A new mammal from the Jurassic Cañadon Asfalto Formation, Chubut Province (Argentina)

ANALIA FORASIEPI; GUILLERMO ROUGIER; AGUSTIN MARTINELLI

Denver

Congreso; 64th Annual Meeting Society of Vertebrate Paleontology; 2004

A new mammal from the Callovian-Oxfordian (uppermost Middle to lowermost UpperJurassic) Cafiadon Asfalto Formation (Chubut Province, Argentina) is presented. This taxon corresponds to a new species clearly different from Asfaltomylos patagonicus from the same locality and stratigraphicl evel. The new species is based on three lower jaws with relatively well-preserved dentitions. The dentaries show a basal morphology having a meckelian groove, a prominent medial flange, a deep dentary trough, which possibly indicates the presence, though reduced, of postdentary bones still attached. The lower dental formula is i4, c 1, p5, m3. The premolars are simple, bearing a single main cusp, while the molars show a fully tribosphenic pattern, with an obtuse to right-angled trigonid, and a fully basined talonid with three cusps. The association of plesiomorphic (dentary) and derived (molars) features was inferred, or more partially documented, in several taxa of the recently proposed Australosphenidag roup. Preliminaryc omparisonsu nderliet he similaritiese xisting among thenew taxon, Asfaltomylos,A mbondro,a nd more distantly,o ther australosphenidansA. cceptingthese tentative phylogenetic affinities, the tribosphenic molar pattem would have evolved twice, and the seemingly homologous middle ear of monotremes and therian would be convergent. Finally, the new specimen emphasizes the differences that exist between the Jurassic mammalian record of South America and the Jurassic faunas of Laurasia on one hand, and the better documented Cretaceous fauna of South America on the other.