The origins of mammal growth patterns during the Jurassic mammalian radiation
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2024-06-28, 2024-06-28
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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.