Comparação da paleoictiofauna do cretáceo inferior (Aptiano/Albiano) da parte ocidental do Mar de Tétis

Abstract

The evolution of the Tethys ocean was a crucial event concerning biogeographical knowledge of marine paleoichthyofauna during the Mesozoic era. This fossil fish fauna comprised mainly Neopterygii, such as Semionotiformes, Amiiformes, Ionoscopiformes, Pycnodontiformes, Aspidorhynchiformes, Ichthyodectiformes, Crossognathiformes, Elopomorpha, Clupeomorpha and stem Euteleostei. Recently, further advances have been achieved regarding the paleobiogeography of the tethyan assemblage, partly due to advances in taxonomic and systematic phylogenetic studies on fossil fishes from paleontological sites of the late Jurassic and lower Cretaceous (e.g. Solnhofen, Cerin, Lerida). However, several inquiries remain open, especially with regard to the origin of the Brazilian Continental Margin Basins (BCMBs) marine fauna, before the entire Gondwana breakup, as well as to how the epeiric sea developed to the western Gondwana interior. The Tlayúa Formation, Lower Cretaceous of Mexico, is one of the most important cretaceous sedimentary units of the world. Besides the predominance of a series of tethyan neopterygian fish in question, it was geolocated at the epicentrum of the surface changes on Earth that occurred during the Jurassic-Cretaceous boundary (e.g. the Pangaea breakup, the marine transgression of the Tethys ocean, the origin of the proto-Atlantic ocean). Thus, there must be puzzle pieces in the paleoichthyological assemblage of Tlayúa to support hypotheses concerning incursions of these epicontinetal oceans into the BCMBs. Within this context, this thesis proposes an anatomical and taxonomic description of two teleost fossil fishes (Elopiformes and Crossognathiformes) from the Lower Cretaceous Tlayúa Formation; including proof of systematic identifications and their phylogenetic positions, a more detailed contextual paleobiogeographic information in which these two evolutionary lineages were inserted alongside other neopterygian fishes during the Mesozoic and, finally, the comparison between the Aptian/Albian paleoichthyofaunas of Mexico and South America for a better biogeographical and chronostratigraphic understanding. The Elopiformes material represent a new genus and species for this order, differing from the others by possessing three autapomorphies. The new mentioned taxon is placed as a basal lineage within Elopiformes, more specifically between the genera Anaethalion (Jurassic of Europe) and Paraelops (Cretaceous of Brazil). As for Crossognathiformes, they can be described as a new genus and new species, based mainly on the existence of 8 autopomorphies. Our phylogenetic hypothesis placed this new taxon as a sister group of Crossognathidae plus Pachthyzodontoidea, remaining at the base, just below the Jurassic Chongichthys (Chile) and Bavarichthys (Germany). Our results agree with the hypothesis previously endorsed in other papers, that the Tethys Sea evolution influenced the colonization of the continental masses formed along the Mesozoic. Both lineages, Elopiformes and Crossognathiformes, dispersed from the North (Laurasia) to the South (Gondwana) Hemisphere, through the Hispanic Corridor. This biogeographic hypothesis is confirmed for other neopterygian groups. The faunal associations distributed in western Tethys (Mexico, Venezuela, Colombia and Brazil) endorse that the faunal exchange between the North and South Hemispheres during the Middle to Late Mesozoic probably began in the Berriasian-Valanginian inverval. After this time interval, epicontinetal seaways regressed, which explains the taxonomic variability between the analyzed areas