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
Triassic period),
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).
Synapsids (also sometimes known as "Theropsids") -
Animals with a single temporal opening (fenestra) in theirn skull
behind each eye. This group includes many reptiles, known as "mammal-like reptiles",
as well as mammals.
Some examples of mammal-like reptiles include
Cynodonts,
Dicynodonts,
Gorgonopsians (including Lystrosaurs),
Dinocephalians like
Estemmenosuchus,
Moschops and
Struthiocephalus,
as well as
Pelycosaurs like
Dimetrodon,
Edaphosaurus,
Sphenacodon, and
Varanosaurus.
Mammal-like Reptiles were one of the main groups of amniotes, including the ancestors of all modern mammals, and lived between 324 and 100 million years ago
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