Data from: Heterochrony and postnatal growth in mammals – an examination of growth plates in limbs

Mammals display a broad spectrum of limb specialisations coupled with different locomotor strategies and habitat occupation. This anatomical diversity reflects different patterns of development and growth, including the timing of epiphyseal growth plate closure in the long bones of the skeleton. We investigated the sequence of union in 15 growth plates in the limbs of about 400 specimens, representing 58 mammalian species: 34 placentals, 23 marsupials, and one monotreme. We found a common general pattern of growth plate closure sequence, but one that is universal neither among species nor in higher order taxa. Locomotor habitat has no detectable correlation with the growth plate closure sequence, but observed patterns indicate that growth plate closure sequence is determined more strongly through phylogenetic factors. For example, the girdle elements (acetabulum and coracoid process) ossify always first in marsupials whereas the distal humerus is fused before the girdle elements in some placentals. We also found that heterochronic shifts (changes in timing) in the growth plate closure sequence of marsupials occur with a higher rate than in placentals. This presents a contrast with the more limited variation in timing and morphospace occupation typical for the marsupial development. Moreover, unlike placentals, marsupials maintain many epiphyses separated throughout life. However, as complete union of all epiphyseal growth plates is recorded in monotremes, the marsupial condition might represent the derived state.哺乳类动物展现出丰富多样的肢体特化类型，伴随各异的运动策略与栖息生境。这种解剖学多样性反映出不同的发育与生长模式，其中包括骨骼长骨内骺生长板（epiphyseal growth plate）的闭合时序。本研究针对约400件标本的肢体15处生长板的愈合时序开展了调查，这些标本涵盖58个哺乳类物种：34种有胎盘类、23种有袋类，以及1种单孔目动物。研究发现存在一套通用的生长板闭合时序模式，但该模式既未在所有物种间通用，也并非在高阶分类群中普遍存在。运动栖息生境与生长板闭合时序未发现可检测的相关性，但观测结果表明，生长板闭合时序更多由系统发育因素决定。例如，有袋类动物的肢带骨骼（髋臼acetabulum、喙突coracoid process）始终最先发生骨化，而部分有胎盘类动物的肱骨远端愈合时间早于肢带骨骼。我们还发现，有袋类动物生长板闭合时序的异时性变化（heterochronic shifts，即时序改变）发生频率高于有胎盘类。这与有袋类发育过程中典型的时序与形态空间占据模式所具备的有限变异形成鲜明反差。此外，与有胎盘类不同，有袋类动物终生保留多处未愈合的骨骺。不过，由于单孔目动物中已记录有所有骺生长板完全愈合的案例，因此有袋类的生长模式或许代表了演化衍生性状。