Dinosaur

The sauropods were a group of large saurischian herbivores that included the world's largest-ever terrestrial animals. This group rumbled along on four, enormous, pillar-like, roughly equal-sized legs, with a long tail trailing behind. Sauropods also had very long necks, and their heads were relatively small, at least in comparison with the overall mass of these immense animals. The teeth were peg-like and were mostly used for grazing, rather than for chewing their diet of plant matter. Digestion was probably aided by large stones in an enormous gizzard, in much the same way that modern, seed-eating birds grind their food. The sauropods were most abundant during the Late Jurassic. They declined afterwards and were replaced as dominant herbivores by different types of dinosaurs, especially the hadrosaurs.

Apatosaurus (previously known as Brontosaurus or the ground-shaking "thunder lizard") was a large sauropod that lived during the Late Jurassic and reached a length of 65 ft (20 m) and a weight of 30 tons (27 metric tons). Diplodocus was a related animal of the Late Jurassic, but it was much longer in its overall body shape. A remarkably complete skeleton of Diplodocus was found that was 90 ft (27 m) long overall, with a 25 ft (8 m) neck and a 45 ft (14 m) tail, and an estimated body weight of 11 tons (10 metric tons). In comparison, the stouter-bodied Apatosaurus was slightly shorter but considerably heavier. Brachiosaurus also lived during the Late Jurassic and was an even bigger herbivore, with a length as great as 100 ft (30 m) and an astonishing weight that may have reached 80 tons (73 metric tons), although conservative estimates are closer to 55 tons (50 metric tons). Supersaurus and Ultrasaurus were similarly large. Seismosaurus may have been longer than 160 ft (50 m), and Argentinosaurus (recently discovered in Patagonia, South America) may set a new weight record of 100 tons (91 metric tons).

Stegosaurus was a 30 ft long (9 m long), Late Jurassic tetrapod with a distinctive row of triangular, erect, bony plates running along its back. These may have been used to regulate heat. Stegosaurus had sharp-spiked projections at the end of its tail, which were lashed at predators as a means of defense. Dacentrurus was a 13-ft-long (4-m-long), Jurassic-age animal related to Stegosaurus, but it had a double row of large spikes along the entire top of its body, from the end of the tail to the back of the head.

The ceratopsians were various types of "horned" dinosaurs. Triceratops was a three-horned dinosaur and was as long as 33 ft (10 m) and weighed 6 tons (5.4 metric tons). Triceratops lived in the late Cretaceous, and it had a large bony shield behind the head with three horns projecting from the forehead and face, which were used as defensive weapons. Anchiceratops was a 7-ton (6.3-metric-ton) animal that lived somewhat later. It was one of the last of the dinosaurs and became extinct 66 million years ago at the end of the Cretaceous period. There were also rhinoceros-like, single-horned dinosaurs, such as the 20-ft-long (6-m-long), 2-ton (1.8-metric-ton) Centrosaurus of the late Cretaceous. Fossilized skeletons of this animal have been found in groups, suggesting that it was a herding dinosaur. The horned dinosaurs were herbivores, and they had parrot-like beaks useful for eating vegetation.

Ankylosaurus was a late Cretaceous animal that was as long as 36 ft (11 m) and weighed 5 tons (4.5 metric tons). Ankylosaurus was a stout, short-legged, lumbering herbivore. This animal had very heavy and spiky body armor and a large bony club at the end of its tail that was used to defend itself against predators.

The duck-billed dinosaurs or hadrosaurs included many herbivorous species of the Cretaceous period. Hadrosaurs are sometimes divided into groups based on aspects of their head structure; they could have a flattish head, a solid crest on the top of their head, or an unusual, hollow crest. Hadrosaurs were the most successful of the late Cretaceous dinosaurs in terms of their relative abundance and wide distribution.

Hadrosaurs apparently were social animals; they lived in herds for at least part of the year and migrated seasonally in some places. Hadrosaurs appear to have nested communally, incubated their eggs, and brooded their young. Hadrosaurs had large hind legs and could walk on all four legs or bipedally if more speed was required—these animals were probably very fast runners.

Hadrosaurus was a 5-ton (4.5-metric-ton), late Cretaceous animal and was the first dinosaur to be discovered and named in North America-in 1858 from fossils found in New Jersey. Corythosaurus was a 36 ft-long (11 m-long), 4 ton (3.6 metric ton), Late Cretaceous herbivore that had a large, hollow, helmet-like crest on the top of its head. Parasaurolophus of the late Cretaceous was similar in size, but it had a curved, hollow crest that swept back as far as 10 ft (3 m) from the back of the head. It has been suggested that this exaggerated helmet may have worked like a snorkel when this animal was feeding underwater on aquatic plants; however, more likely uses of the sweptback helmet were in species recognition and resonating the loud sounds made by these hadrosaurs. Edmontosaurus was a large, non-helmeted hadrosaur that lived in the Great Plains during the late Cretaceous and was as long as 40 ft (13 m) and weighed 3 tons (2.7 metric tons). Anatosaurus was a 3 ton (2.7 metric-ton) hadrosaur that lived as recently as 66 million years ago and was among the last of the dinosaurs to become extinct. The hadrosaurs probably were a favorite prey for some of the large theropods, such as Tyrannosaurus rex.