The fourth trochanter is a shared characteristic common to

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

s. It is a protrusion on the posterior-medial side of the middle of the

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

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 The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the ...

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

lizards Lizard is the common name used for all squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The ...

. 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, large basal archosauriform predators of the early

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

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

Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W. K. Parker) is an Extinction, extinct genus of archosauriform reptile from the Triassic of South Africa. ''Euparkeria'' is close to the ancestry of Archosauria, the reptile gro ...

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

References

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