Sauropods become giants

Sauropods are the long-necked, long-tailed plant eaters that include Brachiosaurus, Diplodocus and later Argentinosaurus. They descend from smaller sauropodomorphs of the Late Triassic, which started small and grew steadily bigger, such as Plateosaurus from Germany and Switzerland, about 8 m long. True sauropods appear near the Triassic-Jurassic boundary, with early forms such as Vulcanodon from Zimbabwe, and by the Middle Jurassic the group includes Shunosaurus and Mamenchisaurus in China, with its extraordinarily long neck.¹²

Several traits together made gigantism possible. Their pillar-like legs carried huge weight, the head was small and swallowed plants without chewing, and bird-style air sacs made the skeleton lighter and the breathing more efficient. They laid many small eggs, grew fast and could eat tall conifers, cycads and ferns that nothing else could reach. After the end-Triassic extinction removed rival herbivores and left forests open, sauropods spread to every continent.¹³⁴

By the Late Jurassic they dominated land habitats such as the Morrison Formation of North America, and in the Cretaceous the titanosaurs reached the largest sizes of any land animal. Dozens of tonnes were routine, and the largest lived on until the asteroid at the end of the Cretaceous ended their reign. Brachiosaurus held its head about 10 m or more above the ground, Diplodocus stretched about 25 m from nose to tail, and the biggest titanosaurs were larger still.¹⁵