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AW: Evolution of dinosaur epidermal structures (free pdf)

I recently came across this study which might be relevant to the question of 
dinosaur integument:

Aiello & Wheeler (2003): Neanderthal thermoregulation and the glacial climate.
In: van Andel & Davies (eds): Neanderthals and Modern Humans in the European 
Landscape during the Last Glaciation: Archaeological Results of the Stage 3 

Presently, we have very little actual data on the larger dinos, because there 
are too few sufficiently complete fossils (and that's not likely to change). 
What Aiello & Wheeler did was to calculate, from morphological data and robust 
assumptions on physiology/thermoregulation, up to what ambient temperature 
Neanderthal and Modern humans could survive without clothing. They found the 
difference to be slight enough to necessitate some form of light clothing or an 
exceptionally hairy (by human standards) integument for Neanderthals to survive 
in a glacial climate.

This approach could probably be used the other way around, to determine whether 
fully feathered dinosaurian megafauna could survive without special adaptations 
to get rid of excess body heat. With actual evidence (say, a fairly complete 
sauropod or tyrannosaurid mummy) not likely to be forthcoming in any quantity 
(or at all), robust inference would be the best approach to tell whether 
dinosaurian megafauna would have been (secondarily, in the case of many or all 
theropods) unfeathered as adults - or more precisely, down to what latitude 
dinosaurian megafauna could survive all year round if fully feathered and 
lacking special heat-shedding adaptations. Possibly, it would even allow us to 
make a pretty robust statement on migratory behavior of polar dinos - if their 
metabolism was effective enough to allow fatal overheating during polar summer 
if feathered, it suggests they a) weren't feathered and b) had to migrate 
during polar winter.

What's nice about the Aiello/Wheeler approach is that it needs no data other 
than what we have (or can reasonably assume) for dinos - just what information 
on body layout, musculature etc you can infer from the fossils, and some robust 
assumptions about metabolism/physiology. The latter is a bit more tricky in 
dinos, but it should by now be possible. The ambient temperature at various 
latitudes during various parts of the Mesozoic is also researched well enough 
to be usable. (In particular, the Triassic is crucial - the basal dinosaurs 
lived under strong hothouse conditions, possibly the hottest/driest climate 
ever experienced by amniotes)

We wouldn't get a definitive value for any particular lineage, but rather 
cut-off conditions which no lineage could physiologically exceed; with these we 
could see if a particular taxon did exceed these at a particular latitude, or 
not. Birds could be used to test the reliability of the approach - if pluggin 
in sandgrouse data (of which there is quite a lot) yields the result that they 
can't avoid fatal hyperthermy *only* at the most extreme temperatures found on 
today's Earth surface, we know the maths are sufficiently reliable.

It would certainly more robust than phylogenetic bracketing, which requires a 
good taxonomic sample to work - but the dinosaurian integument sample is 
extremely biased towards small avialans (which we know to be fully feathered), 
i.e. the least interesting/critical taxa; we'd need data on medium-sized basal 
theropod integument we don't have, resulting in a bracket that is too wide and 
too biased to be confident.

Of course, we might find that the heat retention caused by plumage is small 
compared to other factors, similar to the original case with Homo. That would 
be interesting nonetheless, because it would mean that thermoregulation has 
been overestimated as a factor in plumage evolution.

Note that this approach is not very informative regarding *hatchling* 
integument - a case could be made for all dino hatchlings to be fully feathered 
from the very start of the lineage, or even for hatchling "plumage" (in a loose 
sense, ie including pycnofibres) to be plesiomorphic for Avemetatarsalia, 
because of metabolic constraints (nonhomothermy) and bracketing (in the absence 
of integument from basal avemetatarsalians). And in crown dinos, well-developed 
plumage in hatchlings is the ancestral condition even though their 
post-hatching ontogeny is accelerated compared to non-ornithuromorph avialans.

Unfortunately, pterosaurian data are not very helpful, because their 
surface/volume ratio was exceptionally high throughout, so whatever their 
metabolism, any heat-retaining mechanism would give them a fitness advantage 
more pronounced than in all other terrestrial vertebrates (except bats, *if* 
pterosaurs were as endothermic as bats. Which they probably weren't.).



Ben Creisler <bcreisler@gmail.com> schrieb am Mi, 3.6.2015:

 Betreff: Evolution of dinosaur epidermal structures (free pdf)
 An: dinosaur@usc.edu
 Datum: Mittwoch, 3. Juni, 2015 17:15 Uhr
 Ben Creisler
 A new paper in open access:
 Paul M. Barrett, David C. Evans & Nicolás E. Campione
 Evolution of dinosaur epidermal structures.
 Biology Letters 11: 20150229
 DOI: 10.1098/rsbl.2015.0229
 Spectacularly preserved non-avian dinosaurs with
 filaments/feathers have revolutionized dinosaur studies and
 the suggestion that the dinosaur common ancestor possessed
 integumentary structures homologous to feathers. This
 hypothesis has
 major implications for interpreting dinosaur biology, but
 has not been
 tested rigorously. Using a comprehensive database of
 dinosaur skin
 traces, we apply maximum-likelihood methods to reconstruct
 phylogenetic distribution of epidermal structures and
 interpret their
 evolutionary history. Most of these analyses find no
 evidence for the appearance of protofeathers in the dinosaur
 ancestor and scales are usually recovered as the
 plesiomorphic state,
 but results are sensitive to the outgroup condition in
 Rare occurrences of ornithischian filamentous integument
 represent independent acquisitions of novel epidermal
 structures that
 are not homologous with theropod feathers.
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