Morphology and function of the cruciate ligaments in diapsids

The extant diapsids (amniotes with two temporal fenestrate) consist of archosaurs (birds and crocodiles) and of lepidosaurs (sphenodon and squamates). The dichotomy of these two lineages occurred in the upper Permian. Originally, the intraarticular knee ligaments were three cruciates. The intermediate cruciate (a posterior one with guiding function) is present in all diapsids, whilst the medial one (an anterior with guiding function) is absent in birds. The lateral cruciate ligament is subdivided into a lateral anterior and a medial posterior shank. The posterior shank disappeared in birds and crocodiles. The anterior shank disposes of a guiding function in archosaurs, and is in the course of reduction in lepidosaurs. The reduction process has progressed least in sphenodons, whereas the lateral cruciate of chameleons is totally absent. The anterior cruciates of birds (the lateral one) and of chameleons (the medial one) are hence not homologous. The morphology and function of the cruciate ligaments can be related with gait evolution.