All procolophonoids share a few unique derived characters that demonstrate that they are closely related to each other. These include (pictured below for your enjoyment):
- A posteriorly enlarged orbit (eye opening). This character is only moderately developed in Upper Permian and some Lower Triassic procolophonoids like Owenetta, and it is most noticeable in Upper Triassic forms like Hypsognathus.
- A medial process in the orbital flange of the prefrontal.
- A concave ventral edge of the skull in the postorbital area.
Skull of the Lower Triassic procolophonoid Owenetta. Abbreviations: O. f., concave occipital flange of the squamosal; P. e., posterior extension of the orbit; Prf. pr., medial process of the prefrontal; V. e., ventral emargination of the cheek.
Characters unique to procolophonoids and turtles that suggest close relationships between these two groups include:
- A wide medial flange of the prefrontal (in the anterior orbital rim).
- A short lacrimal bone that does not reach the external naris.
- A concave occipital flange of the squamosal bone that extends medial to the tympanic ridge.
A skull of Procolophon from the UCMP collections. The large hole between the orbits is a pineal foramen, a primitive character lost in extant anapsids.
Gauthier, J., A. G. Kluge, and T. Rowe. 1988. The early evolution of the Amniota. In: M. J. Benton (ed.) The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds: 103-155. Oxford: Clarendon Press.
Lee, M. The turtle's long-lost relatives. Natural History, April 1994 1994, 63-65.
Reisz, R. R. and M. Laurin. 1991. Owenetta and the origin of turtles. Nature 349: 324-326.