A Late Cretaceous abelisaurid from La Rioja Province, NW Argentina.

HAROLD JIMÉNEZ VELANDIA; MARTÍN EZCURRA; MARTÍN HECHENLEITNER; AGUSTÍN MARTINELLI; GIORGIO BASILICI; LUCAS FIORELLI

La Rioja

Jornada; 36º Jornadas Argentinas de Paleontología de Vertebrados; 2023

CRILAR - Ministerior de Turismo y Culturas

Abelisauridae is a group of ceratosaurian theropods that thrived mainly in Gondwana during the Cretaceous. They are currently represented by more than twenty species and their most complete and abundant records are from the Argentinian Patagonia. Outside Patagonia, the record of named Argentinian ceratosaurians has been limited to the Cretaceous of the Salta Province: the noasaurid Noasaurus–represented by a fragmentary skeleton–and the brachyrostran abelisaurid Guemesia– represented by a nearly complete braincase and partial skull roof–. Here, we report a new abelisaurid theropod from the Late Cretaceous of La Rioja Province, northwestern Argentina. The remains were found at the locality of Tama, in outcrops of the Campanian Los Llanos Formation. This unit is characterized predominantly by a sandstone succession throughout cumulative palaeosol profiles. In the vicinity of Tama, the Los Llanos Formation fossil record is dominated by titanosaurian remains (bones and egg-clutches). The new abelisaurid is represented by cranial and postcranial remains (CRILAR-Pv 501 and 506): left frontal, periotic bones, two cervicodorsal vertebrae, left scapula, partial sacrum and left ilium, pubis and ischium, and other indeterminate elements still under preparation. The partial left frontal is unfused to its counterpart, dorsoventrally thick, and has an ornamented external surface interrupted by a deep supratemporal fossa with a rounded anteromedial corner. The frontal resembles those of Viavenator, Llukalkan and Abelisaurus. The periotic regions are severely damaged, but the semicircular canals of the inner ear are partially preserved as natural casts and resemble the morphology of other abelisaurids. The floccular mould is shallow, as in Guemesia, but contrasting with the longer, tongue-like flocculus of Patagonian abelisaurids (e.g., Carnotaurus, Llukalkan, Niebla, MAU-Pv-LI-582). The cervicodorsal vertebrae resemble those of Majungasaurus and the Patagonian brachyrostran MAU-Pv-LI-665 in the presence of a transverse groove on the top of the neural spine. This groove is absent in the cervicodorsal vertebrae of Carnotaurus. The fragmentary left scapula is similar to that of other abelisaurids, while the pelvis resembles that of brachyrostrans. Our comparisons confirm the presence of abelisaurids in the Late Cretaceous of the Los Llanos Formation and this is the first record for the group in La Rioja Province based on more than isolated teeth. This represents the most complete abelisaurid from northwestern Argentina and further research on this specimen would shed light on the phylogeny and palaeobiogeography of the clade, helping to bridge the gap between the richer Patagonian and Brazilian abelisaurid assemblages.