Early synapsids from the Cisuralian (lower Permian) Pedra de Fogo Formation, Parnaíba Basin, Brazil: the first definitive South American “pelycosaurs”

Abstract

The paleotropics of Euramerica provide nearly our entire picture of Permo–Carboniferous terrestrialtetrapod evolution. The geographic sampling bias inherent in this record obscures important events, such as the turnoverbetween “pelycosaur”- and therapsid-dominated assemblages. The Cisuralian Pedra de Fogo Formation (PdF) of Brazil isa unique window into Gondwanan tropical to subtropical vertebrate assemblages of this time, including abundantchondrichthyans, actinopterygians, sarcopterygians, and temnospondyls, but the terrestrial tetrapod assemblage reportedto date is limited to acleistorhinid stem reptiles and captorhinids. Here we describe the first two specimens of Synapsidafrom the PdF. One specimen is a natural mold of the cranial face of a posterior dorsal or anterior caudal vertebra. Itslarge size, amphicoelous centrum, and narrow neural arch with closely set, dorsolaterally angled anterior zygapophysesconfirm that it is a synapsid. The second specimen is a natural mold of the medial surface of a partial maxilla. The largecaniniform tooth, presence of a supracaniniform buttress lacking a dorsal process, and ventral convexity of the elementpermit its identification as a member of Sphenacodontia, but the straight, dagger-like caniniform is unusual within theclade. Phylogenetic analysis of the maxilla corroborates its sphenacodont affinities. Besides expanding the phylogeneticdiversity of the PdF tetrapod assemblage, the specimens add a new size class (very large terrestrial amniote) andecological guild (medium-size terrestrial faunivore) to the PdF terrestrial fauna. Together, these fossils hint at thepresence of a complex terrestrial ecosystem between expansive, perennial water bodies of sufficient depth to support largeaquatic predators.