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RE: Eodromaeus, new basal theropod from Triassic in Argentina
--- On Sat, 1/15/11, Jaime Headden <firstname.lastname@example.org> wrote:
> Â The argument for non-squamous integument is not a
> fanciful one when dealing with any basal dinosaur. The issue
> with regards to *Tianyulong* especially evokes an integument
> much akin to that of *Sinosauropteryx*, preferring then an
> argument of parsimony: It is easier to conceive that
> integument shifted from scales to "fuzz" once than it is to
> assume it did so repeatedly throughout Dinosauria. Whether
> parsimony forms the better argument over any other is to be
> seen; the next contravening hypothesis, which I assume
> Jura/Jason may prefer for the moment, is that all Dinosauria
> should be seen to be squamous (like crocs and other
> archosaurs for which integument is known) unless proven
> otherwise, but this is not the null-hypothesis. Instead,
> based evenly on Phylogenetic Bracketing and historical
> assumption, we can i
Jaime: Not sure what you mentioned here as the truncation demon appears to have
snipped your post at the most inopportune place.
> This inference becomes greater than 50% in favor of
> "fuzz" when pterosaurs are placed between birds and crocs on
> the avian lineage, or even between Archosauria and the base
> of Archosauropmorpha. In the latter scenario, with the
> presence of feathers at one end, "fuzzy" taxa along that
> lineage, and "fuzzy" taxa at the base, it can be argued that
> without contradicting evidence, "fuzz" would be the likelier
> inference, including for stem-crocs. This is based solely
> upon Phylogenetic Bracketing. Placing pterosaur4s closer to
> birds than crocs should only increase the likelihood for one
> lineage of -- rather than the whole of -- Archosauria, but
> weighs greater preponderance on the argument that "fuzz" is
> the null hypothesis of integument.
Phylogenetic bracketing only works if the structures you are looking for
mology (Patterson 1982). While many authors have entertained the idea of these
various filaments being homologous, none so far have attempted to actually test
for said homology. A structure is considered homologous if it passes the tests
of similarity (same topographical relationships between each other and with
corresponding other structures), congruence (make up a monophyletic group), and
conjunction (both possible homologs can't occur simultaneously in the same
organism [Witmer 1995]).
While the case for homology is fairly strong for maniraptoran filaments
(complete with a remarkably good stepwise evolution of complexity in the
structure), the similarity between maniraptoran fuzz and the filaments seen in
pterosaurs, or the stuff associated with _Tianyulong_ and _Psittacosaurus_ are
crude at best. Just from cursory observations (and author notes) the
pycnofibres of pterosaurs are thicker than those in maniraptorans, and lack any
sign of branching. They may not even be hollow (Kellner et al 2010). While
Zheng et al (2009) mentioned that _Tianyulong_ possible filaments are closest
to _Sinosauropteryx_ in shape, it was only in the context of known fuzzy
dinosaurs. Short of that, the filaments are much thicker and grow to be
substantially longer than anything on _Sinosauropteryx_. The authors even go so
far as to mention that the chance of homology with maniraptor fuzz is suspect.
The material associated with _Psittacosaurus_ SMFR 4970 are
larger and thicker still, found on one spot of the body only, and may not even
be filamentous at all (i.e. they could be more akin to the dorsal spikes of
I would argue that, at least on the outset, the filaments seen in all these
taxa fail the first test of homology. Since tests of homology are sequential,
this would suggest that these structures are not homologous at all.
Still, even if one is willing to be lenient with the results of the first test
(and keeping in mind that this is all based off of reproduced pictures and
statements in the literature, and not on an actual rigorous analysis), a
congruence test would also suggest non-homology as there are large gaps of taxa
-- that we have integument impressions for -- which do not show these
Your argument that it is more parsimonious to assume that filamentous
integument evolved only once in Ornithodira is a bit one sided. You are only
looking at the filaments. As I have stated ad nauseum on this list and
elsewhere, scales are integument too. A plucked bird is not scaly, and despite
anecdotes (and pictures) of derived taxa such as snowy owls with feather/scale
mixing, the presence of scales in birds is highly restricted. Current evo-devo
studies suggests that feathers evolved by hijacking the scale development
pathway (Sawyer et al 2003, contra Prum 1999 & Brush 2000), and that scales
re-evolved by suppressing this (Tanaka et al 1987, Zou and Niswander 1996). In
order for filamentous integument to have evolved only once, it would have to
have been lost (and scales re-evolved) a minimum of seven times!
Independent reversal/reacquisition in:
Compare that to the alternative where a filamentous integument is a convergent
feature. We get independent acquisitions only four times.
maniraptorans/coelurosaurs close to them
Neither one is particularly pretty, but the latter at least has less steps.
As for the
viviparity in squamates is 100 times! I'd argue that the changes it would take
to accommodate a new mode of reproduction are likely to be a heck of a lot more
complicated than the changes it would take to make a new integument. Nature has
no qualms with reinventing the wheel.
Jaime Headden <email@example.com> wrote:
> I would caution direct evidence as the best explanation for integument >
> type, but logical inference projects two bases for reasoning an assumption >
> that "fuzz" can be assumed _a priori_ for the basal Dinosauria, and > would
> permit one to assume basal sauropodomorphans might actually be > "fuzzy" and
> not squamous.
I think you might need to reexplain this point. Are you really saying that a
logical construct should outweigh actual evidence? How is this any different
from closing one's eyes, sticking fingers in one's ears and just shouting: "I'm
not listening!" ?
Brush AH. 2000. Evolving protofeather and feather diversity.
Am Zool 40:631â639.
Kellner, A.W.A., Wang, X., Tischlinger, H., Campos, D.A., Hone, D.W.E., Meng,
X. 2010. The Soft Tissue of Jeholopterus (Pterosauria, Anugrognathidae,
Batrachognathinae) and the Structure of the Pterosaurs Wing Membrane.
Patterson, C. 1982. Morphological Characters and Homology. In Joysey, K.A.
Friday, A.E. (eds): Problems of Phylogenetic Reconstruction. New York: Academic
Press. pp. 21-74
Prum RO. 1999. Development and evolutionary origin of
feathers. J Exp Zool 285:291â306
Sawyer, R.H., Salvatore, B.A., Potylicki, T.F., French, J.O., Glenn, T.C.,
Knapp, L.W. 2003. Origin of Feathers: Feather Beta Keratins Are Expressed in
Discrete Epidermal Cell Populations of Embryonic Scutate Scales. J.Exp.Zool.
(Mol Dev Evol) Vol.259B:12-24
Tanaka, S., Sugihara-Yamamoto, H., Kato, Y. 1987. Epigenesis in
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Witmer, L.M. 1995. The Extant Phylogenetic Bracket and the Importance of
Reconstructing Soft Tissues in Fossils. In: Thomason, J.J. (ed): Functional
Morphology in Vertebrate Paleontology. New York: Cambridge University Press.
Xiao-Ting, Z. , Hai-Lu, Y., Xing, X. and Zhi-Ming, D. 2009. An Early Cretaceous
Heterodontosaurid Dinosaur with Filamentous Integumentary Structures. Nature.
Zou, H., Niswander, L. 1996. Requirement for BMP Signaling in
Interdigital Apoptosis and Scale Formation. Science 272:738â741.