Phylogeny and Origins

Most scientists assume that all life evolved through a succession of stages from a common ancestor, generally thought to be a single-celled simple organism, like a bacterium, or a blue-green alga, that lived over 3500 million years ago. There is a great deal of evidence to relate all present-day organisms to each other, and that they all arose from this single ancestor.

Many scientists are interested in what these relationships would be like and they reconstruct them to look very much like a branching tree. The pattern of these branching relationships is called a phylogeny. Because the pattern of a phylogeny is essentially branching, it is possible to arrange all species of plants and animals into a hierarchical arrangement, where species fit into genera, genera into families, and so on, up to kingdoms.

An example is shown below, for the classification of a human being, Homo sapiens:-


Kingdom = Animalia
Phylum = Chordata
Class = Mammalia
Order = Primates
Family = Hominidae
Genus = Homo
Species = sapiens


Dinosaurs have generally been divided into two orders, the Saurischia and the Ornithischia, on the basis of the anatomy of their hip bones.

Indeed the Class DINOSAURIA, which includes all the dinosaurs, has been defined on the basis of hindlimb anatomy. Typical reptiles are sprawlers, which means that they turn their elbows and knees out sideways, and their limbs are bent out to the side. Unlike their ancestors, the dinosaurs had a fully erect posture, in which the legs were tucked in under the body. This led to major rearrangements of all the hindlimb joints, with straightening of the ankle and knee, and a mjor change at the hip joint.

Saurischian dinosaurs possessed a 'lizard-like' hip, which meant that the pubis bone points forward and the ischium bone points backwards (see the diagram below).

Saurischian dinosaurs included all the meat-eating dinosaurs, as well as the enormous plant-eating Sauropods, like Diplodicus.

Ornithischian dinosaurs, on the other hand, possessed a 'bird-like' hip, which meant that the pubis bone ran backwards, parallel to the ischium bone (see diagram below).

Ornithischian dinosaurs were all plant-eating and they divide iinto two groups, the Cerapoda, which included the bipedal ornithischians, the Pachycephalosaurs, the bone-headed dinosaurs, the mighty horned Ceratopsia, such as Triceratops, and they also included all the armoured plant-eaters, the Stegosauria, such as Stegosaurus and the Ankylosauria, such as Ankylosaurus.

So what does the phylogenetic tree for the dinosaurs look like? The diagram below should explain all

Diagram taken from Benton (1997) "Vertebrate Palaeontology". For the referrance see; Links and further reading
Many thanks to Mike Benton for his permission to use this diagram.

The dinosaurs originated during the Triassic period, some 230 million years ago. It is clear that dinosaurs are archosaurs. The Archosauria, or 'ruling reptiles' arose over 250 million years ago at the end of the Permian period, and they beecame important vcarnivores in the Triassic.

The archosaurs split into two lines in the Triassic, one that led ultimately to crocodiles, and the other ulktimately to birds. Dinosaurs fit on the bird line, the Ornithodira. Some basal ornithodirans, like Scleromochlus from Scotland and Lagosuchus from Argentina, show many dinosaur-like characters, such as a lightly built skeleton, a fully bipedal posture, and long hindlimbs that are fully erect.

Lagosuchus is one of the closest relatives of Dinosauria, but it is not itself a dinosaur. It is known from the end of the Middle Triassic. This proves that dinosaurs had arisen by then, even though the oldest dinosaurs are from the beginning of the Late Triassic.

To Huw Boulton's Home Page

To learn more about Archosaurs and other animals that lived around at the same time as the Dinosaurs, please follow this link:-

To Alistair McGowan's Home Page


Byron's Dinosaurs Home Page