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Re: [dinosaur] Fossorial Origin of the Turtle Shell

On Mon, Jul 18th, 2016 at 12:30 PM, Tim Williams <tijawi@gmail.com> wrote:

> This is a fascinating idea - that digging behavior can lead to
> swimming behavior.  In both activities the forelimbs are used to
> propel the body forward by the displacement of the surrounding media
> (soil vs water), as Lyson &c put it.  So the osteological adaptations
> associated with one behavior (digging) were exapted toward the other
> behavior (swimming) in the line leading to turtles.
> The same transition, in the opposite direction, was previously
> proposed for another group of oddball tetrapods: the monotreme
> mammals.  The echidnas (or spiny anteaters), which are fossorial and
> terrestrial, are hypothesized to have evolved from aquatic
> platypus-like ancestors, thereby making echidnas secondarily
> terrestrial (Phillips et al., 2009; www. pnas.org cgi doi 10.1073
> pnas.0904649106.)

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.

> I don't mean to imply that stem turtles were arboreal(!). 

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.


Dann Pigdon
Spatial Data Analyst               Australian Dinosaurs
Melbourne, Australia