A New Prozostrodontia (Therapsida, Eucynodontia) from the Late Triassic Riograndia Assemblage Zone (Santa Maria 2 Sequence) of Southern Brazil.

MARINA BENTO SOARES; AGUSTIN MARTINELLI; TEO OLIVEIRA

Vitória

Simposio; IX Simpósio Brasileiro de Paleontologia de Vertebrados; 2014

The diversity of Prozostrodontia (Therapsida, Eucynodontia) in the Upper Triassic Santa Maria 2 Sequence of Southern Brazil is remarkable when compared to its global record. Until now, two prozostrodontians were described from theHyperodapedon Assemblage Zone (AZ) of this Sequence while five were recovered from the younger Riograndia AZ. Within Prozostrodontia, a clade whichincludes Therioherpeton, Prozostrodon, ictidosaurs, brasilodontids, among others, plus Mammaliaformes, several mammalian features first evolved. Here we report a new find prozostrodontian from the Riograndia AZ, collected in the Botucaraí outcrop, Candelária Municipality, Rio Grande do Sul. The specimen MMACR-PV-003-T is represented by a left lower jaw (50 mm long), without postdentary bones, bearing the canine and four postcanines, and two maxillary fragments, one with a broken canine and another with one postcanine. The horizontal ramus is relatively slender and the broad and tall coronoid process rises behind the pc4. A wide diastema is placed between the canine and the postcanines. The second and fourth postcanines are better preserved and both have a larger main cusp (a), buccally aligned with one mesial (b) and two distal accessory cusps (c and d), being c the tallest. The cusps are not bulbous and are slightly curved backwards. In lingual view, there is one mesial small cusp (e) followed by a series of tiny cuspules, forming a cingular shelf. The upper postcanine is smaller than the lowers and bears three symmetrically aligned cusps in the buccal side, being the central one the largest. In its lingual side, there are two mesial and two distal cuspules symmetrically positioned, but not forming a real cingulum. The postcanine roots are incipiently bifurcated. The upper canine is more robust than the lower one. The postcanine pattern of the MMACR-PV-003-T is quite different from those of ictidosaurs from the Riograndia AZ. It resembles more the lower sectorial dentition of brasilodontids due to the presence of four aligned cusps in the buccal side and a lingual cusp e. However, the cuspidated cingular shelf of MMACR-PV-003-T is not present in brasilodontids, resembling more the pattern exhibit by Prozostrodon from the Hyperodapedon AZ. In addition, the reduced number of postcanines, only four, and the accessory cusps in the upper poscanine are unique features of MMACR-PV-003-T, representing a new taxon. A cladistic analysis with 16 cynodont terminal taxa and 93 characters was performed, resulting in two most parsimonious trees, in which MMACR-PV-003-T is positioned inside Prozostrodontia, closer to brasilodontids than to Prozostrodon.