The cynodonts of the Upper Triassic Los Colorados Formation (La Rioja, Argentina): on the purported presence of Tritylodon

Only two cynodont taxa have been reported from the Upper Triassic Los Colorados Formation, northwestern Argentina. One of them is represented by a few postcranial elements (PVL 3849) assigned to confer Tritylodon. Tritylodontids are specialized herbivorous non-mammaliaform cynodonts, exceptionally well represented in Laurasia but not so diverse in Gondwana. If correctly identified, the Norian Los Colorados specimen would represent the oldest record of tritylodontids and the only documentation of this clade in South America. This specimen includes the proximal portion of a femur, tibia and fibula, the distal portion of a humerus, and two sets of two articulated dorsal vertebrae. The notorious size difference between some of the bones suggests that two individuals are represented under the same collection number. After a thorough comparative exercise of the limited remains of PVL 3849 with abundant African specimens of the monospecific Tritylodon longaevus, we consider that the material from the Los Colorados Formation should be regarded as an undetermined cynodont different from Tritylodon longaevus. Comparisons with Pachygenelus show that tritheledontid affinities for PVL 3849 cannot be discarded. The only other cynodont record for the Los Colorados Formation is represented by two imperfectly preserved skulls of the tritheledontid Chaliminia musteloides. Postcranial remains of PVL 3849 are interpreted to be much larger than what is expected for the skull size of known individuals of Chaliminia; thus, co-specific affinities do not seem to be supported. The evidence available points to the presence of a still unrecognized cynodont taxon from the Los Colorados Formation. Fil: Gaetano, Leandro Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina Fil: Abdala, F.. University of the Witwatersrand; Sudáfrica