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 coracoi...