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Re: PROBLEM WITH AMPHIBIAN GILLS
On Mon, 18 May 1998 firstname.lastname@example.org wrote:
> I'd be interested in any comments on this, especially if you are an
> amphibian expert and know the answer. Where is Adam Yates when you
> need him...
(delurk mode) I'm right here. Just been way to busy to write anything
intelligent over the last few months. There are two osteological
corellates we can use to infer the presence of gills in
fossil amphibians (well temnospondyls at least). The first is large,
elongate and well ossified ceratobranchials that are longitudinally
grooved along the ventral surface. In modern fish the grooves carry the
afferent branchial aortic arches, associated with paired gill filaments.
It was the presence of these coupled
with the post-brancial lamina of the cliethrum (which in fish forms the
rear wall of the internal gill chamber) that led Coates and Clack (1991)
to conclude that the stem tetrapod Acanthostega retained internal gills.
Several temnospondyls (eg. Trimerorhachis, Dvinosaurus, Plagiosauridae)
ossified ceratobranchials (though they lack the postbranchial lamina so
presumably the gills were external). BTW the number of gills appears to
have been a primitive four (as in lungfish larvae) rather than the three
seen in lissamphibians.
The other correllate appears to have been branchial denticles. These
denticles (tiny tooth-like structures) were held in groups of 1-5 on small
pieces of bone. These structure form tracts in the branchial region of
several larval temnospondyls (Branchiosaurids, Sclerocephalus, Onchiodon)
and may have been used as filament supports. They are lost early in
ontogeny, indeed Boy (1974) uses their disapearance as a marker for
metamorphosis. There some suggestion that some Mesozoic temnos may have
retained these branchial denticles into adulthood but this is not
published and is complicated by the fact that the palates of several
temnos (Siderops is a classic example) were covered by little
osteoderm-like bones bearing denticles that if displaced could easily be
confused with branchial denticles.
In short I think the answer to Darren's question is that some early
anamniotes (for me amphibian is a dirty word) certainly did have external
gills but most probably did not.
Boy, J. A. 1974. Die Larven der rhachitomen Amphibien (Amphibia:
Temnospondyli; Karbon-Trias). Palaont. Z. 48, 236-268.
Coates, M. I. & Clack, J. A. 1991. Fish-like gills and breathing in the
earliest known tetrapod. Nature 352, 234-236.