New fish remains from the Araçatuba and Adamantina formations (Upper Cretaceous), Bauru Group, southwestern São Paulo State

Abstract

The paleoichthyofauna of the Bauru Group is generally represented by isolated remains. This fossil record includes actinopterygians such as Lepisosteidae, Amiidae, Osteoglossiformes, Characiformes, Siluriformes, and a few Dipnoi occurrences. However, given the scarcity of more complete specimens, the Bauru Group fish diversity is poorly known when compared to other vertebrate groups of the same unit. We report new fish remains, including scales and skull fragments, from the lacustrine Araçatuba Formation and the fluvial Adamantina Formation from the Presidente Prudente region (cities of Coronel Goulart and Pirapozinho), State of São Paulo. Some skull remains were analyzed by microtomography and the ganoid scales were studied by morphometric, thin sectioning, and SEM analyses. The cycloid scales of the Bauru Group have not yet been sufficiently investigated due to the incompleteness of the material found so far. Therefore, we better characterize new cycloid scale findings with the aid of comparisons with extant taxa, which indicate that some of them can be assigned to Characiformes. We also report a dentary attributed to the genus Atractosteus (Lepisosteidae) based on unique features such as robust conical teeth with well-developed plicidentine, and the absence of a row of smaller lateral teeth. The underdeveloped morphology of the mandibular canal indicates that this is a juvenile specimen. The paleoichthyofauna of both formations are similar, with remains of Characiformes and Siluriformes showing nearly the same morphology. Lepisosteidae remains are also alike, including the presence of the genus Atractosteus. The remains of Lepisosteidae are the most abundant material found in both units, indicating either this was the predominant group of fish in the Bauru Group or that their remains are easily preserved, as ganoid scales are much more resistant to transportation than cycloid scales.