Douglas Palmer - Dinosaurs

Scientists divide dinosaurs and all other organisms into groups of related forms. Traditionally, the groups have been placed in classes, orders, families and so on, but a new system of classification called cladistics has supplemented these names. Here, branches (clades) are defined by characters that evolved at, or immediately before, their origin. For instance, the bird clade, called Aves , is defined by the possession of wings and primary flight feathers.

Each group has a number of unique designated characteristics that should be found in each of its members. For instance, the ankylosaurs were just one of several groups of four-legged plant-eating dinosaurs but they can be distinguished from the others by rows of bony plates embedded in the skin of their backs, a distinctive bony skull form and small denticulate teeth. The ankylosaurs can be grouped together with the stegosaurs (characterized by their prominent bony back plates) as thyreophorans because of the extensive nature of the armour they had on their backs. The thyreophorans include all dinosaurs that are more closely related to Ankylosaurus than to Triceratops , which belongs in another related group called the marginocephalians, consisting of bone-headed and horned plant-eating dinosaurs.

Both thyreophorans and marginocephalians had distinctive bird-like hip bones that characterize them as belonging to the ornithischians, one of the two main dinosaur groupings. The animals in this group were all plant eaters and most moved around on all four legs. They include such well-known groups as the ankylosaurs, stegosaurs, ceratopsians, and ornithopods (with birdlike feet).

Caption:

A tree diagram showing ornithischian relationships. From right to left:

Genasaurs and the Lesothosaurus belong to the ornithischian group. .

From the genasaurs follow two distinct groups: cerapodans and thyreophorans..

Following on from cerapodans:

There are two groups included in marginocephalians: ceratopsians and pachycephalosaurians..

Ceratopsians lead to ceratopsids and psittacosaurids..

From ornithopods follow iguanodontians, leading to hadrosaurids and igaunodontids..

From ornithopods follow hypsilophodontids and heterodontosaurids..

From thyreophorans follow ankylosaurians, stegosaurians and Scutellosaurus ..

The other main group of dinosaurs is the saurischians, which had a hip structure more like that of lizards. A major subdivision within this group separates the plant-eating, four-legged sauropods and the meat-eating, two-legged theropods. In recent decades the classification of the theropods has been transformed by new evidence indicating that the birds originated within a theropod group known as the maniraptorans.

The theropods group contains the most subdivisions of any dinosaur group, which reflects the considerable range in form and size of its members. These include some of the largest land-dwelling animals, the allosaurs and tyrannosaurs, which reached lengths of 14m (45ft) and weights of up to 6 tonnes (5.9 tons). At the other end of the scale the theropods include tiny dinosaurs such as Microraptor , which had a body length of around 47cm (1ft 7in). Aves also belongs to the theropods and numbers Archaeopteryx among its dinosaur members; it also includes all of the modern-day birds.

Caption:

A tree diagram showing saurischian relationships. From right to left:

Eoraptor, herrerasaurids, and the two groups of sauropodomorphs and theropods belong to the saurischian group..

From the sauropodomorphs follow two distinct groups: prosauropods (leading to anchisaurids and plateosaurids) and sauropods..

From sauropods follow Vulcanodon and eusauropods..

From eusauropods follow Mamenchisaurus , cetiosaurids and neosauropods..

From neosauropods follow two distinct groups: diplodocoids (leading to discraeosaurids and diplodocids) and macronarians..

From macronarians follow camarasaurids, brachiosaurids, Euhelopus , saltasaurids, and titanosaurians..

A final group of small to medium-sized meat-eating bipedal saurischians is still difficult to categorize. These include Herrerasaurus , Eoraptor and Staurikosaurus , which have been found in Late Triassic rocks, especially in South America. Although it is still not clear what their relationships are to the main groups of saurischians, these dinosaurs were certainly more like the theropods than the sauropodomorphs.

Caption:

A tree diagram showing theropod relationships. From right to left:

Tetanurans, ceratosaurians and coelophysoids belong to the theropods group..

Ceratosaurians include neoceratosaurians and Ceratosaurus ..

Following on from tetanurans:

From tetanurans follow avetheropodans and spinosauroids (leading to spinosaurids and megalosaurids)..

From avetheropodans follow coelurosaurians and carnosaurians (leading to carcharodontosaurids and allosaurids)..

From coelurosaurians follow maniraptorans, compsognathids, ornithomimosaurians and tyrannosauroids..

From maniraptorans follow the group known as eumaniraptorans, as well as oviraptorosaurians and therizinosauroids..

From eumaniraptorans follow the group known as avialans (leading to modern birds and Archaeopteryx ), as well as dromaeosaurids and troodontids..

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