Notes on the origin and function of the artiodactyl tarsus. American Museum novitates ; no. 1356

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New York City : The American Museum of Natural History
"The definitive artiodactyl tarsus as it first appears in Lower Eocene deposits represents the abrupt introduction of a basic ordinal character. There are no known intermediate or transitional stages between this tarsus and the presumed ancestral type. Predator pressure must have been an important factor in the apparent rapid evolution of this structure, which had virtually reached the biomechanical limit of specialization by the Lower Eocene. 2. Available evidence indicates that the artiodactyls evolved from the primitive hyopsodontid condylarths, although the tarsus of these early ferungulates is very similar to that of the arctocyonid creodonts. In these types, the astragalus has a shallow tibial trochlea, a wide interarticular sulcus, and a rounded head. The calcaneum has a well-developed interosseous fossa and a robust sustentaculum supporting, in part, the astragalus. 3 The axes of the upper and lower ankle joints and the transverse tarsal joint have been determined for the arctocyonid creodont Claenodon. These axes are oriented in several different planes, suggesting the condition occurring in the tarsus of the chimpanzee and man, which may be close to the generalized eutherian arrangement. The probable motion at each of these joints is discussed. 4. The primitive artiodactyl tarsus differs markedly from the generalized ferungulate type. The astragalus has a deep tibial trochlea, the interarticular sulcus has disappeared in connection with a reorientation of the astragalocalcaneal and sustentacular facets, and the head is trochleated. The calcaneum has lost the interosseous fossa, and the sustentaculum no longer offers direct support to the astragalus. 5. The axes of the three ankle joints have been worked out for Ovis. With the changes in the astragalus and calcaneum noted above, these axes are now oriented in such a manner as to permit rotation of the astragalus in a vertical plane. This motion is demonstrated in a series of X-rays of the foot of a living goat. 6. The ligaments of the artiodactyl tarsus have been modified to permit vertical astragalar rotation. The fibers of the calcaneotibial and superficial calcaneal ligaments are somewhat spiraled in order to maintain effectively the integrity of the ankle joint during this rotation. 7. The transformation of the artiodactyl astragalus and calcaneum from the Claenodon type is described in detail by the use of the deformed coördinate method. The limitations of the method are discussed. 8. An attempt is made to describe the function of the artiodactyl tarsus, particularly in relation to astragalar rotation. The astragalus is essentially a cam-like structure that may be considered as a velocity mechanism. It also aids in increasing the angle of application of the crural flexor musculature at the start of the propulsive phase when two tarsal fulcra are probably present, at the upper ankle joint and the transverse tarsal joint"--P. 22-23.
24 p. : ill. ; 24 cm.
Includes bibliographical references (p. 23-24).