Re: Marsupial forelimbs... or rather hindlimbs

[David Marjanovic <david.marjanovic@gmx.at>]

> 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...