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Re: feathers and WWD
Ruben Safir <firstname.lastname@example.org> schrieb am Di, 12.11.2013:
Betreff: Re: feathers and WWD
An: "evelyn sobielski" <email@example.com>
Datum: Dienstag, 12. November, 2013 20:40 Uhr
On Tue, Nov 12, 2013 at 04:29:21PM
+0000, evelyn sobielski wrote:
> ...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;
> Ostriches don't live in amazingly hot conditions?
Totally do, they are close to outright desert birds. But an adult male ostrich
weighs shy of 150 kg (twice the weight of _Deinonychus_), and even so they have
extremely slender and little-insulated necks and legs (appreciably different at
the cervical vertebra level) for shedding excess heat. They are no
tyrannosaurids but close enough to be interesting.
As I said: I would like to see the math. Ostriches are avian metabolism models
of some significance, the raw data are there. Since ostriches are tropical, you
don't even need to correct for warmer climate. Just plug in a range of
reasonable estimates of mass, surface area, and metabolic rates for
_Tyrannosaurus_ (or whatever theropod you like) into the equations, and see how
much excess heat the theropod had to shed to avoid overheating. Then do the
math assuming no integument. Then assuming the theropod was fully covered in
Tetraonidae- or Mergini-type plumage (these are among the most insulating we
get, and also conveniently plesiomorphic by avian plumage standards).
It should be possible if you are a physiologist and know what equations and
variables you need, but nobody seems to have done it yet and I am no
physiologist. But TBH it's only 10 years or so that we actually have the
results from the computer models that allow reliable estimates of dinosaur
surface area and (to a lesseer extent) mass
http://www.youtube.com/watch?v=5RG0yLeJE_U. Any earlier calculations would have
ben not very informative.
> 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.
> So, if I understand you correctly, you're saying that a trex
feathers is unlikely because the climate was too hot?
No, I think it's unlikely because from what we know in birds and mammals, and
from what basic physics tells us, maintaining evolutionary fitness (ability to
survive as a species = gene pool) in the face of heatstroke would have been a
major issue of concern for any megafauna that walk around all year thickly
insulated under average temperatures of 20+°C and are at least well advanced
Conservation of energy. Like with spaceships: Accelerating is not the problem,
if you have time (They had a few million years). Slowing down is. the problem.
Climate is a factor insofar as the ambient temperature (in space flight, your
starting speed) is higher, and if you are *only* "well advanced towards
autothermy" it may be a major factor. It's relevant evolutionarily insofar that
it did not make the challenge of shedding excess heat any *easier* and that's
for shure, you can't cheat thermodynamics. If there were a physical limit in
body size/metabolism/integument combinations today, there was a *higher* limit
in the Mesozoic.
But the ostrich data should be good enough to allow disregarding climate at
One aspect of the warmer climate argues in favor of much plumage even on large
theropods tho: it makes an excellent raincoat, and you don't even need an
uropygial gland do keep it in shape. Tropical rainforest mammals tend to have
sleeker, smoother and shorter fur than subtropical relatives, but rainforest
flightless birds (as far as we know; there weren't so many) were/are all
notably shaggy. And the main consequence of a warm climate is an accelerated
water cycle, ie a global increase in precipitation (more water in the
atmosphere and it gets turned over more quickly). So even though there were
vast Cretaceous deserts, a widely successful radiation of large land animals at
that time would more often than not have to cope with strong downpours on a
But if plumage is lethally dense if dry, or if waterlogged, makes no
difference. Dead is dead.
> So elephants and Hippos are virtually hairless?
Not hairless, *fur*less. Not featherless, *plumage*less. (Mammalian skin is
rarely hairless, but plumage grows in tracts, the areas between being entirely
nude; it's an interesting but probably minor factor in the whole issue)
Hippos are aquatic, theropods weren't (until _Gansus_ or so, and perhaps with a
few specialist exceptions other than that). Technically it would not be
unsurprising if one found hippos thickly furred (considering the niche), but
evolution usually discovers blubber first, as it did in their case. Hippos are
a poor analogue except for... other hippos I guess. And a bunch of Paleogene
mammals of more or less disputed affiliation.
Any case, It's not just one line of evidence. The maintenance needs of plumage
are another, the lack of actual fossil *proof* another etc. Altogether I find
the reasons to assume full plumage should not be lost and re-evolved in a
size-constrained fashion within a short time and multiple times (in fact any
time a size increase above some threshold happened) to be very weak. The
density of mammalian fur is so plastic phylogenetically, and avian pterylosis
is also very much noninformative as regards phylogeny if you go back beyond the
Neogene (arranging avian "orders" by pterylosis is hopeless), and the genetics
are obviously plastic too (think show fowl breeds, alopecia and hirsutism
phenotypes). The Mesozoic hypodigm of isolated fossil feathers is not
indicative of any large-bodied animals and in general (especially as regards
pennaceous feathers) closely follows the radiation of avians and parallel small
volant theropod lineages. And so on.
That being said, for any theropod of dog or smaller size I would assume that a
comprehensive "plumage" is the null hypothesis. Maybe not with feathers like
those we usually think of first, but then again, the stuff cassowaries grow
from their skin doesn't look very much like such feathers either. Very early in
theropod evolution there seems to have emerged something only a little less
complex, and visually probably not that much different.