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RE: Eodromaeus, new basal theropod from Triassic in Argentina
Erik Boehm wrote (in response to my message which is appended below in full):
<So then it can be definitively concluded they weren't merely elongated
scales, analogous to the "ridge" on the backs of most iguanas?>
No. In my response to Jura/Jason, I argued that even were the morphology
different, structural features of the development of the follicular bud and
B-keratins in crocs and birds are more comparable than in non-archosaurs. This
would imply that scales (in crocs) and scales, feathers, and "hair" or bristles
in birds are all homologous. As the squamation of crocs is assumed to be
homologous to that of, say, the common garter or red-eared slider, we could
assume then that squamation in Reptilia (_sensu lato_) and birds (and that of
feathers) are also homologous. Other work also indicates that keratin
structures in mammalian hair and lacertilian scales are related, making them
more homologous than morphology implies.
What has been recently argued is that the follicular bod develops variously
among these animals, either as a scale OR as a filament, but not one into the
other. This means that "No," the feather is not an elongated scale, but arises
from a spine-like papilla (forming an invaginated follicle) that splits and
branches, forming barbs; this is structurally similar, then to scale formation:
placode to papilla to follical, to mature form.
Finally, "homology" can be used in both an absolute manner, where it is
strictly used to designate objects which share a common origin; but also for a
spectrum of similarity throughout development, through form, and via structural
composition. In this, I favor the more absolute approach when dealing with
absolute questions; when dealing with homology of development, the sliding
scale agrees with its use here between feathers and scales, but disagrees on
the absolute use because of the form and structure conditions.
Jaime A. Headden
The Bite Stuff (site v2)
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion
> Date: Sun, 16 Jan 2011 01:56:22 -0800
> From: firstname.lastname@example.org
> Subject: RE: Eodromaeus, new basal theropod from Triassic in Argentina
> To: Dinosaur.Mailing.List@listproc.usc.edu; email@example.com
> > Eventually, this issue rests on tiny *Tianyulong,* an
> > ostensible heterodontosaurid with "quills" like that in
> > *Psittacosaurus* on the base of the tail (suggesting less
> > than a preservation restriction in the latter); here, the
> > animal is also covered from neck to further down the tail
> > than the "quills" in much finer filaments. They also appear
> > to be hollow.
> So then it can be definitively concluded they weren't merely elongated
> scales, analogous to the "ridge" on the backs of most iguanas?
> (like in this picture)
> I haven't read the paper or seen pictures of the fossils.
> Only the abstract http://www.ncbi.nlm.nih.gov/pubmed/19295609
> Describing "long, singular and unbranched filamentous integumentary (outer
> skin) structures"
> So I was worried it came down to basically just taking any "bristle" looking
> thing, and concluding it is a form of "fuzz".
> I certainly wouldn't look at birds and iguanas, and conclude bristles/fuzz
> are ancestral to diapsids, and widespread throughout diapsids.