Revisão anatômica e posicionamento filogenético das espécies de Ionoscopiformes (Actinopterygii: Holostei) do Cretáceo do nordeste do Brasil e de Puebla, México

Abstract

The fossil genera Oshunia and Placidichthys are holosteans, belonging to the Order Ionoscopiformes, from the Early Cretaceous of the Araripe and Tucano basins in Brazil. Ionoscopiformes sensu Grande & Bemis (1998) includes the families Ionoscopidae and Ophiopsidae, however the inner relationships of this clade are still uncertain. Oshunia and Placidichthys belong to the families Ionoscopidae and Ophiopsidae, respectively, and while the genus Oshunia is considered to be monospecific (cf., O. brevis), Placidichthys presents two nominal species (cf., P. bidorsalis and P. tucanensis). Especially due to the fact that these species where described from just a few specimens, there were still several gaps on the knowledge about them, as for example, the possible existence of additional species from the genus Oshunia and the lack of anatomical information, notably from the skull roof, occipital region, check bones, and caudal fin of Placidichthys species. Another debatable matter was the phylogenetic position of these two genera. Given these issues, the objective of the present thesis was to make a revision of the genera Oshunia and Placidichthys, with the aim of increasing the anatomical and taxonomic knowledge about them, and also to make a phylogenetic analysis of Ionoscopiformes, based on data matrixes from the literature, in order to achieve a stronger hypothesis over the relationships of these Brazilian genera. Granted the easy access to Mexican specimens from the Cretaceous of the Tlayúa Formation, the genera Teoichthys and Quetzalichthys were also reviewed on the present work. From a taxomomical point of view, it was not possible to conclude over the existence of new species from the genus Oshunia, however it was evident the presence of a new species belonging to the Mexican genus Teoichthys. The present revision brought a series of new information over the anatomy of these Ionoscopiformes species, such as a first description of the circumorbital and skull roof bones and a reinterpretation over the dorsal fin of Placidichthys bidorsalis, or about the shape of the rostral bone of Teoichthys kallistos. Likewise, this revision also offered new data for the construction of a new phylogenetic hypothesis for Ionoscopiformes, which was considerably distinct from the anterior ones (cf., inner relationships of Ionoscopidae and the placement of the genus Teoichthys). The low support for many clades makes evident the fragility of the phylogenetic hypotheses of Ionoscopiformes, as well as the necessity of a more detailed revision of other species from this group and also of the characters to be used on future phylogenetic analyses.