The stapes of Thrinaxodon Seeley, 1894 and Galesaurus Owen, 1859: a case of study for intraspecific variability in basal cynodonts

We surveyed the stapedial anatomy of Thrinaxodon liorhinus Seeley, 1894 and Galesaurus planiceps Owen, 1859, two iconic Early Triassic basal cynodonts. The complete characterization of this bone and the analysis of its morphological variability along the ontogeny within each taxon were possible through the study of eleven elements of Thrinaxodon Seeley, 1894 and nine of Galesaurus Owen, 1859. Our results highlight notable qualitative and quantitative differences among specimens of the same species. Stapes growth model in Galesaurus shows a positive allometry on the lateromedial length and on the anteroposterior width, whereas in Thrinaxodon there is a negative allometry in the anteroposterior width but the lateromedial length is isometric. This study offers a counterintuitive result of inordinate high intraspecific morphological variation of the stapes of basal non-mammaliaform cynodonts.