Evolutionary and functional morphology of the knee in fossil and extant horses (Equidae)

The hindlimb of modern Equus caballus is distinguished by the presence of a passive stay-apparatus, i.e., a system of bones, muscles, and ligaments that facilitate long periods of standing with reduced muscular activity. Gravitational forces normally collapse (flex) the knee (stifle) joint. Thus, either muscular activity in the knee extensors must counter this action, or there must be a passive “locking” of the knee in extension. The latter situation has evolved through a complex arrangement of patellar ligaments, medial enlargement of the patella, and significant enlargement of the medial trochlear ridge (MTR) of the femur. Together, these elements facilitate a passive “lock” of the knee joint in an extended position without continued high levels of muscular activity. Primitive horses, such as Mesohippus from about 25 to 35 Ma, have a distal femoral morphology similar to that seen in outgroups, including tapirs, primitive rhinoceroses, and most artiodactyls. Specimens of Protohippus from about 12 Ma represent the first equid clade in which there is an enlarged MTR. This derived condition of the MTR is observed thereafter in the family Equidae, despite evolutionary experiments that included dwarfing or the tendency towards monodactyly. The enlarged MTR may be used as an indicator of the existence of a passive knee locking mechanism in the evolution of horses. Thus, morphologic characters correlated with the knee locking mechanism were present early (about 11–12 Ma) relative to an analogous locking mechanism proposed in the shoulder region of the Equidae (about 3–5 Ma). We propose that biomechanical differences in the weight-bearing of forelimbs versus hindlimbs may account for the earlier appearance of a “locking mechanism” of the equid hindlimb.