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Darren wrote:

> According to new evidence presented recently by Sundell (1997),  
> mercoidodontoids were, in fact, burrowers. Whole families of 
> merycoidodontoid have now been found preserved in burrows, and this 
> lifestyle is supported by the stocky merycoidodontoid body, their 
> broad, clawed feet and big heads with robust canines. I think 
> Sundell compared them to living warthogs.
> One thing that may strike you as odd is that mammalogists are always 
> adding to the list of fossorial/semi-fossorial mammal taxa, yet the 
> Mesozoic seems strangely depauperate in its number of archosaurian 
> burrowers.

While we're talking about burrowing in fossil taxa, we should give 
dicynodonts their due.
In the Beaufort beds of the South African Karoo, R. Smith of the 
South African Museum has discovered several helical burrow casts, 
very similar to the ichnofossil Daimonelix from the Miocene of 
Nebraska. Daimonelix has been attributed to the digging activities of 
the primitive beaver Paleocastor. In the Karoo, the maker and 
inhabitant of the spiral-like burrow were dicynodonts of the genus 
Diictodon; several specimens have been found coiled, and sometimes 
intertwined at the terminal end of the cast, which appears to have 
been a widened, terminal chamber in the burrow.  Scratch marks at the 
sides of the burrow cast suggest that the burrow was made by the aid 
of both the claws and the beak of the dicynodonts.
Next to  Diictodon, the genera Cistecephalus (Kistecephalus), 
Cistecephaloides and Kawingasaurus have been considered as probably 
fossorial animals, based on the morphology of the body and especially 
the forelimbs. Cistecephalus shows a striking general resemblance to 
the extant moles and molerats.
Apart from the dicynodonts, there is also some evidence for burrowing 
in some small parareptiles, the procolophonids.  Bones of the genus 
Procolophon have been identified in an elliptical burrow cast in 
fossil riverbanks of the Early Triassic.  Broadening of the three 
inner digits of the manus of Procolophon has been interpreted as an 
adaptation to digging. Von Huene was already pointing to digging 
habits of procolophonids and Sclerosaurus in some publications in the 
Recently, an assemblage of juvenile Youngina has been found in 
Permian strata from the Karoo,  intertwined and apparently killed 
together while sheltering in a den.

Apart from the Karoo basin, in which taphonomic circumstances 
probably favoured the preservation of fossorial structures, 
there are very few mentions or remains of digging vertebrates in the 
Paleozoic or Mesozoic.

A recently discovered diapsid from the Early/Middle Jurassic  Boca 
formation of Mexico, Tamaulipasaurus, displays a morphology 
strikingly adapted to a fossorial way of living.
And if I remember correctly, the first amphisbaenid has been 
recovered from Late Cretaceous sediments of the Gobi some years ago.

For the completeness of matters, the Italian Triassic 
archosauromorph(?) Drepanosaurus was described initially as fossorial 
animal (with the enlarged claws and hooked tail considered as 
adaptations to burrowing) until new material and new descriptions 
allowed correction: drepanosaurs are now considered as tree-dwellers.

Drifted far away from dinosaurs but still arriving at 

Pieter Depuydt