in taxon Archosauria trochanter fourth trochanter A knob-like feature on the medial side of the femur that serves as a muscle attachment. UBERON:0008787 The fourth trochanter first appeared in the Erythrosuchidae, large basal archosaurian predators of the early Triassic period. This seemingly insignificant detail may have made the evolution of dinosaurs possible (all early dinosaurs and many later ones were bipeds), and may also be connected with the ability of the archosaurs or their immediate ancestors to survive the catastrophic Permian-Triassic extinction event[WP]. Wikipedia:Fourth_trochanter