Tuesday, June 23, 2009

Sauropods: The Biggest Dinosaurs that Ever Lived

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 bipedal predators like T. Rex or Velociraptor. By far the largest creatures ever to roam the earth, sauropods branched into numerous genuses and species over the course of 100 million years, and their remains have been dug up on every continent, including Antarctica.

So what, exactly, is a sauropod? Some technical details aside, paleontologists use the term to describe large, four-legged, plant-eating dinosaurs with bulky bodies, 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 the smallest "encephalization quotient"). 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 truncatedBrachytrachelopan), 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 classical sauropods--familiar beasts like Diplodocus and Apatosaurus (the dinosaur previously known as Brontosaurus)--follow the sauropod body plan to the letter.

Sauropod Evolution

According to current knowledge, the first true sauropods (such as Vulcanodon andBarapasaurus) arose about 200 million years ago, in the early to middle Jurassic. The exact evolutionary relationships are unclear, but it's possible that these huge herbivores were directly descended from smaller "prosauropods" like Anchisaurus and Massospondylus.

The 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 were 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 constant supply of food.

The Cretaceous period saw a slow slide in sauropods' fortunes; by the time the dinosaurs went extinct 65 million years ago, only the titanosaurs (such as Titanosaurus andRapetosaurus) were left. Frustratingly, while paleontologists have identified dozens of genuses of titanosaur, 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 had rudimentary armor plating--clearly an evolutionary adaptation to predation by carnivores.

Sauropod Behavior and Physiology

As befitted their size, sauropods were eating machines: an adult 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 inCamarasaurus and Brachiosaurus), or thin and peglike (as in Diplodocus). Presumably, the spoon-toothed sauropods subsisted on tougher vegetation that required more powerful grinding and chewing.

Reasoning by analogy with modern giraffes, most paleontologists believe sauropods evolved their ultra-long necks in order to reach high leaves. However, this raises as many questions as it answers, since pumping blood to a height of 40 or 50 feet would strain even the most robust heart. One maverick 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. 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 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.



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