Thomas D. Carr

Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria)

ABSTRACT A study of ontogenetic variation is used to clarify aspects of tyrannosaurid taxonomy and investigate the supposed phenomenon of dwarfism in the clade. A hypothetical ontogenetic trajectory is described for the relatively well-represented taxon Albertosaurus libratus. The type specimen of the purported “pygmy” tyrannosaurid Nanotyrannus lancensis was compared with specimens of A. libratus and found to share many morphological characters that exemplify immature specimens of the latter taxon. Most of the cortical surface of the Cleveland skull of N. lancensis has immature bone grain. Also, the skull shares unique derived characters with mature specimens of Tyrannosaurus rex, suggesting that the specimen is a young T. rex and not a dwarf tyrannosaurid. An increase in tooth width, accompanied by loss of tooth positions, and a global shift from an immature gracile to a mature robust morphotype in the craniofacial skeleton typifies the ontogenetic changes in T. rex. Similarly, on the basis of immature ...

Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms

Tyrannosaurs Revisited Tyrannosaurs represent some of the most successful and largest carnivores in Earths history. An expanding fossil record has allowed studies of their evolution and behavior that now allow broader comparisons with other groups, not just dinosaurs. Brusatte et al. (p. 1481) review the biology and evolutionary history of tyrannosaurs and update their phylogenetic relations to include several new fossils. The analysis suggests that tyrannosaurs remained relatively small (less than about 5 meters long) until the Late Cretaceous (about 80 million years ago). Tyrannosaurs, the group of dinosaurian carnivores that includes Tyrannosaurus rex and its closest relatives, are icons of prehistory. They are also the most intensively studied extinct dinosaurs, and thanks to large sample sizes and an influx of new discoveries, have become ancient exemplar organisms used to study many themes in vertebrate paleontology. A phylogeny that includes recently described species shows that tyrannosaurs originated by the Middle Jurassic but remained mostly small and ecologically marginal until the latest Cretaceous. Anatomical, biomechanical, and histological studies of T. rex and other derived tyrannosaurs show that large tyrannosaurs could not run rapidly, were capable of crushing bite forces, had accelerated growth rates and keen senses, and underwent pronounced changes during ontogeny. The biology and evolutionary history of tyrannosaurs provide a foundation for comparison with other dinosaurs and living organisms.

A NEW GENUS AND SPECIES OF TYRANNOSAUROID FROM THE LATE CRETACEOUS (MIDDLE CAMPANIAN) DEMOPOLIS FORMATION OF ALABAMA

Abstract The discovery of a new genus and species of tyrannosauroid from the Demopolis Formation (middle Campanian) of Alabama increases the known diversity of the clade, although it does not elucidate the place of initial dispersal. This subadult tyrannosauroid is the most complete non-avian theropod collected and described from the Cretaceous of eastern North America. In contrast to tyrannosaurids, the new taxon possesses several plesiomorphic characters, including lacrimals that lack a distinct peaked cornual process, and a dorsoventrally shallow horizontal ramus of the maxilla. Autapomorphies include a wide jugal process of the ectopterygoid, a caudal pneumatic foramen of the palatine that pierces the rostral half of the vomeropterygoid process of the bone, an articular surface for the lacrimal on the palatine that is distally positioned on the dorsolateral process, and pedal unguals that have a distinct proximodorsal lip over the articular surface. Cladistic analysis indicates the new taxon is a basal tyrannosauroid and its presence in eastern North America suggests that the recent common ancestor of Tyrannosauridae probably evolved following the transgression of the Western Interior Seaway. Cladistic analysis indicates that Dryptosaurus aquilunguis is also a basal tyrannosauroid but is less derived than the new genus.

Bistahieversor sealeyi, gen. et sp. nov., a new tyrannosauroid from New Mexico and the origin of deep snouts in Tyrannosauroidea

Skeletal remains of Late Cretaceous (Campanian and Maastrichtian) tyrannosauroids are rare in southwestern North America (Carr and Williamson, 2000). Historically, the identity and diversity of southwestern tyrannosauroids was unclear because most of the fossils were isolated teeth and bones that are not diagnostic of known genera or species (Carr andWilliamson, 2000). One partial skull and skeleton (OMNH 10131) from the upper Campanian of NewMexico was referred to the problematic tooth taxon Aublysodon cf. A. mirandus, a referral that was later falsified (Lehman and Carpenter, 1990; Carr and Williamson, 2004). Recently, two fairly complete skulls and skeletons were collected that enabled a review of tyrannosauroid fossils from the Campanian of New Mexico. These specimens provide the opportunity to accurately characterize Campanian tyrannosauroids of the southwest, and recover their phylogenetic relationships with wellknown species (Carr and Williamson, 2000). We report the presence of a new genus and species of deep-snouted tyrannosauroid from the upper Campanian of New Mexico, represented by several specimens including the partial skeleton of an adult and a juvenile. This new taxon is part of the diversification of deepsnouted tyrannosauroids and emphasizes the high species richness of this widespread clade in the upper Campanian of western North America. Institutional Abbreviations—NMMNH, NewMexico Museum of Natural History and Science, Albuquerque; OMNH, Sam Noble Museum of Natural History, Oklahoma; TMM, Texas Memorial Museum, Austin; TMP, Royal Tyrrell Museum of Paleontology, Drumheller. Anatomical Terminology—We follow Anglicized versions of the Nomina Anatomica Avium terminology in Baumel et al. (1993) and in Witmer (1997).

A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia

Tyrannosaurid theropods are characterized by a generalized body plan, and all well-known taxa possess deep and robust skulls that are optimized for exerting powerful bite forces. The fragmentary Late Cretaceous Alioramus appears to deviate from this trend, but its holotype and only known specimen is incomplete and poorly described. A remarkable new tyrannosaurid specimen from the Maastrichtian (Late Cretaceous) of Mongolia, including a nearly complete and well-preserved skull and an extensive postcranium, represents a new species of Alioramus, Alioramus altai. This specimen conclusively demonstrates that Alioramus is a small, gracile, long-snouted carnivore that deviates from other tyrannosaurids in its body plan and presumably its ecological habits. As such, it increases the range of morphological diversity in one of the most familiar extinct clades. Phylogenetic analysis places Alioramus deep within the megapredatory Tyrannosauridae, and within the tyrannosaurine subclade that also includes Tarbosaurus and Tyrannosaurus. Both pneumatization and ornamentation are extreme compared with other tyrannosaurids, and the skull contains eight discrete horns. The new specimen is histologically aged at nine years old but is smaller than other tyrannosaurids of similar age. Despite its divergent cranial form, Alioramus is characterized by a similar sequence of ontogenetic changes as the megapredatory Tyrannosaurus and Albertosaurus, indicating that ontogenetic change is conservative in tyrannosaurids.

A new genus of derived pachycephalosaurian from western North America

Abstract Pachycephalosaurian specimens from the Upper Cretaceous (upper Campanian) Kirtland Formation of northwestern New Mexico include a partial skull that preserves much of the basicranium. It represents a new genus and species, Sphaerotholus goodwini. A new species, S. buchholtzae, from the upper Maastrichtian Hell Creek Formation of Montana is referred to this genus. Additional New Mexico pachycephalosaurians include a partial dentary with associated skull material that is tentatively referred to S. goodwini and a partial frontoparietal that is referred to Pachycephalosauridae incertae sedis. On the basis of a hypothetical growth series of Stegoceras, we excluded characters based on dome development and suture closure from a cladistic analysis of pachycephalosaurian relationships. Ornatotholus browni, the only putative “flat-headed” pachycephalosaurian from North America, is considered a nomen dubium and may represent a juvenile Stegoceras. Gravitholus albertae is an adult Stegoceras sp.; Stegoceras edmontonense is a nomen dubium and is referred to cf. Sphaerotholus sp. Based on the results of a quantitative cladistic analysis, Stegoceras (including Stegoceras breve, S. lambei, S. sternbergi, and UCMP 130051) is the sister taxon to all other domed pachycephalosaurians. Derived pachycephalosaurids consist of two principal clades: a lineage that includes Stygimoloch, Pachycephalosaurus, and Sphaerotholus and a lineage represented by the Asian taxa, Tylocephale and Prenocephale. With biogeographic occurrences mapped onto the phylogeny, a single dispersal event from Asia into North America, followed by dispersal of Prenocephale and Tylocephale into Asia prior to the late Campanian is indicated (ACCTRAN) or two independent dispersals into North America prior to the late Campanian is indicated (DELTRAN). Pachycephalosaurian phylogeny does not support Asian-North American contiguity throughout the Campanian and Maastrichtian.

The Osteology of Alioramus, A Gracile and Long-Snouted Tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia

Abstract The Late Cretaceous tyrannosaurid theropod Alioramus has long been one of the most puzzling large carnivorous dinosaur taxa, largely because for several decades it has been represented only by a single, fragmentary specimen that seems to represent a long-snouted and gracile individual but is difficult to interpret. The discovery of a substantially complete skeleton of Alioramus at the Tsaagan Khuushu locality in the Maastrichtian Nemegt Formation of Mongolia, recovered during the 2001 American Museum–Mongolian Academy of Sciences expedition and described as a new species (Alioramus altai) in 2009, definitively shows that this mysterious taxon is a distinct form of longirostrine tyrannosaurid that lived alongside the larger and more robust Tarbosaurus. Here we describe and figure this remarkably preserved skeleton in detail. We provide exhaustive descriptions and photographs of individual bones, and make extensive comparisons with other tyrannosauroids. This monographic description provides further evidence that Alioramus is an unusual long-snouted, gracile, and slender-limbed taxon with an unpredecented degree of cranial ornamentation among tyrannosaurids and an extremely pneumatized skeleton. Anatomical comparisons indicate that the long skull of Alioramus is an autapomorphic feature that is proportionally longer (relative to femur length) than in any other known tyrannosaurid specimen, including juveniles, and that Alioramus is morphologically distinctive relative to similarly sized individuals of the contemporary and sympatric Tarbosaurus. The holotype specimen of A. altai belongs to a young individual, and many differences between it and the other known specimen of Alioramus (the holotype of A. remotus) may represent ontogenetic variation. The unusual longirostrine skull of Alioramus was largely produced by lengthening of the snout bones (maxilla, nasal, dentary, lacrimal, jugal), rather than the orbiotemporal bones (frontal, postorbital, squamosal, quadratojugal). The long snout, gracile skull bones, comparatively small attachment sites for jaw muscles, and lack of interlocking sutures and a robust orbital brow would have precluded the holotype individual from employing the characteristic “puncture-pull” feeding style of large-bodied adult tyrannosaurids, in which the muscular jaws, thick teeth, and interlocking sutures enabled individuals to bite with enough force to fracture bone. Whether adult Alioramus could utilize “puncture-pull” feeding awaits discovery of mature individuals of the genus. The coexistence of the long-snouted Alioramus and robust and deep-snouted Tarbosaurus, which are found together at the Tsaagan Khuushu locality, demonstrate that multiple large tyrannosaurids were able to live in sympatry, likely because of niche partitioning due to differences in craniofacial morphology and functional behavior.

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.

The phylogeny and evolution of Cretaceous–Palaeogene metatherians: cladistic analysis and description of new early Palaeocene specimens from the Nacimiento Formation, New Mexico

Metatherian mammals were the most diverse mammalian clade in North America through the Late Cretaceous, but they underwent a severe extinction at the Cretaceous–Palaeogene (K-Pg) boundary. In order to clarify the origin of Palaeogene metatherians and the pattern of metatherian survivorship across the K-Pg boundary we conducted an inclusive species-level phylogenetic analysis of Cretaceous and early Palaeogene metatherian taxa. This analysis includes information from new Palaeocene specimens from south-western North America. Both the phylogenetic topology and information from new specimens support the validity of the genus Thylacodon and justify the recognition of a new species, T. montanensis. Thylacodon is closely related to Swaindelphys and Herpetotheriidae, which must have diverged by the latest Cretaceous due to its close relationship with late Campanian Ectocentrocristus. Pediomyidae and ‘Peradectidae sensu lato’ together comprise a major metatherian clade. Maastrichtidelphys, from the Late Cretaceous of the Netherlands, is the oldest member of ‘Peradectidae sensu lato’, indicating a Cretaceous origination for this group. Therefore, the major groups Herpetotheriidae and ‘Peradectidae sensu lato’, represented almost completely by Palaeocene taxa, must have originated in the Late Cretaceous. The lineages leading to these clades include at least four lineages that must have crossed the K-Pg boundary and therefore confirm that the K-Pg boundary marked a profound extinction event for metatherians and suggests that Palaeogene taxa originated from only a few clades of Cretaceous species, all of which were relatively minor or very rare components of known Cretaceous mammalian faunas.

The phylogeny and evolutionary history of tyrannosauroid dinosaurs.

Tyrannosauroids—the group of carnivores including Tyrannosaurs rex—are some of the most familiar dinosaurs of all. A surge of recent discoveries has helped clarify some aspects of their evolution, but competing phylogenetic hypotheses raise questions about their relationships, biogeography, and fossil record quality. We present a new phylogenetic dataset, which merges published datasets and incorporates recently discovered taxa. We analyze it with parsimony and, for the first time for a tyrannosauroid dataset, Bayesian techniques. The parsimony and Bayesian results are highly congruent, and provide a framework for interpreting the biogeography and evolutionary history of tyrannosauroids. Our phylogenies illustrate that the body plan of the colossal species evolved piecemeal, imply no clear division between northern and southern species in western North America as had been argued, and suggest that T. rex may have been an Asian migrant to North America. Over-reliance on cranial shape characters may explain why published parsimony studies have diverged and filling three major gaps in the fossil record holds the most promise for future work.