Re: [dinosaur] Fossorial Origin of the Turtle Shell

[Tim Williams <tijawi@gmail.com>]

Dann Pigdon <dannj@alphalink.com.au> 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;
doi:10.1371/journal.pone.0048213).