New thyreophoran (Dinosauria, Ornithischia) remains from the Lower Cretaceous Bajada Colorada Formation (Neuquén, Argentina)

RIGUETTI, FACUNDO; GALLINA, PABLO ARIEL; APESTEGUÍA, SEBASTIÁN; CANALE, JUAN IGNACIO

La Plata

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2019

Thyreophoran fossil record from Gondwana is scarce. In South America, the whole osteological remains come from Argentina, including records of both Stegosauria and Ankylosauria clades. New remains were found in the Bajada Colorada Formation (Berriasian-Valanginian) at the homonym locality, Neuquén Province. They include a wide diversity of osteoderm types assigned to thyreophorans. MMCh-PV 153 and MMCh-PV 227 are interpreted as lateromedially compressed spines. They share an enlarged, ventrally concave base that inclines from the main axis of the spine (like parascapular spines of eurypodans). MMCh-PV 253 and MMCh-PV 254 are strongly lateromedially compressed plates, with rounded edges and a strong crest on one of its sides that run distally from its base (similar to the La Amarga stegosaurian plates MACN-PV-N-86). MMCh-PV 250 is a conical osteoderm with a rounded apex and a very asymmetrical base: the left basal edge is projected ventrally and the right one is a tiny lateral prominence. MMCh-PV 249 is a small conical spine with an acute posteriorly directed apex (typical thoracic ankylosaur osteoderm shape). It bears a symmetrical and ventrally plane base. MMCh-PV 72-4, MMCh-PV 251 and MMCh-PV 252 are very irregular depressed pieces with a polygonal contour and low asymmetrical crests on both sides. This variety of osteoderm shapes could suggest a diversity of sympatric thyreophorans in the Lower Cretaceous of North Patagonia still no diagnosed by non-osteoderm skeletal remains, or the presence of an unknown thyreophoran lineage that contain characters from both stegosaurs and ankylosaurs.