November 7, 2012
Sauropods were extreme dinosaurs. From the relatively small dwarfed species–still a respectable 12 feet long or so–to giants that stretched over 100 feet long, these small-headed, column-limbed, long-necked dinosaurs were among the strangest creatures ever to walk the earth. Don’t be fooled by the familiarity of species like Apatosaurus and Brachiosaurus; the anatomy of sauropods was so strange that paleontologists are still debating basic issues of their biology. How sauropods mated, fed, pumped blood from their hearts to their heads and even how they held their necks have all provided rich grounds for debate among specialists. Among the longest-running mysteries is how such enormous and undoubtedly active animals prevented themselves from overheating. Perhaps the solution lies in an anatomical quirk shared with birds.
Diplodocus and kin might have had a problem with body temperature. Multiple lines of evidence, from histology to limb proportions, have indicated that extinct dinosaurs had physiological profiles more like those of avian dinosaurs and mammals than any reptile, but maintaining an active metabolism and high body temperature came at a cost for gigantic dinosaurs. The bigger the dinosaur, the more difficult it would have been to dump excess heat. If a hot-running sauropod had to hoof it to catch up with a mate or escape a stalking theropod, the dinosaur could run the risk of overheating through exercise.
The difficulty big sauropods must have faced with shedding heat has sometimes been cited as a reason that these dinosaurs must have had an ectothermic, crocodile-like physiology, or that they were “gigantotherms” that only maintained relatively high body temperatures by virtue of their size and therefore had a little more leeway with heat generated through exercise. As paleontologist Matt Wedel argued in a 2003 review of sauropod biology, though, these positions are based upon assumptions about dinosaur respiratory systems and physiology that used crocodylians as models. Not only has evidence from bone microstructure indicated that sauropods grew at an extremely rapid pace on par with that of mammals, but paleontologists have found that sauropods had birdlike respiratory systems that combined lungs with a system of air sacs. Such a system would have been attuned to cope with an active, endothermic lifestyle, including a way to dump excess heat.
We know sauropods had air sacs because of their bones. In the neck, especially, air sacs stemming from the core of the respiratory system invaded the bone and left distinctive indentations behind. (While not always as extensive, theropod dinosaurs show evidence of these air sacs, too. To date, though, no one has found solid evidence of air sacs in the ornithischian dinosaurs, which includes the horned ceratopsians, shovel-beaked hadrosaurs and armored ankylosaurs.) In addition to lightening the skeletons of sauropods and boosting their breathing efficiency, this complex system may have played a role in allowing sauropods to dump heat through evaporative cooling in the same way that large birds do today. The concept is similar to what makes a swamp cooler work–the evaporation of water in the moist tissues of a sauropod’s trachea during exhalation would have helped the dinosaur dump heat into outgoing air.
But the role of air sacs in such a system, much less an animal 80 feet long or more, is unclear. The inference is obvious–like birds, sauropods had the anatomical hardware to cool themselves–but the mechanics of the process are still obscure given that we can’t observe a living Mamenchisaurus. Earlier this fall, however, biologist Nina Sverdlova and colleagues debuted research that may help paleontologists more closely examine sauropod breathing.
Using observations from living birds, Sverdlova created a virtual model of a chicken’s trachea and air sac with an eye towards simulating heat exchange. The researchers found that their relatively simple model was able to approximate experimental data from living birds, and so similar models may help paleobiologists estimate how sauropods dumped heat. We’ll have to wait for what future studies find. This line of evidence won’t totally resolve the debate over sauropod physiology and body temperature, but it may help paleobiologists more closely investigate the costs and benefits of being so big.
Sander, P., Christian, A., Clauss, M., Fechner, R., Gee, C., Griebeler, E., Gunga, H., Hummel, J., Mallison, H., Perry, S., Preuschoft, H., Rauhut, O., Remes, K., Tutken, T., Wings, O., Witzel, U. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews 86: 117-155
Sverdlova, N., Lambertz, M., Witzel, U., Perry, S. 2012. Boundary conditions for heat transfer and evaporative cooling in the trachea and air sac system of the domestic fowl: A two-dimensional CFD analysis. PLOS One 7,9. e45315
Wedel, M. 2003. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. Paleobiology 29, 2: 243-255
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