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.