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

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