Thefourth trochanter is ashared characteristic common toarchosaurs. It is a protrusion on the posterior-medial side of the middle of thefemur shaft that serves as a muscle attachment, mainly for themusculus caudofemoralis longus, the main retractor tail muscle that pulls the thighbone to the rear.
The fourth trochanter is consideredhomologous with theinternal trochanter, an asymmetrical ridge-like structure that extends down from thefemoral head and is edged by anintertrochanteric fossa in other reptiles such aslizards. The fourth trochanter can be characterized by its position further down the shaft, symmetrical nature, and lack of an intertrochanteric fossa. Thecaudofemoralis attachment crest first separated from the femoral head in theErythrosuchidae, largebasalarchosauriform predators of the earlyTriassic period. Shortly afterwards,eucrocopodan archosauriforms (such asEuparkeria) evolved, losing the intertrochanteric fossa and acquiring a symmetrical fourth trochanter.[1][2][3]