Anapsid

An anapsid is an amniote whose skull does not have openings on the sides behind the eyes.^[1] This is

1. A primitive ('basal') condition for early amniotes in the Mississippian (early Carboniferous).
2. A secondary ('derived') condition of some other reptile groups, probably including turtles (Chelonia).

General diagram of the anapsid skull.

Some other groups were anapsids, such as the mesosaurs.

References

1. Pough F.H. et al. 2002. Vertebrate life, 6th ed. Prentice Hall, NJ. ISBN 0130412481