The fourth trochanter is a shared characteristic common to archosaurs. It is a knob-like feature on the medial side of the femur that serves as a muscle attachment.
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.
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