Description of Arundel Clay ornithomimosaur material and a reinterpretation ofNedcolbertia justinhofmannias an “Ostrich Dinosaur”: biogeographic implications

Abstract

The fossil record of dinosaurs from the Early Cretaceous of Eastern North America is scant, especially since a few stratigraphic units from the east are fossiliferous. Among these stratigraphic units, the Arundel Clay of the eastern seaboard has produced the best-characterized dinosaur faunas known from the Early Cretaceous of Eastern North America. The diverse dinosaur fauna of the Arundel Clay has been thoroughly discussed previously, but a few of the dinosaur species originally described from the Arundel Clay are still regarded as valid genera. Much of the Arundel material is in need of review and redescription. Among the fossils of dinosaurs from this stratigraphic unit are those referred to ornithomimosaurs. Here, the researcher describes ornithomimosaur remains from the Arundel Clay of Prince George’s County, Maryland which may be from two distinct ornithomimosaur taxa. These remains provide key information on the theropods of the Early Cretaceous of Eastern North America. Recent discoveries of small theropod material from the Arundel Clay possibly belonging to ornithomimosaurs are also reviewed and described for the first time. The description of the Arundel material herein along with recent discoveries of basal ornithomimosaurs in the past 15 years has allowed for comparisons with the coelurosaur Nedcolbertia justinhofmanni, suggesting the latter animal was a basal ornithomimosaur rather than a “generalized” coelurosaur as it was originally described. Comparisons between the Arundel ornithomimosaur material and similar Asian and European specimens suggest that both extremely basal ornithomimosaurs and more intermediate or derived forms may have coexisted throughout the northern hemisphere during the Early Cretaceous.

The biogeography and ecology of the Cretaceous non-avian dinosaurs of Appalachia

“…have also been recovered from the formation (e.g., Langston, 1974;Weishampel et al, 2004). As noted, both ornithomimosaurs and dromaeosaurs are known from Aptian to Albian deposits in the American west (Ostrom, 1969;Ostrom, 1970;Ostrom 1976;Cifelli, 1997;Cifelli andGardner, 1997, Weishampel et al, 2004;Signac and Mackovicky, 2010;Oreska and Carrano, 2013;Brownstein, 2017a). Notably, "Laelaps" (=Dryptosaurus sp.)…”

“…Acrocanthosaurus and Deinonychus specimens have also been collected from the Twin Mountain and Antlers formations of Texas and Oklahoma (Stovall and Langston, 1950;Cifelli, 1997;Harris, 1998;Currie and Carpenter, 2000;, and in addition to the common occurrence of Tenontosaurus remains (e.g., Ostrom, 1970;Forster, 1984;Forster, 1990;Winkler et al, 1997;Weishampel et al, 2004), the ankylopollexian iguanodontians Hippodraco and Theiophytalia are present during the Aptian in the American west (Brill and Carpenter, 2006;. The lack of reported ornithomimosaur material from the Ruby Ranch member of the Cedar Mountain Formation is not regarded as significant, as the possible ornithomimosaur Nedcolbertia justinhofmanni is known from the slightly older Yellow Cat Member of the Cedar Mountain Formation (Brownstein, 2017a;Kirkland and Hunt-Foster, 2017).…”

“…More recently, the specimens were assigned to Archaeornithomimus (Russell, 1972), to indeterminate theropods (Smith and Galton, 1990), and to Ornithomimosauria indet. (Makovicky et al, 2004;Weishampel, 2006;Brownstein, 2017a). All but one of the original specimens of Arundel ornithomimosaurs were found at the same site near Muirkirk, Maryland (Gilmore, 1920).…”

“…The original fossils found nearby Muirkirk likely come from the Dinosaur Park site, which has yielded the tibia and other pedal elements as well as a variety of other elements including many pedal unguals (Brownstein, 2017a). Notably, two morphotypes of ornithomimosaur pedal unguals (diagnosed as such based on a single flexor fossa on the ventral surfaces of the elements) are found at this site: the shorter and smaller recurved unguals like that described by Gilmore (1920), and elongated, larger unguals with more flattened ventral surfaces in lateral view and less expansive dorsal and ventral faces over their proximal articular facets (Brownstein, 2017a). This latter morphology is more akin to the pedal unguals of more derived ornithomimosaurs, suggesting the possibility of two distinct ornithomimosaurs coexisting within the Arundel facies.…”

“…Notably, ornithomimosaurs and neoceratopsians are absent from the Ruby Ranch Member, though they are present in both the Arundel and the Cloverly Formation along with a large nodosaurid (Sauropelta), Deinonychus, Acrocanthosaurus, Tenontosaurus, ornithomimosaurs, and titanosauriforms (Ostrom, 1969(Ostrom, , 1970(Ostrom, , 1976Kranz, 1996;Chinnery et al, 1998;Makovicky and Sues, 1998;Gignac and Makovicky, 2010;Woodruff, 2012;Oreska et al, 2013;Farke et al, 2014;Brownstein, 2017a). Nevertheless, the Arundel ceratopsian material remains problematic (e.g., Farke et al, 2014), and so future work on the Arundel fauna will be needed to better comparisons between these formations.…”