New occurrences of Atractosteus (Ginglymodi: Lepisosteoidea: Lepisosteidae) from the Bauru Group (Upper Cretaceous, Brazil) and paleobiogeographic implications

Abstract

The superfamily Lepisosteoidea, popularly known as gars, is the only group of extant ginglymodian fishes that has its history traced back to the Mesozoic. The extant diversity of the superfamily comprises one family, two genera and seven species. The oldest fossils of Lepisosteoidea are known from Upper Jurassic deposits in Mexico. From the Early Cretaceous onwards, lepisosteoids diversified into two major lineages: Lepisosteidae, that encompasses modern gars, and the currently extinct family Obaichthyidae. In this paper, we report new lepisosteid remains, attributed for the first time to Atractosteus sp., from the Turonian–Santonian Adamantina Formation, Bauru Group, Brazil. The anatomical description and assessment of taxonomically relevant characters was employed with the support of data from Scanning Electron Microscopy (SEM) and histological analyses. The presence of plicidentine tooth structure and lepidosteoid-type scales, without an intercalated dentine layer, allows the assignment of these remains to Lepisosteidae. The size and density of the microornamentations of the scales of the new remains were also compared to measurements of other lepisosteoids, which allowed their assignment to Atractosteus. These new fossils allow biogeographical inferences regarding Lepisosteoidea origins and their distributions along the Mesozoic. The origin of Lepisosteoidea may have occurred in the Sinemurian-Toarcian of Tethys, whereas the origin of Lepisosteidae can be explained by a cladogenetic event associated with the break-up of the Pangaea supercontinent. Both vicariance or dispersal events could explain the presence of lepisosteids, originally from Africa or North America, in South America.