J. Paleont., 81(6), 2007, pp. 1301–1307 Copyright ᭧ 2007, The Paleontological Society 0022-3360/07/0081-1301$03.00 A POORLY KNOWN RODENTLIKE MAMMAL (PACHYRUKHINAE, HEGETOTHERIIDAE, NOTOUNGULATA) FROM THE DESEADAN (LATE OLIGOCENE) OF ARGENTINA. PALEOECOLOGY, BIOGEOGRAPHY, AND RADIATION OF THE RODENTLIKE UNGULATES IN SOUTH AMERICA MARCELO A. REGUERO,1 MARI´A TERESA DOZO,2 AND ESPERANZA CERDEN˜ O3 1Divisio´n Paleontologı´a de Vertebrados, Museo de La Plata, B1900FWA La Plata, Argentina, Ͻ
[email protected]Ͼ, 2Laboratorio de Paleontologı´a, Centro Nacional Patagonico, Consejo Nacional de Investigaciones Cientificas y Te´cnicas, 9120 Puerto Madryn, Chubut, Argentina, Ͻ
[email protected]Ͼ, and 3Departamento de Geologı´a y Paleontologı´a, Instituto Argentino de Nivologı´a, Glaciologı´a, y Ciencias Ambientales, Centro Regional de Investigaciones Cientı´ficas y Te´cnicas, Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Avda. Ruiz Leal s/n, 5500 Mendoza, Argentina, Ͻ
[email protected]Ͼ ABSTRACT—The cranial anatomy of the Deseadan species Medistylus dorsatus (Ameghino, 1903) is described based on new and complete material from Cabeza Blanca (Chubut, Argentina). Medistylus is the largest of the Pachyrukhinae and the specimen described here is probably the best-preserved pachyrukhine skull known in the Paleogene of South America. Previously, the validity of the species and its phylogenetic affinities with Interatheriidae (Notoungulata, Typotheria) were ambiguous and not conclusive. The syntypes, now reported lost, were isolated teeth poorly described by Ameghino in 1903. This almost complete skull with teeth provides more diagnostic features in order to complete the knowledge of genus. Details about cranial and dental morphology allow the reassessment of Medistylus dorsatus and its inclusion within the subfamily Pachyrukhinae (Hegetotheriidae, Notoungulata).