[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: T. rex overbite



Donna Braginetz wrote:

"Adult T. rexes seem to have a noticeable overbite. This malocclusion begs the 
question of how the fleshy, upper-teeth-covering lip model might have worked. A 
*lot* of extra gum and lip tissue would be required on the mandible to meet the 
upper lip."

Some rex skulls have a noticeable overbite: Stan and Scotty are good examples 
of this. Others do not: AMNH FR5027, Sue are good examples. PIN 551/1 
(Tarbosaurus bataar) lacks a distinct overbite, but is relatively incomplete 
though based on other, more complete skulls probably. The first dentary tooth 
in most tyrant skulls occurs at or about the first maxillary tooth, and 
depending on the curvature of the premaxillary "arcade" this overbite can be 
minor or almost irrelevant.


"Does anyone know of other animals -- living or extinct -- that have/had a 
pronounced over- or underbite that could be used as a model for the rex mouth?"

I cover this here: 
http://qilong.wordpress.com/2014/01/23/something_about_overbites/

The short answer is that all rodents, many insectivorans, lagomorphans, a fair 
number of carnivorans, pigs and hippos, horses ... a lot of mammals; many birds 
with a hooked beak, including parrots, terrorbirds, gulls, pelecaniforms of a 
variety of stripes, etc.; and a host of sauropsidans: tuataras, and other 
sphenodontians/rhynchocephalians, rhynchosaurs, Clarazia and Hescheleria among 
thalattosaurs, Atopodentatus, proterosuchids like Archosaurus rossicus, etc, in 
which the entire seven-toothed premaxilla overhangs the rostral dentary. This 
appears to be natural.


"OR!!!!!!!!!!!!!!



Does simple bracketing answer the question:



crocodilians (no lips) -->  theropod dinosaurs (no lips) -->  living birds (no 
lips)"

No. The answer is far more complex. The tissue of the extra-oral tissues in 
crocs differ from that of birds, and both differ from THEIR ancestors, meaning 
that the case for either being the key to theropod "lips" (best use is 
"extra-oral tissues") question -- or both together -- is not made. In these 
cases, either make
, as the surface anatomy of their skulls differ so much.

I posted to another medium a more explicit statement regarding use of the EPB 
as a method to reconstruct extra-oral tissues. That is posted at the bottom of 
this post.

This is why I argue that they make poor analogies, though their specific 
morphologies may be found amongst dinosaurs (work by Hieronymus et al. PDF for 
ceratopsian facial integument here from Witmer Lab: 
http://www.oucom.ohiou.edu/dbms-witmer/Downloads/2009_Hieronymus_et_al._Pachyrhinosaurus_skin.pdf;
 go here for more: http://www.oucom.ohiou.edu/dbms-witmer/publications.htm).

"Or does the fact that living crocs are all aquatic and the condition in 
ancestral crocs is unknown muck this up?"

Unclear. The argument that the oral seal provided by extra-oral tissues 
prevents moisture loss, and data regarding drying out of the enamel of croc 
teeth (which causes fracturing and increases other damage to teeth). More data 
is needed.

I will have a post out tomorrow that will be addressing a host of issues in 
"lips" as they relate to tyrannosaurids specifically, as a response to Tracy 
Ford's 1997 paper on the subject. There's a novel argument in there that 
doesn't see much in the way of responses that needed addressing.

--

Broader statement regarding "lips" in dinosaurs.


To infer correct tissues in theropods,
we must look to living relatives for direct homologies. This is birds
(descending from within dinos) and crocs (living sister taxon).
Conditions in both show closely applying cornified skin (crocs) or
cornified (keratinized) plates covering the facial bones, conditions
which aren't apparent in fully-toothed dinosaurs. Thus, neither group
establishes a strong inference for tissue homologies. Looking
outward, we find that turtles, which may be archosauromorphans, are
beaked and have similar tissues to birds. But again, dinosaurs
differ. Further outward we have lizards and snakes and tuataras, and
bone structure is similar to fully-toothed dinosaurs, which provides
a stronger inferences of homo
we can extend this premise to lissamphibians. All of the facial
tissues present are sessile. Alternately, mammals present facial
tissues with muscles in them, and novel muscles attaching to the
bones of the jaw for which no homologies to theropods are present.
Thus, we can infer these structures are not present in fully-toothed
dinosaurs.



When I say things like “extend the
EPB to include lizards and snakes,” and “we must use lizards and
snakes as providing better inference for homology,” this is what I
mean. Crocs and birds are not adequate on this score because, by the
inference of them alone, their tissues are distinct and not useful.
Else, we might be implying crocs had beaks (turtles (crocs, birds))
which is defeated by direct examination. We do the same when
inferring muscle anatomy and tissues, for similar reasons. The deeper
back your homologous structures can be found, and the more taxonomic
variance you can capture where you can explain why differences occur
and how they aren't useful, the better your inference.


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)