Re: Defining the Beak

[David Marjanovic <david.marjanovic@gmx.at>]

> Defining the beak... 
> 
> One of the most important aspects in the debate over whether or not 
> certain theropods did or did not possess a beak, is that many people in  
> the debate may be using varied definitions for the term "beak". 
 
That's the case. 
 
> I do not know if anyone has tried to straighten this problem out, 
 
Not that I know of. 
 
> Though mammals are defined by a multitude of traits, 
 
:-) Not under phylogenetic nomenclature, where so far all proposed 
definitions are node- or stem-based, and an apomorphy-based feature can 
only involve one trait. 
 
> According to many a person, mammals are 
> supposed to have only one bone for their lower jaw. 
 
Namely the dentary, splenial and coronoid. :-) The latter 2 are not 
present in any living mammal, or in any Cenozoic mammal I know of, but 
basal LK eutherians retain one or both, for example. But that's no 
problem, because the definition is meant to mean that the postdentary 
bones -- articular, prearticular, angular and surangular -- are no longer 
part of the lower jaw. 
 
This appears to be the case throughout the mammalian crown group, but the 
trough on the dentary of *Ausktribosphenos* might indicate that monotremes 
evolved that condition independently from the rest. 
 
> If they have more, then they are not actually considered mammals. 
> However, there are quite a few problems with using this method as a 
> defining characteristic. The old fashioned articular-quadrate type of 
> joint is not totally lost in many animals that are considered to 
> be mammals in the fossil record. 
 
In fact, all close relatives to crown-group mammals have both jaw joints, 
and both are functional. This condition is beginning in the 
Tritheledontidae (aka Ictidosauria), which are usually considered 
just-not-mammals. 
 
> Also, some marsupials today are born with 
> this joint still in place, but lose it as they develop. Therefore,  they 
> technically have more than one bone as part of their lower jaw. Does  
> this mean that marsupials that possess this trait are not mammals until  
> they become adults, even though they are feeding on milk produced by 
> their mother? 
 
Simply add "in adults" somewhere to the definition. 
 
> For one, it is highly likely that the modern beak was not just there in 
> one generation. There was most definitely several steps involved, and 
> many people seem to already have opinions about what these steps were. 
> How do we refer to these steps, in either a hypothetical sense or an 
> actual sense, if one is found in place on a fossil? 
 
There seem to be many different implications and no publications about 
this. 
 
> One problem with this issue is that beaks are supposed to be toothless 
> by at least some definitions. Therefore, if there were teeth, then there 
> can be no beak. If beak always means toothless, well then fine... When 
> it came to the beak, evolution took only one step. So by definition, a 
> beak wasn't in place until the final tooth was lost, regardless if some 
> regions or the total region was covered in a single keratinous covering. 
 
So you think *Hesperornis* and *Ichthyornis* only had a beak on their 
(toothless) premaxillae and nowhere else? 
 
> The issue is how we define a beak in a stepwise evolutionary context.  
>  
> People like to throw around the term rhamphotheca, but by definition a 
> rhamphothca is just the keratinous covering of a region already called a  
> beak by the defining aspect that it "has no teeth". 
 
Does this mean you imply that the teeth disappear before the rhamphotheca 
appears? I can't imagine that, because a vertebrate with neither teeth nor 
a beak is very limited in what it can eat... 
 
> Would it be legitimate to 
> call an upper and lower jaw that was covered with a complete and  
> singular keratinous covering a beak if those jaws also possessed teeth  
> which were used as the biting surface? 
 
I say yes. Others probably say no. Again others imply such a thing never 
existed (explicit mentions are hard to find). 
        The problem here is that there is, so far, no fossil occurrence of 
a preserved rhamphotheca on a tooth-bearing part of a bone. 
 
> To restate this question another way... Does one 
> require a tomial crest before one can call the region a beak? 
 
Is this the difference between "beak" and "bill"? Only English seems to 
have 2 different words like that, to my continuing confusion. 
 
> If we accept that the presence of teeth does not stop one from calling  
> the region a beak, other questions logically come about... Do teeth need  
> to protrude through the keratinous region where the tomia is present, 
 
so that no lips are present and the tomial edge is homologous to the gums, 
 
> or can they protrude through true fleshy gums medial to a keratinous 
> covering yet still qualify the lateral region as a beak 
 
which is in that case homologous to the lips? 
Nobody seems to know which of these possibilities has ever occurred. 
 
> Does the entire region need to be 
> covered, or can we settle with just singular bones in those regions? A 
> popular example could be that the premaxilla and nasal bones were 
> covered with one continuous cornified covering, while the true maxilla 
> was left uncovered. If this, or something similar to it, is found to be 
> the case, can we call such an arrangement a beak? A partial beak? A 
> prerhamphotheca? A pseudorhamphotheca? 
 
Yes. :-) 
        Some birds seem to have a beak only at the tip of the jaws, and 
bare skin elsewhere. So I would call suchlike beak/rhamphotheca. But there 
is of course no standard nomenclature. 
 
"I know that I know nothing" 
-- Socrates 

-- 
+++ GMX - Mail, Messaging & more  http://www.gmx.net +++
Bitte lächeln! Fotogalerie online mit GMX ohne eigene Homepage!