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

Abstract

ABSTRACT 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 1...