First Jurassic Triconodont from South America

ROUGIER, GUILLERMO W.; GARRIDO, ALBERTO; GAETANO, LEANDRO C.; PUERTA, PABLO; NOVACEK, MICHAEL

Ottawa, Ontario, Canada

Congreso; Sixty-sixth Annual Meeting Society of Vertebrate Paleontology; 2006

The Queso Rallado quarry in the Middle Jurassic Cañadon Asfalto Formation, Chubut province Argentina is so far the only South American locality providing Jurassic mammalian remains. The australosphenid Asfaltomylos patagonicus and a closely related form have been reported from that locality. We report here on the first discoveries of a triconodont from that locality. The first specimen and type of a new species is an isolated lower molariform. The tooth is dominated by five mesiodistally aligned cusps. The main cusp (a) is the tallest, while b and c are subequal in height; two small accessory cusps (d and e) occupy the mesial and distal extremes of the molariform. The largest cusps (a-c) are recumbent, in particular a and c. There is a poorly differentiated cusp g but no distinct cingulum is present. The roots or crown are not grooved for molar interlock, which as suggested by the flaring of cusp d was probably in echelon. The whole tooth is extremely buccolingually compressed, determining a very gracile cusp morphology. The compression of the tooth, and recumbency of cusps is reminiscent of the Early Cretaceous Ichthyoconodon jaworowskorum from Morocco, but the relative cusp proportions are different. The new taxon is similar to the very incomplete Triconolestes curvicuspis from the Jurassic of Utah in cusp proportions and recumbency, but far more compressed. The retention of a primitive cusp g and the derived nature of the remainder of the crown suggest a peculiar taxon possibly related to the generalized Jurassic tricodonts of the northern landmasses, but exhibiting a certain degree of endemic development; the same conclusion can be drawn for the australosphenidans from Queso Rallado. The molariform shows a mosaic of features present among Triconodontidae and the plesiomorphic ?amphilestidae? Previously reported Cretaceous triconodonts from South America are probably best interpreted as dryolestoid premolars. New Jurassic specimens have just been collected, including cranial and postcranial remains, that will provide a much broader comparative basis for this enigmatic triconodont.