PALPEBRALS (not palpabrals)

[darren.naish@port.ac.uk]

On the possible function of palpebrals ('eyebrow' or 'eyelid' 
bones, called supraorbitals in the older literature), Pete Buchholz 
said...

> Peter Galton has made the best attempt at explanation that I can get 
> my hands on (and believe me I've looked). ........

> This whole palpabral setup seems to have been there to 
> keep the head wide enough so that the eyes a) could be big and fit 
> into the head and b) so the eyes wouldn't fall out.

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?

Also, ornithischian palpebrals recall the expanded lateral processes 
of the frontals seen in predatory birds. The only function I have 
ever seen proposed for these is that they shade the eyes when the 
birds are hunting in sunny, open environments. Maybe ornithischian 
palpebrals first evolved for the same function.

> WHY this happened is beyond me.. and in fact seems to have lost 
> "favor" because every derived ornithischian group lost its 
> palpabrals and widened its frontals (ceratopians, hadrosaurs, 
> ankylosaurs etc etc etc).

Of course, not all ankylosaurs lost their palpebrals, for in some 
ankylosaurids they became modified into apparently mobile 'shutters'.

"None of us nor any blade of grass is alone; death and distinction 
are illusions of limited vision"

DARREN NAISH
darren.naish@port.ac.uk