The ankylosaurid neomorphic skull elements

Abstract

Ankylosaurids but not nodosaurids possess a suite of neomorphic elements fused to the posterior border of the skull roof. Previous authors have offered conflicting descriptions of these elements, leading to ongoing confusion over their morphology, developmental origin, and homologies. They have been described both as secondary dermal additions and as outgrowths of known skull elements. However, they are demonstrably neomorphic and were incorporated into the skull early in ankylosaurid evolution. The six elements—three per side—are herein termed the supranuchal, postsquamosal, and postquadratojugal and form the posterior margin of the skull on each side (in some taxa, the parietal also reaches the posterior border). They do not overlie the squamosal or quadratojugal but rather articulate with them. Other neomorphic elements including the supraorbitals and supranarial ossifications have been described previously. The squamosal proper is a small element in the skull roof with a well-defined cotylus ventrally for articulation with the quadrate. The three neomorphs likely originated as a cervical half-ring or simply as a series of six cervical osteoderms situated just posterior to the skull in ancestral ankylosaurs.