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Physiological Adaptations of the Dinosauria



Hello fello dino enthusiasts, i'm new here, but i'm just going to jump right in with an article I wrote recently, and one which i'd like your thoughts on. Pardon the simplistic terminology, but anyway, here it is:

In 1969, John Ostrom, a paleontologist working at the Peabody Museum of Natural History, discovered the remains of a small carnivorous dinosaur known as Deinonychus antirrhopus. Its body was light and compact, with a 5-inch-long claw on its second toe used for eviscerating its prey. Ostrom believed this small dinosaur was a close relative of birds, citing many characteristics it shared with the earlier forms, like Archaeopteryx lithographica. He hypothesized that the dinosaurs were endothermic (or warm-blooded) like their flying descendants. These two ideas became widely accepted over the next decade and a half until John Ruben, a physiologist working at the University of Oregon in 1996, scanned several dinosaur skulls and showed that they lacked any trace of scroll-shaped bones (or cartilages) in their nasal cavities known as respiratory turbinates. In both mammals and birds, these structures are used in heating air as it is inhaled, and recovering water from the air when exhaled. In birds, the middle and anterior turbinates perform this task, whereas the posterior turbinates are used in olfaction. They are critical for maintenance of a high metabolism where an organism would otherwise quickly lose water through respiration. He concluded that dinosaurs could not have been endothermic, and suggested that they were instead "turbo-charged" reptiles with a high lung ventilation rate. This discovery cast doubt on the dinosaurian ancestry of birds, and studies of bone material indicating endothermy (oxygen isotope ratios, well-vascularized bone, upright posture). However, new genera of dinosaurs unearthed in China's Liaoning Province, showed that many small theropod dinosaurs possessed a coat of feathers or hair-like filaments. One of the most intriguing fossils was Sinosauropteryx prima, a compsognathid, which is a primitive coelurosaurian dinosaur, a group including Tyrannosaurus rex. Feathers in such a large and diverse group as the coelurosaurs indicated that they were endothermic, using them as insulation like a modern bird. While not direct confirmation that they were warm-blooded, it did lend some credence to the hypothesis. These mixed messages from the fossil record provoked many paleontologists to view dinosaurs as heterothermic (both warm and cold blooded at different times), or possibly gigantothermic, using their immense size to store accumulated body heat. Personally, I didn't see why they couldn't have been endothermic with a different system of moisture recovery. The alternative I propose is the antorbital fenestra, a window-like opening in front of the orbits and behind the external nares. This feature of the skull is not unique to dinosaurs, it is also present in primitive archosauriforms (e.g. Proterosuchus). Current thinking is that it served to lighten the skull, but I think it had an important participation in respiration. Cool air from the environment would enter through the nostrils and pass along the underside of the nasal bone, where it would reach the olfactory bulbs beneath the frontal bones. The nasal passage would then make a sharp curve and open into the dorsal region of the antorbital fenestra posteriorly. Above the nasal bone are vascular foramina for the superior nasal artery. Many dinosaurs had prominent crests on their snout, which may have helped cool the blood in this artery, where it would travel down to the lateral wall of the antorbital fossa. This cool air would be heated by the warm walls of the chamber, then pass into the throat via the choanae. Upon exhalation, the warm, moist air would rise up into the antorbital fossa and condense on the cooled surfaces, exiting through the nasal passage proper. Thus, the fossa functioned as a simple condenser, analogous to the respiratory turbinates of modern endotherms. Other structures found in dinosaurs could have also performed similarly, such as the crests in lambeosaurines like Corythosaurus casuarius, the enlarged narial openings in ceratopsians, and the long necks and large external nares in many sauropods. The difficulty in testing this hypothesis is mainly due to not knowing enough about predicted metabolic rates in dinosaurs, and not having the proper equipment to test the idea. One way to go about it is to take samples of bone from animals like "Dilophosaurus" sinensis, and subject them to oxygen istopic studies. Knowing the approximate difference in temperatures between the crests and the bone lining the antorbital fossa would give some clues to the heat dissipation rate. The paleoclimatology of the Early Jurassic would also be a crucial piece in the puzzle. While this has the potential to solve many questions about the physiology of dinosaurs, it may also explain their extinction. Since this system relies on the ambient air temperature being somewhat lower than the animal's core body temperature, it would likewise be drastically impaired by a global increase in temperature. Mammals, birds, and reptiles are oblivious to such factors. Reptiles would bask in such warmth, and both bird's and mammal's respiratory turbinates would function in spite of the changes. If, as suspected, some dinosaurs migrated, they may have been doing so to reach cooler climates where they could better survive the hot summer months during much of the Mesozoic. Many dinosaurs from the southern hemisphere (e.g. Amargasaurus cazui, Spinosaurus aegyptiacus, Ouranosaurus nigeriensis) have dorsal sails which would have boosted body temperature relative to air temperature if used as a thermoregulatory device, or enlarged antorbital fossae which would enhance the condensation rate (e.g. Carcharodontosaurus saharicus, Giganotosaurus carolinii, Abelisaurus comahuensis). Perhaps a gradual increase of temperature due to volcanic greenhouse gases during the Late Cretaceous caused the slow decline of the dinosaur. Further testing is required to validate my suspicions, but I hope that two of the most fascinating and controversial theories in dinosaur palaeontology have been resolved.


Any comments? Does it get your stamp of approval? thanks in advance!


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