The holes at the back of the skull in all amniotes are called temporal fenestrae. Amniotes are classified into groups by the configuration of these fenestrae.
Anapsids have no temporal fenestration. This is the primitive amniote condition, and is found in turtles.
Synapsids have a single large temporal fenestra. The synapsid skull condition is found in mammals.
Euryapsids have a single, small upper fenestra. This condition is found in several groups of extinct Mesozoic marine reptiles: ichthyosaurs, plesiosaurs, nothosaurs and placodonts.
Diapsids have two temporal fenestrae. This condition is found in many reptiles, including dinosaurs and their relatives.
It was once thought that the Euryapsida evolved from synapsids, as both have only one temporal fenestra. It is now known that euryapsids are in fact a derived form of diapsid in which the lower temporal fenestra has been lost, and euryapsids have generally been reclassified as belonging to the Diapsida. 'Euryapsid' is therefore not a taxonomic name, but a descriptive one.
Euryapsids are a group of extinct Mesozoic marine-adapted reptiles. The loss of the lower fenestra of the skull is part of their adaptation to marine life.
Despite their adaptation to aquatic life, euryapsids were air-breathing creatures, and there is evidence that some groups gave birth to live young, much like modern dolphins, instead of laying eggs.
The generalised anatomy of the euryapsids remains recognisably reptilian, but the different groups show a varying degree of adaptation to aquatic life. For example, the more primitive nothosaurs looked much like lizards with webbed feet. The advanced ichthyosaurs, however, had paddles rather than feet, and appeared (at least superficially) very fish-like in their morphology.
Author: Rachel Jennings
Last updated: 22/11/2005
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