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. Morganucodon also 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.

中文翻译：

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侏罗纪早期干哺乳动物的爬行动物样生理

关于哺乳动物吸热的发生时间和化石种类尚无定论，关于吸热的关键方面（例如基础代谢率或最大代谢率）或将它们置于活体脊椎动物代谢范围内的干哺乳动物的研究很少。牙骨质增量的同步X射线成像显示，两个侏罗纪早期干哺乳动物Morganucodon和Kuehneotherium的寿命（基础代谢率的替代物）比同等大小的活体哺乳动物更长，但与爬行动物相似。Morganucodon在活着的哺乳动物和爬行动物之间，股骨的血流速度（最大代谢率）也处于中间水平。这表明最大代谢速率在进化之前先于基础速率增加，并且与先前有关三叠纪起源的建议相反，侏罗纪早期干哺乳动物缺乏活体哺乳动物的吸热代谢。