Identificação taxonômica dos dentes de Theropoda e Sauropoda da Formação Açu (Albiano-Cenomaniano), Bacia Potiguar, Nordeste do Brasil

Abstract

The Açu Formation (Albian – Cenomanian) is one of the lithostratigraphic units of the Potiguar Basin, located between the states of Ceará and Rio Grande do Norte, Northeastern Brazil. Various studies from the last decade have described a diverse paleofauna for the Açu Formation, with it being mainly composed of dinosaur specimens. This dissertation aims to identify 61 isolated dinosaur teeth (43 theropod teeth and 18 sauropod teeth) prospected from the Açu Formation, Potiguar Basin. To that end were used the most recent quantitative (morphometry) and qualitative (cladistics) methods available in the literature in order to identify isolated theropod teeth, together with a multivariate morphometric analysis idealized for the identification of sauropod teeth in this study. Through this set of techniques, it was possible to identify four dinosaur clades: Abelisauridae, Paraves, indeterminate Titanosauriformes, and Lithostrotia. While abelisaurid teeth are already known in this unit, it was possible to observe for the first-time variations referent to their position in the dental arcade, separating them in four distinct morphotypes. By allometric comparisons between those specimens and other abelisaurid teeth it was possible to recognize at least two distinct types of abelisaurids in the Açu Formation, one congruent to the smaller Early Cretaceous abelisaurids and a larger one similar to Late Cretaceous taxa. Specimen UFRJ-DG 671 Rd (morphotype 5) was solely identified as a non-dromaeosaurid paravian, a poorly known clade in “Mid'' Cretaceous of Gondwana. This implies that the gondwanan paravian diversity might have been greater than previously expected. The unprecedented application of quantitative methods for the taxonomic identification of sauropod teeth was not shown to be efficient, obtaining low reclassification ratios (RR) in comparison to those obtained in analyses performed on theropods. Through a series of detailed morphological comparisons, it was possible to identify that part of the specimens (morphotype 6) presented convergent characteristics between Brachiosauridae and Titanosauria, allowing the latter to be identified only as indeterminate Titanosauriformes. The remaining three sauropod morphotypes (7 to 9) were identified as lithostrotians, with morphotypes 7 and 8 presenting broad crowns, similar to basal representatives of the group, while morphotype 9 presents narrow crowns, similar to derivate lithostrotian taxa. The coexistence between different types of megaherbivores in the locality implies that there was a possible niche partitioning between these groups. However, in order to firmly affirm this, more detailed studies on the enamel microwear of those animals are needed. The discovery of these three new clades for the Açu Formation considerably increased the known dinosaur diversity for this unity, making it one of the most valuable locations for the study of this group in Brazil