Reversing the Trend: The Evolution of Cranial Akinesis in the Terror Birds (Cariamiformes, Phorusrhacidae)

Cranial kinesis in Neornithes is enabled by a complex system of bones, muscles, ligaments, and joints, allowing variable degrees of movement between the upper jaw and the neurocranium. Particularly, cranial prokinesis—mobility of the upper jaw relative to the neurocranium—is the most widespread form. It has been stated that Phorusrhacidae (Aves, Cariamiformes) were incapable of performing cranial kinesis. To assess potential functional differences within the clade, all known cranial flexion zones were analyzed and compared with the kinetic systems of extant Neornithes. In Psilopterinae and Mesembriornithinae, fusion of the ventral palatal flexion zones indicates a rigid, immobile palate. In larger taxa such as Patagornithinae and Phorusrhacinae, the pronounced thickening and fusion of the craniofacial flexion zone confirms the loss of prokinetic capabilities. The functional implications of such akinesis include simplification of food processing and a significant increase in bite force, which correlates with the predatory skills of the group. In a phylogenetic framework, phorusrhacids evolved from a kinetic ancestral condition, representing the only truly akinetic group among Neornithes.