The oldest known member of Anapsida is Acleistorhinus,
a small reptile from the lower Permian of Oklahoma, about 270 million years ago. However, the presence of
(the sister-group, or closest relatives, of anapsids) in the upper
Carboniferous indicates that Anapsida originated no later than this,
around the same time as the very first amniotes were found - Hylomonus and Paleothyris
from Nova Scotia, around 310 million years ago. Anapsids can therefore
be traced right back to the dawn of the reptiles. This implies that
there is a long period of ancient anapsid history unknown from fossils.
Most known ancient forms of anapsid originated
in the mid to late Permian and became extinct at the Permian-Triassic
boundary, at about 250 million years ago (with the exception of the Procolophonoidae), a range of up to about 20 million years.
The fossil record of the anapsids is good, as their typically
unfenestrated, strong skulls are generally well preserved. In later
forms, the shells of the Testudines are also easily preserved.