A new ornithomimid dinosaur from the Upper Cretaceous Packard Shale formation (Cabullona Group) Sonora, México
Introduction
Ornithomimosaurian dinosaurs were a group of lightly built, medium to large size, cursorial theropods with rather long forelimbs. These organisms lived in the Northern Hemisphere during Cretaceous times and are common elements of many Northamerican and Asian dinosaur faunas (Barsbold & Osmólska, 1990; Choiniere, Forster, & de Klerk, 2012; Lee et al., 2014). The earliest and most basal taxa, Pelecanimimus and Shenzhousaurus were relatively small in size, in comparison with Gallimimus bullatus who could reach a body length of eight meters (Ji et al., 2003, Makovicky et al., 2010).
Most ornithomimid genera are known from Asia and only Ornithomimus and Struthiomimus, are currently known from the Upper Cretaceous of North America (Marsh, 1890, Marsh, 1892, Osborn, 1917, Makovicky et al., 2004, Longrich, 2008). Ornithomimus is characterized by gracile forelimbs, elongate first metacarpals (Russell, 1972) and a straight ventral edge of the maxilla (Parks, 1928). The genus includes two species: Ornithomimus edmontonicus (Sternberg, 1933) and Ornithomimus velox (Marsh, 1890). On the other hand, Struthiomimus has robust limbs, short first metacarpals (Russell, 1972) and a strongly convex maxillary ventral edge (Osborn, 1917). Struthiomimus altus (Osborn, 1917) is the only valid species included in this genus at present.
Several ornithomimid fossils have been collected along the interior margin of Western North America, which cannot be assigned to either one of these genera because of their distinctive morphology (Longrich, 2008, Aguillón-Martínez, 2010). Particularly in Mexico, ornithomimid material is common among continental vertebrate faunas, but it consists of isolated bones (Torres-Rodríguez, 2006). In this work we described and analyzed a new genus and species of ornithomimosaur from the Packard Shale Formation, Cabullona Group (Upper Cretaceous) in northeastern Sonora, Mexico. This formation is characterized by a rich assemblage of fresh-water invertebrates, plants and dinosaurs.
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Geological setting
The Cabullona basin is a continental depocenter at least 80 km long and 30 km wide that formed in northeastern Sonora during Late Cretaceous (Fig. 1a). Its fluvial to lacustrine sedimentary fill is near 4 km thick and has an age between 80 and 70 Ma (González-León, Solari, Madhavaraju, & Villanueva-Amadoz, 2014). It extends into southeastern Arizona where it correlates with the Fort Crittenden Formation. The 2.5-km thick Cabullona Group that was first described by Taliaferro (1933) crops out
Systematic Paleontology
Order Dinosauria Owen, 1842.
Saurischia Seeley, 1888.
Theropoda Marsh, 1881.
Ornithomimosauria Barsbold, 1976.
Family Ornithomimidae Marsh, 1890.
Genus Tototlmimus gen. nov.
Type species: T. packardensis sp. nov., by monotypy.
Etymology: The generic name is based on Tototl which is the Náhuatl word for bird; Greek mimus refers to mimic.
Diagnosis: As for the type and only species.
Species T. packardensis sp. nov.
Etymology: Referring to its occurrence in the Packard Shale Formation.
Holotype: ERNO 8553.
Ornithomimid features of Tototlmimus packardensis
This specimen was referred to the Ornithomimidae family on the basis of the presence of an arctometatarsalian pes (Holtz, 1995), and a pedal ungual that has a triangular articular surface in cross-section, relatively straight ventral edges, a ventrolateral edge developed into a keel (closer to the articular end), and a flat ventral depression instead of a flexor tubercle (Barsbold & Osmólska, 1990).
Morphological comparisons to other ornithomimosaurs
In general, it has been assumed that postcranial elements are not diagnostic; however, this is
Conclusions
The specimen described here provides evidence for the recognition of a new genus and species of ornithomimid in the Late Cretaceous of southern North America. This new taxon adds to the diversity of Late Campanian ornithomimids and represents one of the southernmost occurrences in the Western Interior Basin. Although the limited diagnostic information of the holotype and only known specimen Tototlmimus packardensis, it possesses a set of particular characteristics that make it unique among the
Acknowledgments
The authors want to thank the Smithsonian Institution National Museum of Natural History for allowing us to use the collections of the Paleobiology Department extensively for comparison. We also want to thank MSc. Brad McFeeters (ROM) and Dr. Brandon Strilisky (TMP) for allowing the reproduction of the photographic material of ornithomimid specimens. Finally, we want to thank the reviewers for their detailed and thoughtful reviews that greatly improved the preparation of this manuscript.
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