RE: Tomia Re: Question: Why did birds lose their teeth?

[Jaime Headden <qi_leong@hotmail.com>]

A tomium serves the same function as a row of sharp teeth, by concentrating 
vertical forces into a single margin, as a knife. Teeth have an additional 
effect that is largely lost, which is the creation of gaps that help increase 
pressure on food items held between the jaws; some birds have these, including 
the falconid "pseudotooth" and, of course, pelagnornithids and mergansers. This 
helps hold slippery prey.

Not all birds have a tomium on the jaw edge. In parrots, for example, the jaw 
edge is largely flat on the upper jaw, but there's a tomium on the lower jaw. 
There's also a "pocket" on the palatal surface of the upper jaw that receives 
the lower beak tip during protraction. Parrots generally are palatal grippers, 
using the lower jaw to hold an object and the palate to apply vertical 
pressure; but it being broad and flat, often with ridges, increases the ability 
to crush food items or grip as food is torn up. Most birds, however, use the 
beak tip, and the formation of a tomium on either side helps enable a sort of 
I-beam effect by which the tomium helps resist torsion along the jaw, and thus 
splay of the tip.

Almost NO work has been done to reconstruct the rhamphothecal morphology of 
extinct birds, though *Diatryma* is a notable exception, and there is more work 
ahead on this. For non-neoavians, the beak morphology is largely assumed to be 
like living birds, and the typical go-to for beak morphology (as they are so 
plentiful) has been passerines. That's sorta stacking the deck.

So, hypothetically, the presence of a curved tip of the jaw and absences of 
other gripping features of the jaw margin should mean the presence of a tomium. 
They wouldn't be mutually exclusive, but the chances that a tomium is present 
would increase if the tip is recurved and there aren't additional gripping 
features. Note that pelagornithid jaw tips are only as recurved as the length 
of the pseudoteeth, so recurvature there may be explained by the absence of a 
need to reduce it from the typical recurved tip of its less serrated relat

And now to test that hypothesis...

Cheers,

  Jaime A. Headden
  The Bite Stuff (site v2)
  http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)


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> Date: Tue, 11 Mar 2014 08:31:55 -0600
> From: rtravsky@uwyo.edu
> To: dinosaur@usc.edu
> Subject: Tomia Re: Question: Why did birds lose their teeth?
>
> What about tomia? For those birds that have them it would indicate there
> is some sort of need for this functionality, and not for those who don't
> have them.