Reptile Skulls

From the Middle Permian Period (about 268 million years ago) through the Triassic Period, the mammal-like reptiles began to change slowly. The synapsid skull began to change into what is now recognized as the therapsid skull. Therapsid animals (mammals) are considered to be a sub-group of the synapsid animals. Instead of a fenestra, the therapsids formed more of an arch, called the zygomatic arch, with their skull bones. The jaw muscles could pass through this arch to their sites of attachment, giving the animals more jaw strength with less work.

The next group of reptiles are the diapsids, whose name refers to the two fenestrae in their skull, one low on both sides of the skull, and the other high toward the back of the skull on both sides. The diapsids lived alongside the early synapsids during the Late Carboniferous Period. The diapsids are represented by many lizard-like reptiles, and also by the dinosaurs, crocodiles, pterosaurs, and birds.

The last group of reptiles that we will look at are the euryapsids. Euryapsid means "wide arch". The euryapsids have one fenestra high on both sides of the skull. The euryapsids are represented by the marine reptiles such as the icthyosaurs, plesiosaurs, and placodonts, and are the only completely extinct group. The euryapsids arose from the diapsids with their two fenestrae, and sealed over one of the openings to strengthen their skulls for life under water.

I will be discussing the marine reptiles in a future article.

More information about fenestration and the classificaion of amniotes

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