Quadrate orientation and joint reaction force underwent correlated evolution during suchian evolution.

As part of the jaw joint, the quadrate is a key skeletal element of the feeding system in nonmammalian vertebrates, which plays a critical role in resisting joint reaction forces (JRF). Some authors have suggested that the quadrate orientation reflects overall muscle anatomy and, by implication, JRF. Here, we quantitatively test the longstanding hypothesis that quadrate orientation is correlated with JRF orientation using the suchian lineage leading to extant crocodylia. The evolution of the characteristic crocodylian skull is a major transformation in vertebrate evolution in which the quadrate played a crucial role. We use detailed, three-dimensional biomechanical modeling to estimate JRF in a sample of eleven fossil and extant suchians and compare these to the orientation of quadrates. We use the cross-product of orientation vectors to quantify similarity in orientation and show that the angle of the quadrate in the sagittal plane is tightly coupled with JRF in the same. These results demonstrate a coordinated evolution between JRF and quadrate anatomy during suchian evolution and provide a framework with which to analyze evolutionary changes in joint anatomy and biomechanics.