Growth patterns of Thrinaxodon liorhinus, a non-mammalian cynodont from the Lower Triassic of South Africa

The growth dynamics of the Early Triassic non-mammalian cynodont Thrinaxodon liorhinus were assessed through bone histology. Several limb bones of various sizes were examined, revealing a rapidly deposited, uninterrupted, fibro-lamellar bone tissue. A region of slowly deposited parallel-fibred bone occurs peripherally in most skeletal elements studied, becoming more extensively developed in the larger limb bones. On the basis of the bone histology, it is proposed that Thrinaxodon liorhinus grew rapidly during early ontogeny, and at a slower rate with increasing age, possibly once sexual maturity was reached. Variation in bone tissue patterns at different stages of ontogeny is noted and discussed. Given that growth rings are generally absent from the skeletal elements studied, and that the environment was seasonal, it appears that Thrinaxodon liorhinus growth was unaffected by environmental fluctuations.