SWIMMING ANKYLOSAURS

[darren.naish@port.ac.uk]

Excuse me here while I spin off at a lengthy tangential. I need to 
write some copious nonsense to take my mind off current concerns 
(like prepping the 2 m long stinking disarticulated  boa skeleton 
that has just arrived in the department).

Matt Bonnan wrote....

> Hippos not only have specially adapted limbs and splayed feet, but 
> also do not have armor and have large, dorsal nostrils.

Also, aquatic hippos have a particularly high SG because of dense 
bones. Of course, the idea that certain ankylosaurs may have been 
amphibious or semiaquatic is not new: Mehl (1936) published on it.

However, the evidence for it is entirely circumstantial: based on 
little more than the presence of certain nodosaur fossils within 
marine sediments. In the MS of a book I'm co-writing, I note that 
aquatic habits for these 'coastal' nodosaurs is very unlikely given 
that they seem pretty much the same as their definitely non-aquatic 
relatives found 1000s of km from the marine environment. A criticism 
set to me by a reviewer was that aquatic/amphibious taxa are _not_ 
always distinguishable from closely related non-aquatic taxa (mink 
and other mustelids were cited as an example). 

In fact, though, this is not correct and, certainly in mammals, there 
_are_ strong indicators of aquatic habits in the skeleton. Aquatic 
hippos have particular limb proportions and a 'gorgops' type skull 
(where the eyes and nostrils are above an imaginary water line) 
whereas terrestrial hexaprotodont hippos don't. Mink, beaver, coypu 
etc etc., _Thalassocnus_ (the now speciose Pisco Fm. 'swimming 
sloth') all have larger transverse processes on their proximal 
caudals and more elongate metatarsals than their close, terrestrial 
relatives. Ergo, the swimming taxa are distinguishable based on 
comparative morphology. However, little work of this kind has been 
done on reptiles. Bummer.

"This is getting out of hand - now there are two of them"
"We should not have made this bargain"

 have no 
adaptations 


While you do find Euoplocephalus in Judith River sediments which suggest a 
deltaic environment, how do we know that they aren't the carcasses of poor 
suckers who drowned in a river or estuary and then were transported to and 
buried in the deltas?  The heavily armored carcass may have prevented it 
from breaking up as quickly as others and may have protected it from 
smashing into rocks on the way to its final burial place.

I dunno, but I would take a long hard look at the limbs and feet (as you 
would predict), but also the teeth to get some ideas about diet.  Also, most 
mammals have relatively flexible vertebral columns that flex in a 
dorsoventral plane which may allow hippos (I don't know for sure, so check 
me on it) to "bound" or "run" under water.  The armored thyreophorans, in 
contrast, seem rather limited in their body movements by their armored 
bodies, squat limbs, and heavy tails.  It would appear to me that the only 
time armored dinos like Euoplocephalus went into the water was when it was 
shallow enough to drink or bathe in, or when they drowned.

Sorry to rain on your Euoplocephalus parade into the deep blue sea,

Matt Bonnan
Dept Biological Sciences
NIU



_______________________________________________________________
Get Free Email and Do More On The Web. Visit http://www.msn.com
DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
Burnaby Road                           email: darren.naish@port.ac.uk
Portsmouth UK                          tel: 01703 446718
P01 3QL                               [COMING SOON: 
http://www.naish-zoology.com]