A primitive Late Triassic 'ictidosaur' from Rio Grande do Sul, Brazil

A primitive 'ictidosaur' from lower Norian beds of southern Brazil, Riogran dia guaibensis gen. et sp. nov., represented by a fragmentary skull and a l ower jaw bearing a complete dentition, shows a more generalized morphology than Chaliminia from the Upper Triassic of Argentina and Pachygenelus and D iarthrognathus from the Lower Jurassic of South Africa, Canada and Greenlan d. The frontal bone borders the orbit, and ventrally contacts the dorsal pr ocess of the palatine. The secondary bony palate extends back to the last p ostcanine. I1 and I2 are reduced, whereas I2-3 and it are hypertrophied. Bo th PC 1-7 and pc 1-7 have blade-like crowns without cingula and with 5-9 sm all sharp cuspules. The upper postcanine crowns are semicircular in labial view with the cuspules around their margins. The lower postcanine crowns ar e asymmetrical with most of the cuspules dorsodistally distributed. The pos sible origin of this peculiar dentition is interpreted as the retention of the juvenile dentition of ancestors. The hypothesis that Riograndia guaiben sis and the so-called 'ictidosaurs' might have been derived from gomphodont cynodonts is presented.