New triconodont specimen from the Middle Jurassic of Patagonia

GAETANO, LEANDRO C.; ROUGIER, GUILLERMO W.

Trelew, Provincia de Chubut, Argentina

Jornada; XXIII Jornadas Argentinas de Paleontología de Vertebrados; 2007

At present, South American Jurassic mammals are found only in the outcrops of the Oxfordian-Callovian Cañadón Asfalto Formation, Chubut, Argentina. Among them, there is a triconodont previously known from a purported lower molar (MPEF-PV1877). We report here on the discovery of new material (MPEF-CC115/04) referable to this triconodont, in particular a partial skeleton including both upper and lower dentitions, which suggest the previously reported molar is in fact an upper tooth. Two lower molars are found in place on the left dentary. The teeth and the dentary are extremely bucolingually compressed. Both teeth are very similar and dominated by five mesiodistally aligned cusps (a-e). The central cusp (a) is slightly taller than b and c, which are subequal in height, and all of them are distally recumbent. There are also two small accessory cusps (d and e). There is no evidence of cingula or cusp g. The molar interlocking was in 'echelon', being cusp b of the posterior molar buccal to the cusp d of the preceding one. The molars of the Patagonian triconodont are similar to Ichthyoconodon jaworowskorum Sigogneau-Russell, 1995 and Triconolestes curvicuspis Englemann and Callison, 1998. The lower molars of our taxon are similar to those of triconodontids, while the uppers resemble ?amphilestids?. This mammal is likely part of a basal stock of triconodonts not yet fully differentiated into the traditionally recognized monophyletic groups and constitutes the best anatomical evidence for basal triconodonts. The bearing the Middle Jurassic triconodont on mammaliaform phylogeny is yet to be explored.