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Journal of Vertebrate Paleontology 25(2):469–472, June 2005 © 2005 by the Society of Vertebrate Paleontology

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TYRANNOSAURUS REX FROM THE UPPER CRETACEOUS (MAASTRICHTIAN) NORTH HORN FORMATION OF UTAH: BIOGEOGRAPHIC AND PALEOECOLOGIC IMPLICATIONS SCOTT D. SAMPSON and MARK A. LOEWEN, Utah Museum of Natural History and Department of Geology and Geophysics, University of Utah, 1390 East Presidents Circle, Salt Lake City, Utah 84112-0050, U.S.A.; ssampson@umnh.utah.edu, mloewen@umnh.utah.edu

The discovery of a Tyrannosaurus rex specimen from the Upper Cretaceous (Maastrichtian) North Horn Formation provides the first documented occurrence of this theropod dinosaur from an upland, intermontane basin, as well as the first example from Utah. The partial skeleton further represents the only conclusive co-occurrence of this giant carnivore with a sauropod, Alamosaurus sanjuanensis. Integration of this find with biogeographic and paleoecological data derived from other specimens indicates that T. rex was a top carnivore—and likely the only large-bodied (>1 ton) theropod—throughout most of the Western Interior of North America during the latest Maastrichtian, spanning habitats from wet lowland coastal plain environments to cooler, alluvial plain settings and semi-arid, upland intermontane basins. The apparently more restricted distributions of coeval dinosaurian herbivores suggest that T. rex exploited multiple prey species. Thus, the current evidence supports the notion that Tyrannosaurus was an ecological generalist, at least in the sense that it possessed an extensive species range that spanned a broad geographic area within the Western Interior of North America, encompassing diverse habitats and a range of prey species.

The Late Cretaceous North Horn Formation in central Utah consists largely of fluvial channel, overbank and floodplain sediments deposited in an intermontane basin (Fouch et al., 1983; Olsen, 1995; Loewen et al., 2001; Difley and Ekdale, 2002). The formation ranges from Maastrichtian through early Paleocene in age, dated largely on the basis of palynology and vertebrate indicators (Cifelli et al., 1999; Yi, 1989). North Horn sediments, typically associated with the onset of Laramide orogeny, were deposited in an upland basin bounded by the remnants of the Sevier orogeny to the west, the San Rafael Swell to the east, and the Uinta range to the north. Sedimentologic and stratigraphic evidence support the notion of a physiographic basin characterized by fluviolacustrine sedimentation (Fouch et al., 1983; Olsen, 1995).

Charles Gilmore conducted the first paleontological expeditions to the North Horn Formation during the 1930’s (Gilmore, 1942, 1946). These efforts produced a diversity of vertebrates, including several dinosaurs: a titanosaurid sauropod (Alamosaurus sanjuanensis), a ceratopsid dinosaur (later assigned to Torosaurus utahensis [Lawson, 1976; Lehman, 1996]), a hadrosaurid and a large theropod. Various institutions have undertaken fieldwork in this formation since the 1950s, most focusing on microvertebrate remains (Cifelli et al., 1999). These efforts revealed evidence of two small theropods—a troodontid and an unidentified taxon— as well as a second unidentified hadrosaurid (Cifelli et al., 1999). With the exception of Alamosaurus, represented by a large portion of the postcranial skeleton (Gilmore, 1946), virtually all of these taxa, including the theropods, were represented by fragmentary remains. Recent work in the same field area (North Horn Mountain) conducted by the University of Utah and the College of Eastern Utah produced additional ceratopsid and theropod materials that further elucidate the dinosaur fauna of the North Horn Formation. Below we describe the most significant of these finds—UMNH VP 11000, a partial skeleton of a large tyrannosaurid—and address the biogeographic and paleoecologic implications of this specimen.

Institutional Abbreviations: BHI, Black Hills Institute, Hill City, South Dakota; FMNH, Field Museum, Chicago, Ilinois; MOR, Museum of the Rockies, Bozeman, Montana; TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta; UMNH, Utah Museum of Natural History, Salt Lake City, Utah; ZPAL Paleobiology Institute, Academy of Sciences Warsaw, Poland.

SYSTEMATIC PALEONTOLOGY DINOSAURIA Owen 1842 SAURISCHIA Seeley 1888 THEROPODA Marsh 1881 TETANURAE Gauthier, 1986 TYRANNOSAURIDAE Osborn, 1906 TYRANNOSAURUS REX Osborn, 1905 DESCRIPTION UMNH 11000 consists of approximately 17% of a tyrannosaurid skeleton, including cranial and postcranial materials (Fig. 1). The cranial elements consist of a nearly complete right postorbital and squamosal (Fig. 1). The axial column appears to be represented by cervical vertebrae 3 and 4, sacral vertebrae 2, 3, and 4, and a series of mid-caudal vertebrae, perhaps ca10, 12, 13, 14, 15 and 16. Chevrons cranial to each of ca6, 13, 14, 15, 16 and 17 are present, along with a single mid-dorsal rib. The specimen includes several incomplete pelvic girdle elements, including part of the distal blade of the right ilium, the left pubic peduncle, and the proximal end of the left ischium. Hindlimb elements consist of left tibia, fibula, and astragalus. Elements of UMNH 11000 are within the size range of other large, more complete Tyrannosaurus specimens (e.g., MOR 555, BHI 3033, FMNH PR 2081). Some evidence of pathologic modification of bones is present in the fibula and four other unidentified elements.

Despite the incomplete nature of UMNH 11000, a combination of diagnostic features on its postorbital and squamosal provide strong evidence that this specimen represents Tyrannosaurus rex. The right postorbital is nearly complete, although the distal tip of the squamosal process was displaced and remained in articulation with the squamosal. As in other specimens of Tyrannosaurus (e.g., MOR 555, TMP 91.36.500, BHI 3033; Brochu, 2003), the caudal margin of the ventral ramus is nearly straight, forming an approximate right angle with the squamosal process. This condition contrasts with that of other tyrannosaurids (e.g., TMP 81.10.1, Albertosaurus; TMP 94.143.1, Daspletosaurus; ZPAL MgD-1/4, Tarbosaurus), in which the ventral ramus tends to be more convex and angled rostrally. Similarly, UMNH 11000 and specimens of Tyrannosaurus share the presence of a subcircular postorbital process or “horn”—sometimes associated with a capping accessory ossification, here termed the “cornual boss” (BHI3303, AMNH 5027)—positioned caudodorsal to the orbit (Fig. 1). A presumably homologous postorbital projection occurs in other tyrannosaurids, but tends to be more elongate and complex in shape (Currie, 2003a).

The squamosal of UMNH 11000 is also indistinguishable from that of Tyrannosaurus, possessing a relatively broad, elongate quadratojugal process (distalmost tip missing in UMNH 11000), a deeply incised contact surface for the postorbital, and a broad pneumatic exacavation of the squamosal body. Daspletosaurus also possesses a pneumatic excavation

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FIGURE 1. A. Skeletal restoration of UMNH 11000, Tyrannosaurus rex, with preserved elements highlighted. Right postorbital and squamosal of UMNH 11000 are shown separately in right lateral view. Abbreviations: co, cornual ossification; ltf, lateral temporal fenestra; o, orbit; PO, postorbital; qjp, quadratojugal process of squamosal; sp, suborbital process; SQ, squamosal. Scale bar equals 10 cm. B. Postorbitals and squamosals of various tyrannosaurids viewed in right lateral view: Gorgosaurus, TMP 91.36.500 (reflected and modified after Currie, 2003a); Albertosaurus, TMP 81.10.1 (reflected and modified after Currie 2003b); Daspletosaurus, combination of NMC 8506 and TMP 2001.36.1 (reflected and modified after Currie 2003b); Tarbosaurus, ZPAL MgD-1/4 (reflected and modified after Hurum and Sabath, 2003); Tyrannosaurus, FMNH PR 2081 (reflected and modified after Brochu, 2003) and UMNH VP 11000.

of the body, but this feature lacks the remarkable depth, or “inflated� condition, characteristic of Tyrannosaurus. Pneumatic excavation of the squamosal is also present in Tarbosaurus, but with a more restricted, circular foramen for passage of the air sac. The quadratojugal process is significantly broader and less curved than those of Gorgosaurus, Albertosaurus and Daspletosaurus, whereas it is longer than in Tarbosaurus (Currie, 2003a; Hurum and Sabath, 2003).

DISCUSSION Taxonomy, Biostratigraphy and Biogeography Stratigraphic evidence collected in the region of North Horn Mountain shows that UMNH 11000 occurs within 10 meters of Alamosaurus remains, and further that materials of the titanosaurid actually bracket those of Tyrannosaurus. Lacking evidence of significant environmental change within this interval, the stratigraphic data are interpreted as evidence that T. rex co-existed with Alamosaurus in this area, together with various other dinosaur taxa (e.g., Torosaurus utahensis, hadrosaurids, and small theropods). A sauropod-tyrannosaurid association is not without precedent, because the giant Asian tyrannosaurid Tarbosaurus bataar has been documented in co-occurrence with titanosaurid sauropods in the Maastrichtian Nemegt Formation (Weishampel, 1990).

The great majority of Tyrannosaurus rex specimens recovered to date occur in the northern portion of the Western Interior of North America—in particular Alberta, Saskatchewan, Montana, Wyoming, and South Dakota. Putative remains of this taxon have also been reported from more southern regions of the Western Interior, including Colorado (Carpenter and Young, in press), New Mexico (Carr and Williamson, 2000, 2004), and Texas (Lawson, 1976). However, these reports generally were based on fragmentary materials, or at least remains that were not confidently ascribed to genus and species.

The incomplete nature of many large theropod specimens raises the possibility that at least two distinct species of large theropod co-existed in western North America during the late Maastrichtian (Lehman, 1987).

However, to date, all of these purported, coeval, large-bodied taxa are founded on fragmentary data. Recent studies suggest that the diagnostic characters of one of these taxa, Aublysodon, are present in juvenile

tyrannosaurs of other genera (Currie, 2003a). Similarly, Stygivenator molnari and Dinotyrannus megagracilis have been regarded as juvenile specimens of Tyrannosaurus rex (Currie 2003b). Nanotyrannus presents a similar problem, as previous studies have questioned the validity of this taxon (Carr, 1999; Currie, 2003a; Brochu, 2003). Nonetheless, even if Nanotryannus represents a separate taxon, its small size almost certainly would have precluded its direct competition with the multi-ton T. rex. In short, current evidence is consistent with the presence of a single large theropod, Tyrannosaurus rex, in western North America during the Maastrichtian.

In contrast to the apparent dominance of a single top carnivore in the Western Interior of North America during the late Maastrichtian, late Campanian dinosaur faunas from the same region include at least two northern tyrannosaurids, Gorgosaurus and Daspletosaurus, and likely additional taxa in the south (Farlow and Pianka, 2003; Carr and Williamson, 2004). Thus, it appears that only one tyrannosaurid lineage persisted into the late Maastrichtian. While previous analyses (e.g., Holtz, 2001) have postulated the Asian form Tarbosaurus as the sister taxon of Tyrannosaurus, recent work (Currie, 2003a; Currie et al., 2003) suggests that a clade consisting of Tyrannosaurus + Nanotyrannus (also from the late Maastrichtian of North America) may be the sister group of Daspletosaurus. If so, this raises the possibility that Tyrannosaurus evolved from smaller-bodied North America ancestors closely allied to Daspletosaurus.

UMNH 11000 represents the most complete of the more southern Tyrannosaurus specimens, indicating that T. rex ranged over a large portion of the Western Interior. Additional, though less complete, specimens suggest an even more southerly range extension into southern New Mexico and Texas (Fig. 2; Lawson, 1976; Carr and Williamson, 2000, 2004). Biogeographic analogues among recent maximally-sized ectothermic and endothermic carnivores argue for an even larger geographic range for the species, perhaps encompassing all of North America (Burness et al., 2001). Moreover, assuming significant overlap in dietary resources, the co-existence of two or more giant theropods is improbable on ecological grounds, given the substantial energetic and geographic requirements of such large-bodied carnivores (Farlow, 1993; Farlow and Pianka, 2003).

NOTES Paleoecology Recovery of UMNH 11000 from the North Horn Formation, combined with the more southerly occurrences, suggest that Tyrannosaurus rex inhabited at least three distinct paleoenvironments (Fig. 2). The first consists of semiarid, upland, intermontane settings, characterized by fluvio-lacustrine sediments, red-beds, caliche and soils, all deposited within restricted basins between active Early Laramide uplifts (Lehman, 1987).

The North Horn and McRae formations are examples of this paleoenvironment. Second are the humid, wet coastal plain environments adjacent to the remnants of the Cretaceous Interior Seaway, exemplified by the Frenchman, Lance and Hell Creek formations. This setting, which has yielded the great majority of Tyrannosaurus specimens, is indicated by rare, organic-rich horizons, and fine-grained sediments consistent with distal fluvial sedimentation proximal to the seaway. Third, additional, but rarer specimens have been recovered from cooler, semiarid alluvial plain settings adjacent to the northeastern margin of the Cordilleran mountains. These facies, present in the Scollard and Willow Creek formations, are characterized by red-beds, caliche horizons, and soils.

Significantly, each of these paleoenvironments has been associated with a distinct herbivorous dinosaur fauna (Lehman, 1987, 1997, 2001).

The intermontane basins, which occur in the south-central portion of the Western Interior, can be characterized by an “Alamosaurus fauna,” in-

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cluding the titanosaurid Alamosaurus, the ceratopsid Torosaurus utahensis, and hadrosaurids (Lehman, 1987, 2001; Sloan, 1969). The nearshore coastal plain environments have been correlated with a “Triceratops fauna,” dominated by the ceratopsid Triceratops horridus but also including the hadrosaur Edmontosaurus annectens and several other dinosaur taxa (Russell, 1967; Sloan, 1969; Lehman, 1987, 2001). Finally, the alluvial plain settings correlate with a “Leptoceratops fauna,” including Leptoceratops, ankylosaurs, hadrosaurs, and rare finds of Triceratops (Russell, 1967; Lehman, 1987, 2001).

Thus, herbivorous dinosaurs apparently were not homogeneous in their distributions throughout the Western Interior of North America during the Late Maastrichtian. Rather, species of plant-eating dinosaurs appear to have been endemic to restricted latitudinal zones and, in at least some cases, to specific paleoenvironments within those zones.

Lehman (1987, 1997, 2001) made the strongest argument for dinosaur provincialism and habitat specificity during the Maastrichtian, extending this hypothesis to late Campanian faunas of the Western Interior. He posited further that this apparent provincialism was likely associated with floral endemism, in turn correlated with latitudinal and ecological variation in rainfall and air temperature (Lehman, 2001). Although a portion of the observed geographic variation in dinosaur distributions may be due to time transgressive sampling of the constituent formations—that is, representing non-overlapping time intervals (Sullivan,

FIGURE 2. Reconstruction of late Maastrichtian paleogeography, paleoenvironments, and dinosaur biogeography for the Western Interior of North America. A. Paleoenvironments mapped onto paleogeographic and biogeographic data: white ⳱ water (Pacific Ocean and Late Cretaceous Interior Seaway); light gray ⳱ seasonally moist coastal plain; white stipple ⳱ semiarid alluvial plain; gray stipple ⳱ semiarid upland, intermontane basins; and dark gray ⳱ upland thrust belts and early Laramide uplifts. Known paleogeographic distribution of three herbivorous dinosaurs are indicated as follows: Alamosaurus sanjuanensis occurrences indicated with 䊏; Leptoceratops gracilis occurrences indicated with a 䊉; and Triceratops horridus occurrences indicated with a 䉱. Numbers denote fossil sites within specific geologic formations. Geologic formations are indicated as follows: 1 ⳱ Scollard, 2 ⳱ Willow Creek, 3 ⳱ Frenchman, 4 ⳱ Hell Creek, 5 ⳱ Lance, 6 ⳱ Evanston, 7 ⳱ Laramie, 8 ⳱ North Horn, 9 ⳱ Denver, 10 ⳱ Kirtland Shale, 11 ⳱ McRae, 12 ⳱ El Picacho, 13 ⳱ Javelina. The North Horn Formation is further indicated by a star. Occurrences of fossil taxa are listed in Gillette et al. (1986), Lehman (1987) and Ryan and Russell (2001). B. Known paleogeographic distribution of the theropod Tyrannosaurus rex. Open circles denote occurrences, except for the North Horn Formation, indicated by a star.

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2003)—some of the observed provincialism, including that postulated for the Maastrichtian, is consistent with available data.

The evidence indicates that, in contrast to coeval species of planteating dinosaurs, Tyrannosaurus rex spanned an array of habitats within the Western Interior of North America. The discordance in geographic range size between T. rex on the one hand and ornithischian herbivores on the other further suggests that this gigantic theropod exploited a range of prey species. The co-occurrence of Tyrannosaurus and Alamosaurus is particularly intriguing. Alamosaurus represents an abrupt, latest Cretaceous reappearance of sauropods in North America, known only from the southwestern region of the Western Interior (Lehman, 2001).

Given the much greater body mass estimates of Alamosaurus as compared to Tyrannosaurus, this finding raises the possibility that, at least in some habitats, T. rex exploited prey of considerably larger body size in the form of titanosaurid sauropods. This is in contrast to earlier tyrannosaurids, which were restricted to prey of approximately the same body size or smaller. Nonetheless, it is noteworthy that ceratopsid and hadrosaurid remains co-occur with those of Tyrannosaurus in all of the above paleoenvironmental settings, opening the possibility that this giant carnivore exploited one or both of these groups preferentially.

With over 25 partial to complete skeletons, plus hundreds of isolated elements, the fossil record of Tyrannosaurus rex far exceeds that of any other giant theropod. The paleoecological data described herein suggest that this famous carnivore was an ecological generalist, inhabiting a vast geographic area together with a diversity of habitats and prey species.

Future discoveries of tyrannosaurid remains will provide valuable tests of the hypotheses presented herein, and materials of other giant theropod taxa will allow us to determine whether these patterns were applicable generally.

Acknowledgments—We thank R. Difley, M. Getty, M. Hayden, J. Kirkland, M. Castro, and volunteers from both the Utah Museum of Natural History and the Utah Friends of Paleontology for their generous assistance in fieldwork and specimen preparation. Thanks also to J. Defreest (Forest Service) for assistance with permitting and field logistics, and to R. Blakey for access to paleogeographic information. T. Carr, P. Currie, M. Carrano, R. Difley, D. Eberth, J. Farlow, M. Getty, M. J. Ryan, and two anonymous reviewers provided helpful discussions and/or reviews of earlier drafts of this manuscript. Access to comparative materials was kindly provided by P. Currie (Royal Tyrrell Museum of Paleontology). UMNH 11000 was discovered by Q. Saharatian and R. Difley (University of Utah) in the Manti La-Sal National Forest. Funding was provided by the National Science Foundation (grant DEB-9904045), Discovery Channel Quest, and the University of Utah.

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Submitted 5 January 2004; accepted 4 August 2004.