October 20, 2008
How did the giant sauropod dinosaurs, the long-necked earth-shakers like Apatosaurus and Brachiosaurus, get to be so big? That has been one of the most vexing questions in all of paleobiology. These dinosaurs were the largest animals to ever walk on the surface of the earth. Some of the largest, like Argentinosaurus and Supersaurus would have stretched over 100 feet from the front of their snouts to the tips of their tails. There was nothing like them before they evolved, nor has there been since.
Sauropods were a very diverse group of dinosaurs. The relatively small Nigersaurus had a head shaped like a Hoover vaccuum, while Amargasaurus had two sails on its neck. Despite these different adaptations, however, the basic sauropod body plan consisted of a small head supported by a long neck, a heavy body held up by column-like legs, and a long tail. To paraphrase the comedian John Cleese, “sauropods were thin at one end, much, much thicker in the middle, and then thin again at the far end.”
Sauropods had to be “thin at the front end.” If they had large heads, they probably would not have been able to lift them at all! To illustrate why, stick your arm out in front of you (after making sure you’re not going to accidentally smack anyone, of course) and hold it there for a moment. Doing so is easy enough, but now pick up something heavy in your hand and do it again. You can feel the strain in your arm as you try to hold up that extra weight, and you have to be careful moving your arm around since the heavy object makes it harder to control. The same was true for the heads and necks of sauropods. If having a long neck was evolutionarily adaptive, heads had to remain small.
But how could such huge animals have eaten enough food to sustain themselves with such tiny heads? We know that they did so (otherwise they wouldn’t have existed in the first place), but how? First, a general rule of body size and nutrition needs to be accounted for. The larger an animal is the less food it needs relative to its body size. A mouse, for instance, eats much less in absolute terms than you or I do, but it needs more food relative to its body size to fuel its body. This same trend would have been true for sauropod dinosaurs just as it is for living animals.
According to a new short communication in the journal Science, the teeth of sauropods provide an important clue to how they digested their food. P. Martin Sander and Marcus Clauss write that sauropods did not chew their food—and this may have been a key to living large.
Just about everyone can recall a time when their mothers admonished them to take “sensible bites” at the dinner table and make sure to chew thoroughly. Wise advice, but it took a long time to properly chew all that food.
Our species, of course, has the dental armament to eat just about anything, but sauropods did not. Many had teeth like pegs or pencils that were found only in the front of their jaw. Other herbivorous dinosaurs, like the horned dinosaurs and hadrosaurs, had row upon row of teeth for grinding food into a pulpy mass before swallowing it, but sauropods had no such equipment.
Instead, sauropods probably swallowed their food quickly and let the rest of their digestive system do the work. Smoothed stones found in association with skeletons of sauropods suggest that they swallowed stones (either intentionally or as they fed), which acted as surrogate teeth in their digestive system, grinding the food they swallowed. The closest living relatives of dinosaurs, birds and crocodiles, swallow stones, too, and these are called “gastroliths.”
Likewise, although the stomachs of these dinosaurs have not been preserved, it is probable that they had special adaptations similar to those seen in animals like cows today, where food goes through a number of digestive chambers to extract the maximum amount of nutrition. All of this would allow the sauropods to munch on vegetation and move on to the next plants as soon as the food was swallowed, rather than chewing every mouthful.
Large size would have presented sauropods with other problems, like how to lose excess body heat and get fresh air to the lungs for the uptake of oxygen. A solution to these problems, the authors of the Science paper point out, may have been a series of air sacs sauropods had in their bodies (particularly their vertebrae), a feature shared with theropod dinosaurs like the recently-announced Aerosteon and birds. These air sacs arose from the lungs and infiltrated bone, which made the skeleton lighter while keeping it strong. Aside from the structural advantage, they may have also helped the sauropods regulate their body temperature and breathe more efficiently.
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