(dinosauria: theropoda) from the upper cretaceous (?campanian)

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EVIDENCE OF A GIANT TYRANNOSAURID (DINOSAURIA: THEROPODA) FROM THE UPPER CRETACEOUS (?CAMPANIAN) OF MONTANA MICHAEL A. URBAN

Department of Botany, 3165, University of Wyoming, 1000 East University Avenue, Laramie, WY 82071; ([email protected])

MATTHEW C. LAMANNA

Assistant Curator, Section of Vertebrate Paleontology, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, PA 15213; ([email protected])

ABSTRACT We report an isolated right lacrimal of a tyrannosaurid theropod dinosaur, probably from the Upper Cretaceous (upper Campanian) Judith River Formation of Fergus County, Montana. The lacrimal was originally associated with the holotype of the giant crocodylian Deinosuchus rugosus, but was later identified as that of a tyrannosaurid. It is of comparable size to the corresponding element in the gigantic Maastrichtian tyrannosaurid Tyrannosaurus. Moreover, comparison of the lacrimal to those of other tyrannosaurids tentatively supports its referral to this genus. Consequently, provided that its stratigraphic provenance has been correctly identified, the specimen represents the oldest-known North American record of a Tyrannosaurus-sized tyrannosaurid, and possibly the most ancient occurrence of this genus yet documented. KEY WORDS:—Dinosauria, Theropoda, Tyrannosauridae, Tyrannosaurus, Judith River Formation, Montana, Late Cretaceous, Campanian, lacrimal

INTRODUCTION In the summer of 1903, John Bell Hatcher, then of the U.S. Geological Survey, discovered a number of fossils in a section of the Upper Cretaceous (upper Campanian) Judith River Formation outcropping on Willow Creek in Fergus County, Montana, not far from the type locality of the unit. Later that year, Hatcher dispatched Carnegie Museum of Natural History collector W.H. Utterback to the locality to recover whatever fossils he could find there (Holland 1909). There Utterback collected what was to become the holotype specimen of the gigantic crocodylian Deinosuchus hatcheri (CM 963 (Holland 1909); now denominated D. rugosus (see Schwimmer 2002)) among other fragmentary fossil material pertaining to hadrosaurs, turtles, and a mosasaur. Apparently associated with the Deinosuchus material was a right lacrimal (Fig. 1) that Holland (1909) did not describe, even though he did note the presence of skull fragments with CM 963. In 1983, this lacrimal was informally identified by Dale A. Russell (then of the Canadian Museum of Nature) as belonging to the tyrannosaurid theropod dinosaur Tyrannosaurus, and was subsequently given a new collection number (CM 9401; McIntosh 1981). Interestingly, Molnar (1990:139) claimed that CM 9401 might in fact pertain to a fragmentary, presently undescribed Tyrannosaurus skeleton in the CM collections that was collected in 1902 from the Lance Formation of Niobrara County, Wyoming (CM 1400; McIntosh 1981). However, Molnar (1990) did not specify how he came to this conclusion.

To date, evidence of gigantic tyrannosaurids (defined here as those attaining a body length of > 12 m) in North America is confined to a single species, Tyrannosaurus rex, definitive records of which are stratigraphically restricted to the latter part of the Maastrichtian stage of the Late Cretaceous (~68–65.5 Ma, per Gradstein et al. 2004; Holtz 2004). CM 9401 is comparable in size to the lacrimals of large T. rex specimens (e.g., CM 9380, MOR 555; pers. obs.; Table 1, Fig. 2). Consequently, if its stratigraphic provenance has been correctly identified, CM 9401 would extend the temporal range of gigantic North American tyrannosaurids into the late Campanian (~75 Ma, Rogers and Kidwell 2000). Moreover, CM 9401 shares putative diagnostic characters with lacrimals of Tyrannosaurus, and may therefore represent the oldest record of this genus yet discovered. INSTITUTIONAL ABBREVIATIONS The following abbreviations are used throughout this paper: BHI—Black Hills Institute of Geological Research, Hill City, South Dakota; CM—Carnegie Museum of Natural History, Pittsburgh, Pennsylvania; FMNH—The Field Museum, Chicago, Illinois; LACM—Natural History Museum of Los Angeles County, Los Angeles, California; MOR—Museum of the Rockies, Montana State University, Bozeman, Montana; TMP—Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; ZPAL:—

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Fig. 1.—Right lacrimal of ?Tyrannosaurus sp., CM 9401, in lateral (A) and medial (B) views. [Figure modified from Hurum and Sabath 2003.]

Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland. SYSTEMATIC PALEONTOLOGY Class Dinosauria Owen, 1841 Order Theropoda Marsh, 1881 Family Tyrannosauridae Osborn, 1906 Genus ?Tyrannosaurus Osborn, 1905 ?Tyrannosaurus sp. (Fig. 1) Referred specimen.—CM 9401, an isolated right lacrimal. Locality and horizon.—Probably Willow Creek, Fergus County, Montana; Upper Cretaceous (late Campanian, ~75 Ma) Judith River Formation (see Discussion below).

DESCRIPTION AND COMPARISONS The tyrannosaurid lacrimal is an inverted L-shaped bone that forms the posterodorsal corner of the antorbital fenestra and separates this fenestra from the orbit. At its most anterior end, the maxillary process (= anterior process of Brochu 2002) of the lacrimal articulates with the maxilla. The lacrimal also articulates medially with

the nasal and posteriorly with the prefrontal and frontal. In some specimens, the prefrontal is not clearly discerned and it is assumed that during ontogeny it fuses with the lacrimal (Brochu 2002). The jugal process (= descending process of Brochu 2002) articulates with the jugal at its most ventral end. CM 9401 (Table 1, Fig. 1) is damaged and consequently lacks the anterior portion of the maxillary process. The preserved portion of this process is a roughly cylindrical structure that appears semilunate in dorsal view. The medial surface of the process is poorly preserved and partially restored. The maxillary process of CM 9401 lacks the prominent “lacrimal horn” (= cornual process) at its posterior apex that is found in many tyrannosaurids (e.g., Albertosaurus, Daspletosaurus, Gorgosaurus; Lambe 1917; Russell 1970; Brochu 2002; Currie 2003; Holtz 2004) (Fig. 2). Instead, as in Tyrannosaurus (Osborn 1912; Molnar 1990; Brochu 2002), Tarbosaurus (Maleev 1974; Hurum and Sabath 2003), and the basal tyrannosauroid Appalachiosaurus (Carr et al. 2005), there is a distinct rugosity along the dorsal surface of the bone that is especially pronounced at this apex. However, the apex appears much more “inflated” in Tyrannosaurus and CM 9401 than it does in Tarbosaurus (Currie et al. 2003). In Tyrannosaurus, the development of this rugosity is highly variable between

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Fig. 2.—Schematic drawings of tyrannosaurid lacrimals in right lateral view, to scale. A, ?Tyrannosaurus sp., CM 940; B, Tyrannosaurus rex, CM 9380; C, Tyrannosaurus rex, MOR 1125; D, Tyrannosaurus rex, MOR 555; E, Tarbosaurus bataar, ZPAL MgD-I/41; F, Daspletosaurus torosus, MOR 590; G, Gorgosaurus libratus, TMP 91.36.5002; H, Albertosaurus sarcophagus, TMP 81.10.12. [Figure modified from Currie et al. 2003.]

individuals, due to intraspecific (e.g., individual, ontogenetic, and/or sexual) variation (Osborn 1912; Carpenter 1990; Molnar 1990; Brochu 2002). As in other tyrannosaurids, the lacrimal foramen (= pneumatopore) of CM 9401 opens anteriorly into a recess (the antorbital fossa) at the posterodorsal corner of the antorbital fenestra, and is connected to a central cavity. The lacrimal foramen is small and subcircular, as in Daspletosaurus (Russell 1970; pers. obs. of MOR 590) and Tyrannosaurus (Molnar 1990; Brochu 2002; pers. obs. of CM 9380 and MOR 1125), being propor-

tionately smaller than that in Albertosaurus (Russell 1970; Currie 2003) and Tarbosaurus (Maleev 1974; Hurum and Sabath 2003) (Fig. 2). The lacrimal recess of CM 9401 is not as pronounced as it is in at least some individuals of Tyrannosaurus (pers. obs. of CM 9380, MOR 555, and MOR 1125). However, the depth of this recess appears to vary within Tyrannosaurus and other tyrannosaurid taxa (e.g., Daspletosaurus, pers. obs. of MOR 590). The jugal process is a thin, flat, and relatively smooth structure that widens ventrally towards its jugal articula-

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TABLE 1. Comparative measurements of tyrannosaurid lacrimals in centimeters. Taxon

Specimen

Anteroposterior Length

?Tyrannosaurus sp. CM 9401 29* Tyrannosaurus rex CM 9380 32 Tyrannosaurus rex MOR 555 24* Tyrannosaurus rex MOR 1125 28 Tarbosaurus bataar ZPAL MgD-I/4 35 Daspletosaurus torosus MOR 590 18* Albertosaurus sarcophagus TMP 81.10.1 16* Gorgosaurus libratus TMP 91.36.500 22 _________________ ¹ Maximum length of the maxillary process. ² Maximum length of the jugal process from the base of the maxillary process. *Specimen incomplete.

tion. The anterior edge of the widened section is convex and its posterior edge is concave. Medially, the process possesses an oblique, posteriorly curved ridge that extends posterodorsally-anteroventrally. Anterior to the dorsal area of the ridge is a small chamber that opens anteriorly and is connected posteriorly to the orbit via a small channel. It is possible that this channel housed the lacrimal duct (Molnar 1990). The jugal process of CM 9401 is noticeably anteroposteriorly thinner than that of some specimens of Tyrannosaurus rex (e.g., CM 9380), but is comparable to others (e.g., MOR 555, MOR 1125) and to Daspletosaurus (e.g., MOR 590) in this regard (Fig. 2). In definitive Tyrannosaurus lacrimals, the ventral portion of the jugal process thins mediolaterally; conversely, it thickens in CM 9401. Moreover, the oblique ridge on the medial surface of CM 9401 is stouter, more robust, and curves over the medial chamber to a much lesser extent than in CM 9380, the right lacrimal of LACM 23844 (Molnar 1990), and the left lacrimal of BHI 3033 (Hurum and Sabath 2003). The oblique ridge does not curve over the medial chamber in Tarbosaurus (Hurum and Sabath 2003). The medial aspect of the lacrimal is not sufficiently figured in other tyrannosaurid taxa. The medial chamber varies greatly in morphology and size among all the tyrannosaurid specimens examined, and even within the same individual. It is not present on a Daspletosaurus lacrimal examined (MOR 590). As in CM 9401, most Tyrannosaurus lacrimals exhibit a small foramen situated directly posteroventral to the medial chamber. However, unlike most other lacrimals studied here, the lateral surface of the jugal process of CM 9401 possesses a small, posteroventrally directed foramen situated within a deep groove on the posterior side. Brochu (2002) mentions a similar structure on a small tyrannosaurid lacrimal found with a Tyrannosaurus skeleton (FMNH PR2081) and argues that it may be vascular in nature.

Dorsoventral Height 33 27 28 23 19 16 17 14

DISCUSSION Regrettably, field documents for Utterback’s 1903 expedition cannot presently be located. Because of this, it is not possible to determine the stratigraphic provenance, and therefore the age, of CM 9401 with absolute certainty. Interestingly, based on correspondence from Hatcher, it appears that immediately prior to visiting the Willow Creek locality, Utterback prospected the Maastrichtian Hell Creek Formation of Musselshell County, Montana. It is therefore possible that CM 9401 was collected there or elsewhere, and later was incorrectly associated with material collected from Willow Creek. If this is the case, then the significance of CM 9401 would be diminished, as tyrannosaurids of comparable size (Tyrannosaurus rex) are already well known from the Maastrichtian of North America. Nevertheless, the preservation of CM 9401 resembles that of other material collected from Willow Creek in 1903, and therefore is consistent with the idea that the lacrimal was collected from this locality. Conversely, its preservation does not resemble that of any Hell Creek material in the CM collection. Unfortunately, however, unless better documentation can be obtained, there may always be uncertainty regarding the age of the specimen. Given its large size and lack of a cornual process, the morphology of CM 9401 is most similar to Tyrannosaurus rex among known tyrannosaurids (Russell 1970; Molnar 1990; Brochu 2002; Currie 2003). All known specimens of other tyrannosaurid species from the Campanian of Montana (i.e., Albertosaurus sarcophagus and Gorgosaurus libratus from the Judith River Formation and Daspletosaurus torosus from the Two Medicine Formation) are considerably smaller and possess this distinctive process (Russell 1970; Currie 2003) (Table 1, Fig. 2). However, if the Campanian age of CM 9401 is correct, then it would seem unlikely that the specimen could pertain to T. rex

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itself, given that indisputable records of this species are currently known exclusively from considerably younger (late Maastrichtian) deposits (Holtz 2004). Moreover, as detailed above, CM 9401 is distinct in several osteological aspects from the definitive T. rex lacrimals examined here. Consequently, although these discrepancies could conceivably represent individual, sexual, or populational variation (e.g., Carpenter 1990; Brochu 2002), it is possible that this lacrimal represents either a previously unrecognized species of Tyrannosaurus or, given its probable temporal distance from undisputed records of this genus, a closely related taxon. However, until additional large tyrannosaurid fossils are discovered from undoubted Campanian sediments of North America, these hypotheses will remain untested. ACKNOWLEDGMENTS We thank Amy Henrici, Yvonne Wilson, and Allen Shaw of Carnegie Museum of Natural History for their valuable assistance and technical advice. Laura Wilson (Montana State University) provided a number of detailed photos of tyrannosaurid lacrimals from the Museum of the Rockies for comparison. MAU also thanks Peter Larson (Black Hills Institute of Geological Research) and Nate Murphy (Judith River Dinosaur Institute) for additional advice. This paper benefited from critical reviews by Christopher Brochu (University of Iowa) and Jerald Harris (Dixie State College), as well as editorial comments by David Berman (Carnegie Museum of Natural History).

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