Re: [dinosaur] Fossorial Origin of the Turtle Shell

[Tim Williams <tijawi@gmail.com>]

Dann Pigdon <dannj@alphalink.com.au> wrote:

> River banks may be ideal places for basal burrowers to cut their teeth on (or 
> claws, as the case may be).
> Soft damp soil that can be dug into horizontally may not require much in the 
> way of burrowing ability,
> unlike drier environments that may require vertical digging into much harder 
> sediments. River banks are
> also highly productive environments.


The semi-aquatic docodont _Castorocauda_ has both swimming and
burrowing skeletal adaptations.  (Although large by Jurassic
mammaliaform standards, it was only <800g.)  Interestingly,
_Castorocauda_ also has expanded (plated) thoracolumbar ribs.

Little _Haldanodon_, another docodont, also has both semi-aquatic and
fossorial adaptations (but no evidence of expanded ribs).  Combined
swimming/burrowing habits appear to have been around for a long time
within mammaliaforms.