Growth pattern and lifestyle habits of the Triassic non-mammalian cynodont Trirachodon are deduced from bone histology and cross-sectional geometry. Several skeletal elements of Trirachodon were examined in order to document histological changes during ontogeny, as well as histovariability in the skeleton. The bone histology of all the elements consists of a moderately vascularized, periodically interrupted, fibro-lamellar bone tissue. This suggests that the overall growth of Trirachodon was probably rapid during the favourable season, but decreased or ceased during the unfavourable season. As the environment is thought to have been semi-arid with seasonal rainfall, it is possible that Trirachodon was sensitive to such environmental fluctuations. Some inter-elemental histovariability was noted where the number and prominence of growth rings varied. Limb bone cross-sectional geometry revealed a relatively thick bone wall and supports earlier proposals that Trirachodon was fossorial.

Key words: Cynodonts, Trirachodon, lifestyles, bone histology, growth patterns.

Jennifer Botha [jbotha@iziko.org.za]], Natural History Collections Division, South African Museum, Iziko Museums of Cape Town, P.O. Box 61, 8000, South Africa (corresponding author); Anusuya Chinsamy [achinsam@botzoo.uct.ac.za], Zoology Department, University of Cape Town, Rondebosch, 7701, South Africa.