A new hypothesis of eudromaeosaurian evolution: CT scans assist in testing and constructing morphological characters

ABSTRACT

Eudromaeosauria is a clade of derived dromaeosaurids that typifies the common perception of ‘raptor’ dinosaurs. The evolutionary history of this clade has been controversial due to conflicting views of taxonomic identity, and because, due to taphonomic bias, several species were diagnosed primarily or solely by the maxilla. The maxilla is therefore crucial in understanding the phylogenetic relationships within the clade. Morphometric characterization has been commonly applied to recognize and distinguish major dromaeosaurid clades. However, morphometrics mainly showed morphological convergence rather than phylogenetic relationships. This approach has made it difficult to get resolution of phylogenetic relationships among eudromaeosaurian taxa, often resulting in large polytomies or inconsistent placement of key species. To test previous character statements, computed tomography was used to analyze the maxillae of Acheroraptor, Atrociraptor, and Deinonychus, and compare them with other eudromaeosaurians from Asia and North America. Morphometric characters were examined, and regressions were used to look for allometric trends in maxillary dimensions and the relationship to topological landmarks within Eudromaeosauria and its outgroups. Characters were improved and implemented to better capture eudromaeosaurian morphological variation and better resolve their phylogenetic relationships. Phylogenetic analysis recovered three well-defined clades within Eudromaeosauria and corroborated occurrence data within the fossil record. Acheroraptor and Atrociraptor were recovered as derived members of Saurornitholestinae. Deinonychus is recovered as a basal eudromaeosaurian, sharing features with dromaeosaurines and saurornitholestines. These results challenge previous biogeographic hypotheses suggesting Asian and North American faunal interchange during the Late Cretaceous and support convergence of traits relating to snout dimensions and proportions.

ACKNOWLEDGMENTS

Funding for this project was in part from the lead author’s Natural Science and Engineering Research Council, Canadian Graduate Scholarship—Master’s (NSERC CGS-M) grant. Additional funding came from author PJC in the form of an NSERC discovery grant (grant #-04674). For access to collections and specimens we thank collections manager C. Mehling (AMNH) for access to Bambiraptor, Dromaeosaurus, Tsaagan, and various Velociraptor spp. specimens; collections manager R. Scheetz and supervising student R. Esplin (BYUVP), curator K. Carpenter and collections staff K. Corneli (CEUM), for access to Utahraptor specimens; collections manager K. Seymour, collections technician B. Iwama, for access to Acheroraptor material; collections manager B. Strilisky, collections staff B. Sanchez, R. Russell, and T. Courtenay, and fossil curator Do. Brinkman (TMP) for access to Atrociraptor and Saurornitholestes material; lab technician/collections manager C. Coy, and technician H. Gibbins (UALVP) for access to Saurornitholestes material; and collections manager Da. Brinkman for facilitating access to Deinonychus specimens. Photos for specimens from the MPC were provided by G. Funston. For CT scan data, Mayfair Diagnostics in Calgary, AB provided scans for Atrociraptor and TMP 1994.012.0844 with the assistance of R. Russell and T. Courtenay (TMP); synchrotron radiation µCT data acquisition, including scanning protocol development, competitive beam time procurement, and along with A. Dyer, assisted with the transport and preparation of UALVP 55700 at the Synchrotron facility. Part of the research (scans for UALVP 55700) described in this paper was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan with a special thanks to S. Gasilov, D. Miller and T. Bond for their extensive assistance in the acquisition of the synchrotron scans. For discussions, we thank M. Caldwell, A. Dyer, G. Funston, M. Hudgins, A. LeBlanc, M. Rhodes, T. Simões, C. Sullivan, O. Vernygora, and Y. Wang. We would like to thank the two reviewers of the first draft of this manuscript, P. Makovicky and S. Laughtenschlager, whose comments and suggestions greatly improved the quality of the manuscript.