Living mammal groups exhibit rapid juvenile growth with a cessation of growth in adulthood¹. Understanding the emergence of this pattern in the earliest mammaliaforms (mammals and their closest extinct relatives) is hindered by a paucity of fossils representing juvenile individuals. We report exceptionally complete juvenile and adult specimens of the Middle Jurassic docodontan Krusatodon, providing anatomical data and insights into the life history of early diverging mammaliaforms. We used synchrotron X-ray micro-computed tomography imaging of cementum growth increments in the teeth^2,3,4 to provide evidence of pace of life in a Mesozoic mammaliaform. The adult was about 7 years and the juvenile 7 to 24 months of age at death and in the process of replacing its deciduous dentition with its final, adult generation. When analysed against a dataset of life history parameters for extant mammals⁵, the relative sequence of adult tooth eruption was already established in Krusatodon and in the range observed in extant mammals but this development was prolonged, taking place during a longer period as part of a significantly longer maximum lifespan than extant mammals of comparable adult body mass (156 g or less). Our findings suggest that early diverging mammaliaforms did not experience the same life histories as extant small-bodied mammals and the fundamental shift to faster growth over a shorter lifespan may not have taken place in mammaliaforms until during or after the Middle Jurassic.

现存的哺乳动物群体表现出快速的幼年生长，并在成年期停止生长 ¹ 。由于缺乏代表幼年个体的化石，阻碍了对这种模式在最早的哺乳动物（哺乳动物及其最近灭绝的近亲）中的出现的理解。我们报告了中侏罗世 docodontan Krusatodon 的异常完整的幼年和成年标本，为早期分化哺乳动物的生活史提供了解剖数据和见解。我们使用同步加速器 X 射线微计算机断层扫描对牙齿中的牙骨质生长增量进行成像 ^2,3,4 ，以提供中生代哺乳动物生活节奏的证据。成虫死亡时约 7 岁，幼虫死亡时约 7 至 24 个月大，当时正在用最后的成年一代替换其乳牙列。当根据现存哺乳动物的生活史参数数据集 ⁵ 进行分析时，在Krusatodon中已经建立了成年牙齿萌出的相对顺序，并且在现存哺乳动物中观察到的范围内，但这种发展被延长了，发生在与成年体重相当（156克或以下）的现存哺乳动物相比，其最大寿命明显更长，这是其最长寿命的一部分。我们的研究结果表明，早期分化的哺乳动物并没有经历与现存小体型哺乳动物相同的生活史，并且直到中侏罗世期间或之后，哺乳动物才可能发生在较短寿命内更快生长的根本转变。