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Re: AW: Tawa hallae: everything you know about basal saurischians is wrong...
> > (And has anybody ever checked GenBank whether the
> > genes have some sort of viral signature? A
> > papillomavirus or similar would explain the
> > different but physiologically somehow related - as
> > integumental hyperplasies triggered by a fairly
> > genetic mechanism, essentially - structures
> > widespread among archosaurs. But its genetic
> > would be obvious, and thus this assumption could be
> > tested.)
> Sorry, I'm having a hard time interpreting what you mean
Like the placenta appears to have evolved by "domesticating" a virus
A papillomavirus would be a good candidate because a) papillomaviri are known
to infect birds, b) they usually cause integumentary outgrowths, sometimes
keratineous, and c) they are rarely debilitating. Infection with such a virus
could easily turn an archosaur with a monitor lizard-like skin into one covered
in "fuzz". Not pretty, but viable.
But as I said, it would need to be tested. The sequence of "feather genes" - or
rather the DNA associated with these - would need to be recognizably viral in
> Doesn't this argue against your point? Big mammals
> have secondarily lost their extensive hairy body
> covering. If the dino-fuzz in _Tianyulong_ is
> homologous to the feathers (and proto-feathers) of
> theropods, then it is pretty much guranteed that this kind
> of integument is primitive for dinosaurs.
IF they were homologous. Were they? And to what degree - were they present in
the LCA, or (like the wings of birds and bats, which are homologous insofar as
they are both formed by the forelimbs) was only the underlying genetic
framework present but the actual trait realised independently?
_Tianyulong_ raises an important point: until its discovery, phylogenetic
bracketing would have predicted it to be nude. It wasn't. So it serves to show
that even type I inference-bracketing is unreliable in the absence of a
comprehensive sample of taxa of close and known phylogenetic position to the
taxon in question.
Any physiological considerations need to take into account that the Mesozoic
climate was quite a bit warmer than today's. So the drawbacks of an insulating
integumentary covering would have been more severe then than they are now.
And then, the known skin imprints of a number of saurischians and
ornithischians that show zero evidence whatsoever for integumentary structures
cannot be dismissed. One could equally "prove" that all dinosaurs were naked by
using _Stegosaurus_ and a sauropod or basal theropod as phylogenetic bracket.
Of course, we know that "all dinosaurs were naked" is wrong. But so is "all
dinosaurs were covered in integumentary fuzz, feathers, bristles or whatnot".
If crocodiles were non-nude endotherms, the case would be pretty clear (but
then there probably wouldn't have been an argument in the first place). But
they aren't. So, have crocs reverted to or preserved the ancestral states? And
if so, why?
Until we have an in-depth analysis of size increase, geographical distribution
(and palaeoclimatology) and evolution of endothermy in archosaurs, I prefer to
assume extensive "fuzz" for small (say < 1 m total length) theropods and larger
ones in a hatchling stage or living at very high latitudes, and lack of
widespread integumentary covering for anything else.
Note that this does NOT imply complete loss/reversal. Some hair is present even
in the largest land mammals and IIRC even whales have remnants of vibrissae.
Mammoths, as it seems, evolved dense fur *again*. At least mammalian hair is a
trait that - as the diversity in humans shows easily - can be suppressed and
unsuppressed in no (evolutionary) time. And chicken/pigeon breeds with
feathered legs suggest it's the same with dinosaurs - that integumentary
covering is better understood as a dynamic range than a yes-or-no thing.
Things may be different in pterosaurs. It is notable that bat "webbings" are
nude and at least their underside is integument that is generally hairless in
mammals. But colugos and gliding squirrels have *not* lost the hair on their
webbings AFAIK. And we know that a surface that is almost but not entirely
smooth can decrease drag. So pycnofibres are likely to be beneficial for
As regards _Tawa_, it lived about 10° north of the Equator, and its environment
was seasonally almost a desert. The temperatures would have been 40°C or more
even in the shade, of which there was seasonally very little; in the sun it
must have been worse than Death Valley. Shedding excess heat would have been a
bigger problem for the incipient endotherm it likely was than heat
conservation. Vibrissae-like bristles, or a crest of fuzz along the spine to
convey mood states to conspecifics - fair enough; perhaps even more likely than
not. Covered in "proto-feathers"? I find that hard to believe.
> Some modern flying birds lack quill knobs.
> _Velociraptor_ has quill knobs, and it didn't fly. So
> again, sorry, your point escapes me.
My point was that quills are useful for non-physiological reasons, and
therefore do not need to follow the same evolutionary requirements. FWIW, even
something as large as _Allosaurus_ might have had them.
In summary, I think that at present each and every individual species of
non-avian dinosaur needs to be carefully evaluated, taking into account
everything that is known about its phylogeny and environment and whatever else
may be significant, to infer which integumentary structures were present and
absent in it.
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