Data from: Monotreme ossification sequences and the riddle of mammalian skeletal development

The developmental differences between marsupials, placentals and monotremes are thought to be reflected in differing patterns of postcranial development and diversity. However, developmental polarities remain obscured by the rarity of monotreme data. Here I present the first postcranial ossification sequences of the monotreme echidna and platypus, and compare these with published data from other mammals and amniotes. Strikingly, monotreme stylopodia (humerus, femur) ossify after the more distal zeugopodia (radius/ulna, tibia/fibula), resembling only the European mole among all amniotes assessed. European moles also share extreme humeral adaptations to rotation digging and/or swimming with monotremes, suggesting a causal relationship between adaptation and ossification heterochrony. Late femoral ossification with respect to tibia/fibula in monotremes and moles points towards developmental integration of the serially homologous fore-and hind limb bones. Monotreme cervical ribs and coracoids ossify later than in most amniotes but are similarly timed as homologous ossifications in therians, where they are lost as independent bones. This loss may have been facilitated by a developmental delay of coracoids and cervical ribs at the base of mammals. The monotreme sequence, although highly derived, resembles placentals more than marsupials. Thus, marsupial postcranial development, and potentially related diversity constraints, may not represent the ancestral mammalian condition.

学界普遍认为，有袋类（Marsupials）、胎盘类（Placentals）与单孔目（Monotremes）之间的发育差异，可通过颅后（postcranial）发育模式与多样性的差异得以反映。然而，单孔目相关数据的稀缺性，导致发育极性始终难以厘清。本文首次呈现了单孔目动物针鼹与鸭嘴兽的颅后骨化序列，并将其与已发表的其他哺乳类及羊膜类（Amniotes）相关数据进行对比。值得注意的是，单孔目的近侧肢段（stylopodia，即肱骨、股骨）的骨化时间晚于更靠远端的中侧肢段（zeugopodia，即桡骨/尺骨、胫骨/腓骨），在所有被评估的羊膜类中，仅与欧洲鼹鼠的骨化模式一致。欧洲鼹鼠与单孔目同样具备适配旋转掘土和/或游泳的极端肱骨适应性特征，这提示适应性特征与骨化异时发育（heterochrony）之间存在因果关联。单孔目与鼹鼠的股骨相对于胫骨/腓骨的晚期骨化特征，表明系列同源的前、后肢骨骼存在发育整合现象。单孔目的颈椎肋（cervical ribs）与喙骨（coracoids）的骨化时间晚于多数羊膜类，但与兽亚纲（Therians）中的同源骨化时序一致——在兽亚纲中，这些结构不再以独立骨骼的形式存在。这种独立骨骼的丢失，可能得益于哺乳类基部类群中喙骨与颈椎肋的发育延迟。尽管单孔目的骨化序列高度衍生，但其与胎盘类的相似度仍高于有袋类。因此，有袋类的颅后发育模式，以及潜在相关的多样性限制，可能并非哺乳类的祖先状态。