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Re: Defining the Beak
> 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
> 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
> 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?
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"
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