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
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.