The fourth trochanter is a shared characteristic common to

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...

s. It is a knob-like feature on the posterior-medial side of the middle of the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates ...

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 homologous with the internal trochanter, an asymmetrical ridge-like structure that extends down from the

femoral head The femoral head (femur head or head of the femur) is the highest part of the thigh bone (femur). It is supported by the femoral neck. Structure The head is globular and forms rather more than a hemisphere, is directed upward, medialward, and a l ...

and is edged by an intertrochanteric fossa in other reptiles such as

lizards Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia altho ...

. The fourth trochanter can be characterized by its position further down the shaft, symmetrical nature, and lack of an intertrochanteric fossa. The ''caudofemoralis'' attachment crest first separated from the femoral head in the

Erythrosuchidae Erythrosuchidae (meaning "red crocodiles" in Greek) are a family of large basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian). Naming The family Erythrosuchidae was named by ...

, large basal

archosauriform Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthier ...

predators of the early

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

period. Shortly afterwards, eucrocopodan archosauriforms (such as ''

Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W.K. Parker) is an extinct genus of archosauriform from the Middle Triassic of South Africa. It was a small reptile that lived between 245-230 million years ago, and was close to ...

'') evolved, losing the intertrochanteric fossa and acquiring a symmetrical fourth trochanter. 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

biped Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...

s. It may also have been a factor in the archosaurs or their immediate ancestors surviving the catastrophic Permian-Triassic extinction event .

References

{{Reflist * Benton, M. J. (2000), ''Vertebrate Paleontology'', 2nd Ed. p.136; (2004) 3rd edition, Blackwell Science Ltd Vertebrate anatomy