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>(re:
Dilophosaurus:
it is not the size of the crests which are relevant, but their functionality
relative to the brain, not necessarily their sexual display
utilizations)<
But, the crests themselves are not located over the brain cavity. Yeah, I
know that blood vessels and what not could help transport cool blood, but the
longer the blood travels inside the body, the warmer it will get. You could
probably make a case for _Cryolophosaurus_ using its crest as a radiator, seeing
as how it was located directly over the brain cavity (geography of the specimen
is something else though).
Also, Dilophosaurus didn't really have much gray matter to cool
down...assuming that all dinosaurs from day one (almost) on were endothermic,
I'm pretty sure that its internal metabolic systems were capable of dealing with
brain temperature.
Furthermore, the work that I have done (that sounds cocky, doesn't it?)
suggests that the postcrania of known _Dilophosaurus_ specimens don't show
notable dimorphism (*remember that it's a small sample size here, so this isn't
gospel or anything, but I think its a good start), but if you'll remember in
your copy of Dinosaur Systematics, Molnar (IIRC) mentions that animals with
large cranial features tend to not show much if any body differences, and tend
to only show dimorphism in the skull...seeing that we only have one really good
skull with a crest, its kinda hard to determine if there's anything going on
there.
It's also perhaps important to consider the
environment. The Kayenta was deposited by a series of large streams, both
perennial and ephemeral. From my understanding of the paleoenvironment, based on
literature, and my own work, a scene from time here in Northern Arizona probably
was wooded, with many large and small streams flowing through the forest, and an
occasional dune field. If you look at modern animals, particularly elephants,
you'll see that while African Elephants possess large ears used for display and
thermoregulation to some extent, their Indian cousins have much reduced ears,
still used for display, but are forest dwellers. Does this imply that even in
hot forests, external thermoregulatory apparatus are less important? I think it
might indicate that, but as I said in a previous post, I can't really think of
any animal that has external ornamentation designed _solely_ for
thermoregulation, so my idea may be hard to test.
Does anyone think that ceratopsians used their
frills as thermoregulatory devices? In the animals with open frill fenestrae,
the flesh would certainly act as a better conductor of heat than
bone.
Anyways, thanks for reading, if you made it all the
way to the bottom of this massive tome...and thanks for the ref's,
Stephan...I'll be sure to check them out.
Peace,
Rob
----- Original Message -----
Sent: Monday, July 15, 2002 2:45 AM
Subject: Skull crests
The literature
on this is quite large -- I have in my files the major studies -- but an
excellent source is Victor Mihalkovics, 1898. Nasenhoehle und Jacobsonsches
Organ. Eine morphologische Studies. Anatomische Studie 11:1-107.
From there, the studies are wide-ranging in specificity. I would recommend
T.M. Crowe and P.C. Withers, 1979. Brain temperature regulation in helmeted
guineafowl. South African Journal of Science 75:362-365.
Brain temperature regulation...this, as you will
recall, is the fundamental reason why some hadrosaurs and theropods have
crests . I believe all theropods, most sauropods, and any number of
ornithischians, possessed the rete ophthalmicum, the paired
vascular heat exchangers found in all living dinosaurs. All extant taxa are
able to keep their brains @2 degrees C lower than their bodies, the blood
cooled from draining of eyes, upper respiratory systems, and the nares. The
most important applicable work is being conducted by Uffe Midtgard, from whom
I draw these comments (cf. his 1986 Univ. Copenhagen dissertation, The
peripheral circulatory system in birds: a morphological and physiological
study of some adaptations to temperature regulation). Among "bird! ! s" and
pre-K/T dinosaurs, the opthalmic rete is homologous to mammalian carotid rete,
and is situated in the temporal fossa. Another role is blood transport to
dinosaur eyes, regulating the ambient temperatures of ocular tissue, as well
as pivotal roles in dinosaur breathing and heart rates and body thermal
neutral zones. One would expect that pre-K/T theropods not possessing feathers
on their heads would, when exposed to high ambient temperatures, extend their
heads, urinate on their legs (urohidrosis), and expose the undersides of
their arms (I am assuming our hypothetical theropod had feathers atop its
arms). Exposure to cold among polar-dwelling theropods meant their heat-loss
reduction mechanisms involved arteriovenous, countercurrent heat exchange in
heads, arms, and legs. Peripheral blood flow in these polar taxa would be
likewise reduced, but, during warmer seasons (as with theropods elsewhere),
one would see increased cutaneous flow of blood so as to prevent hea! ! t
stress. (Among living dinosaurs, much work needs to be done on arteriovenous
heat exchangers.
A crested dilophosaur would
possess finely tuned, as it were, ophthalmic retia, with blood vessel number
reduced because of the crests, these cooling arterial blood to the brain,
reducing possibility of heat stroke. The opthalmic retia, coupled with AV
heat-exchange systems in the legs, would reduce heat loss when the animal was
in colder regions (this has been documented rather well, by the way, in the
living Spheniscus demersus, and might be a key to how polar
dinosaurs survived, and is documented also in owls, ibises, and cranes). One
would expect terrestrial theropods, particularly those in semi-arid regions,
to have arms with venae comitantes system surrounding the ulnar and radial
arteries, and a large vena basilica (a shunt vein). The legs would have a
large tibial vein for venous return in order to dissipate heat (bypassing the
heat exchanger), as is discernable among turkey vultures.
All of these systems -- including skull crests --
would prevent damage to a dinosaur's central nervous system, enable the animal
to store heat (and release heat without evaporation during running), and,
equally important, the body to store water. Cf. D.L. Kilgore, M.H. Bernstein,
D.M. Hudson, 1976. Brain temperatures in birds. Journal of Comparative
Physiology 110:209-215 (which is still a fine paper). The skull
crests, thus, would be more than decorative. (I have, of course, not mentioned
their other role: infrasonic communication.)
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