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Mamenchisaurus, one of the longest-necked dinosaurs of all time, perfectly represents the bizarre nature of sauropods. Art by Steveoc 86, image from Wikimedia Commons.

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.

References:

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

Contrary to a popular myth, Stegosaurus did not a have a butt brain. Photo by the author at the Utah Field House of Natural History in Vernal, Utah.

There’s no shortage of dinosaur myths. Paleontologist Dave Hone recently compiled a list of eight persistent falsehoods over at the Guardian–from the misapprehension that all dinosaurs were huge to the untenable idea that Tyrannosaurus could only scavenge its meals–but there was one particular misunderstanding that caught my attention. For decades, popular articles and books claimed that the armor-plated Stegosaurus and the biggest of the sauropod dinosaurs had second brains in their rumps. These dinosaurs, it was said, could reason “a posteriori” thanks to the extra mass of tissue. It was a cute idea, but a totally wrong hypothesis that actually underscores a different dinosaur mystery.

Dinosaur brain expert Emily Buchholtz outlined the double brain issue in the newly-published second edition of The Complete Dinosaur. The idea stems from the work of 19th-century Yale paleontologist Othniel Charles Marsh. In an assessment of the sauropod Camarasaurus, Marsh noticed that the canal in the vertebrae over the dinosaur’s hips enlarged into an expanded canal that was larger than the cavity for the dinosaur’s brain. “This is a most suggestive fact,” he wrote, and, according to Buchholtz, in 1881 Marsh described a similar expansion in the neural canal of Stegosaurus as “a posterior braincase.”

Sauropods and stegosaurs seemed like the perfect candidates for butt brains. These huge dinosaurs seemed to have pitiful brain sizes compared to the rest of their body, and a second brain–or similar organ–could have helped coordinate their back legs and tails. Alternatively, the second brain was sometimes cast as a kind of junction box, speeding up signals from the back half of the body up to the primary brain. That is, if such an organ actually existed. As paleontologists now know, no dinosaur had a second brain.

There are two intertwined issues here. The first is that many dinosaurs had noticeable expansions of their spinal cords around their limbs–a feature that left its mark in the size of the neural canal in the vertebrae. This isn’t unusual. As biologists have discovered by studying living species, the enlargement of the spinal cord in the area around the limbs means that there was a greater amount of nervous system tissue in this area, and dinosaurs with larger expansions around the forelimb, for example, probably used their arms more often than dinosaurs without the same kind of enlargement. The expansion of the neural canal can give us some indication about dinosaur movement and behavior.

But the so-called “sacral brain” is something different. So far, this distinct kind of cavity is only seen in stegosaurs and sauropods and is different than the typical expansion of the neural canal. There was something else, other than nerves, filling that space. Frustratingly, though, we don’t really know what that something is.

At the moment, the most promising idea is that the space was similar to a feature in the hips of birds called the glycogen body. As sauropod expert Matt Wedel has pointed out, this space stores energy-rich glycogen in the hips. Perhaps this was true for the sauropods and stegosaurs, too. Again, though, we hit a snag. We don’t really know what the glycogen body does in birds–whether it helps with balance, is a storehouse for nutritious compounds that are drawn upon at specific times or something else. Even if we assume that the expansion in dinosaurs was a glycogen body, we don’t yet know what biological role the feature played. Dinosaurs didn’t have hindbrains, but the significant spaces in the hips of stegosaurs and sauropods still puzzle paleontologists.