The earliest ornithischians, apart from the very early Pisanosaurus, are represented by animals such as Lesothosaurus (named after the place where it was found). It was about one metre (3 ft) long, and its skeletal remains suggest that it was a fast runner; in appearance it would have resembled a large lizard walking bipedally.
Thyreophorans (‘shield bearers’) were armoured herbivorous dinosaurs living from the Early Jurassic until the end of the Cretaceous. Scutellosaurus (‘small shield lizard’) was 1.3 metres (3 ft) long. Its back and sides were covered with rows of conical bony plates, and there was evidence that the sides of its skull were also developing thickened bone. In Early Jurassic Britain the heavier and more solid armoured Scelidosaurus (‘limb lizard’) was quadrupedal and probably slower moving.
Stegosaurs (‘roof lizards’) were herbivorous dinosaurs that lived during the Jurassic and Early Cretaceous. Stegosaurus had a distinctive double row of kite-shaped plates rising vertically along its arched back. It ranged from 7-8 metres (23-26 ft) in length. The plates may have been defensive but they could have been used to regulate the animal’s heat.
Ankylosaurs (‘stiff lizards’) were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the Early Jurassic, and persisted until the end of the Cretaceous. Euoplocephalus (‘well-armoured head’) from the Early Cretaceous was about the size of a small elephant.
Ornithopods (‘bird feet’, referring to their three-toed foot-prints) began as small, bipedal running grazers and became one of the most successful groups of herbivores in the Cretaceous world. During the Early Cretaceous, Iguanodon (‘iguana tooth’) reached lengths of ten metres (33 ft) and was particularly abundant. It had large jaws and more teeth to process larger quantities of food. From the Late Cretaceous a smaller animal Hypsilophodon (‘high-ridged tooth’) at 3-5 metres (10-16 ft) long, had slender sprinter’s legs and a stiffened tail for balance when running.
Hadrosaurs (‘bulky lizards’) also known as the ‘duckbilled’ dinosaurs, appeared in the Late Cretaceous. Closely related to iguanodons, the hadrosaurs had literally thousands of teeth suitable for grinding food before it was swallowed. Saurolophus (‘ridged lizard’) had a spike-like crest projecting up and back from the skull. Parasaurolophus (‘like Saurolophus’) had a distinctive tubular crest. The consensus is that major functions of the crest include visual recognition of species and sex, acoustic resonance, and thermoregulation.
Pachycephalosaurs (‘thick-headed lizards’) were all bipedal animals with thick skulls, the analysis of which suggests that this was possibly associated with an ability to withstand head butting. Stegoceras (‘roof horn’) lived during the Late Cretaceous. It grew to about two metres (6.5 ft) long
Ceratopsids (‘horned faces’) were a group of beaked dinosaurs that lived during the Cretaceous. Ceratopsids ranged in size from one metre (3 ft) and 23 kg (50 lb) to over nine metres (30 ft) and 5400 kg (12,000 lb). Later members, like Triceratops (‘three-horned face’) became very large quadrupeds and developed elaborate facial horns and a neck frill.
Bony ray-finned fishes became faster and more lightweight in the Triassic, the main groups are often called holosteans (‘whole bone’). Major developments took place in the Jurassic among the bony fishes and the sharks. The holosteans were still important, but then a new group, the teleosts (‘complete bone’) started to spread. Today the teleosts dominate the seas, with 20,000 extant species in about forty orders.
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