The origins of mammal growth patterns during the Jurassic mammalian radiation

We use synchrotron X-ray tomography of annual growth increments in the
dental cementum of mammaliaforms (stem and crown fossil mammals) from
three faunas across the Jurassic to map the origin of patterns of
determinate growth, which is intrinsically related to mammalian
endothermy. Although all fossils studied exhibited slower growth rates,
longer lifespans, and delayed sexual maturity relative to comparably sized
extant mammals, the earliest crown mammals developed significantly faster
growth rates in early life that reduced at sexual maturity (determinate
growth), compared to stem mammaliaforms. Estimation of basal metabolic
rates (BMRs) suggests some fossil crown mammals had BMRs approaching the
lowest rates of extant mammals. We suggest mammalian determinate growth
first evolved during their mid-Jurassic adaptive radiation, although
growth remained slower than in extant mammals.