Think of the word "dinosaur," and two images are likely to come to mind: a snarling Velociraptor hunting for grub, or a giant, gentle, long-necked Brachiosaurus lazily plucking the leaves off trees. In many ways, the sauropods (of which Brachiosaurus was a prominent example) are more fascinating than famous predators like Tyrannosaurus Rex or Spinosaurus. By far the largest creatures ever to roam the earth, sauropods branched into numerous genera and species over the course of 100 million years, and their remains have been dug up on every continent, including Antarctica. (See a gallery of sauropod pictures.)
So what, exactly, is a sauropod? Some technical details aside, paleontologists use the this to describe large, four-legged, plant-eating dinosaurs with bulky trunks, long necks and tails, and tiny heads with small brains (in fact, sauropods are believed to have been the dumbest of all the dinosaurs, with a smaller "encephalization quotient" than even stegosaurs or ankylosaurs). The name "sauropod" itself is Greek for "lizard foot," which oddly enough was among these dinosaurs' least distinctive traits.
As with any broad definition, though, there are some important "buts" and "howevers." Not all sauropods had long necks (witness the oddly truncated Brachytrachelopan), and not all were the size of houses (one recent discovery, Europasaurus, seems to have only been about the size of a large ox). On the whole, though, most of the classical sauropods--familiar beasts like Diplodocus and Apatosaurus (the dinosaur previously known as Brontosaurus)--followed the sauropod body plan to the letter.
As far as we know, the first true sauropods (such as Vulcanodon and Barapasaurus) arose about 200 million years ago, during the early to middle Jurassic period. Preceding, but not directly related to, these plus-sized beasts were smaller prosauropods ("before the sauropods") like Anchisaurus and Massospondylus, which were themselves related to the earliest dinosaurs. (In 2010, paleontologists unearthed the intact skeleton, complete with skull, of one of the earliest true sauropods, Yizhousaurus, and another candidate from Asia, Isanosaurus, straddles the Triassic/Jurassic boundary.)
Sauropods reached the peak of their eminence toward the end of the Jurassic period, 150 million years ago. Fully grown adults had a relatively easy ride, since they would have been virtually immune to predation (although it's possible that packs of Allosaurus might have ganged up on an adult Diplodocus), and the steamy, vegetation-choked jungles covering most of the Jurassic continents provided a steady supply of food.
The Cretaceous period saw a slow slide in sauropod fortunes; by the time the dinosaurs went extinct 65 million years ago, only the lightly armored titanosaurs (such as Titanosaurus and Rapetosaurus) were left to speak for the sauropod family. Frustratingly, while paleontologists have identified dozens of titanosaur species, the lack of fully articulated fossils and the rarity of intact skulls means that much about these beasts is still shrouded in mystery. We do know, however, that many titanosaurs possessed rudimentary armor plating--clearly an evolutionary adaptation to predation by large carnivorous dinosaurs.
Sauropod Behavior and Physiology
As befitting their size, sauropods were eating machines: adults had to scarf down hundreds of pounds of plants and leaves every day. Depending on their diets, sauropods came equipped with two basic kinds of teeth: either flat and spoon-shaped (as in Camarasaurus and Brachiosaurus), or thin and peglike (as in Diplodocus). Presumably, the spoon-toothed sauropods subsisted on tougher vegetation that required more powerful methods of grinding and chewing.
Reasoning by analogy with modern giraffes, most paleontologists believe sauropods evolved their ultra-long necks in order to reach the high leaves of trees. However, this raises as many questions as it answers, since pumping blood to a height of 40 or 50 feet would strain even the biggest, most robust heart. One maverick paleontologist has suggested that the necks of some sauropods contained strings of "auxiliary" hearts, but lacking solid fossil evidence, few experts are convinced.
This brings us to the question of whether sauropods were warm-blooded, or cold-blooded like modern reptiles. Generally, even the most ardent advocates of warm-blooded dinosaurs back off when it comes to sauropods, since simulations show that these oversized animals would have baked themselves from the inside, like potatoes, if they generated too much internal metabolic energy. Today, the prevalence of opinion is that sauropods were cold-blooded homeotherms--that is, they managed to maintain a near-constant body temperature because they warmed up very slowly and cooled off equally slowly.
Next Page: Sauropod Paleontology, and a List of Major Genera