Introducing Dinosaurs

Dinosaurs (from the Greek, 'deinos', meaning powerful or terrible, and 'saura' meaning reptile or lizard) were the dominant vertebrate animals of terrestrial ecosystems from the late Triassic period (about 230 million years ago) until the end of the Cretaceous period (65 million years ago), when most of them became extinct.

What Are Dinosaurs?

Dinosaurs can be generally described as terrestrial archosaurian reptiles that held their limbs erect beneath their body, first appearing in the Carnian faunal stage of the Late Triassic period, and becoming extinct at the end of the Maastrichtian stage of the Late Cretaceous era. The term, coined by Sir Richard Owen in 1842, is also often used informally to describe other prehistoric reptiles, such as the winged pterosaurs, the pelycosaur Dimetrodon, and the aquatic mosasaurs, ichthyosaurs and plesiosaurs, although none of these animals are classified scientifically as dinosaurs. For example, pterosaurs were archosaurs but not terrestrial, marine reptiles such as ichthyosaurs, mosasaurs and plesiosaurs were neither terrestrial nor archosaurs, and Dimetrodon was a Permian animal more closely related to mammals.

The following set of animals are defined as dinosaurs:

• ankylosaurians (armoured herbivorous quadrupeds)
• ceratopsians (herbivorous quadrupeds with horns and frills)
• ornithopods (bipedal or quadrupedal herbivores including 'duck-bills')
• sauropodomorphs (large herbivorous quadrupeds with long necks and tails)
• stegosaurians (plated herbivorous quadrupeds)
• theropods (mostly bipedal carnivores)

Although popularly known for their large size, many dinosaurs were in fact human-sized or smaller. Through they were initially believed to have been unintelligent, slow cold-blooded animals, recent research has supported the view that dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction. Dinosaur remains have been found on every continent on Earth, including Antarctica.

Dinosaurs were an extremely varied group of animals; according to a recent study, over 500 dinosaur genera have been identified with certainty so far, and 1,047 different species of dinosaurs have been named; some herbivorous, some omnivorous and some carnivorous. Many had bony armour, or cranial modifications such as crests and horns. Some dinosaurs were quadrupeds, some were bipeds, and others, such as Iguanodon and Ammosaurus, could walk just as easily on 2 or 4 legs.

Dinosaurs and their Relationship to Birds

In 1861, the discovery of the primitive bird Archaeopteryx suggested a close relationship between birds and dinosaurs, with Archaeopteryx very similar to the contemporary small predatory dinosaur Compsognathus. Theropod dinosaurs have since been identified as the most likely direct ancestors of birds, and the majority of palaeontologists now regard birds as the only surviving dinosaurs, with some suggesting that the two be grouped into one biological class. Using the strict cladistical definition that all descendants of a single common ancestor must be included in a group for that group to be natural, birds would thus be dinosaurs and dinosaurs are, therefore, not extinct. However, is also technically correct to refer to dinosaurs as a distinct group under the older Linnaean classification system, which accepts paraphyletic taxa that exclude some descendants of a single common ancestor. In almost all writings about dinosaurs, the term 'dinosaur' is used as a synonym for 'non-avian dinosaur'.

Major Features of Dinosaurs

The majority of dinosaurs discovered so far share certain features and traits that are considered typical across Dinosauria. Such common features across a taxonomic group are known as synapomorphies.

Dinosaur synapomorphies include:

• a shelf at the rear of the ilium (main hip bone)
• an elongated crest on the humerus (upper arm bone) to accommodate the attachment of deltopectoral muscles
• a tibia (shin bone) featuring a broad lower edge and a flange pointing out and to the rear
• an ascending projection on the astragalus, one of the ankle bones, which secures it to the tibia

There are also a number of other skeletal features shared by many dinosaurs, which are not considered to be synapomorphies; these include:

• an acetabulum (hip socket) with a hole at the centre of its inside surface
• a sacrum composed of 3 or more fused vertebrae
• an elongated scapula (shoulder blade)
• 2 pairs of temporal fenestrae (openings in the skull behind the eyes)
• additional openings in the lower jaw and snout

Unlike most other reptiles, whose limbs sprawl out to either side, dinosaurs stood erect in a manner similar to most modern mammals. Their erect posture enabled them to breathe easily while moving, which likely permitted activity levels and stamina that surpassed those of 'sprawling' reptiles. Erect limbs may have also helped support the evolution of large size by reducing bending stresses on limbs. This erect posture was due to the development of a laterally facing recess in the pelvis (usually an open socket) and a corresponding inwardly facing distinct head on the femur.

Modern Relatives of the Dinosaurs

Apart from birds, the only other close relatives of dinosaurs to have survived until the present day are crocodilians. Like birds and dinosaurs, crocodilians are members of Archosauria, a group of reptiles that first appeared in the very late Permian era, and going on to predominate in the mid-Triassic period.