Dinosaur

Dinosaurs Fossil range: Upper Triassic – Upper Cretaceous
Triceratops skeleton
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Subclass: Diapsida Infraclass: Archosauromorpha Superorder: Dinosauria Owen 1842
Orders and suborders
Saurischia Sauropodomorpha Theropoda Ornithischia Cerapoda Thyreophora

Dinosaurs, a type of Archosaur reptile, were the dominant land animals of the Mesozoic era.

Dinosaurs became the top land vertebrates in the Upper Triassic, 230 million years ago. By the early Jurassic they dominated most environments on land. They continued until the sudden K/T extinction event 65 million years ago.

Birds are the descendants of theropod dinosaurs; all the terrestrial dinosaurs are extinct.

Types of dinosaurs

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

However, as soon as dinosaur fossils appear (late in the Middle Triassic), the group has 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 sructure.

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

Dinosaur classification

• Saurischia: this is the order of dinosaurs with the ancestral pelvis (hips).
  • Theropods: this is the group of carnivorous dinosaurs.
  • Sauropods: these were large herbivorous dinosaurs with long necks.
• Ornithischia: this is an order of beaked, herbivorous dinosaurs with bird-like hips.
  • Armoured dinosaurs: these are dinosaurs with their backs protected by plates of bone.
    • Ankylosauria: dinosaurs with flat plates of armour
    • Stegosauria: the stegosaurs
  • Cerapoda: three rather different groups:
    • Ornithopoda: the duck-billed dinosaurs
    • Pachycephalosauria: the thick-headed dinosaurs
    • Ceratopsia: the horned dinosaurs, like Triceratops

Dinosaur origins and evolution

Archosaurs

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

• Archosauria
  • Crurotarsi: clade of the crocodiles and their relatives.
  • Avemetatarsalia: clade of the dinosaurs, pterosaurs, birds and relatives.
    • Ornithodira clade of the pterosaurs and dinosaurs
      • Pterosaurs
      • Dinosaurs

Earliest dinosaurs

The first known dinosaurs were bipedal predators that were one to two metres long.

Spondylosoma, from the Middle Triassic, may or may not be a dinosaur. The skull is not known, and the remains are dated at about 230–232 million years ago (mya).

The earliest confirmed dinosaur fossils include saurischian ('lizard-hipped') dinosaurs Saturnalia 225–232 mya, Herrerasaurus 220–230 mya, Staurikosaurus possibly 225–230 mya, Eoraptor 220–230 mya and Alwalkeria 220–230 mya. Saturnalia may be a basal saurischian or a prosauropod. The others are basal saurischians.

Among the earliest ornithischian ('bird-hipped') dinosaurs is Pisanosaurus 220–230 mya. Although Lesothosaurus comes from 195–206 mya, skeletal features suggest that it branched from the main Ornithischia line at least as early as Pisanosaurus.

It is clear from this figure that early saurischians resembled early ornithischians, but not modern crocodiles. Saurischians are distinguished from the ornithischians by retaining the ancestral configuration of bones in the pelvis. Another difference is in the skull, the upper skull of the Ornithischia is more solid and the joint connecting the lower jaw is more flexible; both are adaptations to herbivory and both can already be seen in Lesothosaurus.

Adaptive radiation

Dinosaurs were a varied group of animals. 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 plant-eaters, such as Apatosaurus and Brachiosaurus. They were the largest animals to ever walk on dry land. Other plant-eaters 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 not as we do. Their body was more towards the horizontal, balanced at the back by their tail. Some were very large, like Tyrannosaurus 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 a dinosaur.

Life style

Locomotion

Dinosaurs were primitively bipedal: their probable ancestors were small bipedal Archosaurs. The date of the early dinosaur genus Eoraptor at 230 million years ago is important. Eoraptor probably resembles the common ancestor of all dinosaurs; its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, pre-dinosaur, types in Middle Triassic strata supports this view. Analysis of their fossils suggests that the animals were indeed small, bipedal predators.

Hip joints and hindlimb postures

Those dinosaurs which returned to four-legged stance kept all four legs under their body. This is much more efficient than the sprawling legs of a lizard.

The big sauropods could never have reached so large a size without their pillar-like legs.

Warm blooded

A major change in outlook came in the 1960s, when it was realised that small theropods were probably warm-blooded. The question of whether all theropods or even all dinosaurs were warm blooded is still undecided.

It is now certain (from fossils discovered in China: see Jehol biota) that small theropods had feathers. This fits well with the idea that they were warm-blooded, and that the origin of birds can be traced to a line of small theropods.

Activity

Warm blooded animals have a high metabolic rate (use up food faster). They can be more active, and for longer, than animals who depend on the environment for heating. Therefore, the idea of warm-blooded dinosaurs insulated by feathers led to the idea that they were more active, intelligent and faster runners than previously thought.

Main-stream palaeontologists have followed this view for small theropods, but not for larger herbivores. Since we know that the size of a Stegosaur's brain was about the size of a walnut, there is good reason to think its intelligence was limited.

Extinction

The extinctions at the end of the Cretaceous were caused by one or more catastrophic events, such as massive asteroid or meteorite impacts (like the Chicxulub impact), or increased volcanic activity.

Several impact craters and massive volcanic activity, such as that in the Deccan Traps in India, have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology.

Dinosaurs in fiction

Books about dinosaurs have been popular, especially with children, but adults have also enjoyed these kinds of books. In Edwardian times, Arthur Conan Doyle wrote a novel about a plateau filled with dinosaurs which he called The Lost World.

Several movies are about dinosaurs, such as Jurassic Park.

Tyrannosaurus and human size difference

Other pages

• List of dinosaurs
• For "dinobirds", see Origin of birds
• K/T extinction event

References

Look up Dinosauria in Wikispecies, a directory of species

Wikimedia Commons has media related to:

Dinosauria