Fossil Record of Diapsids


Diapsids first appeared in the Late Carboniferous, with arrival of the small, terrestrial insectivore Petrolacosaurus. It displays features that are similar to the prororothyridids, a paraphyletic group that might have given rise to all three amniotic reptile groups (Diapsida, Anapsida and Synapsida). Petrolacosaurus is a member of the basal diapsid order Araeoscelidia along with the aquatic Spinoaequalis, also from the Late Carboniferous.

Diapsid history is not well recorded in the Early Permian. It was not until the Late Permian that the weigeltisaurids and younginiforms appeared. The weigeltisaurids were the first flying vertebrates - they had extended ribs that carried skin membranes, and they took extended leaps from tree to tree. The younginiforms included ground-dwellers and swimmers with deep tails. These two groups were primitive in various ways, and came before the major diapsid clade, the Neodiapsida, which includes the lepidosauromorphs and archosauromorphs.

Fossil record of the neodiapsids

Basal Archosauromorpha: Mid Permian-Late Triassic. The prolacertiforms, trilophosaurids and rhynchosaurs. Flourished mostly during the Mid Triassic but were all extinct by the Late Triassic.

Ichthyosauria: Early Triassic-Upper Cretaceous Reached greatest diversity in the Early Jurassic, continued through the Jurassic, but diminished in diversity in the Cretaceous. Although they showed great range in size, there was little variation in morphology during their existence.

Sauropterygia: Mid-Triassic-End-Cretaceous All three subgroups - the nothosaurs, placodonts and plesiosaurs - arose around the same time in the Mid-Late Triassic, but were also all wiped out by the end-Cretaceous mass extinction event.

Lepidosauria: Mid-Triassic-Today The Squamata (which includes the lizards and snakes) has a poor early fossil record. Half of the six lineages within Squamata are seen in the Late Jurassic, which indicates that the other three were present then as well. The oldest snake fossils are from the Early Cretaceous.

Crocodylia: Late Triassic-Today Began as lightly built, bipedal organisms in the Late Triassic. The "true" crocodylian form first seen in the Early Jurassic, after which there was not much deviation from the main features that first characterised them. There are 23 species of crocodilian today.

Dinosauria: Late Triassic-Today First appeared as medium-sized bipedal carnivores, but diversified rapidly to include quadrupedal herbivores towards the end of the Triassic. Dinosaurs were wiped out by the end-Cretaceous mass extinction event, except for the dinosaur descendants, the birds (Class Aves).

Aves: Late Jurassic-Today Descendants of the theropod lineage of dinosaurs, and survivors of the end-Cretaceous mass extinction event, birds have diversified into nearly 9000 species living today.

Pterosauria: Late Triassic-End Cretaceous Showed rapid diversification of forms during the Jurassic and Cretaceous, but then were wiped out by the end-Cretaceous mass extinction event.



Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2004-5