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Re: Marsupial forelimbs... or rather hindlimbs



It will be interesting to see their
methods: do they hold timing of development of hindlimbs constant
relative to a third event, such as brain development? Or constant to
time/developmental stage?

They talk about ossification sequences. I've seen such literature using the order in which bones all over the body appear to make inferences about phylogeny (especially the origin of lissamphibians); in fact, I think all literature I've seen about ossification sequences talks about implications for phylogenetics. Because most of it is about temnospondyls, and (unless some or all lissamphibians are temnospondyls) all temnospondyls are dead, there is no information about brain development. There also is no "third" event because more like 20 events are compared. Often large parts of the sequence are identical throughout large clades, so most of the sequence can be held constant and compared to at which points the remaining events happen in different animals.


In several cases (e.g., Monodelphis, Dasyurus) you get
development and ossification of the facial and forelimb bones (and
significant development of their associated muscles) while the brain
is still undifferentiated neuroepithelium (and while big chunks of the
rest of the body is undifferentiated mesenchyme).

Now that is interesting! But then all I have for comparison are free-swimming larvae in which ossification starts after hatching and the brain presumably has to be functional at hatching...