RECONSTRUCTING GROWTH PATTERNS IN LATE TRIASSIC DICYNODONTS FROM NORTH AMERICA USING BONE AND TUSK HISTOLOGY

Dicynodonts were non-mammalian therapsids that survived the end-Permian extinction and thrived until their eventual disappearance near the end of the Triassic. Late Triassic dicynodonts lived in a world of ever-changing environmental conditions, and analyzing growth patterns in these animals may help elucidate how they were adapted to this dynamic setting. However, our understanding of growth in Late Triassic dicynodonts is currently poor relative to that of their Permian and Early Triassic predecessors. This study addresses this deficiency through histological analysis of limb bones and tusks from Late Triassic dicynodonts from North America. First, bone microstructure is compared between two tibiae, one from Placerias (UCMP 24864) from the Placerias Quarry (Chinle Formation, Arizona) and the other from an unnamed, non-Placerias taxon (NCSM 21719) from the Deep River Basin (Pekin Formation, North Carolina). Microstructure at the mid-shaft of tibiae reveals zonal fibrolamellar bone, supporting periodic rapid growth in Late Triassic dicynodonts, similar to previously sampled Permian and Early Triassic taxa. Peripheral parallel-fibered tissue in Placerias supports an evolutionary trend toward slowed growth in later ontogeny in Triassic kannemeyeriiform dicynodonts. The lack of parallel-fibered tissue in NCSM 21719 suggests that this animal may be a young individual.

Dicynodont tusks were continuously-growing without replacement or remodeling and thus may preserve a more complete growth history than limb bones. The second part of this study analyzes thickness and periodicity of dentine growth increments from two isolated dicynodont tusks (NCSM 19585, 21735) from the Pekin Formation. Growth increments with an average thickness of 18.79 µm were identified in one tusk, which is consistent with dentine increments deposited under a daily periodicity in living vertebrates. This result is promising because it provides a measure of relative growth rate in dicynodonts. However, poor preservation precludes accurate reconstruction of growth line periodicity in the other tusk. These results serve as a platform for future research in Triassic dicynodont growth and help us better understand growth strategies and adaptation in non-mammalian therapsids from the early Mesozoic.