Fernandez, V. | bioRxiv Science

bioRxiv2019

Reptile-like physiology in Early Jurassic stem-mammals

There is uncertainty regarding the timing and fossil species in which mammalian endothermy arose, with few studies of stem-mammals on key aspects of endothermy such as basal or maximum metabolic rates, or placing them in the context of living vertebrate metabolic ranges. Synchrotron X-ray imaging of incremental tooth cementum shows two Early Jurassic stem-mammals, Morganucodon and Kuehneotherium, had lifespans (a basal metabolic rate proxy) considerably longer than comparably sized living mammals, but similar to reptiles, and that Morganucodon had femoral blood flow rates (a maximum metabolic rate proxy) intermediate between living mammals and reptiles. This shows maximum metabolic rates increased evolutionarily before basal rates, and that contrary to previous suggestions of a Triassic origin, Early Jurassic stem-mammals lacked the endothermic metabolism of living mammals.One Sentence Summary Surprisingly long lifespans and low femoral blood flow suggest reptile-like physiology in key Early Jurassic stem-mammals.View Full Text

paleontology↗

bioRxiv2019

Three-dimensional characterisation of osteocyte volumes at multiple scales, and its relationship with bone biology and genome evolution in ray-finned fishes

Osteocytes, cells embedded within the bone mineral matrix, inform on key aspects of vertebrate biology. In particular, a relationship between volumes of the osteocytes and bone growth and/or genome size has been proposed for several tetrapod lineages. However, the variation in osteocyte volume across different scales is poorly characterised, and mostly relies on incomplete, two-dimensional information. In this study, we propose to characterise the variation of osteocyte volumes in ray-finned fishes (Actinopterygii), a clade including more than half of modern vertebrate species in which osteocyte biology is poorly known. We use X-ray synchrotron micro computed tomography (SRCT) to achieve a three-dimensional visualisation of osteocytes and direct measurement of their volumes. Our specimen sample is designed to characterise osteocyte variation at three scales: within a bone, between the bones of one individual and between taxa spanning actinopterygian phylogeny. At the intra-bone scale, we find that osteocytes vary noticeably in volume between zones of organised and woven bone (being larger in the latter), and across cyclical bone deposition. This is probably explained by differences in bone deposition rate, with larger osteocytes contained in bone that deposits faster. Osteocyte volumes vary from one bone to another, for unclear reasons. Finally, we find that genome size is the best explanatory variable of osteocyte volume at the inter-specific scale: actinopterygian taxa with larger genomes (polyploid taxa in particular) have larger osteocytes. Our findings corroborate previous two-dimensional observations in tetrapods, and open new perspectives for actinopterygian bone evolution, physiology and palaeogenomics.

zoology↗