Protorothyris archeri (Price 1937) Early Permian ~290 mya, ~15 cm snout to vent length. It was derived from a sister to Paleothyris and Coelostegus. Protorothyris phylogenetically preceded Aerosaurus, Archaeothyris and Heleosaurus and represents the base of the split between the synapsids (today's mammals) and the protodiapsids (today's birds and crocs, see below).
Distinct from Coelostegus, Protorothyris had a relatively larger head and a deeper skull with support from a taller cervical series. The canine tooth was several times longer than the other teeth and the rostrum came down to a distinct point in lateral view. The jugal was deeper, which reduced the orbit size. The preorbital portion of the skull was longer than the postorbital portion. The nasals narrowed toward the naris. The supraoccipital was broader than the exoccipitals.
The scapulocoracoid was well ossified. The chest was twice the depth of the pelvis. The limbs remained slender.