Amniotes (tetrapod vertebrate animals with a terrestrially adapted eggs)
are traditionally divided into three categories:
Anapsids - Animals with a box-like skull without openings.
This group includes the early reptiles of the Carboniferous and Permian
periods (and to a limited extent the
and includes animals such Scutosaurus.
This group may also include turtles, however many scientists believe
that turtles are actually descended from Diapsid reptiles which lost
their skull openings.
Diapsids - Reptiles with two temporal holes (fenestra) in their skulls,
and their descendents. This group includes archosaurs (including
crocodilians, dinosaur species
and birds), as well as lepidosaurs such as lizards, snakes, and sphenodonts
- even though many of these groups have lost one hole (lizards), both holes (snakes),
or have heavily modified skulls (birds).
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