the primitive sprawling gait. the lesser trochanter is a unique development of mammals, which lack both the internal and fourth trochanters. the adductor crest is also often absent in mammals or alternatively reduced to a series of creases along the surface of the bone. structures analogous to the third trochanter are present in mammals, including some primates. some species of whales, snakes, and other non-walking vertebrates have vestigial femurs. in some snakes the protruding end of a pelvic spur, a vestigial pelvis and femur remnant which is not connected to the rest of the skeleton, plays a role in mating. this role in mating is hypothesized to have possibly occurred in basilosauridae, an extinct family of whales with well-defined femurs, lower legs and feet. occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (atavism). one of the earliest known vertebrates to have a femur is the eusthenopteron, a prehistoric lobe-finned fish from the late devonian period. viral metagenomics a recent study revealed that bone is a much richer source of persistent dna viruses than earlier perceived. besides parvovirus 19 and hepatitis b virus, ten additional ones were discovered, namely several members of the herpes- and polyomavirus families, as well as human papillomavirus 31 and torque teno virus. invertebrates in invertebrate zoology the name femur appears in arthropodology. the usage is not homologous with that of vertebrate anatomy; the term "femur" simply has been adopted by analogy and refers, where applicable, to the most proximal of (usually) the two longest jointed segments of the legs of the arthropoda. the two basal segments preceding the femur are the coxa and trochanter. this convention is not followed in carcinology but it applies in arachnology and entomology. in myriapodology another segment, the prefemur, connects the trochanter and femur. additional media 3d image muscles of thigh. lateral view. muscles of thigh. cross section. distribution forces of the femur