BURROWING ARCHOSAURS

[darren.naish@port.ac.uk]

Merycoidodontoids have raised their cute stubby little faces on this 
list. Once again we're way off topic, but, in the spirit of 
deconstructing traditional textbook archetypes (cf. the amphibious 
lystrosaur, the beach-basking plesiosaur, the erect-necked super-tall 
moa, the vulture-like azhdarchid), it's interesting that some 
mammalogists are now arguing that the old image of sheep-like herding 
merycoidodontoids is unsubstantiated. 

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.

I'm also intrigued by the fact that palaenodonts, an enigmatic group 
of Palaeogene pangolin-like mammals that may be related to 
xenarthrans, may also have been burrowers. Apparently they exhibit a 
suite of characters suggestive of this lifestyle (a classic paper on 
this subject was published in _Journal of Morphology_ some years 
back, but I haven't yet gotten round to finding it). 

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. Only one dinosaur, to my knowledge, has ever been regarded 
as a burrower, but all I have to go on is a brief bit of blurb Bakker 
came out with on a TV show broadcast in 1993. The crew were filming 
in his house, and in his sink were a load of alligator legs, with a 
hypsilophodontid tarsus and foot on the drainage board. He said, and 
I am sure of this, that this hypsilophodontid represented the first 
evidence for a dinosaur 'that lived in a burrow'. A little later I 
thought he might be referring to _Drinker_, but in everything else 
Bakker has written about _Drinker_  he maintains that it was a 
marsh-dweller adapted for walking around on wet ground. Not the right 
habitat to dig burrows. 

I suppose you could also say that heterodontosaurs have been 
considered as burrowers by some. 

Crocodyliforms to the rescue. Gomani's (1997) recent work on 
_Malawisuchus_ shows that this robust little animal denned in 
burrows, and am I right in thinking that there is also some evidence 
indicating that _Chimaerasuchus_ did the same? These crocs 
(notosuchids) clearly do have burrowing adaptations, including 
well-muscled forelimbs and stout, deep heads with big neck muscles.

And of course, what is funny is that these crocs are the only ones 
that mammals ever managed to half-successfully mimic. Notosuchids are 
not, you see, mammal-like crocs; it is mammals that are 
notosuchid-like synapsids. Perhaps;)

"Insects live in a very ancient world, led by evolution along paths 
that have been closed to us, until our own evolution has led us to 
invent the means of exploring them"

DARREN NAISH
darren.naish@port.ac.uk