III. Ancestors and Higher Groups
Paleontologists have not yet discovered fossils of the ancestral dinosaur, but they have found fossils of animals quite close to it. These animals, called lagosuchians (“rabbit crocodiles”) or dinosauromorphs, lived during the Middle Triassic Period. They were small (about one meter long, weighing about one to five kilograms), perhaps bipedal, and had upright limbs with many features in common with dinosaurs. They are quite rare, with four species known from Argentina (Lagosuchus, Lagerpeton, Lewisuchus, and Marasuchus), and one from England (Agnosphytis).

Evolutionary tree showing major groups of dinosaurs

Along with dinosaurs, these animals belong to a larger group of reptiles called ornithodirans (“bird neck”). Ornithodirans also include Scleromochlus and the flying pterosaurs, which also appeared in the Triassic Period. They share a unique, hinge-like ankle structure and tend to have long, slender limb bones.

Ornithodirans are part of the group Archosauria, or “ruling reptiles.” Archosaurs have two additional openings in their skull, one between the eye and nose (the antorbital fenestra), and another in the lower jaw (the mandibular fenestra). Along with dinosaurs (and their descendants, birds) and pterosaurs, archosaurs include the crocodilians and their relatives. The fossil record has revealed a great diversity of archosaurs throughout the Mesozoic, although only birds and crocodilians remain today.

Thus dinosaurs are members of a succession of ever-more-inclusive groups. They are ornithodirans (along with pterosaurs), and they are also archosaurs (along with crocodilians). This chain continues: archosaurs are diapsids, diapsids are reptiles, and reptiles are amniotes. Therefore dinosaurs are also diapsids, reptiles, and amniotes. The unique features of each of these groups can be found in every dinosaur, except in rare cases where features were secondarily lost.


To Learn more about the divisions of Dinosaurs into different groups, please visit our Tree of Life Interactive