparietal, with peripheral elements incorporated into the dome, and lack of parietosquamosal shelf. These two species are distinguished from one each other based on the possession of paired nodes positioned on the down-turned portion

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of the parietal, and are known solely from late Campanian and Maastrichtian strata, respectively. In addition, Sullivan (2000a) noted that morphologies expressed among various frontoparietals and corresponding adjacent peripheral elements, in specimens previously attributed to Stegoceras validum (sensu lato), remain consistent irrespective of size (ontogenetic stage). This observation suggested that there are a number of different genera and species within the Campanian and Maastrichtian pachycephalosaurs assigned to this taxon. Moreover, previous interpretations, such as the ability to recognize ontogenetic stage, sexual dimorphism (gender identification), along with developing some standard methodology to quantify the highly variable dome shape in small to medium-sized pachycephalosaurs, presented by earlier workers (notably Chapman et al., 1981; Goodwin, 1990), were abandoned. In the same paper, Sullivan (2000a) removed Yaverlandia bitholus from the Pachycephalosauria because it lacks synapomorphies that would allow inclusion in this group. Consequently, all known pachycephalosaurs, with the possible exception of the basal taxon Stenopelix, are of Late Cretaceous age. Sereno (2000) recently reviewed the pachycephalosaurs and their sister-group the ceratopsians. Although he focused mainly on the Asian taxa, Sereno (2000) did review North American pachycephalosaurs, including Stegoceras (sensu lato), and commented on the various shared-derived (cranial and postcranial) characters that demonstrate phylogenetic relationships among these taxa, some of which are reviewed and revised below. In addition, Sereno (2000) named the subfamily Pachycephalosaurinae for the fully-domed taxa. Not all studies of pachycephalosaur dinosaurs have concerned themselves with identifying characters for the purposes of taxon recognition. Chapman et al. (1981) used morphometric analyses in attempts to clarify relationships and proposed methods for determining gender identification based on a priori taxonomic assumptions. Goodwin (1990) followed later, attempting to standardize a landmark measuring system for the purposes of identifying specimens from the Judith River Formation of Montana. Early workers (Nopsca, 1929, 1931; Russell, 1932) briefly debated the relationships, nature of the material and likely life stance of Stegoceras, focusing on the skull and incomplete postcranial skeleton of UALVP-2. Nopsca (1929) originally believed the specimen to be a composite, having the skull of a nodosaur and the postcrania of an unnamed ornithopod, while Russell (1932) correctly defended its integrity as a single specimen. Other osteological studies have addressed the topics of paleoneurology (Hopson, 1979; Giffin, 1989a) and ontogeny (Chapman et al., 1981; Giffin, 1989b). Histological analysis of some pachycephalosaur domes has been the focus, in part, of investigations by Horner and Goodwin (1998) and Goodwin et al. (1998). Regarding pachycephalosaur behavior, Colbert (1955) was the first to proposed ‘‘head-butting’’ behavior in these domeheaded dinosaurs. Subsequent workers, notably Galton (1970, 1971), Sues (1978), Sues and Galton (1987) and Carpenter (1993), further supported this inferred behavior. Gabriel and Berghaus (1988) suggested agonistic behavior in Stygimoloch spinifer based largely on the presence of the enlarged nodes on the squamosals that formed display structures. More recently, Horner and Goodwin (1998) and Goodwin et al. (1998) supported the interpretation of pachycephalosaurs having agonistic behavior and refuted the hypothesis of ‘‘head-to-head butting’’ in the flat-headed pachycephalosaurs, Stegoceras validum, Stygimoloch spinifer and other ‘‘highly domed’’ pachycephalosaurs. Futhermore, Goodwin et al. (1998) supported the suggestion, previously offered by Gabriel and Berghaus (1988), that the hypertrophied nodes and orientation of the squamosal

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SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS in Stygimoloch spinifer functioned as display structures. Giffin (1989b) was the first to recognize that some taxa (Pachycephalosaurus wyomingensis and Prenocephale [5Stegoceras, in part]) brevis were co-eval and thus lived sympatrically. SYSTEMATIC PALEONTOLOGY DINOSAURIA Owen, 1842 ORNITHISCHIA Seeley, 1887 PACHYCEPHALOSAURIA Maryan´ska and Osmo´lska, 1974 PACHYCEPHALOSAURIDAE Sternberg, 1945 STEGOCERAS Lambe, 1902 Troodon Leidy, 1856 (in part), p. 72. Stegoceras Lambe, 1902, p. 68. Stegoceras Lambe, 1902; Hatcher, Marsh and Lull, 1907, p. 97. Stegoceras Lambe, 1902; Lambe, 1918, p. 23. Troodon (Leidy, 1856); Gilmore, 1924 (in part), p. 8. Troodon (Leidy, 1856); Brown and Schlaikjer, 1943 (in part), p. 126. Stegoceras Lambe, 1902; Sternberg, 1945 (in part), p. 536. Stegoceras Lambe, 1902; Wall and Galton, 1979 (in part), p. 1, 177. Ornatotholus Galton and Sues, 1983 p. 469. Stegoceras Lambe, 1902; Sues and Galton, 1987 (in part), p. 5. Type Species Stegoceras validum (Lambe, 1902); emend. Sues and Galton, 1987. Diagnosis Same as for species. Remarks Stegoceras (sensu stricto) is herein considered a monotypic taxon comprised of a single species S. validum. The monotypic genus Ornatotholus is recognized as a junior subjective synonym (see below). STEGOCERAS VALIDUM (Lambe, 1902); emend. Sues and Galton, 1987, p. 5. (Figs. 1A–F, 2, 4G–I, 6E–G) Stegoceras validus Lambe, 1902, p. 68, pl. 21, figs. 1–2. Stegoceras validus Lambe, 1902; Hatcher, Marsh and Lull, 1907 (in part), p. 98, pl. 2, figs. 1–2. Stegoceras validus Lambe, 1902; Lambe, 1918 (in part), p. 35, pls. 1–2. Troodon validus (Lambe, 1902); Gilmore, 1924, p. 11, pls. 1– 6, pl. 8, figs. 3–4; pls. 9–15. Troodon validus (Lambe, 1902); Gilmore, 1931, pl. 1, fig. 2. Troodon validus (Lambe, 1902); Brown and Schlaikjer, 1943 (in part), p. 128, pl. 33, pl. 34, figs. 1–6, pl. 37, figs. 4– 5, pls. 43–44. Stegoceras validus Lambe, 1902; Sternberg, 1945 (in part), p. 536. Stegoceras validus Lambe, 1902; Galton, 1971 (in part), textfigs. 4B, 5–6. Stegoceras validus Lambe, 1902; Wall and Galton, 1979 (in part), p. 1,177, fig. 1J–L, P–R, fig. 2I. Stegoceras browni Wall and Galton, 1979, p. 1,178, figs. 3G, 4D. Ornatotholus browni (Wall and Galton, 1979); Galton and Sues, 1983, p. 469, fig. 1A–J. Stegoceras validum (Lambe, 1902); emend. Sues and Galton, 1987, p. 5, text-figs. 1–14, p. 32, pl. figs. 1–3; pl. 3; pl. 4, fig. 4; pl. 8, figs. 1–4, 11–12.

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Ornatotholus browni (Wall and Galton, 1979); Sues and Galton, 1987, p. 28. Stegoceras validus Lambe, 1902; Sereno, 2000 (in part), fig. 25.3. Revised Diagnosis Differs from all other pachycephalosaurs in having a pronounced parietosquamosal shelf with an incipient frontoparietal dome; nasals inflated; postorbital situated posterolaterally on the dome; ornamentation consisting of numerous minute tubercles on lateral and posterior sides of squamosals with a prominent dorsal row of up to six tubercles on each squamosal and as many as two nodes on the medial posterior projection of the parietal. Lectotype CMN (formerly NMC and previously GSC) 515, nearly complete frontoparietal (Fig. 1a, b). Type Locality East side of the Red Deer River below the mouth of Berry Creek, Alberta, Canada. Horizon Dinosaur Park Formation (5‘‘Belly River beds/ Formation,’’ of earlier workers, David Eberth, pers. comm., 2000, see below). Referred Material See Appendix 1. Remarks Our understanding and characterization of Stegoceras is largely based on the famous specimen (UALVP-2, previously UA-2), which consists of a nearly complete skull, both lower jaws and parts of the postcranial skeleton. This specimen was previously described, in detail, by Gilmore (1924) and more recently by Sues and Galton (1987). The lectotype frontoparietal (CMN 515) conforms readily to UALVP-2, therefore leaving no doubt that UALVP-2 is the same taxon. Unfortunately, numerous other small to medium-size pachycephalosaur specimens, that lack distinctive features of Stegoceras validum (sensu stricto), have been placed in this taxon. Stegoceras validum, as defined here, can usually be distinguished by the following cranial characters: (1) prominent parietosquamosal shelf with open supratemporal fossae; (2) incipient doming of frontoparietal; and (3) minute nodes in clusters on postorbitals and squamosals. Comparison of UALVP-2 to the holotype of Prenocephale prenes (PAL MgD 1/40) shows that the postorbital is more posterior in position (on the dome), the orbital peripheral elements are low on the dome, there is a shorter diastema between the premaxillary and maxillary (as noted by Sues and Galton [1987]), and the nasals are highly inflated in UALVP-2. Unfortunately, most specimens of Stegoceras validum do not preserve peripheral, nasal, maxillary and premaxillary elements, and in some instances there are exceptions to the primary characterizations mentioned above. Of all characters cited by Sues and Galton (1987) in their diagnosis of Stegoceras validum, only two are here considered valid (prominent parietosquamosal shelf and postorbital without tubercles). The others cranial characters are variable and occur in other pachycephalosaur genera. The prominent parietosquamosal shelf is preserved in a number of specimens, including CMN 138, CMN 8816, TMP 84.5.1, TMP 99.62.1 and UALVP2. The supratemporal fossae in Stegoceras validum vary in size. In addition, the regions surrounding these openings are often subject to damage due their fragile nature (see Fig. 2D–G). In a few instances, the supratemporal fossae are completely fused closed, forming enclosed infratemporal (temporal) fossae, an example best seen in an incomplete skull (TMP 99.62.1) from the Dinosaur Park Formation (Fig. 2A, B). Here the parietosquamosal shelf is at least 1 cm thick with no hint of there ever being a supratemporal fossa. Viewed laterally, the sutures of

← FIGURE 1. Stegoceras validum. CMN 515, lectotype, frontoparietal: A, dorsal view and B, ventral view; TMP 95.12.147, parietal: C, dorsal view and D, ventral view; TMP 78.19.4, isolated right frontal: E, ventral view and F, dorsal view. Pachycephalosaurinae, indet. TMP 72.21.1 (holotype of Gravitholus albertae, see text) incomplete frontoparietal dome: G, dorsal view; H, ventral view; I, right lateral view; and J, posterior view. Bar scale equals 1 cm.

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FIGURE 2. Stegoceras validum. TMP 99.62.1, incomplete frontoparietal with left squamosal: A, dorsal view; B, ventral view and C, right lateral view (anterior is up). CMN 8816, incomplete frontoparietal with parts of both squamosals and right peripheral elements: D, dorsal view and E, ventral view. TMP 84.5.1, incomplete frontoparietal with parts of both squamosals and left peripheral elements: F, dorsal view and G, ventral view. CMN 138, incomplete frontoparietal with parts of both squamosals and right peripheral elements: H, dorsal view and I, ventral view. Bar scale equals 1 cm.

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TABLE 1. The evolution of nomenclature for peripheral skull elements by various authors. The nomenclature proposed by Sereno (2000) is adopted herein (see text).

right parietosquamosal and anterior peripheral elements are exposed. The infratemporal fossa is completely enclosed, or roofed-over, by the parietal medially and postorbital, laterally. The left supratemporal is also closed in CMN 8816 (see Fig. 2D) but may be present on the right side. However, its presence cannot be confirmed with certainty because this area is damaged. Specimens previously referred by Galton and Sues (1983) to the monotypic taxon Ornatotholus browni have nearly flat parietals that are identical to the parietals of many specimens referable to Stegoceras validum. Flat frontals, such as TMP 78.19.4 (Fig. 1E, F) have also been attributed to O. browni (Galton and Sues, 1983). Although there is some variability in these isolated elements, in general, they conform to the holotype (AMNH 5450). The distinctive node-like surface texture, seen in TMP 78.19.4 (Fig. 1F), is unworn and is interpreted as belonging to a subadult. The fact that the frontals are not fused to each other, and in turn, not fused to the parietal, further supports this interpretation. Moreover, the prominent node-like surface texture further suggests immature individuals, as this unabraded surface texture is known to occur only on small, to medium-sized individuals. Larger specimens of Stegoceras validum tend to have incipiently inflated frontals. The surface expression of individual nodes is interpreted by me to be related to the internal prismatic columnar structure, first described by Gilmore (1924) for specimens he referred to Stegoceras (sensu lato). These columnar structures may possibly be the Sharpeys fibers of the frontoparietal bone, as noted by Horner and Goodwin (1998). Evidence for this interpretation is clearly seen in TMP 2000.26.01, where each columnar structure is capped by single node. These columnar bundles have an irregularly polygonal cross-section, as do their corresponding capping nodes. Therefore, the small surface nodes, or tubercles, reflect the internal columnar structure of the dome and skull. The distinctive pitting, seen in many pachycephalosaur frontoparietals, is formed by spaces between adjacent nodes and the upper part of their respective columnar bundles and probably the exit for the longitudinal canals described by Goodwin et al. (1998). These spaces usually are situated at the corners of the polygonal nodes and were first interpreted by Gilmore (1924) as evidence of vascularization, a conclusion accepted by Goodwin et al. (1998). Some isolated parietals, such as TMP 95.12.147, are completely flat, while other (larger) isolated parietals, such as CMN 1594, are stouter and are characterized by an upwelling of the anterior portion of the parietal, suggesting they are from more mature individuals. It is interesting to note that in a few incipiently domed specimens of this species, the frontoparietal suture is visible in dorsal aspect. One relatively large specimen (CMN 1108), compared to other very small (presumably from very immature individuals) isolated parietals of unknown affinities

(CMN 8829 and CMN 12351), is distinct, separated at the frontoparietal surface. The frontoparietal suture is discernable in the lectotype (CMN 515) and TMP 99.62.1 and compares readily to an isolated parietal (CNM 1108). The anterior edge of the parietal is distinguished by a relatively pronounced, anteriorly directed, medial prong. Parenthetically, Gilmore (1924) believed the lectotype (CMN 515) represents a ‘‘nearly mature individual.’’ I agree that the specimen represents a subadult based on the size and development of the frontoparietal dome. The suggestion that Ornatotholus browni represents an earlier growth stage of Stegoceras validum (sensu stricto), first proposed by Goodwin et al. (1998), is supported and accepted here. The suggestion that it is a nomen dubium (Williamson and Carr, 2003) is rejected based on the observations just presented. Therefore, I place the former taxon in synonymy with the latter. Goodwin (1990) identified two specimens (UCMP 130048 and 130050) as Stegoceras validum. These specimens lack the diagnostic characters that would allow reference to this taxon. Moreover, the frontoparietal domes differ from one another in that UCMP 130048 has a distinctive down-turned parietal as in Colepiocephale lambei (see below), whereas UCMP 130050 is characterized by a frontoparietal dome that is very high, nearly symmetrical (anterior to posterior), with sutures for lateral peripheral elements that are positioned low on the dome. It is further distinguished by a well-developed medial (nasal) lobe. This specimen is very similar to another (TMP 97.99.2) that shares these same features. However, I am not able assign it to any presently recognized genus and species. In summary, morphotypes of Stegoceras validum run the spectrum of frontoparietals being flat (as in TMP 78.19.4), to intermediate (as in ROM 803), to more incipiently-domed forms (as in UALVP-2). The frontoparietal dome is never fullyinflated (as in other genera), and there is always a distinct lateral shelf on both sides of the skull made of the peripheral elements (postorbital and posterior supraorbital). Therefore, morphologic variation in frontoparietal dome development appears to be, in part, ontogenetic. In general, ornamentation consists of minute tubercles on the posterior and posterolateral sides of the squamosals. In addition, some specimens, notably TMP 99.62.1 (Fig. 2), have a well-developed row of larger dorsal nodes on the posterior-most part of the parietal and the (left) squamosal (see Fig. 2A). These large nodes have their apices directed upwards. Numerous, randomly positioned, minute nodes lie below this prominent row of large nodes. CMN 38428, also has well-developed nodes on the posterior margin of the squamosals. Another specimen, TMP 2000.26.01 (Fig. 6E–G), recently collected from the Oldman Formation, is especially noteworthy, as it preserves the anterior portion of the left postorbital and the greater posterior part of the left posterior supraorbital (Fig. 6E) and conforms to UALVP-2. The

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SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS skull is incomplete, with a large part of the parietal damaged on the left side. The posterior squamosal margin, as with most other specimens of this species, is missing. The left postorbital, and left posterior supraorbital, are low on the frontoparietal and are ornamented with irregular, incipient nodes, which have apices that extend dorsolateral to the dome. The trace of their sutures is clearly discernable both laterally and dorsally, so these elements are not fully incorporated into the dome. The peripheral elements and their respective sutures have been the focus of much attention. Lambe (1918), Gilmore (1924), Maryan´ska and Osmo´lska (1974), Sues and Galton (1987), and most recently, Sereno (2000), have all interpreted the osteology differently (Table 1). Isolated frontoparietals, and more complete specimens, notably CMN138 (Fig. 2H, I), CMN 8816 (Fig. 2D, E), and TMP 84.5.1 (Fig. 2F, G), allow for the discrimination, and to a lesser extent, identification of the peripheral elements. The greatest contention concerns the existence of the ‘‘SO II’’/‘‘palpebral II’’ of Maryan´ska and Osmo´lska (1974) and Sues and Galton (1987), respectively. The primitive condition among dinosaurs is one where peripheral elements remain distinct from adjacent elements. Maryan´ska and Osmo´lska (1974) reported that the anterior supraorbital (SO I) and prefrontal fused to form a single element (SO I 1 prefrontal) in Tylocephale gilmorei. They also believed these elements to be fused in Stegoceras validum based on UALVP-2. Sues and Galton (1987) corroborated this observation and commented that their fusion resulted in a single large element that formed much of the (anterior) dorsal margin of the orbit. Gilmore (1924) interpreted this same large element as the prefrontal 1 supraorbital. Sereno (2000) re-interpreted this region, and recognized, from anterior to posterior, a prefrontal, anterior supraorbital (5SO I) and posterior supraorbital and postorbital. Moreover, Sereno (2000) did not recognize the existence of a fused prefrontal 1 SO I in Tylocephale gilmorei nor a ‘‘supraorbital II’’ or ‘‘palpebral 2’’ wedged ventrally between the ‘‘prefrontal 1 palpebral 1’’ and postorbital for Stegoceras validum as illustrated and interpreted by Maryan´ska and Osmo´lska (1974) and Sues and Galton (1987). Lateral sutural surfaces of the peripheral elements on frontoparietals of Stegoceras validum, and most other pachycephalosaurs, support the presence of distinct elements as summarized by Sereno (2000). Moreover, I cannot confirm the presence of a ‘‘supraorbital II’’ or ‘‘palpebral 2’’ in any of the material of Stegoceras validum. Futhermore, the posterior supraorbital of Sereno (2000) is not equivalent to the ‘‘SO II,’’ so reference to this latter bone should be avoided. Therefore, I adopt the terminology for the peripheral elements of Sereno (2000). Sereno (2000) suggested that Stegoceras validum ‘‘bridges a morphological gap between flat-headed forms and those with a fully developed dome.’’ In general, this statement is true. However, it does not necessarily suggest, or demonstrate, that there is a linear progression from flat-headed, to incipiently domed, to fully-domed forms. The lectotype of Stegoceras validum comes from a place along the Red Deer River where most of the fossiliferous exposures are in the Dinosaur Park Formation (formerly the ‘‘Belly Rivers beds,’’ in part), although there is a remote possibility that the specimen came from the Oldman Formation (sensu Eberth and Hamblin, 1993) according to Eberth (pers. comm., 2000). Most of the specimens are from the Dinosaur Park Formation, while others are from the Oldman Formation. One im-

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portant specimen (CMN 38428) of Stegoceras validum, allegedly from the Bearpaw Shale, is actually from the Dinosaur Park Formation (Eberth, pers. comm., 2001). None are known from the Foremost Formation, the oldest unit of the Judith River Group (Eberth and Hamblin, 1993). PACHYCEPHALOSAURINAE Sereno, 2000 Definition Colepiocephale, Prenocephale, Tylocephale, Hanssuesia, Pachycephalosaurini (5Pachycephalosaurus 1 Stygimoloch), their most recent common ancestor and all descendants (Sereno, 2000, in part). Remarks Sereno (2000) defined the Pachycephalosauridae to include Stegoceras validum (sensu lato) and Pachycephalosaurus wyomingensis, their most recent common ancestor and all descendants as a monophyletic group. Fully-domed forms were placed in the Pachycephalosaurinae (Sereno, 2000), a name change for taxa originally included within the ‘‘Domocephalinae’’ of Sereno (1986). This rank now includes the additional taxa noted below. COLEPIOCEPHALE, gen. nov. Stegoceras Lambe, 1902; Sternberg, 1945, p. 537; Sues and Galton, 1987 (in part), p. 32; Baszio, 1997, p. 129; Williamson and Carr, 2003, p. 794. Type Species Stegoceras lambei Sternberg, 1945, p. 537. Etymology From the Latin ‘‘colepium,’’ meaning knuckle (of beef or pork) used here in reference to the curled ‘‘knuckle’’-like appearance (in lateral view) of the posterior part of the frontoparietal dome; and ‘‘cephale’’ from the Greek meaning ‘‘head.’’ Diagnosis Same as for species. Remarks The distinctive, posteriorly-directed down-turned parietal in lateral view, which resembles a flexed knee or knuckle, departs from all other pachycephalosaurs. The generic name means ‘‘knuckle head.’’ COLEPIOCEPHALE LAMBEI (Sternberg, 1945) (Fig. 3) Stegoceras lambei Sternberg, 1945, p. 537, pl. 70, figs. 4–5. Stegoceras validum Sues and Galton, 1987, p. 32 (in part), pl. 2, figs. 5–6. Stegoceras sp. Baszio, 1997, p. 129, fig. 2. Stegoceras lambei Sternberg, 1945; Williamson and Carr, 2003, p. 794, fig. 10. Revised Diagnosis Nearly fully-domed pachycephalosaur; differs from all other pachycephalosaurs in: having no lateral (posterior supraorbital and postorbital) and no posterior squamosal shelf; squamosals restricted to lateral sides and much reduced, with only a hint of their margins lying ventrally under the parietal; parietal strongly down-turned, forming a steeply inclined border across entire posterior breadth of skull; occiput reduced; frontoparietal mass thick and broad posteriorly, medial frontal lobe high and relatively narrow; lateral frontal lobes recessed posteriorly; frontoparietal dome with pronounced constriction at contact of postorbital and posterior supraorbital (viewed dorsally); peripheal elements (postorbital, posterior supraorbital, anterior supraorbital and prefrontal) reduced and sit-

← FIGURE 3. Colepiocephale lambei (Sternberg), gen. nov. TMP 92.88.1, nearly complete frontoparietal dome: A, dorsal view; B, ventral view; C, right lateral view and D, posterior view. CMN 8818 (holotype of Stegoceras lambei), nearly complete frontoparietal dome: E, dorsal view; F, ventral view and G, right lateral view. ROM 3632, nearly complete frontoparietal dome: H, dorsal view; I ventral view; J, right lateral view and K, posterior view. Bar scale equals 1 cm.

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SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS uated low along the frontoparietal; differs further from Stegoceras validum in lacking supratemporal fossae; differs further from Prenocephale edmontonense in having incipient anterior supraorbitals, which are not incorporated into the anterolateral (frontal) part of the dome. Holotype CMN 8818 (Fig. 3E–G), nearly complete frontoparietal dome with peripheral skull elements. Referred Material CMN 29419, nearly complete frontoparietal dome; CMN 29420, skull fragment; CMN 29421, incomplete frontoparietal; ROM 3632 (Fig. 3H–K), nearly complete frontoparietal dome; TMP 70.2.1, nearly complete frontal parietal dome; TMP 86.146.1, incomplete frontoparietal dome with fused prefrontals; TMP 86.146.2, incomplete frontoparietal dome with complete right prefrontal; TMP 92.88.1 (Fig. 3A– D), nearly complete frontoparietal dome with fused prefrontals; TMP 97.99.2, nearly complete frontoparietal dome with prefrontals; TMP 97.99.3, incomplete frontoparietal dome; TMP 2000.57.1, nearly complete frontoparietal; UALVP-349, incomplete frontoparietal; and UALVP-31471, nearly complete frontoparietal. Type Locality South Saskatchewan River, 4.8 km (3 miles) below Bow Island ferry, Alberta. Horizon Foremost Formation (formerly ‘‘Oldman’’ Formation, in part) (Eberth, pers. commun., 2000). Remarks Sternberg (1945) correctly recognized the distinct nature of this species, and most of the characters that he noted are still valid. The nasal boss of the frontal is well-developed and is flanked on the right and left by incipient anterior supraorbitals. The anterior slope of the frontals is steep, but is the same as seen in Stegoceras validum based on UALVP-2. Colepiocephale lambei is a very distinctive species characterized by a strong down-turned parietal, squamosals nearly or fully absent, and the lateral (peripheral) elements consisting of a much reduced postorbital, posterior supraorbital, anterior supraorbital and prefrontal, as evidenced by their corresponding suture surfaces. In lateral profile there is a gap, or diastema, between the surfaces of the postorbital suture and the anteriorlying posterior supraorbital. Dorsally, the regions between these two elements appear to be constricted or pinched on each side of the skull. Moreover, the suture surfaces are positioned low on the frontoparietal dome. The prefrontal (which may also include lateral sides of the nasals) lateral lobes of the frontal are strongly recessed, with the medial nasal lobe reaching far forward (as viewed dorsally). The medial nasal node is also relatively narrow and high compared to other taxa such as Stegoceras validum. Of particular interest is the nature of the parietal and squamosals. One specimen, (TMP 92.88.1) in particular, is very well preserved and allows discrimination of these elements, in part. In ventral view (Fig. 3B) the parietal wraps around the posterior margin of the skull. Close inspection of the right side reveals what appear to be the remnants of two vestigial nodes. These are somewhat flattened, spade-shaped structures reminiscent of the welldeveloped squamosal nodes seen in some specimens of Stegoceras validum and Prenocephale edmontonensis (Sullivan, 2000a). If this interpretation is correct, one must infer the presence of a much reduced squamosal (on each side of the skull). In the holotype (CMN 8818, Fig. 3F), there is no evidence of the squamosals and any associated vestigial ornamentation, suggesting that it may be lost in larger, more mature individuals. Baszio (1997) briefly, and inadequately, described three spec-

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imens of this taxon and was unable to refer them to this species. Moreover, he did not have the holotype of C. lambei at his disposal to compare the Foremost Formation specimens, a fact which I believe would have made reference to this species certain. Understandably, Baszio accepted the work of Sues and Galton (1987) and thus considered it to be Stegoceras validum (sensu lato). Also, Baszio (1997) attempted to identify these specimens using the morphometric procedures of Chapman et al. (1981) and Goodwin (1990) and concluded that these specimens form ‘‘the morphometric link between the Monogolian [presumably Prenocephale] and North American Pachycephalosauridae’’ (see below). He cited three features that he believed link the Foremost Formation specimens to the Asian taxon Prenocephale prenes: (1) ‘‘skull bones proximal to the frontoparietal seem to be involved in the formation of the dome’’; (2) ‘‘lateral and squamosal shelves cannot be expected to be as distinctive as in Stegoceras validum’’; and (3) ‘‘supratemporal fossae absent.’’ The first feature Baszio (1997) cites refers to the peripheral bones of the skull (i.e., postorbitals, posterior supraorbitals, anterior supraorbitals, prefrontals) that become incorporated with the dome in derived forms. However, close examination shows this is not the case in Colepiocephale lambei. The peripheral bones are distinct and are missing in all specimens of this species based on the existence of distinct suture surfaces exposed low along the lateral edges of the frontoparietal dome. Moreover, the posterior parietal portion in wrapped downward where it meets the superior posterior edges of the paired temporal regions (as noted in the diagnosis above). The second character pertains to the parietosquamosal shelf that is not present in many taxa, not just Prenocephale. Finally, the last is a plesiomorphic character and cannot be used in establishing relationships. It is interesting to note that all the specimens of this species, including the holotype (CMN 8818), are from the Foremost Formation. No other pachycephalosaur species are known from this stratigraphic interval based on my comprehensive study of the specimens in the collections of the Canadian Museum of Nature, Royal Ontario Museum, Royal Tyrrell Museum of Palaeontology, and the University of Alberta Laboratory of Vertebrate Paleontology. I have verified all the stratigraphic occurrences of these specimens with David Eberth (pers. comm., 2000). Based on the distinct morphology, and restricted stratigraphic occurrence, it is apparent that this taxon has some biostratigraphic utility. Baszio (1997) stated that the specimens from the Foremost Formation are the stratigraphically oldest in North America known to date. However, an indeterminate pachycephalosaur (ROM 2962) described below, first noted by Russell (1964) from the upper part of the Milk River Formation, has the distinction of being the oldest known North American pachycephalosaur. However, Colepiocephale lambei is presently the oldest diagnostic pachycephalosaur from North America. The unique morphology of Colepiocephale lambei (i.e., loss of squamosals and much reduced lateral elements) suggests a derived condition, one that antedates the appearance of more primitive taxa such as Stegoceras, Homalocephale and arguably, Prenocephale and related taxa (see below). Many of the specimens of this species are known from small individuals (such as TMP 92.881, Fig. 3A–D), with the holotype (CMN 8818, Fig. 3E–G) being representative of the largest example.

← FIGURE 4. Hanssuesia sternbergi (Brown and Schlaikjer), gen. nov. CMN 8817 (holotype of Troodon sternbergi), nearly complete frontoparietal dome: A, dorsal view; B, ventral view and C, right lateral view. CMN 192, incomplete frontoparietal dome: D, dorsal view; E, ventral view and F, right lateral view. Stegoceras validum CMN 2369, incomplete frontoparietal dome: G, dorsal view and H, ventral view. CMN 1108, isolated parietal: I, dorsal view. Bar scale equals 1 cm.

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SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS cf. Colepiocephale lambei Referred Material CMN 9952, incomplete frontoparietal; CMN 29420, incomplete parietal? fragment; CMN 29421, left side (medial section) of frontoparietal with three fragments, postorbital and prefrontal; TMP 80.20.1, incomplete parietal; TMP 86.146.3, posterior part of frontoparietal dome; TMP 86.146.4, fragmentary frontoparietal dome; TMP 86.146.5, fragmentary right side of frontoparietal dome; TMP 86.146.6, posterior medial fragment with down-turned part of parietal (ventral-supraoccipital); TMP 87.7.7, anterior part of left frontal; TMP 97.99.2, incomplete frontoparietal; TMP 99.31.1,?left side of frontoparietal; and TMP 99.31.2, fragmentary frontoparietal. Remarks Most of these specimens are too fragmentary to identify precisely. However, they most readily conform to C. lambei and therefore are included here. All are from the Foremost Formation. HANSSUESSIA, gen. nov. Troodon Leidy, 1856; Brown and Schlaikjer, 1943 (in part), p. 131. Stegoceras Lambe, 1902; Sternberg, 1945 (in part), p. 537; Wall and Galton, 1979, fig. 1d–f; Sues and Galton, 1979 (in part), p. 32. Gravitholus Wall and Galton, 1979; Sullivan, 2000b (in part), p. 72A. Type Species Troodon sternbergi Brown and Schlaikjer, 1943, p. 131. Etymology Named in honor of Hans-Dieter Sues for his many contributions on dinosaurs, and in particular, for his work on pachycephalosaurs. Diagnosis Same as for species. Remarks The sole species of this taxon is not referable to Stegoceras, Colepiocephale, Prenocephale or any other known pachycephalosaur, thus necessitating a new genus for its reception. HANSSUESIA STERNBERGI (Brown and Schlaikjer, 1943) (Figs. 4A–F, 5) Troodon sternbergi Brown and Schlaikjer, 1943, p. 131, p. 36. Stegoceras sternbergi (Brown and Schlaikjer, 1943); Sternberg, 1945, p. 537. Stegoceras validus Lambe, 1902; Wall and Galton, 1979, fig. 1d–f. Stegoceras validum Sues and Galton, 1987 (in part), p. 32, pl. 2, figs. 1–4, figs. 7–8. Gravitholus sternbergi Sullivan, 2000b, p. 72A. Stegoceras sternbergi (Brown and Schlaikjer, 1943); Sternberg, 1945, p. 537. Stegoceras validum Sues and Galton, 1987, p. 32 (in part). Gravitholus sternbergi Sullivan, 2000b, p. 72A. Revised Diagnosis Differs from all other pachycephalosaurs in having a low, depressed parietal region of the frontoparietal dome, anterior (frontal) and posterior (parietal) portions of the frontoparietal wide; nasal boss of frontal broad with reduced, more inflated, lateral prefrontal lobes; and parietosquamosal shelf reduced.

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Holotype CMN 8817, nearly complete frontoparietal dome. Type Locality Southeast of Steveville, Alberta. Horizon Dinosaur Park Formation (5‘‘Belly River beds’’), although there is a remote possibility that the holotype is from the Oldman Formation (Eberth, pers. comm., 2001). Referred Material See Appendix 1. Remarks Brown and Schlaikjer (1943) were largely correct in their characterization and assessment of this taxon, and it is to their credit that the species is herein resurrected. However, until recently this taxon has been most difficult to characterize because it rested solely on characters of the frontoparietal. None of the frontoparietals, including the holotype, preserved the posterior medial part of the parietal intact with the exception of MOR 480 (see below), if properly referred. However, Goodwin (1990), in his paper concerning the morphometric landmarks of pachycephalosaurid crania, reported on an interesting specimen (UCMP 130051) from the Judith River Formation of Montana. This specimen preserves all the peripheral elements of the skull. In dorsal view, it conforms readily to Hanssuesia sternbergi in that the frontoparietal portion of the dome is wide both anteriorly and posteriorly. Moreover the slope of the parietal is consistent with some specimens of H. sternbergi, notably the holotype (CMN 8817). Based on examination of an unnumbered cast of UCMP 130051 in the collection of the Museum of the Rockies, I refer this specimen to Hanssuesia sternbergi. This referral is extremely significant because no other specimens of this taxon preserve the peripheral elements and their respective ornamentation. Moreover, the stratigraphic occurrence of this specimen is consistent with other specimens of this taxon from the upper Campanian of Alberta. Hanssuesia sternbergi is also distinguished, in part, by reduced sutural surfaces for the attachment of the left and right squamosals along the posterior edges of the frontoparietal dome. This condition differs from that seen in Colepiocephale lambei, where the parietal portion of the frontoparietal dome makes up the entire posterior edge of the skull, and the squamosals are restricted to the lateral sides. As previously mentioned, the squamosals are known only in one recently described specimen (UCMP 130051), which has a reduced parietosquamosal shelf (Goodwin, 1990). Here, the incipient parietoquamosal shelf differs from the well-developed shelf of S. validum and lacks evidence of supertemporal fenestrae. Previously, based solely on frontoparietal material, Brown and Schlaikjer (1943) speculated that the parietosquamosal shelf in this species was ‘‘not-prominent,’’ a fact that is born out by this specimen. Brown and Schlaikjer (1943) also noted that the ‘‘frontals [are] proportionally low and broad where in contact with the nasals’’ a condition similar to that in Prenocephale brevis. However, in the latter taxon, the anterior portion of the frontals has a ‘‘trilobate’’ appearance (Sullivan, 2000a) two lateral lobes, and one medial (nasal boss) lobe, that are near equal in prominence and anterior extent, whereas in Hanssuesia sternbergi, the two lateral lobes are not well developed and lie posterior (recessed in position) to the medial (nasal boss) lobe of the frontoparietal. The lateral (prefrontal) lobes are even more recessed and distinct in Colepiocephale lambei. The lateral profile of Hanssuesia sternbergi varies in that sometimes the peripheral elements attach high on the frontal

← FIGURE 5. Hanssuesia sternbergi (Brown and Schlaikjer), gen. nov. CMN 9148, nearly complete (water worn) frontoparietal dome: A, dorsal view; B, ventral view and C, right lateral view. TMP 79.14.853, nearly complete frontoparietal dome: D, dorsal view; E, ventral view and F, right lateral view. CMN 38079, nearly complete frontoparietal dome: G, dorsal view; H, ventral view and I, right lateral view. Bar scale equals 1 cm.

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SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS parietal (see Fig. 4F) compared to others in which the peripheral elements are sutured low relative to the apex of the dome (Fig. 4C). High lateral sutures are also seen in some other taxa, notably ROM 2962 (Fig. 7C) where the postorbital suture is positioned high on the sides of the frontoparietal dome. The significance of this variation is unclear. The frontoparietals of this species have been noted to be rather smooth at their dorsal-most extent (see Fig. 5A, D, G), and are rather heavily pitted along their margins (Brown and Schlaikjer, 1943). However, I interpret this pitting as an artifact of weathering of the dorsal surface of the dome rather than having any taxonomic significance. This pitting is also seen in specimens of Stegoceras validum (Figs. 2D, F, H, 4G, I; 7F), Colepiocephale lambei (Fig. 3A, H), Prenocephale brevis, and Prenocephale sp. (Fig. 6A). As noted previously, these pits have been interpreted by Gilmore (1924) as evidence for blood vessels, but they may be an artifact that is related to the internal structure and packing of the columnar bundles, as previously mentioned. Finally, I (Sullivan, 2000b) initially referred ‘‘S.’’ sternbergi to the genus Gravitholus, but because the former taxon differs significantly from the monotypic Gravitholus albertae in the anterior expression of the frontal portion of the frontoparietal dome, coupled with the fact that the holotype of G. albertae is not considered valid (see below) along with the points just discussed, I no longer consider this to be a correct assessment. cf. Hanssuesia sternbergi Remarks Two new additional specimens are provisionally referred to Hanssuesia sternbergi: MOR 453, a somewhat large frontoparietal dome (sectioned for histological analysis) and MOR 480, a nearly complete frontoparietal, both from the Two Medicine Formation, Glacier County, Montana. Both specimens are water worn and details of the ventral and lateral surfaces are obscured. They are referred to this taxon based on the characteristic posterior sloping of the parietal portion of the frontoparietal (see Appendix 2 for other material provisionally referred to this species). GRAVITHOLUS Wall and Galton, 1979 Gravitholus Wall and Galton, 1979, p. 1, 180. Gravitholus Wall and Galton, 1979; Sues and Galton, 1983, p. 28. Stegoceras Lambe, 1902; Williamson and Carr, 2003 (in part), p. 798. Type Species Gravitholus albertae Wall and Galton, 1979, p. 1,181. Remarks Since its original description, Gravitholus albertae has been problematic. The holotype, TMP (previously PMA) 72.27.1, and only known specimen (Fig. 1G–J), is from the Oldman Formation (recently verified by David Eberth, pers. comm., 2000). Of the hundreds of small to medium sized pachycephalosaur specimens from the Oldman and Dinosaur Park formations of Alberta, no other specimen has been referred to this taxon. Gravitholus albertae was diagnosed as a large pachycephalosaur (larger than Stegoceras (sensu lato), having a wide dome, parietal with a large depression and many smaller pits, small braincase relative to dome size, and lacking node ornamentation

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(Wall and Galton, 1979). Unfortunately, the diagnosis of this taxon is inadequate, and my recent examination of the holotype does not support its validity. First, size is irrelevant as a character, because there are an array of sizes among, what I would consider, medium-sized pachycephalosaurs, in which Stegoceras (sensu stricto), some species of Prenocephale, Colepiocephale and Hanssuesia also fall. Second, the large depression on the parietal is an artifact, as is the pitting; the latter is seen in other pachycephalosaur taxa and appears to be related to dorsal surface wear. The only possible defining character (small brain size relative to dome size) is ambiguous. The endocranial cavity is just as small in similar size specimens of Hanssuesia sternbergi. Gravitholus albertae is thus considered a nomen dubium. There are a few features worth noting in the holotype of Gravitholus albertae, but they are not enough to allow a precise taxonomic identification. The holotype is similar to that of Colepiocephale lambei in having incipiently developed postorbitals, posterior supraorbitals and anterior supraorbitals 1 prefrontals. However, it differs from the holotype of Colepiocephale lambei in retaining the squamosals posteriorly, based on the presence of distinct posteriorly positioned parietosquamosal sutures that are separated by the downward projecting posterior medial part of the parietal. There is some minor ornamentation on the posteriomedial part of the parietal. The rest of the specimen lacks any ornamentation and is relatively smooth. In addition, the holotype appears to have a broad medial nasal lobe of the frontal, similar to that of Hanssuesia sternbergi and, to a lesser extent, seen in Stegoceras validum. But, it differs from the latter taxon in that it clearly lacks a well-developed parietosquamosal shelf. Despite the recent characterization of the holotype by Williamson and Carr (2003), the specimen cannot be assigned to the monotypic taxon Stegoceras validum. Moreover, it also compares more favorably to Hanssuesia sternbergi in the apparent presence of squamosals positioned posteriorly, separated by a medial, posteriorly directed extension of the parietal. Unlike H. sternbergi, the anterior lateral portions of the frontals are not inflated. This condition results in the rather odd and distinct appearance of the peripheral skull elements. The holotype of Gravitholus albertae preserves the right postorbital and posterior supraorbital and possibly the anterior supraorbital and the posterior portion of the right prefrontal. Only the right and left postorbitals seem to be distinct, the right posterior and anterior surpraorbitals, along with the right prefrontal, appear to be indistinguishably fused. I believe this fusion to be an aberration, as I also interpret the non-inflated anterior part of the frontal parietal dome. If these features were lacking, reference to Hansuessia sternbergi would be certain. In her review of the pachycephalosaurs, Maryan´ska (1990) believed the holotype of Gravitholus albertae to be pathologic. My examination of the holotype, in part, supports this view. In addition to the aforementioned cranial features, the skull (viewed dorsally, Fig. 1G), seems to be slightly asymmetrical (left to right), but whether this is due to pathology it is not certain. Some of this distortion may be due to postmortem deformation. The skull is weathered and bears a few large cracks. In addition, the left posterior part (parietal) is marked by what appear to be borings of some sort. In summary, Gravitholus albertae displays all the principal characters seen in Hanssuesia sternbergi, but departs from that taxon in having the anterior part of the frontoparietal not in-

← FIGURE 6. Prenocephale sp., TMP 87.50.29, incomplete frontoparietal dome with large section of dorsal surface missing: A, dorsal view; B, ventral view; C, right lateral view and D, posterior view. Stegoceras validum, TMP 2000.26.01, incomplete frontoparietal with attached peripheral elements: E, left lateral view showing postorbital and posterior supraorbital (SO II); F, dorsal view and G, ventral view. Bar scale equals 1 cm.

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flated, resulting in the aberrant expression of the anterior peripheral elements (notably the posterior and anterior supraorbitals and prefrontal). Moreover, given that none of the preserved characters are especially diagnostic, coupled with the fact that the holotype is badly damaged (possibly distorted), I consider the taxon a nomen dubium. It is not possible to synonymize Gravitholus albertae with any of the better-known pachycephalosaurs due to the lack of common diagnostic features. PRENOCEPHALE Maryan´ska and Osmo´lska, 1974 Troodon Leidy, 1856; Brown and Schlaikjer, 1943 (in part), p. 132. Stegoceras Lambe, 1902 (in part), p. 68. Sphaerotholus Williamson and Carr, 2003, p. 779. Pachycephalosauridae incertae sedis Williamson and Carr, 2003 (in part), p. 798. Type Species Prenocephale prenes Maryan´ska and Osmo´lska, 1974, p. 53. Remarks Sullivan (2000a) recently placed the North American species ‘‘Stegoceras’’ breve and ‘‘S.’’ edmontonense into the Asian genus Prenocephale based on the possession of a strong down-turned medial projection of the parietal, which in turn is ornamented with a single node, straddling the left and right parietosquamosal suture, on each side. The presence of these paired nodes indicates the presence a linear row of nodes on the posterior edge of each squamosal, as exemplified by Prenocephale edmontonensis (TMP 87.113.3), illustrated previously by Giffin (1989b:fig. 5) and more recently Sullivan (2000a:fig. 2). The newly erected species Sphearotholus goodwini (Williamson and Carr, 2003) shares some of these features and is here considered to belong to the genus Prenocephale. The distinctive morphology of this taxon precludes reference to the genera Stegoceras, Colepiocephale and Hanssuesia. The Asian taxon Tylocephale gilmorei is very similar to its counterpart Prenocephale prenes in that they share a single ‘‘unique oval fossa on the quadratojugal’’ (Sereno, 2000). The fact that there are nearly identical in every other way suggests that these two may be synonymous. PRENOCEPHALE BREVIS (Lambe, 1918) Stegoceras brevis Lambe, 1918, p. 35. Troodon validus (Lambe, 1902); Brown and Schlaikjer, 1943, p. 128, pl. 35, figs. 1–6. Stegoceras brevis Lambe, 1918; Sternberg, 1945, p. 536. Stegoceras brevis Lambe, 1918, p. 35. Troodon validus (Lambe, 1902); Brown and Schlaikjer, 1943, p. 128, pl. 35, figs. 1–6. Stegoceras brevis Lambe, 1918; Sternberg, 1945, p. 536. Stegoceras breve (Lambe, 1918); emendation Sues and Galton, 1987, p. 32, pl. 1, figs. 4–13. Prenocephale brevis (Lambe, 1918); Sullivan, 2000a, p. 184, fig. 3B–I. Stegoceras breve (Lambe, 1918); Williamson and Carr, 2003, p. 797. Holotype CMN 1423, complete frontoparietal. Horizon Oldman Formation. Referred Material See Appendix 1. Remarks Sullivan (2000a) recently resurrected this species

and transferred it to the genus Prenocephale based on the presence of distinct nodes on the medial extension of the parietal, which in adult individuals, is strongly down-turned. All specimens of this species are among the smallest known pachycephalosaurs and may in their own right be considered ‘‘dwarfs.’’ However, I cannot confirm their relationship, if any, to the undescribed ‘‘North American dwarf taxon’’ cited by Sereno (2000). Moreover, I have not seen any additional specimens among the hundreds of cranial material that I have examined that would suggest the presence of yet another small taxon. Recently Williamson and Carr (2003) rejected the placement of this species in Prenocephale based on the ‘‘common position of grooves in the frontal above the orbit’’ that gives it the ‘‘trilobate’’ appearance along the anterior border of the frontal region, one of the diagnostic characters noted by Sullivan (2000a). This feature, couple with the distinctive, single medialmost (4th) nodes situated entirely on the parietal and anterior portion of the frontoparietal sloped, were characters used to diagnose this species. None of these small pachycephalosaurs have open supratemporal fenestrae, therefore they cannot be referred to Stegoceras (sensu stricto), and are consider mature individuals. Moreover, Williamson and Carr (2003) stated that Prenocephale brevis was the most basal member, yet interpreted them as subadults (of Stegoceras), an interpretation which is not only inconsistent, but is also indefensible in the light of the data presented by Sullivan (2000a) and here. cf. Prenocephale brevis Referred Material See Appendix 1. Remarks Most of the material has been reviewed by Sullivan (2000a) and need not be reevaluated here. However, there is one specimen (TMP 85.43.68) that is especially worth noting, as it consists of an isolated parietal that has a slender, medial, posteriorly projecting extension with two small, sharply pointed nodes, one on each edge where it articulates with the left and right squamosals. Viewed laterally, the infratemporal fossae are elongate and shallow. The upper margins are smooth, not rugose, suggesting a dorsal supratemporal slit. I believe that this parietal is from an immature individual of P. brevis. If this identification is correct, then it provides evidence for an ontogenetic change in the morphology of the parietal from immature to adult. This isolated parietal is distinctly different from those of Stegoceras validum and Hanssuesia sternbergi, and differs in a number of features including, size, shape, and sculpturing. PRENOCEPHALE EDMONTONENSIS (Brown and Schlaikjer, 1943) (Fig. 7E–G) Troodon edmontonensis Brown and Schlaikjer, 1943, p. 132, pl. 37, 1–3. Stegoceras edmontonensis (Brown and Schlaikjer, 1943); Sternberg, 1945, p. 537. Stegoceras edmontonesis (lapsus calami) (Brown and Schlaikjer, 1943); Wall and Galton, 1979, p. 1,178, fig. 3a–b. Stegoceras edmontonesis (lapsus calami) (Brown and Schlaikjer, 1943); Sues and Galton 1983, p. 470. Stegoceras edmontonenis (lapsus calami) (Brown and Schlaikjer, 1943); Sternberg, 1945, p. 535 [sic]; Sullivan, 2000a, p. 178. →

FIGURE 7. Pachycephalosaurinae Indet. ROM 2962, incomplete frontoparietal from the Upper Milk River Formation, Dead Horse Coulee, Alberta. A, dorsal view; B, ventral view; C, right lateral view; D, left lateral view. Prenocephale edmontonensis, CMN 8830 (holotype), frontoparietal dome. E, right lateral view; F, dorsal view; and G, ventral view. Bar scale equals 1 cm.


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Stegoceras edmontonense (lapsus calami) (Brown and Schlaikjer, 1943); Giffin, 1989, p. 528, fig. 5. Stegoceras edmontonense (lapsus calami) (Brown and Schlaikjer, 1943); Sues and Galton, 1987, p. 32, pl. 6, figs. 1–4. Prenocephale edmontonensis (Brown and Schlaikjer, 1943); Sullivan, 2000a, p. 178, figs. 1–2, fig. 3A. Sphaerotholus buchholtzae Williamson and Carr, 2003, p. 791. cf. Sphaerotholus sp.; Williamson and Carr, 2003, p. 792. Pachycephalosauridae incertae sedis (in part); Williamson and Carr, 2003, p. 798. Holotype CMN (formerly GSC [Geological Society of Canada] and NMC [National Museum of Canada]). Horizon Horseshoe Canyon Formation (Edmonton Group). Referred Material See Appendix 1. Remarks Specimens of this species were recently re-evaluated by Sullivan (2000a), who placed them in the genus Prenocephale based on the occurrences of three nodes on the squamosals, with a fourth lapping onto the medial extension of the strongly down-turned parietal. The node arrangement is most similar to P. prenes, rather than Stegoceras validum (sensu stricto), or any other pachycephalosaur genus, and it is the reason for inclusion in Prenocephale (see Sullivan, 2000a, for detailed discussion). Williamson and Carr (2003) believed that the holotype was not diagnostic and considered it a nomen dubium. This topological assessment belies the fact that TMP 87.113.3 (holotype of Sphaerotholus buchholtzae), and the other referred material cited and discussed by Sullivan (2002), is clearly the same taxon as the holotype Prenocephale edmontonensis, based on strong morphological autapomorphies (downturned parietal coupled with medial-most nodes on the parietal) that distinguish it from other pachycephalosaur taxa. Therefore, S. buchholtzae is a subjective junior synonym of Prenocephale edmontonensis. PRENOCEPHALE GOODWINI (Williamson and Carr, 2003) Sphaerotholus goodwini Williamson and Carr, 2003, p. 779, figs. 1–6. Holotype NMMNH P-27403, incomplete skull, lacking facial and palatal bones. Horizon and Locality Kirtland Formation (De-na-zin Member), Willow Wash, San Juan Basin, New Mexico. Remarks This specimen was briefly described by Williamson (1999) and subsequently by Williamson and Seeley (1999). More recently, Williamson and Carr (2003) described the holotype in great detail but failed to produce a robust diagnosis for this species, diagnosing it solely on the basis of having a downward, not greatly, ‘‘reduced parietosquamosal bar.’’ They also referred an unassociated dentary, squamosal fragment and unidentified skull fragments (NMMNH-P-30068) from another locality and lower stratigraphic interval (Farmington Member) to this species. Curiously, they made comparisons to ‘‘Stegoceras’’ specimens from the Judith River Group, but not to the holotype (NMMNH P-27403), presumably because the lack of common elements. Consequently, they failed to justify their reference of the referred material to P. goodwini. Suffice it to say, the holotype of P. goodwini differs from all other species of Prenocephale in having 4 nodes in a row on the posterior edge of each squamosal with a large, inferior corner node, on each side. In addition, it differs from Colepiocephale lambei in having a more rounded frontoparietal dome, and peripheral elements distinct and high on the lateral sides of the skull. The small skull is clearly referable to the taxon Prenocephale based on the fact that the skull ornamentation consists on a linear row of nodes on each of the squamosals. The species is distinct from others of the genus and is distinguished by have

four prominent nodes on the each squamosal, with a large inferior positioned corner node. The specimen is rather small and has a very rounded dome. In left lateral view the suture surface of the left squamosal shows a downward projection of the parietal as in the taxa Prenocephale brevis and Prenocephale edmontonensis. The medial extension of the parietal (the parietosquamosal bar of Williamson and Carr, 2003) lack nodes as does the holotype of the Asian taxon Prenocephale prenes. The squamosal, postorbital and posterior supraorbital are distinct, and are not fully incorporated into the dome, and are subequal in size in lateral view. Differences in node ornamentation on the posterior margins of the squamosals and medial extension of the parietal are the only distinct characters that set it apart from other species of Prenocephale. These morphological differences justify the recognition of a distinct species, but not a distinct genus. I regard the Sphaerotholus goodwini as a subjective junior synonym of Prenocephale therefore it is P. goodwini. Prenocephale sp. Referred Material TMP 87.50.29 (Fig. 6A–D), incomplete frontoparietal dome. Locality Steveville area, Alberta. Horizon Dinosaur Park Formation. Collector and Date K. Mowat, 1987. Remarks This incomplete skull (Fig. 6A–D) is referable to the genus Prenocephale based on the strong down-turned, medial projection of the parietal, which is clearly evident in lateral and posterior views (Fig. 6C and D, respectively). At first glance the medial part of the parietal seems to be devoid of any trace of paired nodes, but careful examination indicates the occurrence of nodes by the presence of irregular depressions, one on each side of the parietal (Fig. 6D). The skull is larger than any of the material referred to Prenocephale brevis and is slightly larger than the skull of Prenocephale edmontonensis (TMP 87.113.3). It is distinguished from the latter in two important features. First, TMP 87.113.3 has a distinct pair of nodes that are situated largely on the medial posterior extension of the parietal (although their respective lateral sides are bisected by the left and right parietosquamosal sutures). Second, TMP 87.113.3 has an incipiently developed frontal (nasal) lobe with the prefrontal and nasal regions fully inflated anteriorly, in contrast to the distinct, although somewhat incipient, trilobate appearance of the prefrontal/nasal region in TMP 87.50.29, which is similar to that in the small taxon P. brevis (Sullivan, 2000a). Some other differences are worth noting, but due to uncertainty in their interpretation, do not figure into the identification of the taxon. The sutural surfaces for the posterior supraorbital, and anterior supraorbital, are marked by a distinct line in TMP 87.50.29. In contrast, these two sutural surfaces are nearly continuous in TMP 87.113.3. In addition, the skull of TMP 87.50.29 is thicker posteriorly, and the enclosed temporal fossa is smaller and more constricted. TMP 87.50.29 cannot be assigned to either Prenocephale brevis or P. edmontonensis. The possibility that is represents an adult of P. brevis, as suggested by Williamson and Carr (2003:798), is not supported. It may represent a new taxon, but until more complete specimens are recovered, only a generic identification is possible. PACHYCEPHALOSAURINI New Taxon Diagnosis Pachycephalosaurids with nodes in large clusters on squamosals (hypernoded forms) and nasals. Remarks This monophyletic group consists of two taxa, Stygimoloch spinifer and Pachycephalosaurus wyomingensis, that are distinguished from all other pachycephalosaurid taxa

SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS by the presence of clusters of hypertrophied nodes on the squamosals and nasals. STYGIMOLOCH Galton and Sues, 1983 Stygimoloch Galton and Sues, 1983, p. 463. Stenotholus Giffin et al., 1987, p. 399. Type Species Stygimoloch spinifer Galton and Sues, 1983. p. 463. Diagnosis Same as for species. Remarks Although the monotypic genus Stygimoloch is not of primary concern in this study, a few observations regarding its morphology are worth noting in the context of the phylogenetic hypothesis of non-crown group pachycephalosaurid taxa (see S. spinifer below). Stygimoloch spinifer Galton and Sues, 1983, p. 463. Stygimoloch Galton and Sues, 1983; Sues and Galton, 1987, p. 33, pl. 7, figs. 1–5; pl. 8, figs. 5–10. Stenotholus kohleri Giffin et al., 1987, p. 399. Revised Diagnosis Differs from all other pachycephalosaurids in having a relatively narrow, laterally compressed skull with a vaulted frontoparietal dome; squamosals robust with three or more clustered hypertrophied nodes; frontals distinct. Holotype UCMP 119433, incomplete left squamosal with cluster of hypertrophied nodes. Horizon Lance (Hell) Creek Formation. Referred Material See Galton and Sues (1983) and Goodwin et al. (1998). Remarks Goodwin et al. (1998) recently provided a revised diagnosis for this taxon based on new material, specifically MPM 8111. The frontoparietals are often distinct, as are the frontals, as in an immature specimen, MPM 7111. Of special interest is the posterior projection of the parietal, which arches posteroventrally and is visible laterally, indicated by the photo and illustration presented by Goodwin et al. (1998: fig. 1), as evidence by the squamosal sutural surface. The entire sutural surface for the squamosal corresponds to the right lateral side of the parietal. Contrary to Sereno (2000), the posterior, tongue-like projection constitutes part of a parietosquamosal shelf, as indicated by Goodwin et al. (1998), to which hypertrophied nodes attach. Therefore, the claim that no such shelf is present in fully-domed pachycephalosaurs is not altogether true. Moreover, this posterior projection of the parietal suggests that members of the Pachycephalosaurini are primitive in this respect. I have come across a reference to the species ‘‘Stygimoloch garbanii’’ on the Internet. Galton (1995) reviewed material pertaining to the genus Thescelosaurus, specifically the holotype of ?Thescelosaurus garbanii, a species based on incomplete cervical and dorsal vertebrae and an incomplete left hindlimb. He postulated that the specimen may pertain to a new genus, or possibly be referred to Stygimoloch, but he did not formally assign it to this genus, or to Bugenasaura infernalis, although he tentatively referred the holoptype specimen of ?T. garbanii to this latter taxon. PACHYCEPHALOSAURIDAE INDET. Remarks There are a number of incomplete pachycephalosaurid specimens (see Appendix 1), one of which is worth special attention. It is from the upper part of the Milk River Formation, Dead Horse Creek, Alberta, and, as previously mentioned, represents the oldest known pachycephalosaur from North America. The upper part of the Milk River Formation is approximately 84–82.5 million years old (Eberth and Hamblin,

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1993), a date that places near the Santonian–Campanian boundary (Obradovich, 1993) and is now considered to fall below this boundary, correlating with the upper Santonian (Braman, pers. comm., 2001). ROM 2962 (Fig. 7A–C) has a gently sloping dome, with a nearly complete frontoparietal, although the left side is severely damaged (especially in the region posterior to the frontal) where it is missing a large part. The dorsal surface is smooth, with a prominent furrow running across the medial nasal lobe of the frontal (from the posterior left to anterior right). This furrow may be pathologic, or may represent a postmortem boring. This specimen, originally referred to as Stegoceras sp. by Russell (1964), differs from specimens of Stegoceras validum in lacking the distinct parietosquamosal shelf. Moreover, it has a more fully developed dome than S. validum. It differs from Colepiocephale lambei in that the lateral anterior prefrontals appear to be more inflated (i.e., incorporated into the anterior part of the frontal). In addition, viewed laterally (Fig. 7D), the postorbital sutural surface is higher than the suture surfaces anterior to it. There is also no gap, or constriction, between the postorbital suture and the posterior supraorbital, as in C. lambei. The height of the postorbital suture rivals that of some specimens of Hanssuesia sternbergi, notably CMN 192 (Fig. 4F), but the skull is much narrower than those referred to this species. Posteriorly, the parietal portion of the skull is eroded. The preserved part of the right lateral side suggests the presence of a squamosal, but, the evidence is not conclusive. The medial down-turned portion of the parietal is eroded and there appear to be two lateral diastemae (one on each side as in H. sternbergi). Ventrally (Fig. 7B), only the cerebrum and cerebellum regions of the frontoparietal are preserved; the rest of the ventral surface is missing. DISCUSSION The following North American pachycephalosaurs are accepted as valid: Stegoceras validum, Colepiocephale lambei, Hanssuesia sternbergi, Prenocephale brevis, P. edmontonensis, P. goodwini, Stygimoloch spinifer and Pachycephalosaurus wyomingensis. It is noteworthy that many of the non-crown group pachycephalosaurs include species that were named by earlier workers, notably Lambe (1918), Brown and Schlaikjer (1943) and Sternberg (1945), are still considered valid. Interestingly, the diversity of the non-crown group pachycephalosaur dinosaurs is greater than previously recognized in recent years. Some of the taxa are known from the same strata, indicating that these dinosaurs were coeval. Pachycephalosaur sympatry, previously known only for Prenocephale edmontonensis and Pachycephalosaurus wyomingensis (Giffin, 1989b), is more common in the Campanian and Maastrichtian than previously thought and suggests different paleoecological niches for these dinosaurs. PHYLOGENETIC SYSTEMATICS A character-taxon matrix for the Pachycephalosauria using 49 characters was run using PAUP 4.0b8 (Swofford, 1998). This matrix is based largely on that published by Sereno (2000) and not that of Williamson and Carr (2003). However, a few of the Sereno’s cranial characters have been re-evaluated and rescored where necessary and are identified below. In Appendix 2, characters 1–27 are the same as in Sereno’s data matrix. Since characters 28 (medial process of iliac blade posterior to acetabular process) and 29 (tapering to a flange) of Sereno (2000) were scored the same and have been collapsed into single character (character 28). Consequently, Sereno’s (2000) characters 30 through 41 are now listed as characters 29 through 40, respectively, in my taxon data matrix (Appendix 2). I continue to refer to Sereno’s original numerical identifi-

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JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 23, NO. 1, 2003 The two groups are united by the lack of minute clusters of nodes and retention of a linear row of nodes (character 41). One of the three generated trees supports monophyly of Prenocephale prenes (with Tylocephale gilmorei as the sister taxon). P. prenes and T. gilmorei may be synonymous, and if so, these two branches would collapse to form a single branch, further corroborating the monophyly of Prenocephale. Hanssuesia sternbergi and the odd taxon Colepiocephale lambei are united at an unnamed node and are the sister taxa to the Pachycephalosaurini (Stygimoloch spinifer 1 Pachycephalosaurus wyomingensis). The taxa within the Pachycephalosaurini are strikingly different from members of its sister taxa based on the presence of hypertrophied nodes primarily on the squamosals and nasal region of the skull. Stegoceras validum is the sister taxon to both these clades. Re-evaluation of Cranial Characters

FIGURE 8. Strict consensus tree of 3 equally parsimonious trees for valid Pachycephalosauria (see text for discussion).

cation of his characters (1–27) in all remarks below. Nine additional cranial characters (41 through 49) were identified (see below), and added to those previously utilized by Sereno (2000), for a total of 49 characters. I deleted three taxa in Sereno’s matrix: Yaverlandia, Ornatotholus and the ‘‘North American dwarf taxon.’’ Because Yaverlandia bitholus is not referable to the Pachycephalosauria (as discussed above, see also Sullivan, 2000a) and because I consider Ornatotholus browni to be a subjective junior synonym of Stegoceras validum, they are excluded from this data set. I have no way of evaluating the putative ‘‘North American dwarf taxon,’’ so it too is excluded from this character-taxon matrix. I added the taxa Prenocephale brevis, Prenocephale edmontonensis, Prenocephale goodwini, Colepiocephale lambei and Hanssuesia sternbergi (which includes UCMP 130051 described by Goodwin, 1990). I retained the original scoring of all characters by Sereno (2000), except where noted. A review and explanation of newly incorporated cranial characters is presented below. RESULTS A branch-and-bound analysis was conducted, with all characters unordered, all having equal weight, and two of which (characters 46 and 47) were determined to be parsimony uninformative and were excluded from the analysis. One of the two parsimony uninformative characters (character 47) is identified as autapomorphy. This search generated 3 equally parsimonious trees, each with a tree length 5 54, C.I. 5 0.9074, H.I. 5 0.0962, R.I. 5 0.9451. The strict consensus tree of these 3 trees is presented here (Fig. 8). The strict consensus tree (Fig. 8) corroborates the thesis that doming is a derived character. However, it is clear that doming occurred early on based on its occurrence in the enigmatic pachycephalosaur ROM 2962 and the slightly younger Colepiocephale lambei, which antedates the occurrence of Stegoceras validum, suggesting that doming may be a homoplastic character. An unresolved dichotomous group, consisting of the Asian taxa (Tylocephale gilmorei, Prenocephale prenes), is the sister group to the monophyletic clade consisting of the North American taxa ((P. edmontonensis 1 P. brevis) 1 P. goodwini).

Nodes in Rows (Character 9 [in part] of Sereno, 2000) Skull ornamentation is highly variable among pachycephalosaurs and must be considered as a series of multiple character states (see below). There are distinct, consistent patterns of ornamentation that are readily seen over and over. As discussed previously, the ornamentation consisting of small nodes, or tubercles, reflects the internal columnar structure of the dome and skull in general. Small, granular nodes are known to occur as the predominant surface texture and ornamentation among flatheaded pachycephalosaurs. Therefore, this background texture is a primitive feature, as correctly stated by Sereno (1986). However, in many of the non-crown group pachycephalosaurs this ornamentation sometimes co-occurs with pronounced nodes that are arranged in a linear fashion along the upper edge of the posterior margin of the squamosals. The presence of rows of nodes, co-occurring with irregularly arranged minute nodes on the squamosals, and peripheral and lateral elements of the skull, is widespread among the basal taxa, including Wannanosaurus, Goyocephale and Homalocephale. Reduction and loss of minute nodes (especially on the lateral and posterior edges of the squamosals) is derived and is seen in Tylocephale and Prenocephale. I retain Sereno’s (2000) scoring for character 9, with the exception of Stygimoloch and Pachycephalosaurus, which I score as (0) because the nodes are clustered and not in a row. Squamosals with Node Rows (Character 16 [in part] of Sereno, 2000) Sereno (2000) only scored for 5–7 nodes on the squamosals (character 16). I have redefined this character to include all large nodes on the squamosals, whether in a row or in clusters. Rows of nodes occur both on the sides and posterior part of the skull and vary in number, prominence, and position. These variations are clearly evident among the different species of Prenocephale (Sullivan, 2000a) and are scored separately (see below). There is an apparent tendency to reduce the number of nodes along the posterior parietosquamosal shelf, losing nodes from the medial to lateral sides. Stegoceras validum is most primitive in this respect, displaying as many as seven prominent nodes on each squamosal (as in TMP 99.62.1). In the holotype of P. prenes (Z. PAL MgD-I/104) there are four nodes in a linear row on the left squamosal and five on the right, suggesting a transition from five to four nodes on each squamosal. Among the species of Prenocephale (Sullivan, 2000a) five nodes on the single squamosal is primitive, four nodes is derived, and three nodes (with the fourth straddling the squamosal/parietal suture) is even more derived (see new characters below). Loss of nodes from the medial extension of the parietal is widespread and may have occurred independently among the flat-headed taxa. Where present, the prominence in the size of the inferior corner node of the squamosal differs among taxa. Total loss of the linear row of nodes and retention

SULLIVAN—REVISION OF THE PACHYCEPHALOSAUR STEGOCERAS of irregular minute nodes, as seen in Hanssuesia, may also represent a derived condition. Also, clustering of nodes in Stygimoloch and Pachycephalosaurus is derived (see below). Frontal Suture Fused (Character 31 5 Character 32, in Part, of Sereno, 2000) Fusion of the frontal suture (dorsal aspect) is considered derived (1), and the unfused state is primitive (0) and is usually associated with the flat-headed taxa and immature individuals of Stegoceras validum (sensu stricto). However, one fully-domed taxon, Stygimoloch spinifer, has paired frontals that are distinct from the parietal (Giffin et al., 1987; Goodwin et al., 1998). This is most likely a character reversal, unique to Stygimoloch, as no incipiently, or fullydomed pachycephalosaur retains this primitive condition. The frontal suture may be visible along the ventral surface of the anterior part of the co-joined frontals in some immature specimens of Stegoceras validum (sensu stricto). Frontoparietal Doming (Characters 32 and 35 5 Characters 33 and 36 of Sereno, 2000, Respectively) The degree of doming is variable among pachycephalosaurs, and ranges from nearly flat (broad and rugose) crania in Homalocephale (character 24 of Sereno, 2000), to highly inflated domes as exemplified by Pachycephalosaurus. Co-ossification of the frontoparietal (character 31 5 character 32 of Sereno, 2000) is the norm for all mature domed pachycephalosaurs, although its expression is variable among subadults. Unfused frontoparietal sutures are primitive (0) whereas fused sutures are derived in adult individuals. Semi-flattened frontal and parietal specimens, previously attributed to the taxon Ornatotholus browni, are now considered examples of immature individuals of S. validum. The extent of doming (character 35 5 character 36 of Sereno, 2000) can be due, in part, to an individual’s ontogenetic stage or phyletic position. Therefore, species recognition cannot rest solely on this single character, but must be considered with other features to properly identify a species. Moreover, this character (35) may be, in part, redundant with respect to character 32 (5character 33 of Sereno, 2000) as they are scored nearly the same except for Stegoceras, which I score as primitive (0) because of its ‘‘incomplete doming.’’ However, I would argue that adult Stegoceras is more closely allied to the fullydomed taxa than to the flat-headed ones. Needless to say, evaluation of the degree of doming is subjective, and attempts to quantify doming through morphometrics have done little to help clarify taxonomic placement of specimens. It has been suggested that doming occurred only once within the phylogenetic history of pachycephalosaurs (Sereno, 2000). Strong Posterior Displacement of the Parietal over Occiput (Character 33 5 Character 34 of Sereno, 2000) This character is difficult to quantify among some individual species of pachycephalosaurs due to the incomplete nature of the cranial material. The character seems to be a feature present in all incipiently and fully-domed pachycephalosaurs. Weak displacement over the parietal is primitive (0), and strong displacement is derived (1). Closure of Supratemporal Fossae (Character 34 5 Character 35 of Sereno, 2000) The presence of supratemporal fossae is a well-known primitive character. Closure of these openings is considered to be derived (1), and is presumably consistent within a single species. However, as noted above, in one specimen of Stegoceras validum (TMP 99.62.1), the left and right temporal fossae are closed. This departure, within specimens that clearly are referable to Stegoceras validum, may be pathologic, or heterochronic, in its expression, and is re-scored here as variable (0,1). Among the vast collection of specimens referable to this species, all have distinct supratemporal fossae, although they do vary in size and prominence. Moreover, large supratemporal fossae (relative to skull size) are known in Wannanosaurus (Sereno, 2000) and Homalocephale (pers. obs.), while those of Stegoceras tend to be somewhat smaller.

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Long Snout (Character 38 5 Character 39 of Sereno, 2000) Sereno (2000) grouped Stygimoloch and Pachycephalosaurus together (in part) as having proportionally long snouts. Unfortunately, few of the non-crown group pachycephalosaurs preserve the anterior, or facial part of the skull, so the broad distribution of this character remains an open question. Nodes as Prominent Clusters on Squamosals, Jugals and Nasals (Characters 39 and 40 5 Characters 40 and 41 [in part] Respectively, of Sereno, 2000) The occurrence of hypertrophied squamosal nodes characteristic of Stygimoloch spinifer, and the well-developed stout nodes on the squamosals of Pachycephalosaurus wyomingensis, have been scored separately (Sereno, 2000) from the well-developed nasal nodes that occur in both taxa. However, because these two ornamental conditions co-occur they probably should be treated as part of the same character complex but are retained as separate for now. Moreover, the enlarged nodes on their respective jugals should also be considered part of this character complex. Additional Cranial Characters The following cranial characters have been identified and added to Sereno’s (2000) data matrix. They are, in consecutive, order: Character 41 Loss of minute nodes and retention of nodes in linear row. Tylocephale and Prenocephale include taxa that are characterized by linear rows of enlarged nodes on the squamosals and lateral sides of the skull. The presence of minute clusters of nodes is considered primitive (0); their absence, coupled with retention of a linear row of nodes is derived (1). Character 42 Four (or less) distinct nodes in a linear row on posterior margin of squamosal (with medial part of parietal reduced and lacking nodes). One specimen, the holotype of P. goodwini (NMMNH P-27403), has only four prominent nodes in a row on the posterior margin of the right squamosal (the left is not preserved). This is less than that seen on Prenocephale prenes, although the holotype of P. prenes is assymetrical it this regard, with four on the left and five on the right, so it is scored as variable (0,1). Four distinct nodes on in a linear row on posterior margin of squamosal with medial part of parietal reduced: absent (0); present (1). Character 43 Three (or less) distinct nodes in linear row on posterior margin of squamosal with medial-most nodes straddling squamosal/parietal. Specimens assigned to Prenocephale edmontonensis are, in part, characterized by three welldeveloped nodes on the squamosal, with the medial-most nodes straddling the parietosquamosal suture. Three distinct nodes in linear row on posterior margin of squamosal with the fourth (medial-most) node straddling squamosal/parietal: absent (0); present (1). Character 44 Medial-most nodes partly, or wholly, on posterior medial extension of parietal. The presence of nodes on the medial extension of the parietal may be equivocal. However, because they occur in some specimens of Stegoceras validum (specifically TMP 99.62.1) and reduction of nodes is the tendency among pachycephalosaurs, specifically among species of Prenocephale, I infer that their absence is a loss, and therefore a derived condition. Nodes partly, or wholly, on posterior medial extension of parietal: present (0); absent (1). Character 45 Medial extension of parietal strongly downturned. The polarity of this character may, at first glance, appear ambiguous. It could be argued that the strongly down-turned parietal is primitive because it appears early in the fossil record, antedating occurrences of the flat-headed taxa Homalocephale, Goyocephale and the incipiently domed Stegoceras. The variable state of flat frontoparietal to incipient developed domes within Stegoceras validum further supports the conclusion that flat-headed taxa are inherently primitive. However, because the

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presence of a down-turned parietal seems to be linked to doming of the skull, it should be considered a derived character. If the former scenario were correct, then some flat-headed pachycephalosaurs would presumably represent a character reversal, and might argue for paedomorphism with among the flat-headed taxa. However, I will take the conventional approach and score all non down-turned parietals as being the primitive condition (0) and strongly down-turned parietals as derived (1). Character 46 Lateral peripheral elements fully incorporated into dome. Most pachycephalosaurs retain the peripheral elements as discrete units. Only in Prenocephale edmontonensis and Pachycephalosaurus wyomingensis are these elements fully incorporated into the dome, making their sutural traces impossible to discern. This condition is derived. Therefore, lateral peripheral elements: discrete (0); fully incorporated into dome (1). Character 47 Squamosals present posteriorly. In the primitive condition (0) the squamosals wrap around the skull and are separated by the medial portion of the parietal. Where the squamosals are restricted to the lateral sides, and not visible posteriorly, that condition is considered derived (1). Character 48 Prominent nasal boss of frontoparietal flanked laterally by incipient recessed lobes. One taxon, Colepiocephale, is distinct in having the lateral lobes of the frontal (primarily the prefrontal and anterior supraorbital lobes) incipiently developed. Their anterior extent lies well posterior to the anterior reach of the medial nasal boss (lobe). Therefore, a prominent nasal boss of frontoparietal flanked laterally by incipient recessed lobes: absent (0); present (1). Character 49 Parietosquamosal shelf reduced posteriorly with minute tubercles. Reduction of the parietosquamosal shelf is considered derived (1) as is exemplified in Hanssuesia sternbergi based on UCMP 130051, conversely, a well-developed parietal shelf as in Stegoceras validum is primitive (0). Other Features Many taxa also have a distinct corner node on each squamosal that lies below the node row. The prominence of these nodes varies among species of Prenocephale and Homalocephale calathocercos. BIOSTRATIGRAPHIC AND CHRONOSTRATIGRAPHIC DISTRIBUTION OF THE NORTH AMERICAN PACHYCEPHALOSAURS Because most of the North American pachycephalosaur specimens come from the Late Cretaceous rocks of southern Alberta, an understanding of their occurrence within these units is critical. Thanks largely to the collective studies of Eberth and Hamblin (1993), Eberth (1996), and Fassett and Steiner (1997) and Lerbekmo and Braman (2002) it is now possible to properly place taxa within a robust stratigraphic framework and to better understand their biostratigraphic distribution with precision. A sound biostratigraphic context allows for intra- and intercontintental correlations and sets the stage for understanding the pattern and timing of paleomigrations between paleocontinents. The Gobi terrestrial sequences that contain pachycephalosaurs (Goyocephale lattimorei, Tylocephale gilmorei and Prenocephale prenes) have recently been interpreted as spanning middle Campanian through ‘‘middle’’ Maastrichtian time (Jerzykiewicz and Russell, 1991). Unfortunately, it is not presently possible to correlate the Upper Cretaceous marine stages with their corresponding terrestrial sequences with precision. While the stratigraphic position of many of the Asian sequences are still in debate there has been a general consensus that the Nemegt and Barun Goyot formations are younger than the North American sequences (Foremost, Oldman and Dinosaur Park formations of Alberta, Canada) and are, in part, co-eval with the younger Fruitland/Kirtland strata of the San Juan Basin, New Mexico. The precise stratigraphic position of the Upper

Cretaceous Xiaoyan Formation of Anhui Province, People’s Republic of China is not known (Lucas and Estep, 1998). Upper Santonian In southern Alberta, the oldest pachycephalosaur-bearing unit is the Milk River Formation. Two pachycephalosaur specimens are known from the upper part of this unit, a single indeterminate pachycephalosaur tooth (TMP 10.2.6) and an incomplete skull (ROM 2962). The upper part of the Milk River Formation is thought to be late Santonian age (Braman, pers. comm., 2001). As noted above, it is not possible to identify this latter specimen to any known genus, but it is clear that it was a near fully-domed derived pachycephalosaur. The Milk River Formation is unconformably overlain by the marine shales of the middle Campanian Pakowki Formation which lacks evidence of pachycephalosaurs. Consequently, there are no known pachycephalosaurs that are early Campanian age. Middle-Upper Campanian The Judith River Group is comprised of three distinct units, in ascending order: the Foremost Formation, Oldman Formation, and Dinosaur Park Formation (Eberth and Hamblin, 1993). The Foremost Formation, which overlies the Pakowki Formation, yields the distinctive pachycephalosaur Colepiocephale lambei. This taxon is the only known identifiable species from this unit. The upper part of the Oldman Formation is marked by a north–south time transgressive discontinuity with the overlying Dinosaur Park Formation (Eberth and Hamblin, 1993). The magnitude of the discontinuity has been interpreted as being slight, with little time missing. Moreover, chronostratigraphically, the Dinosaur Park Formation is contemporaneous with the upper part of the Oldman Formation in southeastern Alberta (Eberth and Hamblin, 1993). Three pachycephalosaur taxa are known from the Oldman and Dinosaur Park formations. These include Stegoceras validum, Hanssuesia sternbergi, and Prenocephale brevis. All three taxa occur in both formations. This part of the Judith River Group is chronostratigraphically equivalent to the Judith River Formation of Montana. Sahni (1972:fig. 9N, O) reported on a number of isolated teeth, and illustrated one (AMNH 8530), which he questionably referred to Stegoceras validum (sensu lato) from the type area of the Judith River Formation of Montana. Goodwin (1990) documented eight pachycephalosaur cranial specimens from the Turner Ranch, Kennedy and Super Coulee regions of Montana. There are also a number of isolated teeth from the Judith River Formation attributed to pachycephalosaurs in the collections of the Museum of the Rockies and the Royal Tyrrell Museum of Palaeontology. One isolated tooth (TMP 87.83.16) is putatively from the Two Medicine Formation, found at ‘‘Jack Horners Site’’ south of Del Bonita, Montana. There are also three frontoparietal specimens from the Two Medicine Formation of Montana (MOR nos. 479, 480, and 550) and to my knowledge no other pachycephalosaur cranial material is known from this unit. In New Mexico, the Kirtland Formation (De-na-zin Member) has produced two incomplete pachycephalosaur skulls. The two incomplete skulls are known and were previously referred to the genus Prenocephale (Sullivan, 2000a). Dinosaurs and other fossil vertebrates from this interval of the Kirtland Formation are slightly younger than the classic dinosaur faunas from the Oldman and Dinosaur Park formations based on ash dates published by Fassett and Steiner (1997). Upper Campanian–Lower Maastrichtian The Edmonton Group in Alberta includes, in ascending order, the Horseshoe Canyon, Whitemud, Battle and Scollard forma-


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FIGURE 9. Biostratigraphic and chronostratigraphic distribution of the North American pachycephalosaur genera (see text for discussion of species distribution). Stratigraphy complied and modified, in part, from Lerbekmo and Braman (2002), Eberth (1996), Eberth et al. (2001), Fassett and Steiner (1997), Jerzykiewicz and Russell (1991), Lewy and Odin (2001), Obradovich (1993) and Rogers et al. (1993). Principal Abbreviations: DMT, Drumheller Marine Tongue; Maastr, Maastrichtian; San, Santonian.

tions, with the first and last formations straddling the Campanian–Maastrichtian and Maastrictian–Danian (K–T) boundaries respectively (Eberth and O’Connell, 1995). The Horseshoe Canyon Formation has been subdivided into five informal units, with the third identified as the Drumheller Marine Tongue (Eberth et al., 2001). The Drumheller Marine Tongue previously occupied the upper part of the ‘‘Edmonton B’’ of Russell and Chamney (1967). The global stratotype for the Campanian– Maastrichtian boundary has been based on a section at Tercisles-Bains, France (Barchi et al., 1997; Odin, 1996, 2001). Recently, Lewy and Odin (2001) placed the Campanian–Maastrichtian boundary within magnetopolarity chron 32n, at the base of a reversed subchron, based on the Bottaccione section in Italy. The Campanian–Maastrichtian boundary in Alberta probably rests at the base of the Drumheller Marine Tongue (Eberth et al., 2001; Lerbekmo and Braman, 2002). Dinosaur remains are known principally from the lower part of the Horseshoe Canyon Formation, below the Drumheller Marine Tongue, roughly corresponding to members A and B of the ‘‘Edmonton Formation’’ of earlier workers (Russell and Chamney, 1967). However, few pachycephalosaur specimens are known from the Horseshoe Canyon Formation. An incomplete (left half) frontoparietal of Prenocephale brevis (CMN 11316) is from the Horseshoe Canyon Formation, at about the level of the Drumheller Marine Tongue (Eberth, pers. comm., 2001). The holotype and paratype specimens of Prenocephale edmontonensis are also from this interval. I know of no pachycephalosaur specimens from any of the remaining units, including the upper part of the Horseshoe Canyon Formation. Upper Maastrichtian Russell (1987) reported on, and figured, a single tooth, which he questionably referred to Pachycephalosaurus sp. from the Scollard Formation. No other pachycephalosaur material has been reported from this formation. The Hell Creek and Lance formations in the United States have yielded specimens of the crown taxa Stygimoloch spinifer and Pachycephalosaurus wyomingensis. A specimen of Pachycephalosaurus wyomingensis (BMNH R 8648), collected by C. H. Sternberg in 1916 and sold to the Natural History Museum

in London, reportedly came from the Oldman Formation (Wall and Galton, 1979:1183). However, given the fact that no other specimens of this taxon are known from the hundreds of pachycephalosaurs recovered from the Oldman (and Dinosaur Park) Formation, it is likely that the collection data are in error, and that the specimen is from the Hell Creek or Lance formations of the U.S. and not from the Oldman Formation or its equivalent. The Hell Creek Formation also has yielded specimens of Prenocephale edmontonensis (Sullivan, 2000a). A summary of the biostratigraphic and chronostratigraphic distribution of the North American pachycephalosaur genera recognized herein is presented in Figure 9. PALEOBIOGEOGRAPHIC CONSIDERATIONS Understanding the biostratigraphic occurrences of the North American and Asian pachycephalosaur taxa in time and space, and their phylogenetic relationships, allows for an assessment of paleobiogeographic distribution, timing and directionality of dinosaur migrations during the Late Cretaceous. Previous interpretations, most recently that presented by Lehman (1997), are partly erroneous as they assume a priori that the ‘‘northern’’ (Judith River/Two Medicine) Campanian dinosaur faunas were coeval with the ‘‘southern’’ (Fruitland/Kirtland) dinosaur faunas. If anything, the faunas are time transgressive, with the southern faunas being younger than those of the north, based on ash dates recently published by Fassett and Steiner (1997). Moreover, the faunas from the Fruitland and Kirtland formations have been misinterpreted not only taxonomically, but also biostratigraphically. Recent re-evaluation of the taxa that formed the once distinctive Alamo Wash local fauna (Lehman, 1981) are now largely known to be late Campanian age, not late Maastrichtian. The present taxonomic composition of the Alamo Wash local fauna is insufficient to date with certainty. It is significant to note that fully-domed pachycephalosaurs first appear early in western North America, approximately 84 million years ago (late Santonian). This occurrence antedates that of the more primitive Stegoceras validum (approximately 77 Ma, middle Campanian) and the Asian Homalocephale calathocercos (which is probably early Maastrichtian age, around

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70 Ma). The age of the Asian taxon Goyocephale lattimorei is uncertain, but it is considered to be near coeval with the age of the Djadokhta Formation (Perle et al., 1982; Jerzykiewicz and Russell, 1991) and thus is probably middle-to-late Campanian age. The premise that many of the Late Cretaceous dinosaur groups originated in Asia and migrated to western North American (Russell, 1993) does not necessarily hold true for the pachycephalosaurs. Although I have tentatively retained Stenopelix valdensis as a basal pachycephalosaur, following Sereno (2000), treating it as an immediate sister-taxon to the pachycephalosaurids for purposes of character assessment, it should be noted that this taxon is based solely on postcranial material. Consequently, there has been much disagreement with respect to its phylogenetic position and placement within this group may be highly questionable (Wall and Galton, 1979; Sues and Galton, 1982; Maryan´ska, 1990). As with previous putative pachycephalosaurs (Yaverlandia and Majungatholus), whose identity and phylogenetic relationships have since been proven wrong (Sampson et al., 1998; Sullivan, 2000a), the same may hold true for Stenopelix valdensis. It is clear that pachycephalosaurids appear rather abruptly in the fossil record, their occurrence is presently restricted to the late Santonian through Maastrichtian. The origin of this group, and the directionality in dispersals of its taxa can only be speculative based on the current information. Moreover, Asian– North American contiguity based on the occurrence of Prenocephale in both regions during middle Campanian through early Maastrichtian time cannot be ruled out (contra Williamson and Carr, 2003). While many Late Cretaceous dinosaur groups have been cited as having likely originated in Asia and migrated to western North America (Russell, 1993), the data for pachycephalosaurids is ambiguous and a reverse directionality (in part) could be argued based on the present evidence. The ambiguity lies in the fact that the more primitive pachycephalosaurs, such as Goyocephale and Homalocephale (see Fig. 9) are coeval with many of the North American taxa; the relative age of the primitive Asian pachycephalosaur Wannanosaurus is unknown. The origin and antiquity of the pachycephalosaurid dinosaurs remains largely a mystery and will only be clarified by discoveries of new and older taxa. ACKNOWLEDGMENTS It is with great pleasure I thank Phil Currie and James Gardner (Royal Tyrrell Museum of Palaeontology, Drumheller); Kieran Shepherd and Margaret Feuerstack (Canadian Museum of Nature, Ottawa); Michael W. Caldwell and Timon Bullard (University of Alberta Laboratory of Vertebrate Paleontology, Edmonton); Sam McLeod (Los Angeles Museum of Natural History, Los Angeles); John R. Horner and David J. Varricchio (Museum of the Rockies, Bozeman); Hans-Dieter Sues and Kevin Seymour (Royal Ontario Museum, Toronto); Mark Goodwin (University of California, Berkeley) for loaning and/ or access to specimens for study. I thank Thomas E. Williamson (New Mexico Museum of Natural History and Science, Albuquerque) for the privilege of examining NMMNH P-27403 and Thomas D. Carr (Royal Ontario Museum, Toronto) for discussions concerning paleobiogeographic distributions. Special thanks are extended to David Eberth (Royal Tyrrell Museum of Palaeontology) for providing me with revised stratigraphic data for many of the Canadian specimens and for taking a keen interest in the biostratigraphy of these interesting dinosaurs. Dennis Braman (Royal Tyrrell Museum of Palaeontology) also provided me with the most recent stratigraphic and biostratigraphic data concerning the Canadian sections, for which I am most grateful. I thank Olivier C. Rieppel (Field Museum of Natural History) for critiquing an earlier version of the data

matrix present herein and for his insights on cladistic methodology. Paul Sereno (University of Chicago) kindly provided a copy of his latest manuscript prior to publication, which is gratefully acknowledged. I thank George Olshevsky for his etymological help and insights to gender agreement. Angela Milner (Natural History Museum, London) for verifying the data regarding BMNH R 8648. Spencer G. Lucas (New Mexico Museum of Natural History and Science, Albuquerque) and Olivier C. Rieppel reviewed an earlier version of the manuscript and I thank them for their suggestions for improvement. Peter Galton (University of Bridgeport, Bridgeport), and an anonymous reviewer, formally reviewed this paper and I am grateful for their comments and insights that helped improve the final version of this work. This study was supported in part by a grant from the Jurassic Foundation that is gratefully acknowledged. The State Museum of Pennsylvania also provided funding for travel. LITERATURE CITED Barchi, P., M. Bonnemaison, B. Galbrun, and M. Renard. 1997. Tercis (Landes, Sud Ouest France): point stratotype global de la limite Campanien-Maastrichtian. Re´sultats magne´tostratigraphiques et premie`res donneés sur la nannoflore calcaire. Bulletin de la Socie´te´ Geólogique de France 168:133–142. Baszio, S. 1997. Oldest occurrence of the genus Stegoceras (Dinosauria: Pachycephalosauridae) from the Foremost Formation (Campanian) of south Alberta, Canada. Palaöntologische Zeitschrift 71:129–133. Bohlin, B. 1953. Fossil reptiles from Mongolia and Kansu. Reports from the Scientific Expedition to the North-Western Provinces of China Under the leadership of Dr. Sven Hedin, The Sino-Swedish Expedition, Publication (37) 6:1–105. Brown, B., and E. M. Schlaikjer. 1943. A study of the troodont dinosaurs with the description of a new genus and four new species. Bulletin of the American Museum of Natural History 82:121–149. Carpenter, K. 1993. Did pachycephalosaurs (Ornithischia: Dinosauria) really butt heads? North Dakota Geological Survey, Marshall Lambert Symposium, p. 24. (abstract) Chapman, R. E., P. M. Galton, J. J. Sepkoski, Jr., and W. P. Wall. 1981. A morphometric study of the cranium of the pachycephalosaurid dinosaur Stegoceras. Journal of Paleontology 55:608–618. Colbert, E. H. 1955. Evolution of the Vertebrates. Wiley, New York, 479 pp. Dong, Z. 1978. A pachycephalosaur from the Wang-shih Formation in Lai-yang (County), Shantung (Province). Vertebrata PalAsiatica 16: 222–228. [Chinese with English summary] Eberth, D. A. 1996. Origin and significance of mud-filled incised valleys (Upper Cretaceous) in southern Alberta, Canada. Sedimentology 43:459–477. ———, P. J. Currie, D. B. Brinkman, M. J. Ryan, D. R. Braman, J. D. Gardner, V. D. Lam, D. N. Spivak, and A. G. Neuman. 2001. Alberta’s dinosaurs and other fossil vertebrates: Judith River and Edmonton groups (Campanian-Maastrichtian); pp. 49–75 in C. L. Hill (ed.), Mesozoic and Cenozoic Paleontology in the Western Plains and Rocky Mountains, Guidebook for the Field Trips, Society of Vertebrate Paleontology 61st Annual Meeting, Bozeman. Museum of the Rockies Occasional Paper No. 3. ———, and A. P. Hamblin. 1993. Tectonic, stratigraphic, and sedimentologic significance of a regional discontinuity in the upper Judith River Group (Belly River wedge) of southern Alberta, Saskatchewan, and northern Montana. Canadian Journal of Earth Science 30: 174–200. ———, and S. C. O’Connell. 1995. Notes on changing paleoenvironments across the Cretaceous-Tertiary boundary (Scollard Formation) in the Red Deer River valley of southern Alberta. Bulletin of Canadian Petroleum Geology 43:44–53. Fassett, J. E., and M. B. Steiner. 1997. Precise age of C33N-C32R magnetic-polarity reversal, San Juan Basin, New Mexico and Colorado. New Mexico Geological Society Guidebook, 48th Field Conference, Mesozoic Geology and Paleontology of the Four Corners Region, pp. 239–247. Gabriel, D. L., and C. B. Berghaus. 1988. Three new specimens of Stygimoloch spinifer (Ornithischia: Pachycephalosauridae) and behavior inferences based on cranial morphology; in International Symposium on Vertebrate Behavior as Derived from the Fossil

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APPENDIX 1 Key Material examined. Stegoceras validum—AMNH 5450 (holotype of Ornatholus browni), CMN 138, CMN 515 (lectotype), CMN 1108, CMN 1594, CMN 2369, CMN 2370, CMN 2371, CMN 2372, CMN 2373, CMN 2374, CMN 2375, CMN 2376, CMN 2377, CMN 2378, CMN 8816, CMN 38428, CMN 38439, CMN 38451, ROM 803, TMP 66.21.66, TMP 66.22.23, TMP 78.19.4, TMP 79.8.37, TMP 79.8.829, TMP 80.16.902, TMP 80.16.1011, TMP 80.16.1168, TMP 81.16.341, TMP P81.16.496, TMP 82.16.113, TMP 82.16.260, TMP 82.16.301, TMP 82.19.28, TMP 82.19.156, TMP 82.20.189, TMP 82.20.386, TMP 83.36.38, TMP 83.67.1, TMP 83.67.19, TMP 84.67.20, TMP 84.36.47, TMP 84.121.6,

TMP 85.36.76, TMP 85.44.25, TMP 85.53.4, TMP 85.56.118, TMP 85.56.121, TMP 85.58.68, TMP 85.87.2, TMP 86.36.87, TMP 86.77.105, TMP 86.77.106, TMP 86.116.7, TMP 87.36.269, TMP 87.157.50, TMP 88.36.65, TMP 88.36.198, TMP 88.116.49, TMP 88.166.32, TMP 89.36.115, TMP 89.36.305, TMP 89.50.156, TMP 89.76.42, TMP 89.116.88, TMP 90.66.2, TMP 90.107.97, TMP 91.36.270, TMP 91.146.10, TMP 92.36.1089, TMP 92.36.1125, TMP 92.93.1, TMP 92.109.9, TMP 93.36.257, TMP 93.85.12, TMP 93.99.7, TMP 94.12.13, TMP 95.12.11, TMP 95.12.147, TMP 96.12.123, TMP 97.12.70, TMP 97.38.1 TMP 98.12.75, TMP 98.93.4, TMP 99.62.1, TMP 2000.26.01, and UALVP-2. cf. S. validum—CMN 1074, CMN 1107, CMN 1914, CMN 9959, TMP 64.4.20, TMP 80.16.1312, TMP 80.16.1740, TMP 81.16.145, TMP 81.23.146, TMP 82.16.108, TMP 82.16.111, TMP 82.16.123, TMP 82.31.13, TMP 84.121.21, TMP 86.39.5, TMP 87.7.2, TMP 87.7.3, TMP 92.2.3, TMP 92.36.286, TMP 93.75.55, UALVP-6, UALVP-8503, UALVP-8505, and UALVP-8507. ?S. validum—TMP 73.8.223, TMP 83.209.3, TMP 86.77.85, TMP 86.146.7, TMP 89.116.29, TMP 96.169.2, and TMP 99.55.281. Colepiocephale lambei—CMN 8818 (holotype), CMN 29419, ROM 3632, TMP 70.2.1, TMP 86.146.1, TMP 86.146.2, TMP 92.88.1, TMP 97.99.3, TMP 2000.57.1, UALVP- 349 and UALVP-31471. cf. C. lambei—CMN 9952, CMN 29420, CMN 29421, TMP 80.20.1, TMP 86.146.3, TMP 86.146.4, TMP 86.146.6, TMP 87.7.7, TMP 97.99.2, TMP 99.31.1, and TMP 99.31.2. Hanssuesia sternbergi—CMN 192, CMN 1953, CMN 2379, CMN 8817 (holotype), CMN 2379, CMN 8945, CMN 9148, TMP 79.14.853, TMP 87.36.363, TMP 89.69.21, TMP 2000.26.01, and UALVP-3. cf. H. sternbergi—CMN 1075, CMN 8944, MOR 453, MOR 480, TMP 85.36.240, TMP 98.93.125, and UALVP-8502. Gravitholus albertae—TMP 72.27.1 (holotype). Prenocephale brevis—CMN 121*, CMN 193*, CMN 194*, CMN 1423 (holotype), CMN 11316, CMN 8819*, ROM 2960, ROM 2961, ROM 31616, TMP 81.16.480, TMP 85.36.292, TMP 91.36.265, TMP 99.55.122, TMP 2000.12.1, UALVP-8501 and UALVP-8508. (*designated as ‘‘plesiotypes’’ by Sternberg, 1945) cf. Prenocephale brevis—CMN 1535, CMN 1593, TMP 74.10.74, TMP 81.27.40, TMP 85.43.68, TMP 98.93.125, UALVP-5, UALVP8504, and UALVP-8506. ?Prenocephale brevis—TMP 85.43.62. Prenocephale edmontonensis—CMN 8830 (holotype), CMN 8831 (paratype), CMN 8832 (paratype), LACM 64000, and TMP 87.113.3. cf. Prenocephale edmontonensis—LACM 15345. Prenocephale goodwini—NMMNH P-27403. Prenocephale sp.—SMP VP-1084 and TMP 87.50.29. Pachycephalosaurus wyomingensis—AMNH 1696 (holotype of P. grangeri), BMNH R 8648, CM 3180, DMNH 469 (holotype of P. reinheimeri), LACM 47507, and USNM 12031 (holotype). Pachycephalosaurinae indet.—ROM 2962. Pachycephalosauria indet.—CMN 8944, CMN 9952, CMN 29420, MOR 391, TMP 81.19.232, TMP 82.20.41, TMP 82.19.288, TMP 87.36.364, TMP 89.69.21, TMP 90.107.47, TMP 91.36.343, TMP 92.125.3, TMP 96.93.79, and TMP 99.62.3. ?Pachycephalosauria—CMN 38440, TMP 82.19.198, TMP 80.16.1185, TMP 80.16.1480, and TMP 94.12.740. Casts examined (referred to by original specimen number): UALVP-2, Stegoceras validum UCMP 130051, Hanssuesia sternbergi MPM 7111, holotype of Stenotholus kohleri (5Stygimoloch spinifer) GI No. SPS 100/51, holotype of Homalocephale calathocercos Z. Pal. MgD-I/104, holotype of Prenocephale prenes BMNH R 8648, Pachycephalosaurus wyomingensis


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APPENDIX 2 Character-taxon matrix for the Pachycephalosauria (modified from Sereno, 2000). Yaverlandia, Ornatotholus and ‘‘NA Dwarf taxon’’ not considered; Stenopelix valdensis provisionally accepted (see text for discussion). 0 5 primitive character; 1 5 derived character; 0,1 5 variable character (both conditions are known to occur). Box outline indicates taxa not included in Sereno’s (2000) data matrix.