Infraorder CYNODONTIA
Family RIOGRANDIDAE fam. nov.
Diagnosis. As for the only known species.
Genus RIOGRANDIA gen. nov.
Type species. Riograndia guaibensis sp. nov.
Derivation of name. With reference to the State of Rio Grande do Sul, Brazil, where it was collected.
Riograndia guaibensis sp. nov.
Text-®gures 2±11
Derivation of name. With reference to the GuaõÂba Basin where the material was collected.
Holotype. MCN-PV 2264, anterior portion of skull, from the tip of snout to the fronto-parietal contact, with complete
dentition.
Hypodigm. In addition to the holotype: MCN-PV 2265, an almost complete left lower jaw with complete dentition
lacking the postdentary bones and MCN-PV 2271, fragment of the middle portion of right lower jaw, with three
postcanines.
Locality. The exposure is 8 km west of CandelaÂria City (Text-®g. 1), in a cutting on the road (BR 287) to Santa Maria,
Rio Grande do Sul State, Brazil.
Diagnosis. Advanced cynodonts with the following associated derived characters: (1) absence of
postfrontal; (2) weak anterior portion of the zygomatic arch; (3) dorsoventrally deep lacrimal; (4) secondary
bony palate extending posterior to the last postcanine; (5) contact of ventral process of frontal with
dorsal process of palatine; (6) reduced upper incisor 1 and hypertrophied lower incisor 1; (7) last upper and
lower postcanine crowns imbricated, teeth implanted at an angle to the long axis of the palate; (8)
postcanines blade-like with 5±9 subequal small, sharp cuspules regularly distributed on the almost
semicircular border of the crown in the upper postcanine crown and in the posterodorsal border of the
lower ones.
The only autapomorphy is 8, which might be a character reversal. Although some derived characters are
present in other advanced cynodonts, the association of the indicated characters appears to be unique to
Riograndia.

A primitive `ictidosaur' from lower Norian beds of southern Brazil, Riograndia guaibensis gen. et sp. nov.,
represented by a fragmentary skull and a lower jaw bearing a complete dentition, shows a more generalized
morphology than Chaliminia from the Upper Triassic of Argentina and Pachygenelus and Diarthrognathus from the
Lower Jurassic of South Africa, Canada and Greenland. The frontal bone borders the orbit, and ventrally contacts the
dorsal process of the palatine. The secondary bony palate extends back to the last postcanine. I1 and i2 are reduced,
whereas I2-3 and i1 are hypertrophied. Both PC 1±7 and pc 1±7 have blade-like crowns without cingula and with 5±9
small sharp cuspules. The upper postcanine crowns are semicircular in labial view with the cuspules around their
margins. The lower postcanine crowns are asymmetrical with most of the cuspules dorsodistally distributed. The
possible origin of this peculiar dentition is interpreted as the retention of the juvenile dentition of ancestors. The
hypothesis that Riograndia guaibensis and the so-called `ictidosaurs' might have been derived from gomphodont
cynodonts is presented.