Dinosaur

Dinosaur Temporal range: (Possible Middle Triassic record)
A collection of fossil dinosaur skeletons. Clockwise from top left: Microraptor (a winged theropod), Apatosaurus (a giant sauropod), Edmontosaurus (a duck-billed ornithopod), Triceratops (a horned ceratopsian), Stegosaurus (a plated stegosaur), Pinacosaurus (an armored ankylosaur)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dracohors
Clade:	Dinosauria Owen, 1842
Major groups
†Sauropodomorpha Theropoda †Ornithischia

Dinosaurs are a group of Archosaur reptiles of the clade Dinosauria. Dinosaurs eventually gave rise to birds.

Dinosaurs were the most powerful land animals of the Mesozoic era. Over 500 different genera of dinosaurs are known. Fossils of dinosaurs have been found on every continent.

Dinosaurs evolved in the Upper Triassic, about 230 million years ago (mya). The earliest date of a fossil is that of Eoraptor and Herrerasaurus from Argentina, and Saturnalia from Brazil, 237 to 228 mya.

By the early Jurassic they were the top land vertebrates, and dominated most environments on land. They continued until the K/T extinction event 66 million years ago.

From the fossil record, it is known that birds are living feathered dinosaurs. They evolved from earlier theropods during the later Jurassic. They were the only line of dinosaurs to survive to the present day.

Dinosaurs had adaptations that helped make them successful. The first known dinosaurs were small predators that walked on two legs. All their descendants had an upright posture, with the legs underneath the body. This transformed their whole life-style. Most of the smaller dinosaurs had feathers, and were probably warm-blooded. This would make them active, with a higher metabolism than modern reptiles. Social interaction, with living in herds and co-operation, seems certain for some types. The existence of communal egg-laying sites is best understood if the adults travelled in herds, as herbivores do today.

The first fossils were recognised as dinosaurs in the early 19th century. Some of their bones were found much earlier, but were not understood. William Buckland, Gideon Mantell and Richard Owen saw these bones were a special group of animals. Georges Cuvier was also important in explaining what dinosaurs were. Dinosaurs are now major attractions at museums around the world. They have become part of popular culture. There have been best-sellng books and movies about dinosaurs. New discoveries are reported in the media.

Evolution of dinosaurs

Features of dinosaurs

Labeled diagram of a typical archosaur skull, the skull of Dromaeosaurus

Dinosaurs are so varied that it is not easy to find what they all share. A reasonable list would include many features of the skeleton which are not familiar to the general reader.

Dinosaurs were, at the start, small and bipedal; they walked on their hind legs. They laid eggs in nests. From the start of their fossil record, there were both carnivores and herbivores.

Changes in the basic set-up of dinosaurs happened because of adaptations to different lifestyles. We now know that birds are the living descendants of dinosaurs, and later adapted to different lifestyles as well.

Adaptive radiation

Adaptive radiation let dinosaurs live in many ecological niches. Paleontologists have identified over 500 different genera and 1,000 species of non-avian dinosaurs. Their descendants, the birds, number 9,000 living species, and are the most diverse group of land vertebrates.

The largest dinosaurs were herbivores (plant-eaters), such as Apatosaurus and Brachiosaurus. They were the largest animals to ever walk on dry land. Other plant-eaters, such as Iguanodon, had special weapons to help them fight off the meat-eaters. For example, Triceratops had three horns on its head shield, Ankylosaurus was covered in boney plates, and Stegosaurus had spikes on its tail.

The carnivores were bipedal (walked on their back legs), though they were not exactly like we are. Their body was more horizontal, balanced at the back by their tail. Some were very large, like Tyrannosaurus, Allosaurus and Spinosaurus, but some were small, like Compsognathus. It was the smaller sized meat-eaters that may have evolved into birds. The first fossil bird, Archaeopteryx, had a skeleton which looked much like that of the dinosaur Compsognathus, as T.H. Huxley commented.

Types of dinosaurs

Dinosaurs are united by at least 21 traits in their skulls and skeletons. These common characters (called 'synapomorphies') are the reason palaeontologists are sure dinosaurs had a common origin.

However, when definite dinosaur fossils appear (early in the Upper Triassic), the group had already split into two great orders: the Saurischia, and the Ornithischia. The Saurischia keep the ancestral hip arrangement inherited from their archosaur ancestors, and the Ornithischia have a modified hip structure.

A. Eoraptor, an early saurischian, B Lesothosaurus, a primitive ornithischian,
C A saurischian pelvis (Staurikosaurus) D Lesothosaurus pelvis

Dinosaur classification

The following is a simplified list of dinosaur groups based on their evolution. Groups with a dagger (†) next to them do not have any living members.

• Dinosauria
  • Saurischia ("lizard-hipped"; includes Theropoda and Sauropodomorpha)
    • Theropoda (all bipedal; most were carnivorous)
      • †Dilophosauridae (early crested and carnivorous theropods)
      • †Ceratosauria (generally elaborately horned, the dominant southern carnivores of the Cretaceous)
      • Tetanurae (meaning "stiff tails"; includes most theropods)
        • †Megalosauroidea (early group of large carnivores including the semiaquatic spinosaurids)
        • †Tyrannosauridae (Tyrannosaurus and its close relatives)
        • †Carnosauria (Allosaurus and close relatives, like Carcharodontosaurus)
        • Coelurosauria (feathered theropods, with a range of body sizes and niches)
        • †Coelophysoidea (small, early theropods; includes Coelophysis and its close relatives)
          • †Compsognathidae (common early coelurosaurs)
          • †Ornithomimosauria (meaning "ostrich-mimics"; mostly toothless; carnivores to possible herbivores)
          • †Alvarezsauroidea (small insectivores with short arms that each had one large claw)
          • Maniraptora (meaning "hand snatchers"; had long, slender arms and fingers)
            • †Therizinosauria (bipedal herbivores with large hand claws and small heads)
            • †Oviraptorosauria (mostly toothless; their diet and lifestyle are uncertain)
            • †Archaeopterygidae (small, winged theropods or primitive birds)
            • †Deinonychosauria (small to medium-sized, bird-like, with a distinctive toe claw.)
            • Avialae (modern birds and their extinct relatives)
              • †Scansoriopterygidae (small primitive avialans with long third fingers)
              • †Omnivoropterygiidae (large, early short-tailed avialans)
              • †Confuciusornithidae (small toothless avialans)
              • †Enantiornithes (primitive flying avialans that lived in trees)
              • Euornithes (advanced flying birds)
                • †Yanornithiformes (toothed Cretaceous Chinese birds)
                • †Hesperornithes (specialized aquatic diving birds)
                • Aves (modern, beaked birds and their extinct relatives)

Size differences of the largest sauropods compared to a human

• †Sauropodomorpha (herbivores with small heads, long necks, long tails)
  • • †Herrerasauria (early bipedal carnivores)
      • †Guaibasauridae (small, primitive, omnivorous sauropodomorphs)
      • †Plateosauridae (primitive, bipedal "prosauropods")
      • †Riojasauridae (small, primitive sauropodomorphs)
      • †Massospondylidae (small, primitive sauropodomorphs)
      • †Sauropoda (very large and heavy, usually over 15 m (49 ft) long; quadrupedal)
        • †Vulcanodontidae (primitive sauropods with pillar-like arms and legs)
        • †Eusauropoda ("true sauropods")
          • †Cetiosauridae ("whale reptiles")
          • †Turiasauria (European group of Jurassic and Cretaceous sauropods)
          • †Neosauropoda ("new sauropods")
            • †Diplodocoidea (skulls and tails elongated; teeth typically narrow and pencil-like)
          • †Macronaria (boxy skulls; spoon- or pencil-shaped teeth)
            • †Brachiosauridae (long-necked, long-armed macronarians)
            • †Titanosauria (diverse; stocky, with wide hips; most common in the late Cretaceous of southern continents)
  • †Ornithischia ("bird-hipped"; diverse bipedal and quadrupedal herbivores)
    • †Neornithischia ("new ornithischians")
      • †Heterodontosauridae (small basal ornithopod herbivores/omnivores with prominent canine-like teeth)
      • †Ornithopoda (various sizes; bipeds and quadrupeds; evolved a method of chewing using flexible skulls and many teeth)
      • †Marginocephalia (dome-like growths on their skulls made of bone)
        • †Pachycephalosauria (bipedal with domed or knobby growth on skulls)
        • †Ceratopsia (quadrupeds with bony frills; many also had horns)
    • †Thyreophora (armored dinosaurs; mostly quadrupeds)
      • †Stegosauria (spikes and plates as primary armor)
      • †Ankylosauria (scutes as primary armor; some had club-like tails)

Origins and evolution

Archosaurs

The archosaurs evolved into two main clades: those related to crocodiles, and those related to dinosaurs.

• Archosauria
  • Pseudosuchia: clade of the crocodiles and their relatives.
  • Avemetatarsalia: clade of the dinosaurs, pterosaurs, birds and relatives.
    • Aphanosauria (small group in middle Triassic)
    • Ornithodira: clade of the pterosaurs and dinosaurs.
      • Pterosaurs
      • Dinosaurs