- A primitive ('basal') condition for early amniotes in the Mississippian (early Carboniferous).
- A secondary ('derived') condition of some other reptile groups, probably including turtles (Chelonia).
Some other groups were anapsids, such as the mesosaurs.
References[change | change source]
- Pough F.H. et al. 2002. Vertebrate life, 6th ed. Prentice Hall, NJ. ISBN 0130412481