|Diapsids may all look very different, but they all
share the pattern of openings in their skull. Diapsids have
two temporal fenestrae (an upper and a lower) located behind the
eye socket. This is unlike their sister group Synapsida which have only
one, and Anapsida with none (Figure. 1).
The first diapsid was Petrolacosaurus, a small, terrestrial insectivore first seen in the Upper Carboniferous. It was clearly a diapsid, as it has two temporal fenestrae in the skull. Petrolacosaurus is a member of the basal diapsid order Araeoscelidia along with the aquatic Spinoaequalis, also from the late Carboniferous.
Fig. 1 Generalised skull morphology of (a) anapsids, (b) synapsids, (c) diapsids, and (d) euryapsids. Abbreviations: j, jugal; p, parietal; po, postorbital; sq, squamosal (from Benton, 2005).
Petrolacosaurus (from Benton, 2005)
Some time in the Permian, the Diapsida split into two major lines, the lepidosauromorphs and the archosauromorphs. These are distinguished by features of the skeleton, as listed below.
Lepidosauria: The only living diapsids today (apart from the birds). Shared features between the squamates (lizards and snakes) include a large amount of skull mobility with several different hinging systems. Snakes have lost their limbs, and their skulls are even more mobile than those of lizards.
Sauropterygia: These consist of the nothosaurs, placodonts, and plesiosaurs. The saurotopterygians are more closely related to the lepidosaurmorphs than the Ichthyosauria. They ranged in body length from 2-14 m, and they were all adapted for underwater locomotion: they had large paddle-like limbs and reinforeced limb girdles.
Prolacertiformes, Rhynchosauria, Trilophosauria: These clades were important in the Triassic. Prolacertiforms orignated in the Permian, and included long-necked, slender forms that were mainly insectivorous. The rhynchosaurs and trilophosaurs were exclusively Triassic, and fed on vegetation with powerful beak-like jaws.
Archosauria:The main archosauromorph clade was the Archosauria, characterised by features of the skull and skeleton. The archosuarian 'trademark' is the antorbital fenestra, a major opening in the side of the snout located between the nostril at the front and the eye socket further back. Archosaurs include dinosaurs, birds, crocodiles, and many extinct groups. After an origin at the very end of the Permian, archosaurs split into two main groups in the Mid Triassic, the Crurotarsi and the Ornithodira.
Crurotarsi: The Crurotarsi are characterised by their complex ankle joints in which the two key ankle bones, the astragalus and calcaneum interlock and rotate against each other. Crurotarsans have large, reinforced skulls and longish snouts. Their skull design allows them to absorb forces created from a very strong bite. Includes various Triassic forms, as well as the crocodiles.
Ornithodira: There was a huge variation in forms within this group, from aerial pterosaurs to some of the largest ever terrestrial herbivores and carnivores. They all share a particular ankle and foot design that arose because of the fact they all stood up high on their tip toes: the astragalus and calcaneum act as a single unit, and the main toe bones, the metatarsals, are very long and act in a bunch.
Deinonychus (Modified from Benton, 2005)
Ichthyosauria: Ichthyosaurs appear similar to sharks and dolphins in body shape, although only through convergent evolution. All generally have a long snout, dolphin-like body, broad flippers, a deep tail fine and fin on the back, and large eyes. Ichthyosaurs have only the upper temporal fenestra, and their skull pattern has been termed 'euryapsid', as seen also in sauropterygians - it is now believed that this evolved from the diapsid pattern.