Gnathovorax cabreirai, um dinossauro excepcionalmente bem preservado do Triássico superior do sul do Brasil

Abstract

The dinosaurs excavated in the Triassic rocks of Rio Grande do Sul are some of the oldest worldwide, and therefore provide crucial data regarding the origin and early evolution of the group. Among the oldest dinosaurs, a lineage that stands out for its particularities is Herrerasauridae, a group of medium to large-sized predators whose first and last records are restricted to a time interval of just 8 million years. The herrerasaurids are not part of the three major lineages of dinosaurs (Ornithischia, Theropoda and Sauropodomorpha), presenting several distinctions in relation to these other dinosaurs. Some studies even indicate the existence of a broader grouping (i.e., Herrerasauria), including herrerasaurids and their closest relatives. The recent discovery of the complete, semi-articulated and exceptionally preserved skeleton of Gnathovorax cabreirai has the potential to reveal a wealth of information about herrerasaurids and early dinosaurs as a whole. In this dissertation, the skeletal anatomy of Gnathovorax was studied in detail, and its implications for dinosaur evolution were discussed. The results revealed a series of anatomical details that could be confirmed or observed in an unprecedented way in Gnathovorax. Several traits present in herrerasaurids could be reassessed due to the excellent preservation of the specimen. One of the anatomical novelties of Gnathovorax was the identification of a sternal plate with sternal ribs, all superbly preserved. This is the oldest record of these elements for dinosaurs, as their delicate nature makes them extremely difficult to preserve. Another of the most notable features in Gnathovorax is an unusual projection present on the right maxilla, which we interpret as a healed wound. Analyzing the context of Gnathovorax and the presence of similar features in other herrerasaurids, we concluded that these were more likely achieved during intraspecific combat, being this also the oldest records for dinosaurs. In this dissertation, the locality that provided the holotype of Gnathovorax is also described and we analyze its attributes in the stratigraphic, geological and taphonomic context, in order to understand the conditions that allowed the type-skeleton of Gnathovorax to present such exceptional preservation. Became understood that a short period of subaerial exposure and low degree of hydraulic transport (i.e., death in situ) contributed to this factor. Furthermore, a new fragmentary specimen belonging to Herrerasauria was described and evaluated, showing that the origin of the sister lineage of Herrerasauridae may have occurred in the southern hemisphere of Pangea, although records of this lineage are concentrated in Triassic rocks from the USA (Northern Pangea).