Tradtional paleontologists consider the Diapsida to be a monophyletic clade. That is not so, according to the results of the large reptile tree. Those on the lepidosauromorph branch developed a diapsid skull independently and distinctly differently than did those on the archosauromorph branch, which we’ll cover tomorrow.
Today the origin and evolution of the lepidosauromorph diapsid skull (Fig.1) is presented.
Several convergent appearances even in the Lepidosauromoropha
In the new Lepidosauromorpha lateral temporal fenestra developed in several clades including the Caseasauria (see Casea and Oedaleops), the Millerettidae (see Bolosaurus and Feeserpeton), Lanthanosuchus and the Owenettidae (including their descendants, all living lizards, as well as extinct Lepidosauriformes, such as Paliguana and Gephyrosaurus, Fig. 1).
The ventral embayment of the cheek in owenettids is what formed the lateral temporal fenestra in derived taxa including those lepidosaurs living today. The upper temporal fenestra occurred rather spontaneously in Paliguana and was retained in all subsequent taxa.
Surrounding bones – an evolution
When the lateral temporal ventral embayment appears in Coletta it is bordered by the jugal and quadratojugal. In derived taxa the squamosal slightly advances and slightly retreats. In Candelaria a second lateral temporal fenestra appears bordered by the postorbital, squamosal and quadratojugal. In Paliguana these two lateral temporal fenestrae merge to become one tall embayment as the squamosal becomes tiny (or was lost during taphonomy). The postorbital loses contact with the ltf in Gephyrosaurus and gains it in Homoeosaurus among taxa shown (Fig. 1). At the same time the quadratojugal is reduced to a vestige or disappears, bringing the quadrate into contact with the lateral temporal embayment. The jugal produces a posterior process that, in some taxa, such as Sphenodon and fenestrasauria (by convergence), anchors a new or larger quadratojugal, completing the lower temporal bar. The lateral temporal fenestra is infilled to become a vestige in Trilophosaurus and, by convergence in Kaikaifilusaurus. The upper and lateral temporal fenestra merge in snakes and several burrowing clades.
A reordering of the reptile family tree is required
Due to these various appearances and disappearances of temporal fenestrae, reptiles must be nested and classified according to their overall morphologies, not strictly according to their cheek and temple architecture.
Lepidosauromorph cheek and temple architectural patterns are distinct from those in the new Archosauromorpha, as we’ll see tomorrow. These two clades share no common ancestor with a diapsid skull architecture, which is a heretical departure from the traditional paradigm.