The paleohistology of †Araripichthys castilhoi from the Lower Cretaceous of Araripe Basin, Northeastern Brazil: a typical case of basal teleost

Abstract

ABSTRACT †Araripichthys castilhoi belongs to the paleo-ichthyofauna from the Aptian calcareous concretions of Romualdo Member, Santana Formation, Araripe Basin, northeast Brazil. It was originally placed within Beryciformes (Acanthomorphata) by a combination of plesiomorphic and apomorphic characters. Later, an anatomical review of †Araripichthys demonstrated the presence of several primitive characters (cf. presence of fin spines, orbitosphenoid, two supramaxillae, seven hypurals, autogenous hypurals 1 and 2, second ural centrum and first uroneural not fused, and cycloid scales) and aligned it with Clupeocephala. †Araripichthys was also considered closely related to †Acanthichthys (primitive Acanthomorphata), basically by the body shape resemblance. Other included †Araripichthys in †Protobramoidei (basal Clupeocephala) based on the absence of pelvic fins, presence of stout neural spines and scales partially covering the dorsal fin. The odd taxonomic history of †Araripichthys is directly related to its scarce number of available well-preserved specimens and concurrently its absence in a comprehensive phylogenetic analysis. Here we present a paleohistological study, an efficient tool to precise the taxonomic position of fossil fishes, especially in cases like that of †Araripichthys. Its paleohistology presents pseudolamellar bony tissues with stellate lacunae, filled by osteocytes with ramified cytoplasmic extensions. The star-shaped osteocytes of †Araripichthys are clearly characteristic of basal teleosts. Conversely, in the scarce Acanthopterygii taxa with cellular bone the osteocytes morphology is clearly distinct since their canaliculi are not regularly localized around these cells. The currently accepted synapomorphies for Elopomorpha, Clupeocephala, Acanthopterygii, and the already known anatomical data for †Araripichthys together with its paleohistology, corroborate its incertae sedis position within basal Teleostei.