Fourth trochanter
The fourth trochanter is a
shared characteristic common to archosaurs. It is a knob-like feature on the posterior-medial side of the middle of the femur shaft that serves as a muscle attachment, mainly for the musculus caudofemoralis longus
, the main retractor tail muscle that pulls the thighbone to the rear.
The fourth trochanter is considered
eucrocopodan archosauriforms (such as Euparkeria) evolved, losing the intertrochanteric fossa and acquiring a symmetrical fourth trochanter.[1][2][3]
This seemingly insignificant detail may have made the evolution of dinosaurs possible as it facilitates a bipedal gait. All early dinosaurs and many later ones were
Permian-Triassic extinction event [citation needed
].
References
- Benton, M. J. (2000), Vertebrate Paleontology, 2nd Ed. p.136; (2004) 3rd edition, Blackwell Science Ltd