The Jurassic period was named after the extensive marine limestone exposures of the Jura Mountains, in the region where Germany, France and Switzerland meet.
Dinosaurs were originally classified in a single order. It was later discovered that two types could be distinguished from structural differences. The pelvis of saurischian (‘lizard-hipped’) dinosaurs resembles that of extant reptiles, but in ornithischian (‘bird-hipped’) dinosaurs the pubic bone of the pelvis has forward and backward extensions that resemble those found in birds. Saurischians include the carnivorous theropods (‘bird feet’) and all the medium to very large herbivorous sauropodomorphs (‘lizard feet forms’). Ornithischians tend to be small to medium-sized herbivorous dinosaurs.
Theropods
The earliest known dinosaurs were the theropods: Eoraptor (‘early plunderer’), Herrerasaurus (named after its discoverer) and Staurikosaurus (‘cross lizard’, named after the Southern Cross) were found in Late Triassic Argentinean rocks. Pisanosaurus (named after an Argentine palaeontologist) was a primitive ornithischian. All were small, bipedal creatures not too far from the almost contemporary Lagosuchus.
Coelophysis (‘hollow form’) was two metres (6 ft) long, of slender build and was a fast runner. Dilophosaurus (‘two-crested lizard’) was six metres (20 ft) long and may have weighed half a ton. Later in the Jurassic, numerous theropods are known, including the large allosaurs (‘other lizards’): Allosaurus had an average length of 8.5 metres (28 ft) and average weight of 2.3 metric tons (2.5 short tons).
Dromaeosaurs (‘running lizards’) appeared in the Middle Jurassic. Deinonychus (‘terrible claw’) – its second toe was a retractable slashing claw – was relatively small at three metres (10 ft) long, but used its claws to tear away at larger animals.
Oviraptorosaurs (‘egg thief lizards’) are distinct for their characteristically short, beaked, parrot-like skulls, with or without bony crests on top of the head. Caudipteryx (‘tail feather’) has impressions of feathers, most notably on the wings and tail.
Ornithomimosaurs (‘bird mimic lizards’) also known as ‘ostrich’ dinosaurs, appeared in the Late Cretaceous. They closely resembled living ground birds such as emus and ostriches and are thought to have had a similar lifestyle. Gallimimus (‘chicken mimic’) with a maximum length of 4-6 metres (13-20 ft) and weighing as much as 440 kg (970 lb) was one of the largest of the ornithomimosaurs.
Tyrannosaurs (‘tyrant lizards’) were bipedal carnivores with massive skulls and proportionally very small arms and hands. Tyrannosaurus rex (‘rex’ meaning ‘king’) measured up to thirteen metres (43 ft) in length, up to four metres (13 ft) tall at the hips, and weighed up to 6.8 metric tons (7.5 short tons).
Sauropodomorphs
The sauropodomorphs appear in the Late Triassic. Euskelosaurus (‘true-limbed lizard’) was heavily built and reached eight metres (26 ft) in length. Unlike earlier animals, it seems to have been essentially a quadrupedal animal with a long neck and tail, and stout legs supporting a heavy trunk. Massospondylus (‘massive vertebrae’) was about four metres (13 ft) long. It had a long neck, very long tail, a small head, peg-like teeth, and large, five-fingered hands with a large thumb claw.
In the Late Jurassic the descendents of these early forms were the dominant herbivorous dinosaurs including Diplodocus (‘double beam’), Apatosaurus (‘deceptive lizard’), Brachiosaurus (‘arm lizard’), Mamenchisaurus (named after the place where it was found) and Seismosaurus (‘earth-shaking lizard’) at over 37 metres (121 ft) long.
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