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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 (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). 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 "bi! rd! ! 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 h! ea! ! 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.)