Paleobiology and geographic range of the large-bodied Cretaceous theropod dinosaur Acrocanthosaurus atokensis
Highlights
► We present the first juvenile specimen of Acrocanthosaurus. ► We investigate the bone histology of juveniles and adults of Acrocanthosaurus. ► Acrocanthosaurus reached adult body size in about two decades. ► North American large-bodied theropod diversity was very low.
Introduction
The Lower Cretaceous Cloverly Formation of Wyoming and Montana records the presence of several abundant herbivorous dinosaurs such as the nodosaurid Sauropelta edwardsi, the ornithopod Tenontosaurus tilletti, and the sauropod Sauroposeidon proteles (Ostrom, 1970, D'Emic, in press). Several dinosaur taxa are shared between the Cloverly Formation and penecontemporaneous strata in Texas and Oklahoma, including Deinonychus antirrhopus, Sauroposeidon proteles, and different species of Tenontosaurus. In contrast, other Cloverly Formation dinosaur taxa are less well known and have not been a substantial source of data for studies of Cloverly Formation paleoecology or dinosaur paleoecology in general. For example, only single exemplars of the oviraptorosaur Microvenator celer and the ornithopod Zephyrosaurus schaffi have been described (Ostrom, 1970, Sues, 1980). Still other Cloverly Formation dinosaurs are only represented by remains that are indeterminate to the genus level, such as ornithomimids and large-bodied theropods (body mass > 1000 kg), which are limited to a handful of teeth and bones reported by Ostrom (1970).
The paucity of large-bodied theropod remains from this time on the continent is not limited to the Cloverly Formation. Only a single genus and species of large theropod has been named from the Lower Cretaceous of North America: the carcharodontosaurid Acrocanthosaurus atokensis (Stovall and Langston, 1950). Acrocanthosaurus is one of the largest known theropods, similar in size and weight to Tyrannosaurus rex (Bates et al., 2009) and Giganotosaurus carolinii (Calvo and Coria, 1998). All known definitive remains of Acrocanthosaurus are from the Aptian–Albian Trinity Group of Texas and laterally-equivalent Antlers Formation of Oklahoma (Eddy and Clarke, 2011).
Apart from material ascribed to Acrocanthosaurus, all other Lower Cretaceous North American large-bodied theropod remains are indeterminate to the genus level (Harris, 1998a). These include fragmentary teeth and bones from the Arundel clay of the Patuxent Formation of Maryland (Lull, 1911), the Cedar Mountain Formation of Utah (Kirkland et al., 1997), and the Cloverly Formation of Wyoming (Ostrom, 1970). Harris (1998a) suggested that some vertebrae from the Cloverly Formation and Arundel clay of the Patuxent Formation represent taxa distinct from Acrocanthosaurus, suggesting that more than one large-bodied theropod was present on the continent at the time. Several small- to medium-sized theropods were also present during the Early Cretaceous of North America, including tyrannosauroids (Cifelli et al., 1997, Zanno and Makovicky, 2011) and deinonychosaurs (Kirkland et al., 1993).
Recent fieldwork has yielded a partial skeleton of a juvenile theropod found in a bonebed in the Cloverly Formation, as well as some isolated large-bodied theropod material. Herein this partial skeleton and its bone histology are described, and its ontogenetic age and taxonomic affinity are discussed. Features supporting the referral of this skeleton to Acrocanthosaurus atokensis are demonstrated, and comparisons of its bone histology are made with that of adult individuals of the taxon. Histological data are used to estimate growth rate and longevity for Acrocanthosaurus. Finally, the taxonomic affinities of isolated large-bodied theropod remains from the Lower Cretaceous of North America are reviewed, and the bearing of this new information on dinosaur faunas of the continent at the time is discussed.
Institutional abbreviations: NCSM, North Carolina Museum of Natural Sciences, Raleigh, USA; SMU, Southern Methodist University, Dallas, USA; UM, University of Michigan Museum of Paleontology, Ann Arbor, USA; UMNH VP, Utah Museum of Natural History Vertebrate Paleontology Collection, Salt Lake City, USA; YPM, Yale Peabody Museum, New Haven, USA.
Section snippets
Systematic paleontology
DINOSAURIA Owen, 1842
THEROPODA Marsh, 1881
ALLOSAUROIDEA Currie and Zhao, 1993
CARCHARODONTOSAURIA Benson et al., 2010
CARCHARODONTOSAURIDAE Stromer, 1931
Acrocanthosaurus Stovall and Langston, 1950
Acrocanthosaurus atokensis Stovall and Langston, 1950
New referred material: UM 20796, a dorsal vertebral centrum, caudal vertebral neural arch, right and left pubes, right femur, proximal right fibula, and several fragments from site YPM 63–18 (see later for further locality information). Other materials
Description
The partial skeleton UM 20796 was collected by MDD in 2008 in an area less than 1 m2, and the bones were nearly in contact. For the following description, nomenclature for vertebral laminae and fossae follows Wilson (1999) and Wilson et al. (2011), respectively.
Discussion
Below, the taxonomic affinities of UM 20796 are discussed and estimates of its growth rate are made via bone histology. The implications of this new data and taxonomic revision of other specimens are discussed in the context of Cretaceous North American dinosaur communities.
Acknowledgements
Thanks to B. Dauksewicz, T. Churchill, B. Foreman, P. Gingerich, A. Hayden, D. Klein, S. Macone, C. Manz, A. Wood, D. Raisanen, A. Tillett, C. Youngs, P. Christenson, and the WY and MT Bureaus of Land Management (permits PA07-WY-155 and M 97866) for field assistance and support. G. Gunnell, A. Pan, L. Ballinger, W. Joyce, D. Brinkman, and V. Schneider are thanked for collection access and assistance. Thanks to R. Cifelli and V. Schneider for permission to thin section specimens. Thanks to S.
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