Comments about the astragalar dorsal foramen and the astragalar flexor groove in Felis, Leopardus and Myocastor

Abstract

The dorsal foramen and the flexor groove of the astragalus are traditionally considered traits of plantigrade and pentadactyl mammals, and therefore basal unspecialized taxa. These features are often mentioned as limiting plantar flexion of the foot. Blood vessels, nerves, ligaments and tendons have been proposed to pass through the foramen. During the last two years I had the opportunity to dissect three roadkills: a Myocastor coypus, a Felis catus and a Leopardus geoffroyi. I found the distinctive presence of the tissue of the astragalotibial joint capsule more dorsal than what is considered the plantar end of trochlea in the three animals. The boundary of the trochlea was marked by the plantar edges of the lateral and medial malleolar facets. The astragalar foramen was present in both felids with an unidentified blood vessel passing through it. No nerves or ligaments were found. Additionally, the flexor digitorum longus muscle of L. geoffroyi had a sesamoid at the level of the tibioastragalar articulation. Although the flexor groove and the astragalar dorsal foramen are considered traits of plantigrade and pentadactyl mammals, they are common in Carnivora, particularly in felids, being observed at least in the genera Felis, Leopardus, Panthera, Prionailurus, Puma, and Smilodon. Future research on the soft tissues of these animals as well as more quantitative data, may provide new insights into this ongoing discussion.