On the possible presence of an unknown group of small-sized abelisaurids with reduction of skull roof ornamentation: evidence from northern Patagonia, Argentina

PAULINA-CARABAJAL, A.; MÉNDEZ, ARIEL H.; FILIPPI, LEONARDO S.; GIANECHINI, FEDERICO A.; LEE, Y.-N.

General Roca, Río Negro

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2023

Abelisaurids are theropod dinosaurs characterized by a thick ornamented skull roof, even presenting domes or horn-like structures on the frontals. The ornamentation consists of sculpturing formed by deep grooves interconnecting valleys between protuberances, crests, and rugosities (e.g., Abelisaurus, Aucasaurus, Carnotaurus, Viavenator). Among abelisaurids, the early diverging, Jurassic taxon Eoabelisaurus presents a skull roof lacking ornamentation, thus the ornamentation observed in Cretaceous forms is considered a derived feature. Recently, two small-to-mid-sized abelisaurid skull roofs were found in Cretaceous outcrops (Bajo de la Carpa Formation, Santonian), Cerro Overo-La Invernada area, northern Patagonia, Argentina. The specimen MAU-Pv-LE-620 corresponds to isolated and broken fused frontals, whereas the specimen MAU-Pv-CO-725, corresponds to fused frontals that were found associated with a natural endocast. Unlike most abelisaurids, the frontals in both specimens are dorsoventrally thin and exhibit a notorious low degree of ornamentation when are compared with other Cretaceous abelisaurids. Furthermore, this particular ornamentation is formed by a few circular pits distributed on an almost flat surface and not by grooves, crests, and rugosities as in most abelisaurids. Similar pits are observed in the small-sized abelisaurid Guemesia, which has a non-engrossed frontal and exhibits a low degree of ornamentation formed by a few pits near the midline and “normal” ornamentation (grooves and rugosities) near the orbital rim. So far, the reduction of the ornamentation in Cretaceous abelisaurid forms seems to be body size related, but this is not the rule. Even though it is difficult to determine if Guemesia, MAU-Pv-LE-620, and MAU-Pv-CO-725 represent juvenile specimens, the lack of ornamentation may represent a paedomorphic character as well. MAU-Pv-LE-620 and MAU-Pv-CO-725 were collected from different sites (apart 13 km away from each other) from the same geological formation, and share a similar size and the reduction of ornamentation. Are MAU-Pv-CO-725 and MAU-Pv-LE-620 juvenile specimens or representatives of sexual dimorphism of known taxa? Or do they represent an unknown group of small-sized abelisaurids characterized by the reduction of the skull roof sculpturing? Further findings and studies are needed to answer these questions.