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Re: PALPEBRALS (not palpabrals)



darren.naish@port.ac.uk wrote:

> Be aware that palpebrals are not exclusive to ornithischians, but are
> also known for some crocodylians. Indeed, Cuvier's dwarf caiman is
> known as _Palaeosuchus palpebrosus_ (have I put an extra 'a' into
> the genus name?). Like those of ankylosaurids, these palpebrals are
> mobile, but I don't know what function they may have, if any. It's
> often said that _Palaeosuchus_ is heavily armoured to protect itself
> from rocks in the fast-flowing upland streams it supposedly
> frequents, but then it is also known that these caimans spend a lot
> of time on land. Perhaps the croc experts can help here. Chris?


Not sure I buy that, as just about any "box head" (i.e. small-bodied,
stocky, blunt-snouted croc), living or extinct, had huge palpebrals. 
This includes living Osteolaemus (African dwarf crocodile), and they
don't occur in fast flowing streams.  

Both species of Paleosuchus have eyelids that open like the doors of a
DeLorean, and they're great for ontogenetic staging.  The palpebrals of
both Paleosuchus and Osteolaemus are compound structures, comprised of
two (Osteolaemus) or three (Paleosuchus) ossifications that suture
together during ontogeny.  A few fossil crocodyliforms also do this. 
All living crocs have palpebrals, but beyond Paleosuchus and Osteolaemus
they're usually irregular little things in the eyelid.  A few other taxa
do have some regular morphology there (non-Paleosuchus caimans,
Gavialis, Borealosuchus), but not anything that would tie it to a given
morphology.

I don't know what they're for.  



-- 
----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

voice:  312-922-9410 x469
fax:  312-922-9566
electronic:  cbrochu@fmppr.fmnh.org