redescription of the ceratopsid dinosaur torosaurus ... - CiteSeerX

J. Paleont., 79(3), 2005, pp. 564–582 Copyright 䉷 2005, The Paleontological Society 0022-3360/05/0079-564$03.00

REDESCRIPTION OF THE CERATOPSID DINOSAUR TOROSAURUS UTAHENSIS (GILMORE, 1946) AND A REVISION OF THE GENUS ROBERT M. SULLIVAN,1 ARJAN C. BOERE,2

AND

SPENCER G. LUCAS3

Section of Paleontology and Geology, The State Museum of Pennsylvania, 300 North Street, Harrisburg 17120–0024, ⬍[email protected]⬎, 2 Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, P.O. Box 94062, 1090 GT, The Netherlands, and 3 New Mexico Museum of Natural History, 1801 Mountain Rd. N.W., Albuquerque 87104, ⬍[email protected]⬎

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ABSTRACT—The holotype of the ceratopsid dinosaur Torosaurus (⫽Arrhinoceratops?) utahensis (Gilmore, 1946) consists of a right squamosal, jugal, quadrate, quadratojugal, epijugal, lacrimal, and postorbital horncore/orbital region. Some elements previously described by Gilmore (1946), notably the epoccipitals and parietals, were not originally included, so they cannot be considered part of the holotype. Associated elements (lower jaws and others), which may pertain to the holotype, are described for the first time; they, too, are not formally considered part of the type material, but they provide additional information regarding the osteology of this rare chasmosaurine. Torosaurus utahensis differs from T. latus (type species) in having a squamosal that is shorter and squared-off at its distal end and an unusually expanded horncore base that lies above and anterior to the orbit. In contrast, T. latus has unusually long, attenuated triangular squamosals and a more restricted horncore base. The otic notch is more open in T. utahensis than T. latus. The genus Torosaurus is distinguished from other chasmosaurine genera by a combination of characters including a broad, thin, sheetlike parietal with relatively small, nearly circular fenestrae and broad median parietal bar; convex posterior margin of parietal; and relatively straight postorbital horncores that are oval (elliptical) in cross section. Bona fide records of T. latus from Montana, South Dakota, and Wyoming are from strata of Lancian (late Maastrichtian) age. Previous reports of Torosaurus from the Naashoibito Member of the Ojo Alamo Formation (Lehman, 1981, 1985, 1996) in the San Juan Basin and the McRae Formation (Lucas et al., 1998), New Mexico, as well as the single Torosaurus record from Saskatchewan (Tokaryk, 1986), are based on specimens that can at best be identified as Chasmosaurinae genus indeterminate, because they lack derived features of the taxon. Putative Torosaurus specimens from the Big Bend region of Texas (Lawson, 1976; Lehman, 1996) are also considered as indeterminate chasmosaurines. All records of Torosaurus are Maastrichtian in age, but records of T. utahensis appear to be older than those of T. latus.

INTRODUCTION

Museum (YPM) paleontologist O. C. Marsh, in two classic articles (Marsh, 1891, 1892), described the remarkable ceratopsid dinosaur fossils that John Bell Hatcher collected between 1889 and 1892 in east-central Wyoming. Most of these specimens belong to Triceratops (Marsh, 1889b; Ostrom and Wellnhofer, 1986), but Hatcher’s collection included two skulls (YPM 1830 and 1831) for which Marsh (1891) coined the name Torosaurus (‘‘bull lizard’’). However, whereas Triceratops, in Marsh’s day and now, is one of the most common ceratopsids known from the Upper Cretaceous of western North America, Torosaurus Marsh, 1891 remains a relatively rare taxon. More than a century after Marsh’s original descriptions (Marsh, 1891, 1892), the genus is known from less than six diagnostic specimens. The two Yale specimens, and the others that have been assigned to Torosaurus, come from Maastrichtian-age strata of South Dakota, Montana, Wyoming, and Utah. Other putative ‘‘Torosaurus’’ specimens, notably from New Mexico and Texas, are too fragmentary for generic assignment and are reexamined in detail below. Another putative specimen from Saskatchewan is also reevaluated below. Although much discussed in recent articles, especially by Lehman (1981, 1985, 1990, 1996), no critical reappraisal of all the material referred to Torosaurus has been undertaken. The purpose of this paper is to: 1) redescribe the holotype of Torosaurus utahensis (Gilmore, 1946); 2) critically review the nature and taxonomic assignment of specimens referred to Torosaurus latus (Marsh, 1891), T. utahensis, and T. sp.; 3) present a revised diagnosis of the genus Torosaurus; 4) reevaluate specimens previously referred to Torosaurus from New Mexico, Texas, and Saskatchewan; and 5) further discuss the biostratigraphic significance of the species of Torosaurus. We note here that there are two undescribed skulls presumably belonging to Torosaurus latus in the collections of the Museum of the Rockies (Bozeman). One of these specimens, a nearly complete skull (MOR 1122), will be especially critical to further understanding

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ALE PEABODY

the genus Torosaurus. Although the senior author has had the opportunity to examine this specimen, it will be described in detail elsewhere in a forthcoming paper by A. Farke. Institutional acronyms: ANSP: Academy of Natural Sciences, Philadelphia; EM: Eastend Museum, Saskatchewan; MOR: Museum of the Rockies, Bozeman; MPM: Milwaukee Public Museum, Milwaukee; NMMNH: New Mexico Museum of Natural History, Albuquerque; ROM: Royal Ontario Museum, Toronto; SMP: State Museum of Pennsylvania, Harrisburg; SSM: Saint Paul Science Museum, St. Paul; TMM: Texas Memorial Museum, University of Texas, Austin; UNM: University of New Mexico, Albuquerque; USNM: United States National Museum of National History, Washington, DC; YPM: Yale Peabody Museum, New Haven. PREVIOUS STUDIES

Marsh (1891) named the ceratopsid dinosaur Torosaurus latus for an incomplete skull (YPM 1830) and (undescribed) associated postcranium (Fig. 1). Marsh (1891) described the skull as having two (paired) ‘‘supra-temporal fontanelles’’ that are bordered laterally by the squamosals. He further characterized the squamosals as long and slender. A second, incomplete skull (YPM 1831) was also described by Marsh (1891) as having even longer and narrower squamosals, so he established the name Torosaurus gladius for that specimen. T. gladius was also characterized by Marsh (1891) as having an upright, short, obtuse nasal horncore and postorbital horncores that are oval in cross section. For many years, the two Yale specimens, which were collected from the Lance Formation, Niobrara County, Wyoming, were the only known specimens of this genus. Marsh (1892), Hatcher et al. (1907), and Lull (1933) reviewed Marsh’s original taxa and provided illustrations of the holotypes of T. latus and T. gladius but provided no new information on these taxa. In 1947, Colbert and Bump described a third skull (ANSP 15192) collected from the Hell Creek Formation of southeastern South Dakota (Harding County) during the summer of

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SULLIVAN ET AL.—TOROSAURUS

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FIGURE 1—Right oblique view of holotype of Torosaurus latus (Marsh, 1891) (YPM 1830). Skull length is 2.4 m.

1944. This specimen (Fig. 2.3), unlike the two described by Marsh, is nearly complete, lacking only the anterior part of the rostrum, right nasal, articular end of the quadrate, proximal, and distal regions of the left parietal, and a small portion of the parietal around the right fenestra (Colbert and Bump, 1947). The skull differs from the Yale specimens most strikingly in its smaller size, lack of a ‘‘preorbital fenestra’’ (present in the holotype of T. latus), and relatively shorter frill. Many of the differences Colbert and Bump (1947) observed among the three specimens were attributed to ontogenetic change, sexual dimorphism, or postmortem deformation. They further asserted that the three skulls are strikingly similar despite particular differences, most of which they considered to be taxonomically insignificant. Colbert and Bump (1947) revised the genus by synonymizing Torosaurus gladius with T. latus and presented a growth gradient for postorbital horncore length and squamosal length in the three specimens, noting an apparent ‘‘progressive backward shift in the base of the horn core in its relationship to the orbit’’ from ANSP 15192 to YPM 1830 to YPM 1831 (Colbert and Bump, 1947:105). During the previous year, C. W. Gilmore’s study on the reptilian fauna of the Upper Cretaceous North Horn Formation, Utah, appeared (Gilmore, 1946). Ceratopsid specimens described were collected over a period of years, starting in 1935, followed by expeditions in 1937 and 1939 (Gilmore, 1946). No additional ceratopsids have been collected from the North Horn Formation since. Parenthetically, his paper was still in preparation at the time of Gilmore’s death in 1945, so it was not completed by him. Gilmore (1946) documented the Utah occurrence of the sauropod dinosaur Alamosaurus sanjuanensis, and described an incomplete ceratopsid skull (Fig. 2.1), together with axial and appendicular elements of a new ceratopsid that he tentatively referred to Arrhinoceratops Parks, 1925, as Arrhinoceratops? utahensis. The Utah ceratopsid fossils encompassed incomplete skeletal material

from 11 individuals, but only one of the specimens was considered complete enough for assignment to the genus (Gilmore, 1946). The holotype specimen (USNM 15583) includes the right squamosal, right quadrate, right quadratojugal, right postorbital horncore, right ‘‘postfrontal,’’ right lacrimal, right jugal, and right epijugal. The posterior portion of a parietal was provisionally associated with the holotype. In his description of the specimen, Gilmore (1946:42) listed additional material that was found in association with the holotype. These elements include a ‘‘pair of lower jaws, fragmentary parts of a maxillary, premaxillary, and pterygoid; 13 epoccipitals; posterior parts of the three parietals; coossified atlas, axis and third cervical (syncervical); 5 dorsal vertebrae; 1 cervical rib, 8 thoracic ribs, and numerous fragments.’’ Much of this supplemental material was initially catalogued (in 1937) as part of the holotype (R. Purdy, personal commun., 2002), but we retain it as separate from the material specified as holotype by Gilmore. Some of the additional material is described here for the first time. Gilmore recognized the posterior segments of three separate frills, indicating that the remains of at least three individuals were collected in a small area of 11 m2. Gilmore (1946) noted that the ‘‘greater relative width of the posterior portion of the squamosal’’ of USNM 15583 (Fig. 3) differs from those of Chasmosaurus (Lambe, 1914), Pentaceratops (Osborn, 1923) and Torosaurus, which have narrow, tapering distal ends, and he further believed that the squamosal of USNM 15583 compares well with that of Triceratops. However, the presence of paired parietal fenestrae precluded its placement within the latter taxon. By default, Gilmore (1946) questionably assigned the North Horn chasmosaurine to Arrhinoceratops, as Arrhinoceratops? utahensis, noting it differed from Arrhinoceratops brachyops Parks, 1925 in having postorbital horncores strongly

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FIGURE 2—Right lateral views of the skulls of: 1, holotype of Arrhinoceratops? (⫽Torosaurus) utahensis Gilmore, 1946 (USNM 15583); 2, cast of the holotype of Arrhinoceratops brachyops Parks, 1925 (ROM 796); and 3, Torosaurus latus, referred specimen (ANSP 15192). Abbreviations: j, jugal; pa, parietal; phc, postorbital horncore; sq, squamosal. Scale bars equal 10 cm.

SULLIVAN ET AL.—TOROSAURUS

FIGURE 3—Torosaurus utahensis Gilmore, 1946. USNM 15583 (holotype), lateral (external) view of right squamosal. Scale bar equals 10 cm.

curved forward. In contrast, the latter species has postorbital horncores that are strongly turned outward (Parks, 1925). Thirty years later, Lawson reevaluated the fenestrated frill shapes in ceratopsians and identified three types: 1) triangular; 2) figure-eight; and 3) broad frill with elliptical or circular fenestrae. He included Torosaurus in the third group, and further characterized the taxon as having a ‘‘cardioid frill, weakly developed epoccipitals, thinner parietals and reduced vascular sulci’’ (Lawson, 1976:162). Lawson reassigned Arrhinoceratops? utahensis to Torosaurus, and characterized T. utahensis as having: 1) anteriorly placed postorbital horns; 2) straight, diagonal, and longitudinal vascular sulci on the parietals; 3) distinct epoccipitals only on the anterolateral end of the squamosal(s); and 4) a squamosal proportionally shorter than in T. latus. He also referred an incomplete parietal (TMM 41480–1) from the lower third of the Javelina Formation (Tornillo Group), Big Bend region of Texas, to T. utahensis. This latter specimen was recently reidentified by Farke (2002) as cf. Torosaurus sp. (see below). Lehman (1981), in his description and discussion of the dinosaurs that comprise the Alamo Wash local fauna from the Naashoibito Member (Ojo Alamo Formation, previously Kirtland Formation), San Juan Basin, New Mexico, referred two specimens to aff. Torosaurus utahensis. These are a partial left squamosal,

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NMMNH P-22884 (formerly UNM B-628), and a postero-‘‘medial’’ part of a parietal, NMMNH P-25074 (formerly UNM FKK013), that he claimed resembled T. utahensis. In a subsequent paper, Lehman (1985) listed several Torosaurus (together with ?Pentaceratops and Alamosaurus [Gilmore, 1922]) specimens and plotted their stratigraphic positions (Lehman, 1985, fig. 3). Later, Lucas et al. (1987) illustrated additional San Juan Basin ceratopsian material, including an incomplete postorbital horncore (NMMNH P-32615, previously UNM FKK-082), which they and Lehman (1985) referred to Torosaurus, cf. T. utahensis. This specimen was recently reevaluated by Farke (2002), who considered it to be an indeterminate chasmosaurine. The other specimens, some of which were referred to Pentaceratops and were noted as coming from the Naashoibito Member, are reviewed below. Tokaryk (1986) assigned a relatively complete frill (EM P16.1), including both squamosals and a parietal lacking the median parietal bar, to Torosaurus sp. The specimen, collected from the Frenchman River Valley (Frenchman Formation), southwestern Saskatchewan, is broad, with relatively large and oblong parietal fenestrae, paired squamosal fenestrae, a median indentation in the posterior parietal border, a thickened squamosal bar along the squamosal-parietal suture, and a lack of epoccipitals. These characters led Tokaryk (1986) to assign the specimen, despite its distinctive morphology, to Torosaurus sp. Tokaryk believed it had closer affinities to T. latus than to T. utahensis based on the lack of epoccipitals and the larger size of the squamosals. However, we review the taxonomic assignment of this specimen (below) and remove it from the genus Torosaurus. Lehman (1990) suggested that Torosaurus utahensis may be indistinguishable from T. latus. In the same year, Dodson and Currie (1990, table 29.1) informally placed T. utahensis in synonymy with T. latus without comment or justification. Lehman (1996) later changed his mind, retained T. utahensis and T. latus as distinct taxa and provided a revised diagnosis for T. utahensis. Lucas et al. (1998) recently described an incomplete skull and postcranium from the McRae Formation in south-central New Mexico that they assigned to T. latus. More recently, Farke (2002) commented on the Texas and New Mexican specimens that previous workers had assigned to Torosaurus, noting that most are too incomplete to be diagnostic of the genus. These specimens, and Farke’s assessments of them, are reviewed below in detail. SYSTEMATIC PALEONTOLOGY

Order ORNITHISCHIA Seeley, 1888 Suborder CERATOPSIA Marsh, 1888 Family CERATOPSIDAE Marsh, 1888 Genus TOROSAURUS Marsh, 1891 Torosaurus MARSH, 1891, p. 266. Torosaurus MARSH, 1892, p. 81. Torosaurus HATCHER ET AL., 1907, p. 149. Torosaurus LULL, 1933, p. 130. Arrhinoceratops? GILMORE, 1946, p. 42. Torosaurus COLBERT AND BUMP, 1947, p. 103. Torosaurus LAWSON, 1976, p. 161. Torosaurus TYSON, 1981, p. 1247. non Torosaurus LEHMAN, 1981, p. 209. non Torosaurus LEHMAN, 1985, p. 77. non Torosaurus TOKARYK, 1986, p. 194, fig. 3. non Torosaurus LUCAS ET AL., 1987, p. 43. Torosaurus DODSON AND CURRIE, 1990, p. 612. Torosaurus (in part) LEHMAN, 1996, p. 501. non Torosaurus LUCAS ET AL., 1998, p. 224. Torosaurus FARKE, 2002, p. 52A.

Type species.Torosaurus latus Marsh, 1891 (⫽T. gladius Marsh, 1891).

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Included species.The type species and T. utahensis (Gilmore, 1946). Revised diagnosis.Large chasmosaurine (adult skull length ⬎2 m) that differs from other fenestrated chasmosaurines (Anchiceratops [Brown, 1914], Chasmosaurus, Pentaceratops) in possession of the combination of the following features: long face; broad, triangular squamosals bearing triangular epoccipitals; broad, thin, sheetlike parietal with near circular fenestrae, broad median bar, convex posterior margin, and lack of upturned paramedian epoccipitals; postorbital horncores relatively straight, strongly oval (elliptical) in cross section. Occurrence.Upper Cretaceous (Maastrichtian) of South Dakota, Montana, Wyoming, and Utah. Discussion.The diagnosis presented here distinguishes Torosaurus from the other fenestrated chasmosaurine genera Anchiceratops, Chasmosaurus, and Pentaceratops. The revised diagnosis of Torosaurus latus presented by Colbert and Bump (1947) is based on only three incomplete skulls, and is insufficient for generic distinction. Only the characters that pertain to the parietal and squamosals are informative, as suggested by Lehman (1996). New material (such as MOR 1122) will provide more information and will allow for a more robust diagnosis of the genus. Because T. utahensis had been previously and provisionally assigned to Arrhinoceratops? (Gilmore, 1946), we present a reassessment of the monotypic genus Arrhinoceratops (Parks, 1925; Tyson, 1981) below. Colbert and Bump (1947) first synonymized T. gladius and T. latus, a decision that has not been challenged and with which we concur. Lawson (1976) first transferred Gilmore’s (1946) species A.? utahensis to Torosaurus, and all subsequent workers have also upheld this decision. However, there has been some disagreement over whether T. latus and T utahensis are one or two species, and we discuss this below. TOROSAURUS LATUS (Marsh, 1891) Figures 1, 2.3 Torosaurus latus MARSH, 1891, p. 266. Torosaurus gladius MARSH, 1891, p. 266. Torosaurus latus MARSH, 1892, p. 81, pl. 2, fig. 1. Torosaurus gladius MARSH, 1892, pl. 2, fig. 2; pl. 3, fig. 1. Torosaurus latus HATCHER, MARSH, AND LULL, 1907, p. 150, fig. 118. Torosaurus gladius HATCHER, MARSH, AND LULL, 1907, p. 152, fig. 119; pl. 2, fig. 7; pl. 3, fig. 3. Torosaurus latus LULL, 1933, p. 130, fig. 42, pl. 15. Torosaurus gladius LULL, 1933, p. 132, pl. 16. Torosaurus latus (⫽T. gladius) COLBERT AND BUMP, 1947, p. 103, fig. 1, pls. 2–5. Torosaurus latus TYSON, 1981, p. 1247. Torosaurus latus (in part) DODSON AND CURRIE, 1990, p. 612, fig. 29.3f. Torosaurus gladius LEHMAN, 1990, fig. 16.2g, 16.12p. Torosaurus latus LEHMAN, 1990, fig. 16.14e. Torosaurus latus LEHMAN, 1996, figs. 5.3, 5.5, 7.7, 7.8. non Torosaurus latus LUCAS ET AL., 1998, p. 224, figs. 3, 4.

Revised diagnosis.A species of Torosaurus distinguished from T. utahensis and all other ceratopsids by a relatively longer, narrower, and distally tapered squamosal; the lateral edge forms an anteroventrally directed squamosal process resulting in a restricted otic notch; and postorbital horncores arising posteriorly behind the orbit so that the horncores form (in part) the dorsal border of the orbit; horncores directed subvertically. Description.In the holotypes of Torosaurus latus (YPM 1830) and T. gladius (YPM 1831), the anterior part of the skull (i.e., the area in front of the orbits) is missing, with the exception of the nasal horncore, which is preserved in both specimens. In YPM 1830, only the bases of the postorbital horncores and a small (less than one-third of the entire length) proximal portion of the left postorbital horncore are present. YPM 1831 was first

described as having only one postorbital horncore (Hatcher et al., 1907), but later as having both postorbital horncores preserved (Lull, 1933). We are unable to confirm this either way due to the reconstructed nature of the holotype. The postorbital horncores are directed anterolaterally. The bases are oval in cross section with the long axes running anteroposteriorly, and are situated above and slightly posterior to the orbit, with their anterior margins directly over the centers of the orbits. There is a pronounced dorsal orbital rim anterior to the each of the postorbital horncore bases. The bases of the postorbital horncores were described as ‘‘hollow’’ (Hatcher et al., 1907; Lull, 1933), but we are unable to confirm this. The jugals are not preserved in YPM 1830, and only part of the left jugal and epijugal are present in YPM 1831. The fragmentary nature of these elements in the latter specimen makes it difficult to compare them to those of the holotype of T. utahensis. Only the distal portions of the squamosals are preserved in YPM 1830 (Lull, 1933, p. 131, fig. 42). They are very long, narrow, and taper gradually to a point. The medial borders of the squamosals diverge anteroposteriorly, but distally the squamosals enclose the lateral sides of the parietal, becoming subparallel. The lateral borders of the squamosals are smooth, with slight undulations along their respective anterolateral edges. No epoccipitals are fused to the squamosals. YPM 1830 is missing the otic notch region (⫽quadratojugal notch of Hatcher et al., 1907), but it is preserved in YPM 1831 and is distinguished by having a welldeveloped anterolateral (squamosal) process (Hatcher et al., 1907, pl. 3, fig. 3). The parietal of YPM 1830 is incomplete; only the anterior portion immediately behind the postorbital horncores is preserved (see Hatcher et al., 1907, fig. 118). In contrast, the parietal of YPM 1831 (the holotype of T. gladius) is nearly complete, lacking only the peripheral regions along the lateral and posterior parts of the paired parietal fenestrae (⫽fontanelles, see Hatcher et al., 1907, pl. 2, fig. 7). We were unable to determine the extent of the reconstructed areas of the parietal in YPM 1830, and the distance between the parietal fenestra and the squamosal contact cannot be determined with certainty. The posterior edge of the parietal is somewhat incomplete, but enough of the element is preserved to indicate that it is convexly expanded posterior to the paired fenestrae. Moreover, the posterior edge is marked by a slight medial concavity, giving it a ‘‘cardioid’’ outline as described by Lawson (1976) for Torosaurus. Type.HolotypeYPM 1830, postcranial elements? and incomplete skull, missing the anterior region, the jugals and lower facial region, postorbital horncores and posterior part of the parietal (Fig. 1), Lightning Creek, Niobrara County, Wyoming, Lance Formation. Referred material.YPM 1831 (holotype of T. gladius): incomplete skull consisting of a relatively complete parietal, left squamosal, a postorbital and a nasal horncore, an epijugal, the occipital condyle, and miscellaneous skull fragments from the type locality (but stratigraphically lower in the Lance Formation), Lightning Creek, Niobrara County, Wyoming; ANSP 15192, nearly complete skull missing the anterior portion of the rostrum, right nasal (horncore?), articular part of left quadrate, most of right quadrate, parts of the parietal including around the fenestrae (Fig. 2.3), from the Hell Creek Formation, 16 km east of Camp Crook, Harding County, South Dakota; MOR 981, partial (unprepared) skull from the Hell Creek Formation, Dawson Co., Montana; MOR 1122, nearly complete skull from the lower quarter of the Hell Creek Formation, Petroleum Co., Montana; MPM VP6841, incomplete skull and postcranial skeleton from the Hell Creek Formation (MPM locality 3291), northeastern Montana; and SSM P74.6.1, incomplete frill, unknown horizon (presumably Hell Creek Formation, no specific locality), Hell Creek, Montana.

SULLIVAN ET AL.—TOROSAURUS Occurrence.Upper part of Lance Formation, Wyoming; Hell Creek Formation, Montana and South Dakota (late Maastrichtian). Discussion.Marsh (1891) described and diagnosed YPM 1830 and YPM 1831 as two separate species: Torosaurus latus and T. gladius. He named the second species T. gladius based on the bladelike shape of the squamosal (Marsh, 1891: 266). Colbert and Bump (1947) assigned a third skull (ANSP 15192) to T. latus and considered T. gladius a junior synonym of T. latus. Lehman (1990) considered T. latus a female and ‘‘T. gladius’’ a male individual of a single species, thus corroborating the synonymy of T. gladius with T. latus. Both holotypes are very similar, and the differences in the morphologies of the skulls perceived by Marsh (1891, 1892) can be attributed to individual variation and, in part, to their incomplete nature and to postmortem deformation. Colbert and Bump (1947) adequately redescribed the Yale material and the referred skull (ANSP 15192), and discussed in detail the differences among these specimens, so we restrict our comments to the discussion. We emphasize that although both holotypes of the Torosaurus species, T. latus (YPM 1830) and T. gladius (YPM 1831), are incomplete and have been extensively reconstructed with plaster (the extent of which is often difficult to determine), it is possible to characterize adequately the specimens and thus allow comparison with the holotype of T. utahensis. Another important neoceratopsian that needs to be considered in the context of Torosaurus is Arrhinoceratops brachyops (Fig 2.2) from the Horseshoe Canyon Formation of Alberta, Canada. Torosaurus latus differs from Arrhinoceratops brachyops in the following characters: narrower shape of the squamosal; a less robust jugal; more elongate rostrum; smaller, near circular infratemporal fenestrae; relatively straight postorbital horncores; and nasal horncore reduced and with dorsally directed apex. Some of these differences are also seen in T. utahensis, but due to its incomplete nature, other differences, notably those that pertain to the anterior portion of the skull (in front of the orbits), are not demonstrable. TOROSAURUS

(Gilmore, 1946) 2.1, 3–7, 8.1

UTAHENSIS

Arrhinoceratops? utahensis GILMORE, 1946, p. 42, fig. 12, pls. 11–13. Torosaurus utahensis LAWSON, 1976, p. 162, text-fig. 5. Torosaurus utahensis TYSON, 1981, p. 1247. non aff. Torosaurus utahensis LEHMAN, 1981, p. 209, text-fig. 9.12, 9.13. non Torosaurus cf. T. utahensis LUCAS ET AL., 1987, p. 43, fig. 5a–d. Torosaurus latus (in part) DODSON AND CURRIE, 1990, p. 612. Torosaurus latus LEHMAN, 1990, fig. 16.14f. Torosaurus utahensis LEHMAN, 1996, p. 501, figs. 5.7, 6.1, 7.1, 7.7, 7.10, 8.1, 8.2, 9.4. non Torosaurus utahensis LEHMAN, 1996, figs. 6.2, 6.3, 7.6, 8.3, 8.4.

Revised diagnosis.Torosaurus utahensis differs from T. latus in having a relatively shorter and broader squamosal in which the posterior end is rounded off (not tapered), giving the squamosal a more rectangular shape; anterolateral process of the squamosal reduced, resulting in a more open, less constricted, otic notch; postorbital horecore bases deep and expanded over the orbit with horncore bases expanded anteriorly, thus forming a broad area of origin over the entire orbit. REDESCRIPTION OF THE HOLOTYPE OF TOROSAURUS UTAHENSIS

Gilmore (1946) only briefly described the holotype of Arrhinoceratops? (⫽ Torosaurus) utahensis and some of the referred specimens from the North Horn Formation. Here, we redescribe this material, and where necessary, point out some inconsistencies in Gilmore’s original descriptions. Lacrimal.The lacrimal (Fig. 6.5) is shaped somewhat like a parallelogram. A thickened rim forms the anterior border of the orbit. Posteriorly, there is a strong suture for contact with the

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jugal. Anteroventrally, the lacrimal contacts the maxilla along its ventral edge. Anterodorsally, there is a strong suture located near the orbital rim for articulation with the postorbital. Postorbital horncore.The postorbital horncore (Figs. 2.1, 4.1) is a very distinctive element in T. utahensis. Although the postorbital horncore is slightly crushed and distorted due to postmortem deformation, the morphology of this element is unique. The base is extremely broad and massive compared to that of other chasmosaurines. It lies superior to the orbit, extending from far behind the orbit to well anterior to it (Fig. 2.1). The anterior border of the postorbital horncore ascends obliquely approximately 45⬚ from the upper half of the orbit. It is oval in cross section, and has a postorbital horncore base circumference of approximately 52 cm, with a preserved postorbital horncore length of approximately 52 cm. The postorbital horncore is highly fractured, especially the region behind the orbit. As with the squamosal, these fractures have been in-filled with plaster; however, much of the detail of the surface texture is preserved. Its surface is partly covered by vascular sulci. The postorbital horncore tapers abruptly to a point, although the tip of the horncore is broken and missing (Figs. 2.1, 4.1). Jugal.The right jugal (Figs. 2.1, 6.2, 6.3) is nearly complete, missing only some peripheral parts along the jugal/maxilla suture. It is roughly T-shaped, the top extending from the posterior squamosal process anterior to the maxillary process. The bone is relatively thin, but has a distinctly thickened rim surrounding the ventral border of the orbit. The orbital rim is pierced by five irregularly spaced foramina. Anterior to the orbit is a suture for contact with the lacrimal. The jugal is slightly convex laterally and concave medially. The distal end of the descending process is marked by a rugose surface where the epijugal articulates with the jugal. On the medial surface there is a sightly thickened rugose surface for contact with the quadratojugal. Medially, the posterior process is marked by a depression where the squamosal and jugal join each other. The anteroventral maxillary process of the jugal has an irregular suture for the jugal-maxilla contact. Epijugal.The epijugal (Fig. 6.1, 6.4) is roughly triangular. It has a broad and bipartite medioventral surface for attachment to the ventral process of the jugal (dorsally) and the quadratojugal (ventrally). The lateral surface is irregular, somewhat rugose, and marked by vascular sulci. The lateral surface is concave, the ventral surface is rounded and expands anteriorly, and the medial surface is slightly convex. The tip is blunt, and when articulated, points posteroventrally. Quadratojugal.Although Gilmore (1946:46) assigned both quadratojugals to the holotype in his description, he only listed the right quadratojugal (Fig. 5.1) as pertaining to the holotype specimen (Gilmore, 1946:42). Therefore, we restrict our description to the right quadratojugal. The lateral surface of the right quadratojugal is highly irregular, with a heavy, slightly bulbous rugose boss, the surface of which articulates with the jugal and the epijugal. A distinct ridge rises dorsally from the boss on the quadratojugal and extends to the upper margin of the element. The dorsal sections of the lateral margins are broken, both anteriorly and posteriorly. The ascending portion of the quadratojugal, which articulates with the shaft of the quadrate, is missing. The medial surface of the quadratojugal, which contacts the quadrate ventrally, is somewhat concave and is, in part, characterized by having a rugose internal surface. Squamosal.The right squamosal (Figs. 2.1, 3, 8.1) is cradled (supported) in plaster along its entire internal (medial) surface. Only the external (lateral) surface is visible, and is highly fractured with the fragments cemented together with plaster. The texture of the external surface is relatively smooth, and we cannot

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FIGURE 4—Torosaurus utahensis. USNM 15583 (holotype), 1, Right postorbital horncore; USNM 494472 (previously referred to, but removed from, the holotype), 2, distal portion of incomplete parietal; USNM 494473, 3, anterior portion of the parietal; USNM 15583 (holotype), right quadrate, 4, posterior view, and 5, anterior view. Scale bars equal 10 cm.

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FIGURE 5—Torosaurus utahensis. USNM 15583 (holotype), 1, Right quadratojugal, lateral view; USNM 494473 (referred material), 2, left surangular, lateral view, and 3, medial view; 4, left articular, posteromedial view, and 5, dorsal view; and 6, left surangular articulated with left articular, superior view. Scale bars equal 5 cm.

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confirm the presence of vascular sulci if they were present, but note that plaster cement may have obscured these features. The squamosal measures 99 cm from its posterior tip to the jugal/postorbital/squamosal junction. It is broad for most of its length and rounded off at its posterior end. The external surface bears two depressions with an incipient ridge running anteroposteriorly midway for the entire length of the squamosal. It is unclear whether this ‘‘dual concave’’ morphology is natural or the result of postmortem deformation. Both the lateral and medial borders are (in part) curved anterolaterally. The lateral border is relatively complete, except along the posteriormost part, where some fragments are missing. The anterolateral border is nearly complete and bears an anteriorly directed squamosal process that is not as robust as in Torosaurus latus (YPM 1831, MOR 1122) and Arrhinoceratops brachyops (ROM 796). This suggests that it had a broadly open otic notch. The lateral border of the squamosal is relatively smooth, with only minor undulations. One partial epoccipital is fused along the lateral margin toward the anterior end. There is no indication of attachment scars for the other epoccipitals. The anteromedial part of the squamosal is incomplete, but bears a distinct depression, or surface scar, for articulation with the jugal along its anterolateralmost edge. Lateral to this depression is the basal part of the broken squamosal process. The medial border of the squamosal is a sutural contact along its distal margin for about 20 cm for articulation with the parietal. Anteriorly, the medial margin of the squamosal is rounded for most of its extent. It becomes thin anteriorly where it articulates with the jugal. Quadrate.The quadrate (Fig. 4.4, 4.5) is nearly complete and somewhat irregular in appearance. It consists of a thickened, expanded transversely, anteroventral portion and a more slender dorsal shaft characterized by a conduit or expanded groove that runs along the entire length of the bone. The dorsal end flares laterally. A prominent lateral edge runs along the upper two-thirds of the quadrate, below which the quadratojugal articulates. Ventrally, the quadrate bears a lateral, concave surface for contact with the quadratojugal. Anteriorly, an articulation surface for the lower jaw is evident. Types.Holotype USNM 15583, right side of skull (Fig. 2.1) consisting of the right squamosal, postorbital horncore (i.e., postorbital ⫹ prefrontal, see below), jugal, quadratojugal, lacrimal, epijugal, and quadrate (Gilmore, 1946, fig. 12, pls. 11–13); paratype USNM 15875, a right squamosal and the posterior part of the parietal (Fig. 3). REFERRED MATERIAL REMOVED FROM THE HOLOTYPE

Parietal.The parietal (USNM 494472) that was provisionally referred to the holotype by Gilmore (1946) consists of an incomplete posterior edge that is missing part of the right side (Fig. 4.2). Parks (1925) noted ‘‘pseudosutural divisions’’ of the parietal in the holotype of Arrhinoceratops brachyops, and similar lines of separation in USNM 16572 were noted by Gilmore (1946). We confirm the existence of these so-called ‘‘pseudosutural’’ edges. One is clearly visible on the right side, and the other can be traced anteroposteriorly along the surface. An extensive part of the prominent suture along the left lateral side is preserved. The parietal is convex dorsoventrally, but shows a slight depression near the left side. The posterior border is broadly convex or curved and relatively smooth with some irregular minor undulations. The parietal surface is relatively smooth but is fractured and in-filled with plaster, so the presence of vascular sulci cannot be established with certainty. The bone is very thin (less than 1 cm), except near the lateral sutures. Contrary to Gilmore (1946), we cannot identify a ‘‘finished edge’’ that indicates the posterior edge of the parietal fenestra. Epoccipitals.Thirteen nearly complete epoccipitals (USNM

494473) found associated with the type material were originally numbered as such. Although Gilmore (1946) described them they were not assigned to the holotype. There are two morphotypes of epoccipitals. One is generally larger (despite some variation in size) elongate and ‘‘dorso-ventrally’’ compressed with a prominent medial apex. The articulation surface is concave for attachment to the frill. The outer surface is covered with vascular sulci. The other is smaller, is not laterally compressed, and lacks the medial apex. This latter morphotype is triangular in cross section and lacks the vascular impressions. Only four of the five epoccipitals reported by Gilmore (1946) remain. There are a number of associated fragments that seem to pertain to the first morphotype, but they are incomplete, making it impossible to determine the number of epoccipitals they represent. Referred material.From the North Horn Formation in Utah: USNM 494472, a posterior part of the parietal (Fig. 4.2) provisionally referred and catalogued as part of the holotype by Gilmore (1946:42); USNM 494473 (previously catalogued as part of the holotype, but not designated as part of the holotype, see Gilmore, 1946:42), left surangular (Fig. 5.2, 5.3, 5.6 [in part]) and articular (Fig. 5.4–5.6 [in part]), co-ossified atlas, axis, and third cervical vertebrae (Fig. 6.6), left and right dentaries with fused coronoids (Fig. 7), fragment of the maxilla margin, premaxilla fragment, pterygoid fragment, thirteen epoccipitals, three posterior parietal sections, five dorsal vertebrae, one cervical rib, eight thoracic ribs, and numerous fragments; USNM 16169, medial part of skull with one complete and one partial postorbital horncore, and parts of both squamosals; USNM 16573, posterior and median parts of parietal. Occurrence.Known solely from the North Horn Formation (lower part), Emry County, Utah (early Maastrichtian). See discussion below. DESCRIPTION OF SELECTED REMAINING REFERRED MATERIAL

The following elements were not referred to the holotype by Gilmore (1946) but were associated with the type material. They have been collectively assigned a single number (USNM 494473). Some of these elements are of interest as they add to our knowledge of T. utahensis. Other included elements, mostly postcranial bones (i.e., five dorsal vertebrae, one cervical rib, and eight thoracic ribs), are less informative, and are viewed by us as having little or no taxonomic value. We consider other associated bones (parts of maxillary, premaxillary, and pterygoid) too fragmentary to merit description. Right dentary ⫹ coronoid.The right dentary ⫹ coronoid (Fig. 7.3–7.5) are indistinguishably fused together and are nearly complete. However, both elements are badly fractured and slightly crushed in places. The tooth row of the dentary is missing the teeth, and most of the tooth-bearing region is badly damaged. The maximum length of the dentary is 51 cm. The Meckelian groove is located ventrally and extends anteriorly approximately halfway to the symphysis. There are four nutrient foramina visible laterally on the anterior portion of the dentary. Medially, the anteriormost edge preserves part of the symphysial surface. Anteriorly, there is a weakly developed surface located on the lateral margin, which may represent the posteriormost attachment for the keratinous sheath that covers the predentary. There are 31–32 tooth positions preserved, but we note that the posteriormost teeth are missing. The coronoid process measures approximately 15 cm above the parapet of the jaw. The top of the coronoid process is expanded anteroposteriorly, as in all ceratopsids. Left dentary ⫹ coronoid.The left dentary ⫹ coronoid (Fig. 7.1, 7.2) is less complete than its counterpart. Moreover, it is badly crushed and distorted. Many of the features described for the right dentary ⫹ coronoid are the same except for where the element has been damaged, crushed, or is missing. The overall

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FIGURE 6—Torosaurus utahensis. USNM 15583 (holotype), right epijugal, 1, lateral view; 4, medial view; right jugal, 2, lateral view, 3, medial view; 5, right lacrimal, (lateral view); and USNM 494473 (referred material), 6, syncervical (co-ossified atlas/axis/third cervical), left lateral view. Scale bars equal 5 cm for 1, 4, 5; 10 cm for 2, 3, 6.

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FIGURE 7—Torosaurus utahensis. Referred material: USNM 494473, left dentary, 1, lateral view; 2, occlusal view; right dentary and coronoid, 3, lateral view; 4, lingual view, and 5, ventral view. Scale bar equals 10 cm.

length of the dentary is 53 cm, which is approximately the same size as the right dentary, and it is probably from the same individual based on size. There are no teeth preserved, and the tooth row is too badly damaged to obtain a tooth count. The dorsal edge of the coronoid process is broken and missing. Surangular.The left surangular (Fig. 5.2, 5.3, 5.6, in part) is

nearly complete and has been repaired with plaster. It appears to articulate with the left dentary so it may be from the same individual and it also articulates with the left articular (see below). Three foramina pierce the left buccal surface. Anteriorly, the surangular is expanded where it articulates with the dentary ⫹ coronoid, and there is a pronounced groove posteroventrally for the

SULLIVAN ET AL.—TOROSAURUS reception of the dorsal border of the angular. Posteriorly, the surangular is squared-off, and wraps partly around the posterolateral part of the articular. In dorsal view, the articulation of the surangular and articular forms a depression for the reception of the distal end of the quadrate. Articular.The articular (Fig. 5.4–5.6, in part) is also from the left side and articulates with the left surangular, so it is likely from the same individual. It is an irregularly shaped element that is in part characterized by a high posteromedial wall. Anteriorly, the articular is characterized by a blunt, irregular surface that has an anteromedial (directly slightly ventrally) protuberance located on its medial side. The articular joins with the surangular laterally and the angular anteriorly. Parietal fragment.An incomplete parietal fragment (Fig. 4.3) was catalogued with the holotype of Torosaurus utahensis, but was not described or illustrated by Gilmore (1946). We interpret this element as part of the left anteromedial region of the parietal. The ventral surface of the element is embedded in plaster. The dorsal surface is fractured and the fractures are in-filled with plaster. Key features on the dorsal surface include a median ridge that becomes more distinct posteriorly. The right side of the median ridge is crushed laterally along its posterior half, and is partly folded under. Anterolateral to the anterior region is a large ovoid supratemporal fenestra that measures approximately 7.5 cm long and 3.3 cm wide at its maximum width. There is an oval pit located within the anteriormost margin of the (left) supratemporal fenestra. Anterior to the supratemporal fenestra, another pit (possibly foramen) opens anterolaterally. The roof of this pit is capped by the anteromedial portion of the parietal; the floor is characterized by a shelf of the parietal that extends 1.5 cm anterolaterally. The edge of this shelf bears suture for articulation with the postorbital horncore. The inferior border of the supratemporal fenestra is characterized by a prominent ridge that rises anterodorsally and contacts the posterolateral edge of the aforementioned parietal self. Behind the supratemporal fenestra there is a prominent ridge, or keel, that extends posterolaterally; inferior and lateral to this ridge is a smooth, shallow channel that leads into the supratemporal fenestra. Syncervical.Three cervical vertebrae—the atlas, axis, and third cervical—are fused to form a single complex (Fig. 6.6). There is no evidence of sutures between the individual vertebrae that make up this fused syncervical. However, some characteristic features are preserved on the atlas and third cervical vertebrae. This vertebral complex is crushed laterally, especially along the posterior two-thirds. Ventrally this region is characterized by a rather broad ridge. The first cervical, or atlas, is strongly procoelous, with a very deep concavity for the reception of the occipital condyle. The diapophysis is preserved on the left side and has been appressed just below the neural spine region. Only the proximal portion of the right diapophysis appears to be preserved on the right side of the atlas. Both the left and right cervical ribs are preserved on the atlas and are short and directed posteriorly. Osteological details of the axis vertebra are not evident due to the complete fusion of this element to the atlas and third cervical. The third cervical preserves basal portions of both cervical ribs, but the rib that lies adjacent to the neural spine has been displaced dorsally due to distortion. On the left side of the third cervical, the complete diapophysis is preserved, lying perpendicular to the axial region and directed lateroventrally. The posterior face of the centrum is nearly circular in its perimeter and is slightly depressed (concave), but for the most part appears to be opisthocoelous. Discussion.Gilmore (1946) formally listed only one paratype and questionably considered one of three individual parietal segments as belonging to the holotype of T. utahensis. Additional skeletal material (listed above and given the new number USNM 494473) was cited as possibly pertaining to the holotype, but was

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not so designated by him, nor was the material designated as a formal paratype. The epoccipitals, one of which is illustrated with the holotype number USNM 15583 (see Gilmore, 1946: pl. 13), were also not considered part of the holotype. In addition, Gilmore assigned a postfrontal to the holotype (Gilmore, 1946: 42) without further describing it, so it is unclear what element he referred to. This may be the part of the parietal described above. Parenthetically, we note here that Romer (1956) subsequently demonstrated that dinosaurs lacked a postfrontal bone. It is also curious that Gilmore listed additional ceratopsid material from the North Horn Formation that he presumably considered conspecific with the holotype and the single paratype, yet did not formally designate these as part of the type series. Here we take a conservative position, recognizing Gilmore’s holotype as consisting only of the elements cited above, minus the parietal. We note, however, that some of the material previously cataloged as USNM 15583 may also pertain to the holotype, but its association with the elements designated as the holotype by Gilmore (1946:42) cannot be demonstrated. We attribute this shortcoming and apparent inconsistencies in Gilmore’s paper as the direct result of it being published posthumously. Some of the observations made earlier by Gilmore (1946) can no longer be corroborated due to subsequent damage to the specimen. Moreover, there are also some inconsistencies, such as the supposed presence of ‘‘pseudosutural divisions’’ within the parietal forming an ‘‘interparietal bone’’ in Torosaurus (Arrhinoceratops?) utahensis that had been previously noted by Parks (1925) as present in the holotype of Arrhinoceratops brachyops (Fig. 2.2). In the same paper, Gilmore (1946:45) subsequently claimed ‘‘there is no indication of these lines of separation in either of the other two parietals (USNM 15583 and 15875).’’ Gilmore (1946:42) tentatively assigned the holotype of T. utahensis to the genus Arrhinoceratops? primarily based solely on three characters: 1) presence of a thin, flattened, subquadrangular ‘‘crest’’ (i.e., frill); 2) large postorbital (supraorbital) horncore that is directed strongly forward; and 3) a long, wide squamosal. He noted that if the holotype were more complete that reference to a genus other than Arrhinoceratops might be possible. He further distinguished this taxon from Chasmosaurus, Pentaceratops, and Torosaurus based on the ‘‘greater relative width of the posterior portion of the squamosal’’ (Gilmore, 1946:46). There are, however, several distinct differences between A. brachyops and T. utahensis worth noting. First, although the parietal is comparably thin in T. utahensis, the shape of the posterior border of the parietal is not straight (subquadrangular) as in A. brachyops. All three parietal specimens found with the holotype of T. utahensis are characterized by a curved posterior border similar to Torosaurus latus. However, the posterior border of the frill of the holotype of T. utahensis is not demonstrably ‘‘cardioid,’’ as indicated by Lawson (1976). Therefore, it is not demonstrable that the Texas (parietal) specimen (TMM 41480–1) is Torosaurus utahensis based on this feature, nor can it be referred to A. brachyops. We also note that the parietal border of TMM 41480–1 is more curved than the three incomplete parietals associated with the holotype of T. utahensis, suggesting that the two specimens are not the same taxon. The second character noted by Gilmore (1946) was the strong, anteriorly directed postorbital horncore in T. utahensis. However, the postorbital horncores are distinctly different from those of A. brachyops and other chasmosaurines. The postorbital horncores in A. brachyops are more conical, especially at the base where they rise just above the orbit, and they are slightly anterodorsally arched. In contrast, the postorbital horncores of T. utahensis each have an expanded base, rising posterodorsally over the orbit, with relatively straight horncores directed anterodorsally. Moreover,

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the anteroventral borders of the postorbital horncores each originate at the lacrimal/prefrontal-supraorbital junction, which is located midway along the orbital margin. Although Colbert and Bump (1947) noted that relative positions of the postorbital horncores in the three (then known) specimens of Torosaurus latus vary slightly in position and direction (attributed to a growth series), Lehman (1990) believed these differences to be the result of sexual dimorphism. The postorbital horncores of T. utahensis are distinctly different in their morphology, which we interpret as being outside the range of individual variation seen in other specimens of T. latus or other chasmosaurines, and we consider this an apomorphic character state of T. utahensis. We note, too, that the orientation of the bones of the skull, as depicted in Figure 2, is correct, based on the articulation of the jugal, lacrimal, postorbital horncore, and squamosal. This is the same orientation as illustrated by Gilmore (1946, fig. 12). The bases of the postorbital horncores are round in cross section in A. brachyops, but are distinctly oval in cross section in T. utahensis, and to a lesser extent, in T. latus. We note that the postorbital horncore of T. utahensis (USNM 15583) is crushed due to postmortem deformation, but this crushing is not responsible for the oval nature of the cross section because the base is so broad. The distal half of the postorbital horncore is also crushed and distorted, but in a more dorsoventral, rather than lateral direction. Finally, the third character of T. utahensis, the long and wide squamosal (Gilmore, 1946), also differs from A. brachyops. The squamosal of T. utahensis is broad for most of its entire length, and rounded off at the distal end. In A. brachyops the squamosal is broad anteriorly, but tapers posteriorly, giving it a more triangular shape. The squamosal of A. brachyops has a more pronounced lateral process and a deeper otic notch. At the distal end, the right squamosal of A. brachyops has a distinct medial hooklike process for articulation with the parietal. This unique feature is not evident on the (somewhat crushed) left side, and we are not able to reconcile its asymmetrical occurrence. Other features are also worth noting. The parietal fenestrae are restricted to the parietal and do not contact the squamosal laterally, and these paired fenestrae are oval to round in Torosaurus latus (based on ANSP 15192). The round morphology is corroborated by a newly discovered and undescribed specimen (MOR 1122: Fig. 1.2). The oval shape in ANSP 15192 is probably due to the lateral crushing of the skull and frill described by Colbert and Bump (1947). The actual geometry of the parietal fenestrae in T. utahensis is not known with certainty, but has been interpreted by some (notably Lawson, 1976) as being similar to T. latus. We note that one specimen (EM P16.1) previously assigned to Torosaurus, has wedge-shaped, not circular, fenestrae (see Tokaryk, 1986: fig. 3), which is more reminiscent of A. brachyops (see below). The squamosal of T. latus (YPM 1830, YPM 1831, and ANSP 15192) is different from other chasmosaurines in that it is narrower anteriorly and has a larger, pointed anterolateral process. In MOR 1122, this process is directed anterolaterally toward the epijugal, forming a more enclosed (constricted) otic notch. In addition, the squamosal has a thickened rim along the squamosalparietal suture, which is more pronounced in T. latus than it is in T. utahensis. These differences further differentiate the two species. As noted by Gilmore (1946), the jugal of T. utahensis differs greatly from that of A. brachyops. This element was described by Parks (1925:8) as ‘‘a very stout [robust] bone characterized by an unusually long anterior process overlapping the maxillary.’’ Although Gilmore (1946) expressed doubt about Parks’s interpretation, he recognized its potential importance for distinguishing

between T. utahensis and A. brachyops. Tyson (1981) later pointed out that Parks not only exaggerated the prominence of the anterior process in his illustration, but also failed to notice that the left jugal did not show the same, unusually long anterior process (Parks’s description was primarily based on the right side of the skull). Tyson thought the left side reflected the natural situation. She also postulated that the unusual pathology of the right jugal may be due to some injury that occurred when the individual was young, and that its aberrant morphology developed during maturity. However, because not only the anterior, but also the posterior process seems incomplete on the left side, it is more plausible that this side was damaged and that the right side reflects the true morphology, though the jugal is not as robust as Parks (1925) described it. The posterior processes of both jugals in the holotype of A. brachyops are extremely robust compared to the right jugal of T. utahensis. The jugal forms the anterodorsal border of the infratemporal fenestra, and differs from T. utahensis in that it is larger, and is squarer in shape, especially along the anterodorsal edge of the fenestra. In addition, the posterior and the descending process of the jugal form a sharp angle. Unfortunately, this region is restored in plaster in the type specimens of T. latus (YPM 1830) and ‘‘T. gladius’’ (YPM 1831), but is preserved in some referred specimens (ANSP 15192, MOR 1122). In these specimens, the infratemporal fenestra has the same rounded anterodorsal edge as T. utahensis. The epijugal in A. brachyops differs from T. utahensis in that it is more conical, directed lateroventrally instead of dorsoventrally, and is attached to the jugal at a distinctive, laterally directed angle. Its sharper apex is not posteriorly curved, as in T. utahensis. The other characters Lawson (1976) used to assign Arrhinocertops? utahensis to Torosaurus are the deep reticulate sculpturing (vascular sulci) and ornamentation of the frill. Both are taxonomically ambiguous. The pattern of vascular sulci on the posterior part of the parietal, which is very pronounced in Arrhinoceratops and reduced in Torosaurus, is variable among the three parietals found associated with the type material of T. utahensis. Two of the specimens, including the type, have smooth upper and lower surfaces, whereas the paratype exhibits longitudinal vascular sinuses on at least one surface (Gilmore, 1946). Furthermore, the vascular sulci are conduits for blood vessels seen in many ceratopsids, so no taxonomic utility can be assigned to this feature. The epoccipitals are pronounced in A. brachyops, especially along the antero-lateral border of the squamosal, giving the edge of the frill an undulating appearance. They are so completely fused that Parks (1925) remarked that the frill is ‘‘slightly indented, but there is no sign of epoccipitals.’’ Only one epoccipital is fused to the type squamosal of T. utahensis, but several epoccipitals were found associated with it. This lack of fusion of the epoccipitals may in part be an indication of immaturity, also reflected in the lack of ossification among other skull elements (Lawson, 1976). No epoccipitals are reported for the type of T. latus (YPM-1830), nor for any of the referred specimens (YPM1831, ANSP 15192, TMM 41480–1), but they are clearly present in the very large specimen MOR 1122. For the present, this renders the degree of marginal ornamentation a nondistinctive character. While we note that the presence of epoccipitals is not distinctive, the shape, coupled with size and textural features, together may prove to be useful for taxonomic purposes. Lawson (1976) distinguished T. latus from T. utahensis based on the proportionately shorter squamosal of T. utahensis, the thinner frill of the Texas and Utah specimens (TMM 41480–1 and USNM 15583, respectively), and the relatively smaller distance between the parietal-squamosal sutures at the posterior edge of

SULLIVAN ET AL.—TOROSAURUS the frill of the Utah and Texas specimens of T. utahensis. He suggested that the differences in squamosal length may in fact be due to differences in ontogenetic age, but that geographic distribution would corroborate recognition of two species. Summary.Various workers have considered T. latus and T. utahensis to be distinct species (e.g., Lawson, 1976; Lehman, 1996), while others have considered them as a single species (Dodson and Currie, 1990; Lehman, 1990). Based on review of the above characters, we agree with Lawson’s (1976) decision to transfer A. utahensis to Torosaurus, as they share diagnostic characters. We also conclude that T. latus and T. utahensis are distinct species based on unique features of the squamosal and postorbital horncore. The morphology of the distal end of the squamosal differs within the two species of Torosaurus. The squamosals of the holotypes of T. latus (YPM 1830) and T. gladius (YPM 1831), as well as those of referred specimens (ANSP 15192 and MOR 1122), are relatively long and narrow and taper distally to a point. In contrast, the squamosal of the holotype of T. utahensis is relatively shorter and broader, with an abruptly rounded distal end, giving it a more ‘‘rectangular’’ shape. The squamosal of T. latus is characterized by a relatively long, flangelike anterolateral process that produces a more enclosed otic notch (as in MOR 1122, see Fig. 8). In contrast, the anterolateral process of the squamosal in T. utahensis is more blunt and broad, forming a more open otic notch. Also, the postorbital horncores in T. latus specimens have their bases immediately dorsal to the orbits, and their bases do not form the entire dorsal border of the orbits. In contrast, the postorbital horncores of specimens of T. utahensis are more anteriorly situated, and the bases form the entire dorsal rims of the orbits. We note that these differences are based on a relatively small sample size, fewer than five incomplete skulls of T. latus and one of T. utahensis. However, among the few specimens of T. latus, while there is ontogenetic variation (Colbert and Bump, 1947) and possible sexual dimorphic variation (Lehman, 1990), the squamosal and postorbital horncore morphology is consistent. This limited evidence suggests that these features can be used to diagnose the two species of Torosaurus. Lehman (1996) reached essentially the same conclusions regarding the differences in squamosal morphology in T. latus and T. utahensis. However, he emphasized differences in length may not adequately distinguish the two species. The external surface of parietal and squamosals of MOR 1122 is highly vascularized and the otic notch is restricted and not open as in the holotype of T. utahensis. As Lehman (1996: 504) concluded, the two Torosaurus species have somewhat different frill morphologies. Differences in postorbital horncore morphology noted here among T. latus and T. utahensis specimens are similar to the differences in postorbital horncore morphology of Chasmosaurus mariscalensis (Lehman, 1989) noted by Lehman (1990: fig. 16.11) as examples of sexual dimorphism. Lehman (1990) proposed that the putative male horncores in C. mariscalensis are more erect, situated directly above the orbit, whereas female horncores are inclined forward and more anteriorly placed over the orbit. The same variation is seen within the three specimens of T. latus, with YPM 1831 representing the male, and YPM 1830 and ANSP 15192 the female morph. Of the two horncores assigned to T. utahensis, USNM 16169 has been interpreted as representing the male morph and USNM 15583 the female morph (Lehman, 1990). However, in the sexually dimorphic horncores of C. mariscalensis the degree to which the horncore forms the dorsal rim of the orbit does not vary (Lehman, 1990: fig. 16.11). In Torosaurus it does vary and we judge the degree to which the horncore forms the dorsal rim of the orbit a taxonomically useful character, not a result of sexual dimorphism.

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REVIEW OF TOROSAURUS OCCURRENCES FROM THE UPPER CRETACEOUS OF NORTH AMERICA

Specimens of Torosaurus have also been reported from Upper Cretaceous rocks of New Mexico, Texas, and Saskatchewan. The incomplete nature of the specimens makes a reassessment of this material necessary. Here, we critically reevaluate the taxonomic assignments of these specimens. Specimens from the De-na-zin and Naashoibito members (Kirtland and Ojo Alamo formations) attributed to Torosaurus (and Pentaceratops, in part).Torosaurus has been reported from Upper Cretaceous rocks in the San Juan Basin and Elephant Butte Reservoir area of New Mexico (Lehman, 1981, 1985; Lozinsky et al., 1984; Lucas et al., 1987, 1998). However, these reports were based on a small sample (five specimens) of fragmentary material. Here, we review all specimens assigned to Torosaurus from New Mexico and discuss some specimens assigned to Pentaceratops by Lehman (1985). NMMNH P-21098 (UNM FKK-081) includes part of a left squamosal and left scapula, and is associated with a horncore, predentary, ribs, dorsal vertebra, and scapulocoracoid. The specimen was identified and illustrated as Pentaceratops sternbergii from the Naashoibito Member by Lucas et al. (1987: fig. 5e–f) and is from the De-na-zin Member according to Lehman (1993: 286) contra Lucas et al. (1987), with which we concur based on our reanalysis of the locality data. NMMNH P-21100 (UNM FKK-031) is an incomplete right squamosal (Fig. 8.4) collected from the De-na-zin Member, and was originally cited as coming from the Naashoibito Member (see ‘‘Appendix’’ in Lehman, 1985). It was identified as ‘‘Torosaurus cf. utahensis’’ by Lehman (1985), and subsequently as Torosaurus utahensis by Lehman (1993). The specimen is strikingly distinct, very large, and somewhat thin (15–42 mm thick). Its dorsal and ventral surfaces are smooth, with no evident vascular sulci. It also lacks epoccipitals. The anterolateral margin of the squamosal is squared-off, forming a distinct right angle. Consequently, the otic notch was not developed as in Torosaurus latus, as exemplified by MOR 1122. Farke (2002) believed reference of NMMNH P-21100 to Torosaurus was impossible due to the fragmentary nature of the specimen and referred it to ‘‘Chasmosaurinae indeterminate,’’ an identification with which we concur. NMMNH P-22884 (UNM B-628) is an incomplete left squamosal (Fig. 8.3). This specimen was first identified as ‘‘aff. Torosaurus utahensis’’ by Lehman (1981, text-fig. 9.12) and was subsequently identified and figured as Torosaurus cf. T. utahensis by Lucas et al. (1987: fig. 5b–c), who stated it came from the Naashoibito Member (now included in the Ojo Alamo Sandstone). This incomplete left squamosal measures 82 cm along its medial edge where it joins the parietal. The medial edge is poorly preserved, fragmented for its entire length. The lateral border is 65 cm long. The dorsal and ventral surfaces are smooth, and there are no indications of vascular sulci that in part characterize the dorsal surfaces of the skull seen in many ceratopsids, including Pentaceratops, Chasmosaurus, Torosaurus (based on MOR 1122), and Triceratops. Farke (2002) noted that the neck, or base of the squamosal, is conspicuously longer than in Torosaurus utahensis and that the nature of the material is too incomplete to allow generic identification. We agree with Farke’s observation that the base is broader than in the holotype of T. utahensis. Moreover, the otic notch is twice the size of that in USNM 15583, and the squamosal/quadratojugal/jugal bar is relatively broad. The infratemporal fenestra is somewhat broad, based on the preserved edge of the squamosal. We again agree with Farke (2002) that the specimen cannot be referred to Torosaurus and that it is best identified as an indeterminate chasmosaurine. NMMNH P-25074 (FKK-013) is a partial right parietal. The

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FIGURE 8—Line drawings (to scale) of selected ceratopsian (right) squamosals (see text for discussion); regions between tick marks indicate broken edges. 1, Holotype of Torosaurus utahensis (USNM 15583); 2, holotype of Arrhinoceratops brachyops (ROM 796); 3, Chasmosaurinae indeterminate (NMMNH P-22884); 4, Torosaurus latus (MOR 1122) showing the enclosed (constricted) otic notch created by the convergence of the jugal (epijugal is not shown) and squamosal; 5, Chasmosaurinae indeterminate (NMMNH P-21100); and 6, Chasmosaurinae indeterminate (NMMNH P-25084). Abbreviations: j, jugal; itf, infratemporal fenestra; on, otic notch; sq, squamosal. Scale bar equals 10 cm.

specimen was referred to aff. Torosaurus utahensis by Lehman (1981: text-fig. 9.13), to Torosaurus cf. T. utahensis by Lucas et al. (1987: fig. 5d), and later to Torosaurus utahensis by Lehman (1996: fig. 7.6). This specimen was originally said to have come from the Naashoibito Member by Lehman (1985), but has since been determined to have come from the stratigraphically lower De-na-zin Member of the Kirtland Formation. Comparison to the holotype of Torosaurus utahensis and to the holotypes of Torosaurus latus (YPM 1830) and T. gladius (⫽T. latus) (YPM 1831), as well as to MOR 1122, shows that this parietal fragment differs significantly from others attributed to Torosaurus. It is thicker, lacks the vascular sulci on the dorsal surface, and the posterior edge has more pronounced scalloped

appearance, possibly due to the complete fusion of the epoccipitals. Moreover, the element is not symmetrical (see Lehman, 1981: fig. 9.13; Lucas et al., 1987: fig. 5d), and a prominent ridge extends posteriorly to the back of the frill, contrary to Lehman (1996), who believed it pertained to, and illustrated it as being, the medial part of the parietal. Curiously, both posterolateral sides are characterized by what appear to be distinct sutural surfaces, suggesting that the parietal contacted the squamosal on the right and was bound by another segment of the parietal medially (the so-called ‘‘pseudo-suture’’). This is consistent with the observation made by Parks (1925) on the frill of Arrhinoceratops brachyops, echoed by Gilmore (1946) for T. utahensis, that sutures extend posteriorly from the parietal

SULLIVAN ET AL.—TOROSAURUS fenestrae to the posterior edge of the parietal. While Tyson (1981) viewed these features as problematic in the holotype of A. brachyops (ROM 796), she concluded that it was unlikely that they were evidence of interparietal bones, because such elements are not known in other ceratopsids. Our assessment is that this element does not pertain to any currently recognized taxon, and despite the presence of this suture, reference to Arrhinoceratops or Torosaurus cannot be made based on other features of the parietal. We thus consider NMMNH P-25074 to be an indeterminate chasmosaurine. NMMNH P-25084 (UNM FKK-035) consists of a nearly complete right squamosal (Fig. 8.5) with associated parietal fragments, originally identified as a juvenile of Pentaceratops cf. P. sternbergii by Lehman (1981, 1985: fig. 12). The specimen, according to Lehman (1993: 287), was believed to be from the Naashoibito Member. However, the squamosal is covered with a distinct hematitic veneer that is commonly found on bones from the De-na-zin Member, suggesting that it is from the Kirtland Formation rather than the lower part of the Ojo Alamo Formation (sensu Bauer, 1916). Moreover, the taxonomic identification is also questionable (illustrated by Lehman, 1981: text-fig. 9.11; 1985: fig. 12) as first suggested by Rowe et al. (1981). Lehman (1993) referred to this squamosal as a Pentaceratops-like specimen, and believed it to pertain to a juvenile individual. However, unlike Pentaceratops it is short and squat, not long and tapered, as is the distinctive squamosal in all known Pentaceratops specimens where this element is preserved (Osborn, 1923; Wiman, 1930; Rowe et al., 1981). In addition, the squamosal is very thick, even taking into account the hematitic covering. Its ventral surface is depressed around the otic notch, and there is a pronounced groove extending along the medial edge, indicating strong attachment to the parietal. The groove is recessed ventrally and proximally for two-thirds of its length. Along the proximal third, the groove is directed medially. Distally, there is a prominent notch. There are no distinct epoccipitals, but the lateral edge of the parietal is characterized by undulations. Dorsally, there is a prominent ridge that runs parallel to the medial border as in NMMNH P-22884. Comparison of the outlines of NMMNH P-25084 to NMMNH P-228844 demonstrates that they differ slightly, especially in the regions of the otic notch and infratemporal fenestra. Clearly, this specimen does not pertain to Arrhinoceratops, Torosaurus, or for that matter, Pentaceratops, based on the morphology of this squamosal. We consider it an indeterminate chasmosaurine. NMMNH P-32615 (UNM FKK-082) is a postorbital supposedly associated with a braincase and skull fragments that were subsequently lost (Lehman, 1985). This specimen was first listed by Lehman (1985) as ‘‘Torosaurus cf. utahensis’’ without justification and subsequently illustrated and identified as Torosaurus cf. T. utahensis (Lucas et al., 1987: fig. 5a), based on its ‘‘relatively large size and [being] very slightly curved outward.’’ Later it was identified and illustrated by Lehman (1996: fig. 8.4) as Torosaurus utahensis. However, NMMNH P-32615 is not at all similar to the holotype of Torosaurus utahensis (USNM 15583) as it lacks an extremely wide base and does not taper to a slender point. However, it does share the general morphology found in a number of ceratopsid dinosaurs and therefore cannot be attributed to any particular genus, an opinion shared, in part, by Farke (2002). Unlike Farke (2002), who identified it as Chasmosaurine indeterminate, we believe the specimen is best referred to Ceratopsidae indeterminate. Specimens attributed to Torosaurus from the McRae Formation.Finally, Lucas et al. (1998) described and figured material from the McRae Formation, Sierra County, New Mexico. All the material is unnumbered and is housed in two collections: the New Mexico State University Museum and a private collection. The

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cranial material was presumed to be from a single individual; the association of the postcranial material is less certain (Lucas et al., 1998). Although plesiomorphically similar to Torosaurus latus, all the material is too incomplete and lacks the necessary features that would allow one to assign it confidently to any particular genus. The supraorbital horncore is unlike the strongly tapered, broadbased element seen in Torosaurus utahensis. Moreover, the horncore morphology is similar to that of Triceratops and other chasmosaurines. Despite the fact that the horncore base has an elliptical cross section, this morphology is not, in itself, diagnostic of Torosaurus. In their description of the jugal, Lucas et al. (1998) stated that the jugal ‘‘closely resembles’’ that of Torosaurus. However, no diagnostic features are present that would preclude reference to other ceratopsids such as Triceratops or Chasmosaurus. Lucas et al. (1998) claimed the squamosal ‘‘matches’’ the squamosals of Torosaurus, based on the ‘‘thin, blade-like’’ nature of the squamosal that ‘‘tapers to a posterior pointed tip.’’ These features, as indicated above, are seen widely among other ceratopsids and thus are not diagnostic of Torosaurus. The quadrate is also not distinctive in Torosaurus, and the morphology, as described by Lucas et al. (1998), is similar to all ceratopsids. We concur with Lucas et al. (1998) that the postcranial material is ceratopsid but disagree that the large size, fenestrated frill, lack of epoccipitals, long- and smooth-edge squamosals, and prominent epijugal process are diagnostic features that would allow generic identification as Torosaurus. Farke (2002) noted that the McRae Formation ceratopsid had a ‘‘thin, blade-like’’ squamosal, thus tentatively permitting assignment to ‘‘cf. Torosaurus sp.,’’ but we assess this material as being an indeterminate chasmosaurine. Specimen attributed to Torosaurus from the Frenchman Formation.Tokaryk (1986) described and illustrated an incomplete frill (EM P16.1) consisting of most of the parietal and both squamosals. The specimen is flattened and fractured, and the parietal bar restored. In fact, a large part of the left parietal and squamosal has also been restored with plaster. Tokaryk (1986) characterized the specimen as broad as and more quadrangular than other specimens of Torosaurus. He referred EM P16.1 to Torosaurus sp., but a few key features suggest otherwise. First, the parietal fenestrae are not circular, but are wedgeshaped, or oblong, although ‘‘they lack a definite edge’’ (as noted by Tokaryk, 1986), and are thus more like the condition seen in Arrhinoceratops brachyops. Second, the squamosals are broad anterolaterally, and distally they do not taper to a point, and thus are unlike those of T. latus. Third, if the restoration of the parietal (median) bar is correct, this narrow parietal bar is unlike the morphology of T. latus, where the median bar is very wide between the fenestrae. Tokaryk (1986) noted the presence of a distinct ridge along the parietal border of the squamosals. In addition, both squamosals are pierced by fenestrae that we interpret as pathological (also see Tanke and Farke, 2002) and thus have no taxonomic utility (as previously noted by Tokaryk, 1986). The specimen also lacks evidence of vascular sulci as noted by Tokaryk (1986), another feature that has no taxonomic value. Although Torkaryk (1986) recognized Arrhinoceratops as the only other similar taxon, he believed that this frill pertained to Torosaurus based on the large parietal fenestrae, large frill, and stout squamosal bar. We note here that none of these features, by themselves, permit reference to Torosaurus. Instead, the anteriorly broad squamosals and the wedge-shaped parietal fenestrae are arguably more like Arrhinoceratops brachyops. However, the specimen seems to display a mosaic of features that are shared by the two taxa. Therefore, we

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believe that generic assignment of this specimen is problematic, and it might represent a new taxon. Specimens attributed to Torosaurus from the Javelina Formation (Tornillo Group).Lawson (1976) identified a specimen (TMM 41480–1) consisting solely of an incomplete right parietal, as Torosaurus utahensis. His assignment of this parietal to T. utahensis was based on its: 1) being extremely thin; 2) having a smooth external (dorsal) surface with the posterior end marked by shallow vascular sulci; 3) having an interparietal bar with a sharp sagittal ridge, flattened internally (ventrally) and triangular in cross section; and 4) lacking epoccipitals. Another specimen, TMM 41835–1, consisting of an incomplete supraorbital horncore base with orbital region, was referred to Torosaurus utahensis by Lehman (1996). The former specimen lacks any diagnostic features, or combination of features, that would permit reference to T. utahensis. Moreover, these features are commonly seen in other ceratopsids, so reference to any specific ceratopsid is not possible. The latter specimen (TMM 41835–1) was referred to T. utahensis based on having ‘‘an extensive sinus in the base’’ (Lehman, 1996). Irrespective of any basal sinus, it is irrefutably clear from the illustrations (Lehman, 1996: fig. 8.1, 8.3) that the morphologies of USNM 15583 (holotype of T. utahensis) and TMM 41835–1 are so distinctly different (in both the morphology of the base as well as the overall shape of the horn) as to preclude reference to T. utahensis. Despite the incomplete nature of TMM 41835–1, it is clear that it is of a generalized ceratopsid. A similar assessment was recently made by Farke (2002) for the same specimen, which he referred to ‘‘Chasmosaurinae Indeterminate’’ based on its overall similarity to other chasmosaurines. We tentatively accept Farke’s assignment, but note that even this rather imprecise identification may be impossible to defend. Summary.Torosaurus utahensis is a distinct ceratopsid, known only from the holotype specimen, and in turn, known solely from its type locality in Utah. Claims of its presence in New Mexico and Texas were based on very fragmentary material and are here rejected. Torosaurus sp. from Saskatchewan is also not supported; the specimen may pertain to Arrhinoceratops, or perhaps to a new unnamed taxon. LATE CRETACEOUS CERATOPSID BIODIVERSITY AND BIOSTRATIGRAPHIC IMPLICATIONS

Many workers have advocated a Lancian (late Maastrichtian) age for the North Horn Formation based on the occurrence of Torosaurus. Because there are no unequivocal occurrences of Torosaurus utahensis outside the type area, ceratopsid-based correlations with the North Horn Formation can be viewed as problematic. Recent studies (i.e., Difley and Ekdale, 1999) have done little to clarify the stratigraphic context of the North Horn Formation vertebrate fauna described by Gilmore (1946). Other studies, notably Cifelli et al. (1999), are inconclusive with regard to the biostratigraphic position of the vertebrates from the Upper Cretaceous part of the North Horn Formation. The North Horn Formation sauropod ‘‘Alamosaurus sanjuanensis,’’ as currently understood, has been cited as having an unusually long range (late Campanian to late Maastrichtian), undermining its utility as an index fossil within the Late Cretaceous (Lucas and Sullivan, 2000). This applies to both the New Mexico and Texas sections, where correlation utilizing both Alamosaurus sanjuanensis and Torosaurus utahensis has been attempted. Clearly, biostratigraphic correlation must be made using unambiguous taxonomic identities for it to be meaningful. Species diversity of ceratopsids during the early Maastrichtian in North America is also not well understood. Moreover, the early Maastrichtian vertebrate faunas of North America are not well documented and therefore largely unknown. The North Horn Formation is a thick sequence of rock (spanning Late Cretaceous–

Paleocene time) that includes the dinosaurs Torosaurus utahensis and Alamosaurus sanjuanensis, as well as the teiid lizard Polyglyphanodon sternbergi (Gilmore, 1940), in the lower part of the section. These taxa are arguably older than, and not coeval with, the vertebrate faunas from the type Lance and Hell Creek formations, which are characterized, in part, by distinctly different taxa and which have been correlated to the late Maastrichtian. Therefore, it is unlikely that the vertebrate fauna of the North Horn is late Maastrichtian, despite claims to the contrary (Cifelli et al., 1999; Nydam, 1999). Moreover, the North Horn Torosaurus/Alamosaurus fauna has been previously identified and dated by other workers (i.e., Gilmore, 1946), in our opinion, correctly as equivalent to the Ojo Alamo Formation (‘‘Montana age’’), which is to say, they are partly coeval with the late Campanian– early Maastrichtian as defined by Lerbekmo and Braman (2002). This assessment is, in part, supported by the occurrence of Polyglyphanodon (Gilmore, 1940), which is unknown from the terrestrial vertebrate assemblages of the Lance and Hell Creek formations of the Western Interior. Despite minor specific differences between the late Campanian taxon Polyglyphanodon bajaensis (Nydam, 1999) (if valid, we note here that Nydam’s P. bajaensis was diagnosed solely on tooth morphology, which has proven to be variable among these fossil teiids [see Estes, 1983]) from the El Gallo Formation of Baja California Norte, Mexico, and Polyglyphanodon sternbergi from the North Horn Formation of Utah (see Nydam, 1999), the latter taxon suggests a time interval older than the type Lancian. Thus, the taxa T. utahensis and A. sanjuanensis, which are also absent in the type Lance faunas and cooccur in Utah with Polyglyphanodon sternbergi, suggest an age older than the type Lance vertebrate faunas (i.e., pre Lancian or pre-late Maastrichtian). Recently, McDowell et al. (2004) reported an age of 69 ⫾ 1.0 Ma, based on a uranium-lead analysis, for the occurrence of Alamosaurus within the Javelina Formation of Texas. This date places Alamosaurus within the late Campanian (Lerbekmo and Braman, 2002) and thus suggests a late Campanian to early Maastrichtian age for the Javelina, Ojo Alamo (Naashoibito Member) and lower part of the North Horn formations. In large part, the putative ‘‘Lancian’’ age of the Upper Cretaceous section of the North Horn Formation hinges on the identification of the Late Cretaceous marsupial Pediomys hatcheri (Clemens, 1961), based on a single worn tooth (M3) recovered from the Polyglyphanodon (⫽‘‘lizard’’ locality) (Clemens, 1961; Cifelli et al., 1999). Clemens (1961) noted in his description of this tooth that although it is ‘‘indistinguishable from the typotype M3’s of Pediomys hatcheri . . . the similarities may not prove to be diagnostic of the species or even the genus.’’ Another marsupial, Alphadon (Simpson, 1927), was also recovered nearby (Cifelli et al., 1999), presumably at a stratigraphically equivalent site. However, we note here that the species of both Pediomys and Alphadon are known to occur in strata that span a significant amount of geologic time (‘‘Aquilan’’ through ‘‘Lancian’’), as do other fossil mammal genera (Cimolomys [Marsh, 1889a], Paracimexomys [Archibald, 1982], Mesodma [Jepsen, 1940]; see Clemens, 1979), that were identified from the North Horn Formation based on very fragmentary material (Cifelli et al., 1999). In addition, faunal similarity, based on turtles including Compsemys (Leidy, 1856), which if correctly identified has been reported from the Campanian Fruitland and Kirtland formations (Armstrong-Ziegler, 1980; McCord, 1996), and eosuchians (which are also well known from other pre-Lancian vertebrate faunas), do not strengthen the correlation of the North Horn Formation vertebrate fauna to that of the Lance Formation as implied by Cifelli et al. (1999). In any case, the uranium-lead date reported by McDowell et al. (2004) clearly calls into question the supposed ‘‘Lancian ‘age’’’ for the Alamosaurus sanjuanensis–Torosaurus utahensis association.

SULLIVAN ET AL.—TOROSAURUS Torosaurus latus records from the Lance Formation of Wyoming, and the Hell Creek Formation of South Dakota and Montana are in strata of late Maastrichtian (Lancian) age. For this reason, most authors have considered all records of Torosaurus to be of late Maastrichtian age, and the genus has been identified as a Lancian index fossil by some workers (e.g., Lucas et al., 1987; Lucas, 1991; Lehman, 1996). However, by accepting the genus Torosaurus as strictly Lancian, rather than specifically the species T. latus, then one cannot properly reconcile the occurrences of the genus in older strata. While it appears certainly true, and acceptable, that Torosaurus latus is an index taxon for the late Maastrichtian, the genus arguably is not. Parenthetically, it has been Triceratops, not Torosaurus, which has been consistently used as the index taxon for the latest Cretaceous terrestrial North American sequence. In summary, we interpret the occurrence of the Torosaurus utahensis–Alamosaurus sanjuanensis association to be coeval with the occurrence of Alamosaurus sanjuanensis of the Javelina Formation of Texas, which places it near the Campanian–Maastrichtian boundary. We also interpret the San Juan Basin occurrence of Alamosaurus sanjuanensis to be the same age. ACKNOWLEDGMENTS

We thank M. A. Turner, M. Fox, and L. K. Murray (Peabody Museum of Natural History, Yale University); and R. Purdy (United States National Museum) for the loan of holotype and paratype material of Arrhinoceratops? utahensis. Special thanks are extended to J. Horner (Museum of the Rockies) for letting RMS examine and study MOR 1122. We thank A. Farke (South Dakota School of Mines) for providing us with the text of his study concerning the frills of the New Mexico and Texas ceratopsid material previously referred to Torosaurus and for the text from Tanke and Farke (2002). Special thanks are extended to J. Gardner and V. Lam (Royal Tyrrell Museum of Palaeontology, Drumheller) for digital photos of the cast of ROM 796. C. Baretto (University of Wisconsin, Milwaukee), T. Tokaryk (Royal Saskatchewan Museum Fossil Research Station, Eastend), and A. Redline (Science Museum of Minnesota, St. Paul) kindly provided us with photos and data of Torosaurus specimens from their respective institutions. We thank R. Difley for sharing her insights regarding the biostratigraphy and correlation of the North Horn Formation. We thank G. Olshevsky for his opinions on things etymological. Discussions with P. Dodson (University of Pennsylvania) were instructive and we thank him for his interest in our study. We also thank P. Dodson and an anonymous reviewer for their comments and criticisms, which improved the manuscript. REFERENCES

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