Anapsids, diapsids andsvnapstds Reptil~s,'both'living and extincthave been' classified partly according to openings inthe skull bone behind the orbits (eye sockets). This is a useful feature often preserved in -tossils, but . some authorities doubt its true"value.!n. '".,_. -- determihing evolutionary relatiohships. "'<'~'" • Anapsids. .chiefly•chelonians -'- turtles/terra'pins and tortoises= have no such openings. This is regarded as the primitive condition .' • Diapsids typically have two openings (fenestrae) on either ,side behind the eye socket, one above the other. However. in some types such as snakes further evolution has merged these two into one.' • Diapsids include most living and extinct reptiles, such as crocodiles, dinosaurs (and, therefore, birds), ichthyosaurs, plesiosaurs, lizards and snakes. • Synapsids had one opening low down on the skull behind the orbit. They include the mammal-like reptiles (pelycosaurs, therapsids) and their descendants, the mammals
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