Knob-like muscle attachment point on the femur of archosaurs
The fourth trochanter (indicated by a red arrow) is visible on this mounted skeleton of Allosaurus fragilis.
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 homologous with the internal trochanter, an asymmetrical ridge-like structure that extends down from the femoral head and is edged by an intertrochanteric fossa in other reptiles such as lizards. 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 period. Shortly afterwards, 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 bipeds. It may also have been a factor in the archosaurs or their immediate ancestors surviving the catastrophic Permian-Triassic extinction event [citation needed].
^Nesbitt, S. J.; Stocker, M. R.; Small, B. J. & A. Downs (November 26, 2009). "The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes)". Zoological Journal of the Linnean Society. 157 (4): 814–864. doi:10.1111/j.1096-3642.2009.00530.x.
^Nesbitt, S.J. (2011). "The early evolution of archosaurs: relationships and the origin of major clades". Bulletin of the American Museum of Natural History. 352: 1–292. doi:10.1206/352.1. hdl:2246/6112. S2CID 83493714.
^Ezcurra, Martín D. (2016-04-28). "The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms". PeerJ. 4: e1778. doi:10.7717/peerj.1778. ISSN 2167-8359. PMC 4860341. PMID 27162705.
The fourthtrochanter is a shared characteristic common to archosaurs. It is a knob-like feature on the posterior-medial side of the middle of the femur...
the trochanter is a part of the femur. It can refer to: Greater trochanter Lesser trochanter Third trochanter, which is occasionally present Fourth trochanter...
of the greater trochanter. The lesser trochanter is a cone-shaped extension of the lowest part of the femur neck. The two trochanters are joined by the...
: 181 The fourthtrochanter is an extensive flange on the posterior surface of the femur, directed rearwards. In dinosaurs, the fourthtrochanter is distinctly...
In human anatomy, the third trochanter is a bony projection occasionally present on the proximal femur near the superior border of the gluteal tuberosity...
front of the eyes and in the jaw, respectively), and a pronounced fourthtrochanter (a prominent ridge on the femur). Being set in sockets, the teeth...
such as the triradiate pelvic girdle, the fourthtrochanter, and the third metatarsal longer than the fourth, indicate that erythrosuchids are closer to...
as a fourthtrochanter. However, in noasaurines and elaphrosaurines (but not necessarily other genera such as Deltadromeus), this fourthtrochanter was...
length Fourthtrochanter (projection where the caudofemoralis muscle attaches on the inner rear shaft) on the femur (thigh bone) is a sharp flange Fourth trochanter...
The lesser trochanter of the femoral head was wing-like, and placed below the greater trochanter, which was short. The fourthtrochanter was large and...
process on the jugal with a wide base, and a derived femur with a weak fourthtrochanter and a shallow broad extensor groove at the distal end. In their 2022...
as well as the appendicular skeleton (such as the position of the fourthtrochanter over the midlength of the femur and an adult femur length exceeding...
more than twice as wide in front or back view than in side view. The fourthtrochanter, a prominent bulge on the back side of the femoral shaft, was more...
iliofibularis muscle attached to a lower position on the fibula, the fourthtrochanter of the femur was enlarged, the transverse processes (rib attachments)...
taxa have an enlarged ridge on the lateral surface of the femur. The fourthtrochanter is apparently absent in Kuru, but there is a ridge on the femur that...
obturatum was closed at the rear. In the hindlimb, the holotype shows a fourthtrochanter present just above the middle of the thighbone shaft; in Stegosaurus...
60%, a lack of expansion of the distal scapula and the lack of a fourthtrochanter on the femur. Despite its size, Gigantoraptor would thus have been...
the tail. This muscle, called the caudofemoralis, attaches to the fourthtrochanter, a prominent ridge on the thigh bone, and pulls the thigh bone backwards...
Taquet concluded that Ouranosaurus was not a good runner because the fourthtrochanter, the attachment point for the large retractor muscles connected to...
ornithischians in having a relatively narrow pelvis and a pendant fourthtrochanter. The manus (the hand/front foot) shows a mixture of traits associated...
inside. On the thighbone the lesser trochanter has a robust ridge on its outer side. On the thighbone the fourthtrochanter has a depression in the form of...
characterized by the absence of a hyposphene-hypantrum, no femoral fourthtrochanter, and osteoderms. A small clade of Alamosaurus, Lirainosaurus and the...
to Alwalkeria) the presence of a highly expanded femoral head the fourthtrochanter is very prominent Several features would make Alwalkeria unique among...
lesser trochanter on its upper end—a bony projection serving as attachment site for muscles that drew the hind leg forwards and inwards. The fourth trochanter...
bone strap. The thighbone is short, robust and straight with a low fourthtrochanter positioned below the midpoint of the shaft. The robust shinbone is...
Elopteryx type but it differs in diagnostic traits, e.g. lacking a fourthtrochanter. Also, neither the ribs nor the tibiotarsi can be compared to the...
femur is poorly preserved, and only observable in posterior view. The fourthtrochanter of the femur is incomplete, but visible as a ridge with a distinct...
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