April 14, 2009
Did Sauropods Hold their Heads High?
The skeleton of Diplodocus on display in Berlin
In museums all over the world, skeletons of sauropod dinosaurs are reconstructed with their heads held high. It seems like the most natural position for these animals, but a short letter recently published in Science has questioned whether it is correct. According to biologist Roger Seymour, sauropods more likely kept their heads low to the ground, swinging them from side to side to vacuum up plant food.
The problem with sauropod posture is that their necks are ludicrously long. It would take a huge amount of blood pressure, generated by a massive heart, to keep blood pumping to the brain. This would be made all the more difficult if the animals held their heads high in the air, as the blood flow would have to work against gravity. For this reason Seymour favors the idea that sauropods kept their heads down and mostly moved them horizontally.
In a reply, paleontologists P. Martin Sander, Andreas Christian and Carole Gee agree that sauropods may have preferentially kept their heads at a relatively low level, but it was still possible that sauropods raised their heads high. That sauropods could do so is known from skeletal evidence, and it is possible that sauropods had physiological mechanisms to solve this problem that are not seen in living animals. It would take a lot of energy for a sauropod to raise its neck up high, but if the food it was reaching was high quality, or provided a large nutritional benefit, the reward might be worth the stress. The ability of sauropods to reach up high may have even benefited them during harsh conditions, when they could physically reach a wider array of resources than other dinosaurs.
Unfortunately, much of how sauropods used their necks, particularly in feeding, remains contentious. So much of the debate rests upon sauropod physiology that without a living animal to study the arguments will continue. That is what makes for compelling science, though, and who knows what new discoveries might shed light on this old problem.
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I haven’t read the study in question, but the idea of all sauropods being low browsers ignores the anatomy of some of these animals. It ignores the variety of sauropod anatomy. I think the neutral posture can tell you something about these animals, like what they might be biased towards interims of foraging preference. Brachytrachelopan and Dicraeosaurus with their downward sloping backs and short necks seem to be biased towards low browsing. However a lot of the longer necked forms seem to be biased towards medium to higher browsing. They nearly all have horizontal and often upwardly inclined backs. Take Mamenchisaurus youngi, I strongly doubt it was primarily a low browser; its neck is inclined up nearly 40 degrees, in neutral position. (Check out Scott Hartmans restoration.) It doesn’t make any sense for that animal to evolve an upward inclined back and neck which is really long if it’s a primarily a low browser. Brachiosaurus is similar, why evolve an upward sloping back (with a shoulder height of nearly 6m!) if your munching on the ground?? Why not have a horizontal back, or a downward sloping back with a shorter, cheaper (in energy terms) neck?
Steve, as long as sauropods kept their heads level with or below their hearts, they’d be fine. The higher body of Brachiosaurus would have allowed it to habitually graze on higher vegetation than its diplodocoid contemporaries. This would avoid competition and spur the evolution of a greater diversity of sauropods. It’s not like these animals were forced to munch on plants at their toes. I’m sure there was a great many plants, all of which had different hights.
It would make sense that the sauropods would be able to hold their heads in a high vertical position because they used their tail as a defensive weapon. To be used in this fashion, the tail would have to be fully visible and therefore, the head would have to be elevated.
Hi Zach, the point I was trying to get across was that the sauropods with steeply inclined backs and long straight necks don’t look comfortable in a low browsing pose. Look at this image I knocked together. It’s traced from Scott Hartman skeletal reconstruction of Mamecnhisaurus Youngi. The straight neck is articulated in the neutral pose, as it is in Scott’s reconstruction. The bottom neck is it showing its neck at heart level. Now, I don’t know how flexible the neck is, so the way I have posed it could be wrong, (I didn’t try and get it precise, I did it quickly) but to me it looks awkward. It could be that the base of neck is more flexible that what I have shown and it wouldn’t look as weird.
http://i208.photobucket.com/albums/bb186/Steveoc_86/mamenchisaurus_youngi_hartman_NECKc.jpg
Steve,
The upward sloping vertebrae in Brachiosaurus and others has a simple explanation other than to ‘aim’ the neck upwards. Because the work of the heart is related to the absolute distance between the heart and the head, by raising the chest on long front legs, the animal could reach higher without getting into cardiovascular problems. The giraffe does this. It raises its heart on legs so long that it is difficult for them to drink. The giraffe neck is about 2 meters above the heart. This is about the distance in large bipedal tyrannosaurs too.
I suggest reading the Science note and a longer paper in Biology Letters to learn that to raise the head 9 meters above the heart would have cost the sauropods about half of their food energy.
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