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.

有袋类、胎盘类与单孔目（monotreme）动物间的发育差异，被认为反映于颅后骨骼的发育模式与多样性差异之中。然而，由于单孔目相关研究数据极为稀缺，其发育极性仍未得到清晰阐明。
本文首次报道了单孔目针鼹与鸭嘴兽的颅后骨骼骨化序列，并将其与已发表的其他哺乳动物及羊膜动物（amniote）相关数据进行对比。
值得注意的是，单孔目的近肢骨（stylopodia，即肱骨、股骨）的骨化时间晚于更远端的肢节中段骨（zeugopodia，即桡骨/尺骨、胫骨/腓骨）——在所有被评估的羊膜动物中，这一特征仅见于欧洲鼹鼠。欧洲鼹鼠同样与单孔目共享适配旋转掘土与/或游泳的极端肱骨特化结构，这提示适应性演化与骨化异时性（ossification 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，表明串行同源的前肢与后肢骨骼之间存在发育整合（developmental integration）。单孔目的颈椎肋与喙骨（coracoids）的骨化时间晚于多数羊膜动物，但其骨化时序与真兽类（therians）中的同源结构一致——在真兽类中，这些结构不再作为独立骨骼存在。这种独立骨骼的丢失，或许得益于哺乳动物演化早期喙骨与颈椎肋的发育延迟。
尽管单孔目的骨化序列具有高度衍征（derived），但其与胎盘类的相似度仍高于有袋类。由此可见，有袋类的颅后骨骼发育模式，以及潜在与之相关的多样性限制，可能并不代表哺乳动物的祖先状态。