NARIAL RETRACTION-PAEDOMORPHOSIS

[darren.naish@port.ac.uk]

Sauropod king Matt Bonnan wrote...

> Good points so far on sauropod nostrils.  There are some reptiles 
> (of course, my references are not at hand since I am writing from 
> home, so give me some time here!) with retracted narials and 
> nostrils for no particular "reason."

Best examples are monitors, in particular the Savannah monitor 
_Varanus exanthematicus_. In this species the nostrils are 
dorsocaudally retracted and located near to the eyes. Seeing as it 
lives in scrub and semidesert habitats, why?? Walter Coombs used this 
and other species as proof that narial retraction in reptiles does 
not = aquatic habits. _V. bengalensis_ also has notably dorsocaudally 
retracted nostrils - in this case they are slit shaped - but is also 
not amphibious or aquatic.

On the issue of heterochrony in sauropods, one thing I have decided 
on this issue is that different heterochronic processes can be going 
on in the same animal at different places, if you will. For example, 
sauropods might appear peramorphic due to their massive size, but 
they may be paedomorphic in their strange ankles. Some of you might 
recall that Matt Troutman and I argued a while back about whether 
ratite birds were paedomorphic or not. Given that ratites have 
down-like 'immature' plumage, rhamphothecal grooves, unobliterated 
cranial sutures, 'juvenile' pectoral girdle etc etc, they are 
typically prime candidates for textbook paedomorphosis. However, at 
the same time they are massively big compared to most carinate birds 
and have tremendously well developed - some might say over-developed 
- hind limbs (the latter is particularly true if you look at the 
femoral shaft thickness in some moa). They may thus be both 
paedomorphic and peramorphic at the same size. Same could be true for 
sauropods. 

Incidentally, you do not have to be physically huge to be 
peramorphic. Contrary to expectations (i.e., that they might be 
paedomorphic), the morphology of dwarfed elephants indicates that 
they are peramorphic - I forget the ref for this but it was a JVP 
paper.

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth, Environmental & Physical Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
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