Sauropod dinosaurs are familiar to most everyone. It is the group commonly called brontosaurs, and with such creatures as Brachiosaurus, it includes the largest animals that have ever lived on land. Like most all really huge animals inhabiting the landscape at a given time, the sauropods were herbivorous. They ate plants and only plants. They were big, and they were not necessarily friendly, but they were not necessarily ferocious either.
Larger size often conveys certain advantages. It stacks the deck in favor of bigger individuals in the competition with other members of the same species. Larger individuals can obtain the best space, resources, mates, and they may be better able to defend themselves. Thus increasing size through a biological lineage is a common trend in evolutionary history. This trend to evolve larger descendant species from smaller ancestors is known as Cope's rule.
The best-known sauropods are of gigantic proportions. The largest of the sauropods might have weighed as much as fifty, sixty, maybe even eighty tons. Most were probably in the twenty-to-thirty-ton range, still highly respect- able. Early in sauropod evolution there had to be even smaller ones, presumably, if the evolution of larger species followed Cope's rule. The dinosaurian ancestors of the known sauropods, the hypothetical first true sauropod, if we had a complete fossil record, would have been much smaller than the giants. Some or all of the evolutionary lineages leading to different species of sauropod giants became larger with geologic time until they went extinct.
The only animals that have ever surpassed the sauropod size record are the blue whales. They, of course, have an advantage because they live in seawater, which is a dense fluid that buoys up their massive bodies. If stranded on land, whales, even the smaller ones, will suffocate from being crushed under their own weight. The structure of their bodies cannot keep their weight off of their lungs.
The problems of supporting weight for any animal are much greater on land than those encountered in an aqueous medium, such as whales inhabit, but the problems are particularly difficult for those land-dwelling animals of exceptionally large size. Being such large animals as most of the sauropods are, shear bulk places a tremendous strain on them. One reason contributing to why sauropod dinosaurs were able to evolve into such large and heavy forms is that at some time in their early history they began to walk on four legs rather than two. Strange as it seems, dinosaurs started out walking, running, and standing on two legs. Bipedal locomotion is primitive for the group as a whole (and for many of the more familiar dinosaurs, such as Tyrannosaurus rex). In sauropods, dinosaurs specialized in obtaining large size, four legs provide a more stable foundation for body mass and double the number of pillars supporting the animal.
There are big ancillary problems associated with Cope's rule. The mass of an animal's body increases at a greater rate than the simple increase in a linear dimension. That means that an animal twice as long as another of the same shape and physical appearance will weigh more than twice the second animal's weight. Larger animals weigh proportionately, as well as absolutely, more than smaller animals. Therefore, if a sauropod lineage is to become gigantic, it must be able to support a spiraling increase in weight with each moderate increment in size. The strength of the bones must be great enough to hold up all that bulk, but if the bones are made bigger to be stronger, they will reach a point where they increase in weight faster than they increase in strength. There is the dilemma.
The solution is to evolve structural modifications in the shape and construction of bones so that they are both light and strong. Birds do it by having thin-walled, hollow bones. Pterodactyls do it the same way. From a paleontological point of view, such bones are fragile. That is why both these groups of flying reptiles are uncommonly preserved as fossils, and when they are, they are usually crushed.
Sauropod heads have a particularly nasty habit so far as field paleontologists are concerned. They are not often found. In life they must have been weakly attached to the end of the neck, because in death the head is quickly separated from the body. I cannot shake the image of a sauropod head rolling off like a soccer ball, but of course it was not like that. The light, thin bones of the head are as lightly built and as thin-walled as the vertebrae are. They are insecurely fastened to each other. As a dead sauropod rotted or was dismembered, the head separated from the neck, and the delicate bones became scattered and destroyed. The problem this makes for paleontologists is that many species of sauropods are known only from bones that come from the neck or farther back in the skeleton, and we have no idea what the head was actually like. Heads are known in only a fraction of the named sauropod species. In fact Apatosaurus, the real name of brontosaurus, suffered for decades with the wrong head until the mistake was caught and corrected.
Sauropod skulls that are known show quite a bit of diversity in their shapes. In all, the bony holes through which the nasal passages go appear excessively large. In life these accommodated, in addition to the air passages, tissue and blood vessels that have been considered speculatively at one time or another to control the temperature of blood flowing to the brain, to resonate the sounds uttered by the beast, or to facilitate a flexible proboscis. The issue is still open.
Some sauropods have long heads, others have heads that are almost incredibly blunt. Elongation of sauropod skulls seems to have evolved in at least two different ways. Brachiosaurus was described as having an absurd, toothy duck's bill. The front of the head is long because the bones that hold the teeth, called maxilla and premaxilla, are exaggerated into the shape of a duck's bill. The nasal bones, which support the nostrils, are looped high up and back, doming the skull. Other sauropods, such as Camarasaurus, have high heads, but the jaws are not drawn nearly so far forward as in Brachiosaurus and the nostrils are not so far back and up. Sauropods such as the familiar Apatosaurus and Diplodocus are different. Their nostrils are back and on the top of their heads, but the nasal bones are not looped and the bony opening in the skull for the nasal passage is much smaller. The skulls in Diplodocus and Apatosaurus are oblong-shaped, somewhat like lozenges. The jaws are long, but taper from the back of the skull to the snout gradually, rather than being drawn out into a duck's bill. And the teeth are restricted to the front part of the upper and lower jaws instead of continuing back along the sides of the mouth. In addition, the front of the jaws is squared off and blunt, rather than curving more gently to meet at the midline.
Jaws and teeth are particularly important when considering how the sauropod giants managed to obtain sufficient nourishment. Besides the variation in jaw shape discussed above, there is also variation among sauropod species in the shape of the teeth. Some sauropods, such as Brachiosaurus and Camarasaurus, have teeth with spoon-shaped crowns. Others, such as Diplodocus, have teeth that are shaped like pencils. They are little round cylinders. Both of these shapes of sauropod teeth, when multiplied by a mouthful of them, are adequate for plucking and nipping sprigs, twigs, fronds, and leaves, but they are not good for chewing them up. In addition, they are not held very strongly in their sockets, and they were replaced in life at frequent intervals by replacement teeth growing in from the roots. After a sauropod dies, the teeth very quickly fall out. Since neither tooth shape is adequate for a good, thorough chewing of the food, the teeth were used primarily to obtain food. The processing of it was done by gizzard stones, called gastroliths, that ground foodstuffs into a mash, after which it was most likely fermented by the action of microbes. The gastroliths are rocks that were selectively swallowed for the purpose. Birds and crocodiles do that now. It is not all that unusual in the world of animal digestion. There can be variation in the shape of teeth within certain species of sauropods. In Brachiosaurus, for instance, the teeth in the front of the mouth are more broadly spoon-shaped than those farther back. Still, even in the back of the mouth they are different from the pencil-shaped teeth found in Diplodocus, a shape that is much more simple and less variable because it is basically an uncomplicated cylinder. By comparison with other, more primitive dinosaurs, it seems most likely that the spoon-shaped teeth are less evolved and more akin to the shape of teeth in hypothetical ancestral sauropods. Pencil-shaped teeth evolved from the more primitive spoon-shaped form, but since one simple cylindrical shape looks pretty much like any other simple cylindrical shape, the possession of pencil-like teeth alone, when considered among all the species of sauropods, does not necessarily reflect a common ancestry. In other words, simple teeth may have evolved from more complex teeth more than once.