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Re: [dinosaur] Fossorial Origin of the Turtle Shell
Dann Pigdon <email@example.com> wrote:
> The echidna's weekly developed electroceptors certainly suggest an aquatic
> origin. However the platypus is also an
> accomplished digger, which raises the possibility that the theoretical
> aquatic platypus/echidna ancestor may have
> itself arisen from burrowers, making the echidna secondarily fossorial as wll.
Interesting. I guess, for small tetrapods, swimming and
digging/burrowing go together quite well - which makes the transition
quite easy in either direction.
> Stranger things have happened. The closest living relative of the highly
> arboreal koala is the entirely non-arboreal
> wombat, a burrower with all the climbing adaptations of a lubricated cannon
> ball. Which lineage evolved from which
> (climber from burrower, or burrower from climber) isn't entirely certain.
Weisbacker and Archer (2008) proposed that the ancestor of extant
vombatiforms (koalas and wombats) may have been arboreal
(Palaeontology 51: 321-338). In doing so, they argued against a
long-held hypothesis that koalas were secondarily arboreal and evolved
from wombat-like terrestrial ancestors. Still, they also allow for
the (highly non-parsimonious) possibility that the weird wombat-like
morphology of koalas is due to the koala lineage (phascolarctids)
passing through a wombat-like terrestrial phase before returning to
the trees (as the modern koala and its closest extinct kin). As you
know, fossil (Oligo-Miocene) phascolarctids are not helpful in
figuring out the locomotor adaptations of koala ancestors. The most
basal/plesiomorphic vombatiforms were likely scansorial/arboreal, like
_Nimbadon_ and _Ngapakaldia_ (despite both being the size of sheep!).
The appendicular skeleton of the diprotodontid _Nimbadon_ is
strikingly koala-like (Black et al. 2012;