Abstract

MARTINELLI, Agustín G.; BONAPARTE, José F.; SCHULTZ, Cesar L.  and  RUBERT, Rogerio. A new tritheledontid (Therapsida, Eucynodontia) from the Late Triassic of Rio Grande do Sul (Brazil) and its phylogenetic relationships among carnivorous non-mammalian eucynodonts. Ameghiniana [online]. 2005, vol.42, n.1, pp.191-208. ISSN 1851-8044.

A new tritheledontid, Irajatherium hernandezi gen. et sp. nov., from the Late Triassic Caturrita Formation of Brazil is described. The specimen consists of a left maxilla bearing the canine, five postcanines, and the alveolus of another one, two fragments of lower jaw, a humerus, and a femur. The association of the following features is unique for this taxon: 1) anterior upper postcanines transversely narrow bearing a higher cusp A and smaller cusp C; 2) last upper postcanines with higher, bulbous cusp A and smaller lingual cusps B and C, differing from Pachygenelus in lacking a buccal cingulum; 3) middle lower postcanines bearing a higher anterior cusp and three consecutively smaller posterior cusps, differing from Pachygenelus in lacking a lingual cingulum; 4) lower postcanines with sharp wear facets; 5) humerus with two thick osseous processes for the teres major muscle; and 6) femur with the greater trochanter at the head level and the lesser trochanter on the medial surface of the shaft. Our phylogenetic analysis placed I. hernandezi together with Riograndia guaibensis Bonaparte et al. , Chaliminia musteloides Bonaparte, Pachygenelus monus Watson and Diarthrognathus broomi Crompton. These taxa represent the family Tritheledontidae. The presence of two kinds of upper postcanines in different replacement waves in Irajatherium could be a useful feature for inference of possible mechanism of dental differentiation among non-mammalian cynodonts. Possible fossorial adaptations are documented in the humerus of Irajatherium.

Keywords : Eucynodontia; Tritheledontidae; Irajatherium hernandezi; Late Triassic; Brazil.