Remarks on the paleoecology of Medistylus dorsatus (Ameghino, 1903) and other Pachyrukhinae (Notoungulata, Hegetotheriidae) from the Deseadan (Late Oligocene) of the Chubut Province, Argentina.

DOZO, MARÍA TERESA; REGUERO, MARCELO; CERDEÑO, ESPERANZA

Plaza Huincul

Congreso; XXI Jornadas Argentinas de Paleontología de Vertebrados; 2005

Museo Carmen Funes

The Pachyrukhinae are small terrestrial herbivorous Hegetotheriidae easily recognized by their specialized ?rabbit-like? anterior dentition. Representatives of this subfamily are well known from the Deseadan SALMA (Late Oligocene) with the genera Prosotherium and Propachyrucos. With the recent recognition of Medistylus dorsatus as a Pachyrukhinae, by means of a very well preserved skull (MPEF-PV 693) from Cabeza Blanca (Chubut), the major diversification of the group is recorded in the Deseadan SALMA of Patagonia. Special emphasis has been put on the hypsodonty (HI), the relative width of the muzzle (RMW) and the premaxillary shape (PSI). These characters have been recognized in different mammal groups as good evidence of paleoenvironmental adaptations concerning diet. Pachyrukhinae constitute the most hypsodont clade within Deseadan notoungulates, even more than the primitive Mesotheriidae and Interatheriidae. Whithin ungulate species, the shape of the premaxillae reflects the degree of selectivity in the diet. The PSI value (87 %) of Medistylus dorsatus is more similar to that of the grazer feeders estimated for modern and extinct camelid species. In Medistylus and Prosotherium, the morphology of the relatively broad muzzle and the great hypsodonty, together with the evergrowing upper incisors, the presence of a large diastema, and the small body size indicate that they would be the most grazer herbivores that lived during the Deseadan SALMA in Patagonia. Finally, the occurrence of three euhypsodont genera of Pachyrukhinae provide evidence of a previously unsuspected early diversity of the subfamily in this area, and suggests a great paleoenvironmental difference between the Deseadan faunas of Patagonia and those from Bolivia and Uruguay where they did not live.