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Re: AW: Tawa hallae: everything you know about basal saurischians is wrong...



> > (And has anybody ever checked GenBank whether the
> "feather"
> > genes have some sort of viral signature? A
> "domesticated"
> > papillomavirus or similar would explain the
> structurally
> > different but physiologically somehow related - as
> > integumental hyperplasies triggered by a fairly
> simple
> > genetic mechanism, essentially - structures
> phylogenetically
> > widespread among archosaurs. But its genetic
> "footprint"
> > would be obvious, and thus this assumption could be
> > tested.)
> 
> 
> Sorry, I'm having a hard time interpreting what you mean
> here.

Like the placenta appears to have evolved by "domesticating" a virus 
(doi:10.1038/35001608).

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 
origin.

> 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 
non-physiological reasons.

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.


Regards,

Eike

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