Evidence for high taxonomic and morphologic tyrannosauroid diversity in the Late Cretaceous (Late Campanian) of the American Southwest and a new short-skulled tyrannosaurid from the Kaiparowits formation of Utah
The fossil record of late Campanian tyrannosauroids of western North America has a geographic gap between the Northern Rocky Mountain Region (Montana, Alberta) and the Southwest (New Mexico, Utah). Until recently, diagnostic tyrannosauroids from the Southwest were unknown until the discovery of Bistahieversor sealeyi from the late Campanian of New Mexico. Here we describe an incomplete skull and postcranial skeleton of an unusual tyrannosaurid from the Kaiparowits Formation (Late Cretaceous) of Utah that represents a new genus and species, Teratophoneus curriei. Teratophoneus differs from other tyrannosauroids in having a short skull, as indicated by a short and steep maxilla, abrupt angle in the postorbital process of the jugal, laterally oriented paroccipital processes, short basicranium, and reduced number of teeth. Teratophoneus is the sister taxon of the Daspletosaurus + Tyrannosaurus clade and it is the most basal North American tyrannosaurine. The presence of Teratophoneus suggests that dinosaur faunas were regionally endemic in the west during the upper Campanian. The divergence in skull form seen in tyrannosaurines indicates that the skull in this clade had a wide range of adaptive morphotypes.
KeywordsDinosauria Theropoda Tyrannosauroidea Kaiparowits Skull
Tyrannosauroidea is a diverse lineage of predatory dinosaurs that includes small species from the Jurassic and Cretaceous, and large species from the Late Cretaceous (Rauhut 2004; Holtz 2004; Brusatte et al. 2010). Derived tyrannosauroids are large in size (∼9+ meters long), and they have massive skulls and thick teeth. Tyrannosaurids are a derived clade of large tyrannosauroids that includes Tyrannosaurinae, a lineage of mostly bulky species that includes Tyrannosaurus rex, the largest tyrannosauroid.
The Kaiparowits Formation is a non-marine sequence that was deposited upon a low-relief inland alluvial plain environment (Roberts et al. 2005). Radiometric dating demonstrates that it spans 76.1–74 Ma and thus dates to the Judithian North American Land Mammal Age (Cifelli et al. 2004; Roberts et al. 2005). It is contemporaneous with a number of well-studied late Campanian terrestrial vertebrate faunas of western North America (Gates et al. 2010).
Material and methods
Theropoda Marsh 1881
Tyrannosauridae Osborn 1905
Teratophoneus curriei gen. et sp. nov.
Teratos [Greek], a monster; phoneus, [Greek], murderer, in reference to the presumed predatory habits of this theropod; the species is named in honor of Dr. Philip Currie, in recognition of his scientific contributions to the knowledge of theropod osteology and systematics.
The holotype includes a partial skull and skeleton of one individual, but more than one specimen number was assigned to the bones. BYU 8120/9396 includes the left lacrimal, right jugal, right frontal, right squamosal, left otoccipital and proötic, right otoccipital, right basisphenoid, both quadrates, left articular, cervical vertebra, left scapula, and left coracoid; BYU 8120/9397 includes the left humerus and left ulna; BYU 826/9402 includes the left maxilla; BYU 9398 includes the left dentary; and BYU 13719 is assigned to the left femur.
Locality and horizon
The fossil specimen described here (BYU, Brigham Young University 8120/9396, 8120/9397, 826/9402, 9398, 13719) was discovered in the upper Campanian Kaiparowits Formation, in what is now the Grand Staircase-Escalante National Monument of Utah.
Maxilla with a steep rostrodorsal margin, maxillary fenestra situated far caudal to the rostral margin of the antorbital fossa; complete overlap on the caudal margin of the frontal by the parietal; knob at the front of the joint surface for the quadratojugal on the jugal; distinct angle in the caudal margin of the postorbital process of the jugal; basioccipital restricted to the midline of the basisphenoid recess as a strut; transversely oriented occiput (where the paroccipital processes extend nearly directly laterally, instead of caudolaterally); accessory pneumatic foramen in the basisphenoid recess; non-invasive basisphenoid foramen; subotic recess on the basisphenoid; ostium of the basisphenoid recess that opens externally; and elevated and pedicle-like joint surface for the squamosal on the proötic.
Teratophoneus is important because it is the first diagnostic tyrannosaurid collected from the Campanian of the American Southwest, and it changes our ideas of the evolution of the tyrannosauroid skull. Until recently, the Campanian tyrannosaurid record of western North American was limited to the Northern Rocky Mountain region, which only consist of tyrannosaurid taxa (Fig. 1; Carr and Williamson 2010). The discovery of new Campanian tyrannosauroids, such as Bistahieversor and Teratophoneus sheds new light on the early diversification of tyrannosauroids in North America, following the emplacement of the Western Interior Seaway. The influx of recent discoveries indicates that tyrannosauroids underwent a complex and endemic diversification in western North America. The unique skull form of Teratophoneus is evidence of the morphological diversity produced by this radiation.
Teratophoneus is the first diagnostic tyrannosauroid to be identified from the Kaiparowits Formation (Weishampel and Jensen 1979; Eaton et al. 1999). Although this dinosaur fauna is well sampled, the identity of its large theropods has been unknown (Stadtman et al. 1999; Gates et al. 2010). Teratophoneus is older than Bistahieversor, a tyrannosauroid recently described from the nearby upper Campanian Fruitland and Kirtland formations of the San Juan Basin, New Mexico (Carr and Williamson 2010). The temporal and geographic proximity of these taxa indicates that high taxonomic and morphological diversity among tyrannosauroids was present in the Southwest that is on par with that found in the Northern Rocky Mountain Region, where tyrannosauroids are represented by coeval taxa, Albertosaurus libratus and Daspletosaurus spp.
Teratophoneus adds to the diversity of the widespread clade Tyrannosaurinae, which is found in Asia and western North America. This clade has a surprising diversity of body forms that includes Alioramus, a medium-sized and long-snouted taxon, and the largest deep-snouted species, T. rex. The short-skulled condition seen in Teratophoneus is a notable addition to these extremes in form, indicating a complex history of cranial evolution in this successful clade (Fig. 2a, b). However, the functional significance of the short condition is unclear, given the incomplete nature of the specimen.
The cladistic analysis (Fig. 3) recovered Teratophoneus as the sister taxon of the Daspletosaurus + Tyrannosaurus clade. The femur of Teratophoneus gives a mass estimate of 667 kg, almost twice that of Alioramus, but it is one tenth the mass of an adult T. rex (Anderson et al. 1985; Brusatte et al. 2009; Christiansen 1999). In addition to their larger size than Alioramus, the bulky skull of Teratophoneus and derived tyrannosaurines is a significant morphological difference from the smaller taxon and less derived tyrannosauroids.
Although the skull of Teratophoneus is incomplete, landmarks in the skull indicate that its rostrodorsal region is unusually short in contrast to all large tyrannosauroids. Its maxilla is foreshortened, as shown by the steep rostrodorsal margin (Fig. 2c) and in the low tooth count of 12; subadult T. rex, which are larger than Teratophoneus, have up to 13 alveoli (CM 79057). In medial view, the bone is so short that the promaxillary recess is nearly eliminated, where it extends over only two dental alveoli, the third and fourth. In other tyrannosauroids (e.g., A. libratus), the recess is not reduced and it extends to the first alveolus.
The form of the jugal fragment is consistent with shortening, where the caudal margin of the postorbital process extends vertically for a short distance and then abruptly turns rostrodorsally, and by its rostrodorsally extending ventral margin (Fig. 2d). This change in direction in other tyrannosauroids (e.g., A. libratus) occurs further dorsally and it is less abrupt.
The stout form is seen in the lacrimal, which has a columnar ventral ramus (Fig. 2e). In other tyrannosauroids (e.g., A. libratus), the lateral surface of the shaft is nearly flat. Shortening is most extreme in the braincase, where the basisphenoid recess is almost closed, the basisphenoid has a steeply rostroventrally inclined ventral margin, and the otosphenoidal crest nearly spans the width of the bone (Fig. 2f). In addition to its short condition, the basisphenoid is derived in that a subotic fossa penetrates its lateral surface (Fig. 2f). Also, the pneumatic excavation in the basisphenoid recess is not invasive, forming a fossa, instead of producing the foramen that is seen in other tyrannosauroids (Fig. 2g). The low number of 14 dental alveoli in the dentary is consistent with shortening (Fig. 2h); subadult T. rex (CM 79057) that are larger than this specimen have 15 alveoli. A comparable condition of the short basicranium is also seen in T. rex, but that species has a long skull.
Based on its size and the development of the skull bones, as described for tyrannosaurids by Carr (1999), the holotype is a subadult. The specimen possesses many subadult features seen in A. libratus: the maxilla is narrow; in lateral view, the rostrolateral surface of the maxilla is not inflated, the promaxillary fenestra is slitlike and not recessed, the antorbital fossa grades into the subcutaneous surface at its rostroventral corner, the base of the interfenestral strut is concave, the marginal foramina are small and the sulcus of the caudal-most foramen does not breach the ventral margin of the bone, and the rostral margin of the antorbital fossa is not developed into a strut. In medial view, the dorsal surface of the palatal process is visible in medial view, and its lower edge extends ventrally. The lacrimal has subadult features, where the antorbital fossa and subcutaneous surface blend next to the lacrimal recess, the rostral margin of the rostroventral ala is straight, the ventral ramus has an evenly convex rostral margin, the cornual process has one apex, and the supraorbital ramus is deep.
The presence of a deep and short snout in a subadult tyrannosaurid is significant because the snout is usually shallow and long in all other subadult tyrannosaurids. Therefore, the deep condition in Teratophoneus is not a transient subadult feature, indicating that it is a truly unusual condition of this taxon. Also, we predict that adults of this species will have proportionately shorter skulls than the holotype if skull depth follows the same trend seen in other tyrannosaurids.
The short skull of Teratophoneus is an unusual specialization among tyrannosauroids, a clade that tends to possess long snouts (Carr 1999). The short condition is additional evidence that the diversification of large tyrannosauroids in western North America was well underway by the late Campanian and it was possibly marked by ecological differentiation. This diversity was preceded by a long and complex history that culminated in multiple top predators in the west, including basal tyrannosauroids (Bistahieversor), tyrannosaurines (Teratophoneus, Daspletosaurus), and Albertosaurus (A. libratus). Moreover, although members of the Albertosaurus clade were restricted to the northern Rocky Mountain Region, Teratophoneus is evidence that tyrannosaurines were widespread across western North America during the late Campanian.
This article is from TDC’s Ph.D. dissertation. We thank S. Modesto for providing the genus name. We thank R. Scheetz for access to the holotype of T. curriei. For access to comparative material we thank K. Shepherd (Canadian Museum of Nature); M. Norell and C. Mehling (American Museum of Natural History); P. Currie (Royal Tyrrell Museum of Palaeontology); K. Seymour (Royal Ontario Museum). TDC thanks his Ph.D. advisor, C. McGowan, and his dissertation committee (D. Currie, P. Currie, H.-D. Sues, R. Reisz) for their review of this part of his dissertation. We thank two anonymous reviewers for their comments that improved this manuscript. This work was supported in part by National Science and Engineering Research Council funding awarded to C. McGowan. We thank D. Pulerà for the carbon dust illustrations. We also thank S. Brusatte for his assistance with formatting.
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