Abstract
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.
Highlights
Thrinaxodon liorhinus Seeley, 1894 and Galesaurus planiceps Owen, 1959 are well-represented basal epicynodonts from the Early Triassic of Gondwana
The present study represents the first comprehensive analysis of the stapedial anatomy variation in an ontogenetic framework considering a large sample of individuals of two species of extinct cynodonts: Thrinaxodon liorhinus and Galesaurus planiceps
Galesaurus shows a positive allometry on the lateromedial length and on the anteroposterior width, in this latter case only under Reduced Major Axis (RMA) method
Summary
Thrinaxodon liorhinus Seeley, 1894 and Galesaurus planiceps Owen, 1959 are well-represented basal epicynodonts from the Early Triassic of Gondwana. The stapedial anatomy of Galesaurus remained obscure apart from the short descriptions provided by Watson (1920), Parrington (1934), and most recently Pusch et al (2019).The stapes of juvenile and adult individuals of Thrinaxodon has been analysed in relatively more detail (Parrington 1946; Watson 1953; Estes 1961; Hopson 1966; Fourie 1974; Allin 1975; Allin & Hopson 1992) These earlier studies centred on the general features of the stapes, its contacts with adjacent bones, and the characteristics of purported links with the hypothesized tympanic membrane. A detailed account of the stapedial anatomy as well as the analysis of its intraspecific variation in Thrinaxodon and Galesaurus is long overdue
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