Comparative bone microstructure of two non-mammaliaform cynodonts from the Late Triassic (Carnian) Chanares formation of Northwestern Argentina

The aim of the present contribution is to examine the bone histology of two non-mammaliaform cynodonts recovered from the Late Triassic (Carnian) Chanares Formation, at Los Chanares locality, La Rioja province, Northwestern Argentina. The taxa analysed here are the Traversodontidae Massetognathus pascuali and the Probainognathidae Probainognathus jenseni, both forms known from multiple skeletons. Our osteohistological results reveal an uninterrupted growth pattern and high intraspecific histovariability in Massetognathus. On the other hand, Probainognathus presents cyclical growth, a cortex dominated by parallel-fibred bone tissue and scarce vascularisation. In spite of being derived non-mammaliaform eucynodonts, Massetognathus and Probainognathus show high growth plasticity, a feature usually regarded as plesiomorphic for cynodonts. Both taxa show a thick cortex in the long bones, a feature that could be linked to counteract the strong flexion forces during digging.