Fourth trochanter

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Fourth trochanter on a mounted skeleton of a theropod dinosaur

The fourth trochanter (English fourth trochanter ) is a button-shaped to comb-like bone protrusion of the thighbone (femur) in terrestrial vertebrates from the group of Archosauria . The fourth trochanter is located on the caudal (back) side of the shaft and usually has a medial indentation (toward the midline of the body). It serves as an attachment point for various muscles, such as the caudofemoralis longus muscle (= caudofemoralis pars caudalis muscle in birds), an important tensile muscle of the thigh, which has its origin in the caudal vertebrae, and the pubo-ischio-femoralis internus 1 (= Iliofemoralis internus muscle in birds), which has its origin in the pelvis and attaches craniomedially to the fourth trochanter, but, unlike the caudofemoralis muscle , does not touch the medial indentation.

Michael Benton (2005) lists the fourth trochanter as apomorphism (newly acquired feature) of a group that includes all archosaurs except for the very original Proterosuchidae . Within the Archosauria, the fourth trochanter is a variable, often differently pronounced characteristic. In the basal representatives of the Dinosauromorpha and in theropods , it is large and comb-shaped, but in the closely related theropods, it was already greatly reduced. In recent birds it is only occasionally present as a greatly reduced rudiment.

The fourth trochanter is not to be confused with the third trochanter , an analogous structure in mammals. Another structure, analogous to the fourth trochanter, is found in original terrestrial vertebrates.

Individual evidence

  1. ^ A b Michael J. Benton: Vertebrate palaeontology. 3. Edition. Blackwell Publishing, 2005, ISBN 0-632-05637-1 , pp. 139, 141.
  2. ↑ a b c d e John R. Hutchinson: The evolution of femoral osteology and soft tissues on the line to extant birds (Neornithes). In: Zoological Journal of the Linnean Society. 131, 2001, pp. 169-197.
  3. ^ Alfred Sherwood Romer: Osteology of the Reptiles . The University of Chicago Press, ISBN 0-226-72487-5 , p. 590.
  4. ↑ a b Louis Dollo: Sur la significance du 'trochanter pendant' des dinosaurs. In: Bull. Sci. France Belg. 8, 1888, pp. 215-224.