Different Types of Dinosaurs
There are several hundred dinosaur species, varying greatly in size and shape. Having to remember each one in detail would be very difficult. Fortunately they are not all completely different, but fall into related groups of families which can be arranged more understandably. Much of the classification is done by studying the creatures' legs, hip bones, and feet. Since dinosaurs, unlike any modern reptiles, walk with their legs tucked underneath their bodies, these bones are highly distinctive.
As the name suggests, the hip bones are arranged in much the same way as they are in other reptiles. The large, blade-like upper bone, called the ilium, is connected to the backbone by a row of strong ribs, and its lower edge forms the upper part of the hip socket. Beneath the ilium there is a large bone which points downward and slightly forward - the pubis - and behind this there is a bone extending backwards - the ischium. All three bones meet at the hip socket, which forms a deep, round opening in the side of the pelvis. Large, powerful leg ·muscles are attached to each of these bones. The dinosaurs which have this type of hip structure fall into two distinct types.
Theropods include all the carnivorous (meat-eating) types; the name means "beast foot" on account of the very sharply clawed three-toed feet of these animals. The group includes such notable dinosaurs as the giant Tyrannosaurus; the small and very agile Deinonychus; the very early form Coelophysis; the mysterious new dinosaur recently excavated in Britain, Baryonyx, with its massive claws; and even some toothless types such as Oviraptor and Struthiomimus.
The body form of all theropods tends to be very similar: long, powerful hind limbs ending in sharply clawed bird-like feet; slender or lightly built arms; a chest which is rather short and compact; a body balanced at the hip by a long, muscular tail; a neck which tends to be sharply curved and very flexible; and a head equipped with large eyes and long jaws, nearly always lined with dagger-like teeth.
Despite this overall similarity there are a number of distinct types. Some of the best known are the carnosaurs, the classically huge predatory animals including Allosaurus, Megalosaurus, Carnotaurus, Tyrannosaurus, Tarbosaurus, Albertosaurus, and an apparent dwarf tyrannosaur which has been named Nannotyrannus. All of these types typically have huge, powerful heads mounted upon very thick, powerful necks, and they often have rather small arms for their size.
Another group is a little more varied, and rather rarer in the fossil record. Known as ceratosaurs, they include the very early theropods Coelophysis and Syntarsus, and Dilophosaurus with puzzling thin, bony crests on its head. Ceratosaurus is also adorned, but this time with a horn on its nose. Another, rather mixed bag of theropods are called coelurosaurs; all of these are generally rather slender, lightly built creatures with long, flexible necks
Sauropodomorphs, in contrast, are all herbivores - that is, plant-eaters. They range in size from the diminutive forms, generally known as prosauropods ( "early sauropods"), which appear in the Late Triassic and Early Jurassic, up to the gigantic sauropods of the Jurassic and Cretaceous Periods. The prosauropods include forms such as Anchisaurus, Massospondylus, Riojasaurus, Mussaurus, Plateosaurus, Lufengosaurus, and Efraasia.
Most of these are medium-sized creatures 13 to 20 ft (4 to 6 m) long which are capable of walking on all fours or on their hind legs alone; but a few show early signs of becoming larger and much heavier, and seem entirely four-footed like the later sauropods. Mussaurus is considerably smaller than the others as its name, which means "mouse lizard," suggests - although only juvenile specimens have been found so far and the adults would have been several times larger. The sauropods are the true giants of the Mesozoic Era and include such notable creatures as Diplodocus, Apatosaurus (better known as Brontosaurus), Dicraeosaurus, and Cetiosauriscus.
All of these seem to belong to the same related group, and tend to have long, slender bodies, whip-like tails, long, shallow faces, and thin, pencil-shaped teeth. Those in another group, which includes Brachiosaurus, Camarasaurus, Euhelopus, and Opisthocoelicaudia, again seem to be related to one another. In contrast, they tend to be rather more compact animals, with high-shouldered bodies, shorter tails, shorter and high-snouted heads, and much larger teeth.
In addition to these types, there are various unusual sauropods: Saltasaurus from Argentina has curious armor plating over its back and sides; Shunosaurus from China seems to have had a bony club on the end of its tail; Mamenchisaurus and Barosaurus seem to have had extraordinarily long necks relative to their bodies; and Magyarosaurus seems to have been a rare miniature sauropod
The arrangement of the bones in the hips of these dinosaurs is, as the name suggests, very similar to that seen in living birds - though, confusingly, there is no family link with birds. While the ilium and ischium bones are arranged very similarly to the saurischian dinosaur the pubis, instead of pointing downward and a little forward, is a narrow, rod-shaped bone which lies alongside the ischium.
This pattern becomes a little obscure in some ornithischians, especially among those from the later Cretaceous Period (such as the ceratopians and ankylosaurs), through shortening of the pubis and growth of a new forwardly pointing part to the bone; but the pattern is still evident. In addition to this difference in hip bones, there are other features of this group which are not found in saurischians. All ornithischians seem to possess a small horn-covered beak perched on the tip of the lower jaw.
Somewhat less obviously, they have rows of long bony rods packed along the sides of the spines on the backbone, which helped to stiffen and strengthen the back; these are sometimes visible on museum skeletons. Ornithischians were, in contrast to saurischians, entirely herbivorous; they are also considerably more varied in appearance than the saurischians. There are five major groups.
Ornithopods include many small to medium-sized animals which ran on their hind legs for most of the time. Examples of these include Lesothosaurus, Heterodontosaurus, Hypsilophodon, Dryosaurus, Rhabdodon and Yandusaurus, all of which are small forms, none more than about 10 ft (3 m) long. This type of dinosaur is found throughout the Mesozoic and seems to have been one of the most successful small herbivore groups.
Medium-sized types included Iguanodon, Tenontosaurus, Camptosaurus, and Ouranosaurus; such creatures reached lengths of about la metres and were particularly abundant in the early part of the Cretaceous Period. However, in the Late Cretaceous there appeared another group known as hadrosaurs or, more popularly, as duck-billed dinosaurs. These grew to lengths of 43 ft (13 m) in some cases. They became very diverse. Some seem to have lived in very large herds, and were evidently highly social creatures.
They were also extremely efficient herbivores, with special grinding teeth and muscular cheeks. In some respects the huge herds of hadrosaurs inhabiting the plains of North America in the Late Cretaceous seem to be equivalent to the hordes of buffalo seen in the past on the North American plains, and wildebeest of the African plains.
Ceratopians, the distinctively horned and frilled dinosaurs, with curiously narrow, parrot-like beaks, appeared very late in dinosaur history, only during the second half of the Cretaceous Period. They ranged from the small, rather ornithopod-like Psittacosaurus, Protoceratops - whose eggs were first found in Mongolia in the 1920s - Leptoceratops, Auaceratops, and Bagaceratops, to the massive, rhinoceros-like Centrosaurus, Triceratops, Styracosaurus, Anchiceratops, Chasmosaurus, and Torosaurus.
The first ceratopians appear in the middle of the Cretaceous Period in Asia and evolved extremely rapidly to become one of the most abundant and diverse groups of Late Cretaceous dinosaur. Like the hadrosaurs, these dinosaurs also became phenomenally abundant, as we know from the existence of massive ceratopian "graveyards" at some localities.
It seems likely that they too lived in large herds which roamed the plains of the Northern Hemisphere. The sharp, hooked beak formed a clean cutting tool for feeding on plants, and behind this the jaws were lined by dense rows of teeth, forming guillotine-like blades which could have sliced up the toughest of plants. The horns and frills which adorn the heads of many of these creatures may have had a number of uses
Pachycephalosaurus are rather poorly known creatures; they resemble ornithopods in their body proportions but have most distinctive, curiously domed and massively reinforced heads. These forms appear to have arisen during the middle part of the Cretaceous and persisted to the close of that period, but they were never particularly abundant as a group. It has been suggested that they lived in relatively inaccessible places, such as upland areas, where their remains are less likely to have become fossilized.
Stegosaurs, the well-known plated dinosaurs of which Stegosaurus is such a famous example, seem to have lived almost exclusively during the Jurassic, with some fragmentary reports from the early part of the Cretaceous Period only. The structure of the spines and plates of these animals has proved to be extremely interesting, and provides evidence in the debate as to whether dinosaurs were "warm-blooded" or "coldblooded"
Ankylosaurs are the armored tanks of the dinosaur world. These amazing creatures were fully covered in thick bony plates in order to ward off the attentions of the larger theropods. They appear early in dinosaur history with Scutellosaurus and Scelidosaurus in the Early Jurassic, but become abundant only in the Late Cretaceous of Asia and North America.