ORNITHISCHIAN EYES

[Tetanurae@aol.com]

I have read some of the posts over the last couple of days about the 
palpabral bones in ornithischians with great interest.  For one thing, I am 
glad the discussion has moved onto something much cooler than spinosaur teeth 
and tyrannosaur sacrals, to ornithschians, the *true* pinacle of all 
vertebrate evolution.  Additionally, I like the discussion because the reason 
I think that palpabrals evolved for hasn't already been discussed, and I have 
the oportunity to put in my opinion.

If you look at an ornithischian's skull in a dorsal view (especially basal 
ones like Lesothosaurus and Hypsilophodon) you'll notice that although the 
snout is pretty narrow the frontals are VERY narrow (as an aside this has 
been used as a synapomorphy of the Hypsilophodontidae, but its presence in 
Lesothosaurus and absence in Thescelosaurus kinda negates its "weight" in 
phylogeny).

Additionally, the Palpabral was not just some bone that stuck out into flesh, 
or a horn or something of that nature, but the bony half of an arch that 
linked the prefrontal to the postorbital.  In most ornithischians, the arch 
was finished with a ligament, but in Agilisaurus louderbacki from the 
Jurassic of China, the palpabral is ossified all the way to the postorbital, 
forming a little super-orbital fossa.

I suggest that the reason ornithischians got palpabrals in the first place is 
so that the eyes wouldn't fall out of their heads-; or more exactly, so that 
the eyeballs and eyelids would have ample attachment areas within the eye 
socket because the frontals were so thin and narrow side-to-side.

As for the question of why the frontals got so narrow in the first place... I 
have no idea.  I really don't think however, that these dinosaurs were 
"eagle-eyed" as that would require some sort of brow ridge or something, 
which is probably not what the palpabral arch was doing in the first place.  
The palpabrals were just as wide as the jugal-max complex, and in fact, if 
the eyes and eyelids were attached to the frontals, the eyes would always be 
looking up about 20-30 degrees.

Anyway...  I'm getting an idea that I have to flesh out before May 8th....


Pete Buchholz
Tetanurae@aol.com

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