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Re: Making Lip of It
2011/9/21 Sim Koning <firstname.lastname@example.org>:
> Though I do have problems with some of the arguments being put forward
> by both sides. For example, in old post on this list Tracy Ford argues that
> the presence of an overbite strongly suggests a lipless morphology, since
> lizards tend to have interlocking teeth and no overbite. However, from what I
> can tell, geckos have an overbite rather similar to that of some dinosaurs.
I think they were referring to a greater degree of overbiting, where
the upper tooth spans most of the height of the lower jaw (as in the
canines of cats or the teeth of Dilophosaurus).
> Arguments that revolve around foramina don't seem to help much. If they
> are diagnostic of anything, it seems to be degrees of sensitivity and
> vascularity of the skin around the jaws. The nutrient foramina found around
> the jaws of spinosaurids follow a single channel until they reach the tip of
> the snout, where they begin spread out or form several rows.
> If you look at the jaws of a dwarf crocodile, you will notice that they
> are practically covered in foramina. I know some have argued that the
> liplessness of crocs is probably due to their aquatic nature, but then I have
> to ask why this did not present a problem for terrestrial crocs such as
> Kaprosuchus or Simosuchus, both of which were fully terrestrial, with one
> likely an herbivore.
Were they entirely terrestrial? Or cannot be their pitted surface a
result of inheritance from aquatic animals?
> This leads me to my next point: I suspect that most, if not all
> ornithodiran archosaurs were 'lipless' and this may have been why so many
> lineages tended to evolve a beak of one form or another. Maybe a lipless
> mouth is a necessary precursor to a beak. I suspect that a beak may have
> often started out has a hard covering on the snout that gradually worked its
> way back along the jaws, replacing the teeth in the process. But as you point
> out, a beak of this nature may not have been able to coexist with lizard like
> lips. This seems to present an argument against dinosaurs having lips since
> we know that many archosaur lineages evolved beaks independently.
Nice argument; it may also explain the condition in turtles if they
turn out to be archosauromorphs as a number of molecular phylogenetic
analyses suggested (and if non-archosaurian archosauromorphs also
lacked lips). If the lip is an impedement to beaks, or at least to the
beaks expanded along the outer surface of the mouth present in birds
and turtles, the loss of lips would certainly permit future
development of this type of beaks. I would not very much say that any
kind of beak cannot exist if lips are present, because some similarly
functioning horny surfaces exist in lipped animals as ruminants.
Lönnberg (1904) compared separate parts of some bird beaks with
different scales in squamates, proposing they were homologous. This
may not apply to the palatal parts of the beak, which are not covered
with scales, unless palatal expansion of scales are hypothesized. I do
not know of recent refutations to Lönnberg's hypothesis. If the beak
is considered as formed by modified scales, we would say both birds
and crocodiles share scales attached directly to the jaws, instead of
the lips. As nothing is more superficial to the scales in lizards and
crocodiles (and likely birds, but let's put them besides, because the
presence of scales is an unknown for them), it seems to me that on the
basis of the EPB we should restore scales in dinosaurs attaching to
the jaw bones directly, so impeding the presence of lips. Anyway,
comparing the osteological/histological correlates of gums and
epidermal cornification attachment areas in dinosaurs and other
archosaurs will surely resolve whether lips were possible or not.
Lönnberg, E. 1904. On the homologies of the different pieces of the
compound rhamphotheca of birds. Arkiv für Zoologie 1:479–512.