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Re: feathers and WWD
...but the case of Mammalia shows that the default position is untenable,
especially if the metabolic rate of theropods was as high as their anatomy
indicates. Whether anything heavier than a horse could maintain a thermal
insulation by a dense coat of integumentary structures (feathers in this case)
in the Mesozoic climate (with subtropical conditions almost up to the polar
circles) is very questionable; phylogenetic reasoning is usually a good
approach, but in this case the constraint is basic physics, and physics always
wins over phylogeny: it's hard to maintain your lineage if you are dead form
heatstroke.
(It is not possible to reconstruct the consequences of *any* major niche,
metabolic or size shift correctly based on phylogenetic bracketing. Elephants,
rhinos, pinnipeds, cetaceans - none of these could be reconstructed correctly
based on data from their relatives.)
Still, it is quite safe to assume that _Tyrannosaurus_ hatchlings were fully
feathered.
And in any case, some physiologist should do the maths. We have enough
paleoclimatological data and anatomical data that one could take a range of
reasonable estimates of basic metabolic/heat exchange rates and calculate how
much of the animal could be covered in how thick an integumentary insulation
before it fatally overheats.
But even so, I think it's problematic to imagine an outright plumage on
nonavian theropods. "Fuzz" or ratite-like plumage, no problem with that at
least in the small and midsized taxa. Avian-type plumage is maintenance
intense; I am not sure if preen glands would leave any fossil trace except
under the most favorable conditions (eg
http://link.springer.com/article/10.1007/BF02990211) but these as well as a
beak (ie a sharply pointed snout with keratinous covering) are probably
mandatory for maintenance of an avian-type plumage.
(What did the avian uropygial gland evolve from? There seems to be very little
published on this topic.)
It may also be relevant that there are no dinosaurian feather parasites known
yet. If ALL theropods (and a lot of other dinos) had full plumage at all ages,
the nonavians can be expected to have been crawling with parasites, since their
ability to maintain plumage was lower than in avians and even avians are often
badly affected (Franklin's comment on the Bald Eagle comes to mind). But then
again, avian lice fossilize even worse than preen glands:
http://vsmith.info/files/papers/fossilReconsidered/download/Fossilsreconsidered.pdf
Molecular phylogenetics of avian lice might be useful. Though the dating would
be based on DNA alone and hence be rather spurious, the evidence from crown
Aves indicates host shifts are common (within multispecies breeding colonies as
in the Mirandornithes vs Anseriformes, between predators and prey such as in
accipitrids and pigeons). Given that molecular-clock age estimates tend to
inflate the presumed age of divergence, and in small r strategists strongly so,
if molecular clocks place the phthirapteran radiation post-mid-Mesozoic, this
would strongly indicate that at least *these* integumentary parasites only
arose when avians did. Any difference between Amblycera and Ischnocera early
radiation patterns may be interesting, because the latter have a much stronger
host association than the former.
A relevant paper (though not incorporating the recent years' insights) is
http://phthiraptera.info/Publications/41865.pdf - suffice to say that lice
almost certainly evolved from Psocoptera-like ancestors inhabiting and feeding
on plant debris used as bedding or nesting material by amniotes. There is no
direct evidence of use of such materials by nonavian dinos, but considering
_Mei long_ it is not unreasonable to assume at least the smaller taxa used some
bedding to line their favorite sleeping sites. In that regard, it is again
interesting to note that large sparsely-haired mammals prefer to rest on soft
sediment (eg sand or ash http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3466804/)
rather than gathering bedding material. But whether this is size-related (too
time-consuming to gather the material) and thus comparable to large dinos, or
integument-related (no need to keep the integument from soiling) and thus not
comparable to large dinos in the scope of
this question, or both, or neither, I don't know. But it's not very relevant,
because even if large carnivorous theropods used no bedding themselves, if
smaller dinos did (possible and perhaps likely) and were preyed upon by the
large guys (certain), host switching would still be expected *if and only if*
the large guys had in fact sufficient plumage.
There are three interesting recent papers pertaining to the question, one by
Grimaldi and Engel (ie entomo guru stuff)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1560062/, a recent review of the
underlying question
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2699.2008.01951.x/abstract
and a 2011 analysis of the molecular data
http://pubmedcentralcanada.ca/pmcc/articles/PMC3169043/pdf/rsbl20110105.pdf
which tentatively points to lice only arising coincident with avians (but see
also http://www.biomedcentral.com/1471-2148/10/292/ for the problems of the
molecular approach - for Anoplura it does not work as well as one would like it
to).
All things considered I find the hypothesis of full plumage (or similar
integument) borne by small theropods very convincing, by large and gigantic
dinosaurs quite weak and possibly even fully untenable.
Display feathers are a possibility even in the largest taxa, but even the
presence of quill knobs is not a very strong argument - these only prove
bird-like remiges somewhere in the ancestry (if remiges are lost, the quill
knobs might persist until the now-useless phene gets eliminated by a chance
mutation; quill knobs and remiges are functionally not firmly related and not
linked genetically; we know this from the non-universal development of quill
knobs in volant crown Aves* but the evo-devo link is still unclear - they might
depend on the physical presence of actual feathers to form properly). The
"phylogenetic bracketing" argument is weak if not fully spurious because a) it
flatly disregards a soft constraint (maintenance effort - not prohibitive, but
potentially debilitating), b) it flatly disregards a possible hard constraint
(thermodynamics - prohibitive if it applies because the resultant evolutionary
fitness is zero) and c) it can be
demonstrated to be false if the same question is applied to crown lineages.
That being said, it's still an open question until someone does the math - we
know that there is a thermoregulatory threshold, but this needs to be
calculated even if this calculation will be accurate only to an order of
magnitude perhaps. In absence of hard figures, it's only notable that extreme
gigantism and dense integument are apparently only compatible in subarctic
climates - and as far as anyone can tell, there was no persistent subarctic
climate *anywhere* during the Mesozoic.**
Regards,
Eike
* Unfortunately, flightlessness in crown avians tends to quickly reduce the
ulna to an extent that any persistent quill knobs are obliterated. But perhaps
_Branta hylobadistes_ has some data to offer in this regard; it's the only
avian taxon I can think off the top of my head that is known from a population
containing the whole spectrum from weakly volant to fully flightless
individuals, and the hypodigm is very good - according to the original
description (Olson & James 1991) there should be dozens, possibly >50 ulnae at
USNM and/or BPBM representing all the anatomical diversity found in the
population. _Branta_ quill knobs are not excessively prominent even in
long-range migratory species however.
** Of course the Mesozoic had the same glacial/interglacial cycle we have
today, ie there were colder and warmer periods within a generally hot climate.
It is interesting to speculate if nonavian theropod integument *within* any
lineage became denser or more extensive during Cretaceous glacials. In mammals
it apparently did, and mammals are a very good proxy for nonavian dinos as
regards their ability to shift range in reaction to climate change. (At least
in the late Mesozoic, after an ecological/ecotrophological equilibrium had been
established. It's different if a newly evolved and competitively successful
lineage/radiation is forced to shift range; this will likely be successful to
the detriment of less sophisticated competitors.)
--------------------------------------------
Thomas R. Holtz, Jr. <tholtz@umd.edu> schrieb am Di, 12.11.2013:
Betreff: Re: feathers and WWD
An: hammeris1@att.net
CC: dinosaur@usc.edu
Datum: Dienstag, 12. November, 2013 03:45 Uhr
Don't forget: Walking with Dinosaurs
came out in 1999, and was being
animated in 1998. So at that time the degree of feathering
among
coelurosaurs was unknown.
At present, every single dromaeosaurid for which integument
is known is
fully feathered, as are the outgroups Troodontidae,
Avialae,
Oviraptorosauria, and Therizinosauria. (And we have feathers
on other,
more distant outgroups).