Cranial pneumatic anatomy of Ornithomimus edmontonicus (Ornithomimidae: Theropoda)

ABSTRACT

Modern archosaurs have extensive pneumatic diverticula originating from paranasal and tympanic sinuses. This complex anatomy is present in many fossil archosaurs, but few descriptions of the complete cranial pneumatic system exist. The cranial pneumatic morphology of birds and non-avian theropods are the best studied, but complete description of this anatomy for an ornithomimid was lacking. We describe the cranial pneumaticity of a well-preserved ornithomimid theropod dinosaur, Ornithomimus edmontonicus (RTMP 95.110.1), from computed tomography (CT) scan data and computer-aided reconstruction with quantitative measurements. New details for ornithomimids include an internal promaxillary recess, an internal palatine recess, pneumatic communications between the supraoccipital recess and posterior tympanic recess, and a complex basioccipital recess that connects to the anterior tympanic recess and median pharyngeal system. The pneumatic morphology of the quadrate is similar to modern birds, which have a complete siphoneal duct. We interpret the jugal diverticulum of non-avian theropods to be homologous or at least morphologically related to the avian suborbital diverticulum. Cranial pneumatic morphologies present in Ornithomimus and tyrannosaurids include an extensive internal palatine recesses and a pneumatic articular that is likely derived from a separate siphoneal diverticulum similar to modern birds. Recent phylogenetic hypotheses of theropods imply two equally most parsimonious solutions: that these shared morphologies are either independently derived in each taxon or diagnose Coelurosauria.

ACKNOWLEDGMENTS

We thank the Royal Tyrrell Museum and Brandon Strilisky for access to RTMP 95.110.1. The specimen was scanned in 2001 with the aid of a Jurassic Foundation Grant to H.C.E.L. We thank Timothy Rowe and Jessie Maisano at DigiMorph, University of Texas at Austin, for access to the scan data sets for Alligator mississippiensis (TMM M-983) and Gavia immer (TCWC 13, 300). Scanning data of TMM M-983 was published by Rowe and colleagues (1999) and scanning of TCWC 13,300 was supported by NSF grant IIS-0208675 to T. Rowe. Critical and constructive comments from S. Brusatte, D. Schwarz-Wings, L. Witmer, and L. Zanno greatly improved earlier drafts of this paper. S. Brusatte provided us with unpublished images of Alioramus altai and both he and L. Zanno were especially helpful with insights into the rapidly evolving terminology of coelurosaur cranial sinuses. We thank the handling editor, J. Wilson, for his patience and editorial skills. For technical advice of usage of Avizo software, we would like to thank Shigeru Yoneyama (MAXNET Co., Ltd.). We thank members of the Larsson and Carroll labs for discussions and critical suggestions. This project was supported by a JASSO Student Exchange Support Program (Scholarship for Long-term Study Abroad) to R.T. and a Canada Research Chair and Natural Sciences and Engineering Research Council of Canada grant to H.C.E.L.

Handling editor: Jeffrey Wilson