The great vegetarian sauropod dinosaurs, such as Brachiosaurus, were the biggest animals to walk the Earth, weighing up to 100 tonnes, as long as 60 m from snout to the end of their tails and more than 10 m tall. So big, indeed, that even the largest contemporary predators would have been unable to get sufficient purchase with their jaws to do them much damage. This vast bulk, unlike even bigger modern whales, was unsupported by water and would have posed major problems had the sauropods not evolved very porous, low-density neck and tail bones and kept their heads small relative to the rest of their bodies. Such small heads needed to take in up to a tonne of vegetation each day to keep the monsters alive and  ambling. Their teeth are not those of a chewer, being peg- or spoon-like and pointed forwards; specialised for raking in leaves and twigs, swallowed unchewed in great gulps. Once that style of eating developed in their precursors, with no need for massive chewing muscles it became possible to evolve necks up to 15 m long with increasingly diminutive heads. Studies of large numbers of some species of sauropod precursors indicate that juveniles grew astonishingly quickly, essential if their initial vulnerability was to be outpaced; newly hatched they would have weighed little more than 10 kg. At the growth rates of modern reptiles, the largest sauropods would only have reached full size in about a century. The estimated growth rates suggest warm bloodedness, research suggesting that they maintained body temperatures up to 12°C higher than do alligators. Clearly, sauropod dinosaurs were highly specialised, and their evolution is now known to have been lengthy.

A major news feature in Nature (Heeren, F. 201. Rise of the titans. Nature, v. 475, p. 159-161) traces that evolution through several surprising stages. The earliest likely ancestors, which appear in the Late Triassic (~230 Ma), were about the size of a turkey and had teeth adapted for shredding fibrous plant material; other early dinosaurs show clear signs of a predatory lifestyle. There is a limit to the size of predators bound up with the energy balance between flesh consumption and the energy expended in casing down prey and killing them. The limits on the size of plant eaters are mechanical: how much they can stuff in and the strength of their bodies, especially legs. In a world dominated in numbers by predatory dinosaurs, the selection pressure for herbivores to outgrow them and become too big to bite would have been substantial.

Little Triassic Panphagia (‘eater of everything’) was also bipedal, but the fossil record of sauropod precursors clearly shows their growth to the order of 10 m by the Early Jurassic, but not yet a four-legged gait though they had evolved relatively short but sturdy legs, signs of mass-saving porous neck and tail bones, and jaws with a large gape suited to gulping rather than chewing. By the mid-Jurassic Period sauropods were big, strong and four-legged, and by the Cretaceous they reached unmatched dimensions with the titanosaurs. This evolutionary path was not the only one adopted for dinosaurian herbivory. The famous Iguanodon discovered in 1822 by Gideon Mantell in the Early Cretaceous of Sussex was a member of a bipedal group of herbivores, including the duck-billed dinosaurs, that spanned more or less the same time range as sauropods. Fredric Heeren’s article is accompanied by an on-line ‘tour’ of sauropod evolution (go.nature.com/c7zlct), while the American Museum of Natural History has a website for a major exhibition of sauropods (www.amnh.org/exhibitions/wld/ and http://www.youtube.com/AMNHorg ) that includes footage of  a full-scale animatronic Mamenchisaurus from China which breathes and moves, (Switek, B. 2011. Living it large: review of The World’s Largest Dinosaurs exhibition. Nature, v. 475, p. 172).

Related articles

• Necks for sex? No thank you, we’re sauropod dinosaurs [Tetrapod Zoology] (scienceblogs.com)
• New Dino Finding: Warm-Blooded, Nimble Beasts (livescience.com)