Gomphodont cynodonts of the Chanares Formation: the analysis of an ontogenetic sequence

A revision of gomphodont cynodonts from the Chanares Formation of Argentina , is presented. Four species in two genera: Massetognathus pascuali, M. ter uggii, M. major, and Megagomphodon oligodens were originally described by R omer in 1967 and 1972. Qualitative characters proposed originally as diagno stic of the species were revised, and a bivariate analysis of allometry of 17 linear skull measurements was performed upon 31 specimens. Allometry rel ationships account for most of the differences among specimens. There are n either size gaps nor systematic trends across the sample. The temporal regi on scales with a 'positive' allometry, whereas components of the neurocrani um and muzzle show 'negative' or isometric scaling. This results in a chang e from short, stocky skulls with large muzzles in small specimens, to slend er skulls with small muzzles and large temporal fossae characteristic nf la rge individuals. The reduction and ontogenetic loss of the parietal foramen in a cynodont is reported for the Erst time. Consideration of qualitative characters together with the allometric results lead us to conclude that al l forms belong to a single species in a growth series. Consequently, Masset ognathus pascuali is recognized as the only valid entity, an idea previousl y suggested but without a justification based upon extensive analysis.