Sauropods - The Biggest Dinosaurs

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 the tops of 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 terrestrial 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 and profiles.)

So what, exactly, is a sauropod? Some technical details aside, paleontologists use this word to describe large, four-legged, plant-eating dinosaurs possessing bloated trunks, long necks and tails, and tiny heads with comparably small brains (in fact, sauropods may 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 counted among these dinosaurs' least intuitive 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 recently discovered genus, 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 Mesozoic letter.

Sauropod Evolution

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, occasionally bipedal 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 these 25- or 50-ton behemoths 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. (Newborn and juvenile sauropods, as well as sick or aged individuals, would of course have made prime pickings for hungry theropod dinosaurs.)

The Cretaceous period saw a slow slide in sauropod fortunes; by the time the dinosaurs as a whole went extinct 65 million years ago, only lightly armored but equally gigantic titanosaurs (such as Titanosaurus and Rapetosaurus) were left to speak for the sauropod family. Frustratingly, while paleontologists have identified dozens of titanosaur genera from around the world, 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--and that the biggest titanosaurs, like Argentinosaurus, were even bigger than the biggest sauropods.

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 in order to fuel their enormous bulk. 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, 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 30 or 40 feet would strain even the biggest, most robust heart. One maverick paleontologist has even suggested that the necks of some sauropods contained strings of "auxiliary" hearts, kind of like a Mesozoic bucket brigade, 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 during the day and cooled off equally slowly at night.

Sauropod Paleontology

It's one of the paradoxes of modern paleontology that the largest animals that ever lived have left the most incomplete skeletons. While bite-sized dinosaurs like Microraptor tend to fossilize all in one piece, complete sauropod skeletons are rare on the ground. Further complicating matters, sauropod fossils are often found without their heads, because of an anatomical quirk in how these dinosaurs' skulls were attached to their necks (their skeletons were also easily "disarticulated," that is, trampled to pieces by living dinosaurs or shaken apart by geological activity).

The jigsaw-puzzle-like nature of sauropod fossils has tempted paleontologists into a fair number of blind alleys. Often, a gigantic tibia will be advertised as belonging to an entirely new genus of sauropod, until it's determined (based on more complete analysis) to belong to a plain old Cetiosaurus. (This is the reason the sauropod once known as Brontosaurus is today called Apatosaurus: Apatosaurus was named first, and the dinosaur subsequently called Brontosaurus turned out to be a, well, you know.) Even today, some sauropods linger under a cloud of suspicion; many experts believe that Seismosaurus was really an unusually large Diplodocus, and proposed genera like Ultrasauros have been pretty much discredited altogether.

This confusion about sauropod fossils has also resulted in some famous confusion about sauropod behavior. When the first sauropod bones were discovered, well over one hundred years ago, paleontologists believed they belonged to ancient whales--and for a few decades, it was fashionable to picture Brachiosaurus as a semi-aquatic creature that roved lake bottoms and stuck its head out of the surface of the water to breathe! (an image that has helped fuel pseudo-scientific speculation about the true provenance of the Loch Ness Monster).