Pelvic morphology of a tritylodontid (Synapsida: Eucynodontia) from the Lower Jurassic of China, and some functional and phylogenetic implications

Tritylodontids are specialised, herbivorous cynodonts whose exact phylogenetic position is controversial, with some authors regarding them as close relatives of mammaliaforms and others as members of the eucynodont clade Traversodontidae. The tritylodontid pelvis has been claimed to resemble that of mammaliaforms in having a narrow, rod-like ilium, but such claims have been strongly challenged because of the incompleteness of previously available tritylodontid pelvic material. However, a partial tritylodontid skeleton from the Lower Jurassic of China preserves nearly complete examples of all three pelvic elements in addition to both femora, providing unprecedented insight into the structure of the tritylodontid pelvis and the configuration of the hip joint. This specimen confirms that the iliac blade is rod-like, adding to the evidence for a close relationship between tritylodontids and mammaliaforms. Furthermore, femoral retraction appears to have been driven partly by gluteus musculature in tritylodontids, as in mammaliaforms.