Disparate life histories in coeval Triassic cynodonts and their implications for the evolution of mammalian life histories

Abstract The evolution of mammalian innovations like elevated growth rates, endothermy, and live birth has been the subject of paleobiological work for decades. Bone histology provides one of the best lines of evidence for assessing growth rates and life-history traits in the fossil record. However, little ontogenetic information is available for nonmammalian cynodonts, the stock lineage that eventually gave rise to mammals. Here, I report the bone histology of two traversodontid cynodonts from the Triassic Manda Formation of Tanzania. Using two femoral size series, I correlate bone tissue composition and limb size in Scalenodon angustifrons and Luangwa drysdalli. Fifteen individuals were analyzed from seven penecontemporaneous localities to assess intraspecific histovariation within traversodontid ontogenetic development for the first time. My results show that Scalenodon and Luangwa have disparate life histories despite being similarly sized contemporaries. Luangwa is characterized by parallel-fibered bone that transitions to woven-parallel bone early in ontogeny, interpreted as a growth spurt. This increase in growth rate is seen in small- and middle-sized individuals but is resorbed and remodeled in the largest, skeletally mature individual. By contrast, Scalenodon is characterized by woven-parallel tissue in early ontogeny. However, femur size is not correlated with changes in bone tissue composition, as multiple individuals show peripheral slower-growing tissue regardless of size, interpreted as highly developmentally plastic growth. Together, these results demonstrate that coeval members of Traversodontidae show disparate life histories. The underlying mechanisms to explain different life histories in these taxa are likely due to (1) intrinsic differences in growth rates and (2) varying degrees of developmentally flexible growth. The implication of this work is that intraspecific variation in growth dynamics may be more widespread than currently understood in cynodonts and that size is not a good indicator of maturity for some species.