School of Physiology, Pharmacology & Neuroscience, University of Bristol, Bristol, UK.
Bioengineering Science Research Group, Faculty of Engineering and Physical Sciences, University of Southampton, Southampton, UK.
Nat Commun. 2020 Oct 12;11(1):5121. doi: 10.1038/s41467-020-18898-4.
Despite considerable advances in knowledge of the anatomy, ecology and evolution of early mammals, far less is known about their physiology. Evidence is contradictory concerning the timing and fossil groups in which mammalian endothermy arose. To determine the state of metabolic evolution in two of the earliest stem-mammals, the Early Jurassic Morganucodon and Kuehneotherium, we use separate proxies for basal and maximum metabolic rate. Here we report, using synchrotron X-ray tomographic imaging of incremental tooth cementum, that they had maximum lifespans considerably longer than comparably sized living mammals, but similar to those of reptiles, and so they likely had reptilian-level basal metabolic rates. Measurements of femoral nutrient foramina show Morganucodon had blood flow rates intermediate between living mammals and reptiles, suggesting maximum metabolic rates increased evolutionarily before basal metabolic rates. Stem mammals lacked the elevated endothermic metabolism of living mammals, highlighting the mosaic nature of mammalian physiological evolution.
尽管人们对早期哺乳动物的解剖结构、生态和进化有了相当大的了解，但对它们的生理学却知之甚少。关于温血性是在哪个时间以及在哪个化石群中出现的，证据相互矛盾。为了确定两种最早的祖哺乳动物——早侏罗世 Morganucodon 和 Kuehneotherium 的代谢进化状态，我们使用基础代谢率和最大代谢率的单独替代物。在这里，我们通过对牙齿增量牙本质的同步加速器 X 射线层析成像报告说，它们的最大寿命比同等大小的现代哺乳动物长得多，但与爬行动物相似，因此它们的基础代谢率可能与爬行动物相当。股骨营养孔的测量表明，Morganucodon 的血流速度介于现代哺乳动物和爬行动物之间，这表明最大代谢率在基础代谢率之前就已经进化了。祖哺乳动物缺乏现代哺乳动物那样高的恒温代谢，这突出了哺乳动物生理进化的镶嵌性质。
