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RE: Land of the Lost review-spoiler?
A reason for the prevalence of beaks in certain groups and not others has
been identified in the presence of an egg caruncle (as in archosaurs,
turtles, _Sphenodon_ and monotremes) vs. egg tooth (as in squamates) used in
hatching. The rhamphotheca does not occur in squamates, consistent with it
always developing from a caruncle, and apparently always suppresses
development of adjacent teeth (e.g. dinosaurs with premaxillary teeth would
not also have an upper-jaw beak, a point not always observed in
reconstructions). I don't know about homology of lip muscles, but agree with
previous commenters on this thread that, because birds and crocs lack lips
in totally different ways, we can't argue from those absences to infer
conditions in dinosaurs. Even if lip muscles are non-homologous in lizards
and mammals, the presence of lips and their correlates in bone in these
groups can be usefully compared with non-beaked (or half-beaked) dinos. The
bills of monotremes superficially resemble rhamphothecae (and are also
associated with loss of anterior teeth) but differ from beaks in numerous
ways (soft and leathery, with specialized sensory structures etc.); maybe a
monotreme-type bill could also evolve convergently in other carunculate
groups. If therians lost the caruncle along with eggshells, marsupials and
placentals might thereby have lost the ability to evolve beaks.
Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa
"Get this $%#@* python off me!", said Tom laocoonically.
From: Augusto Haro [mailto:email@example.com]
Sent: 05 June, 2009 9:58 PM
Subject: Re: Land of the Lost review-spoiler?
According to Bakker 86', cocodiles lack lip muscles, but lizards have
them. Lizard lip muscles seem to be non homologous with ours, however,
and not striated, unlike the lip musculature of ours, and likely not
innervated by nerve VII, unlike our facial muscles.
We should go down to turtles and lissamphibians, and even "fishes" to
see if they have the muscle kind present in lizards (I suppose turtles
may not have the muscles because they have a beak, except perhaps for
tryonichids); if not, the lizard condition may be autapomorphic.
Noticeably, beaks appeared many times in Archosauria, including
poposaurs, some pterosaurs, Silesaurus, Ornithischia, some basal
Sauropodomorpha, and many coelurosaurs. May it be that archosaurs have
something which made it easy for them to acquire beaks? I would think
of lack of lip muscles even in beakless, toothed archosaurs, as
before, but do not really know.
There are two main points in which beaked archosaurs emerged, one in
the early history of Dinosauriformes -Late Trias, Early Jura- and
other in Coelurosauria. I do not know if this means anything.