A Primitive Trait in Two Breeds of Equus Caballus Revealed by Comparative Anatomy of the Distal Limb

Understanding the complexities and evolutionary links between extinct and extant equids has been vital to genetic conservation and preservation of primitive traits. As domestication of the equid expanded, the loss of primitive traits that ensured survival in a wild environment has not been documented. In this study, the presence of functional interosseous muscle II and IV in the distal limb has been reported, and yet its existence could only be confirmed in relatives and two closely bred descendants of the extinct Tarpan. The morphology described was ligamentous in structure displaying clear longitudinal fibres with a skeletal origin and soft tissue insertion into the medial and lateral branches of the interosseous muscle III (suspensory ligament) dorsal to the sesamoids, similar in orientation to the flexor digitorum profundus ligamentum accessorium (inferior check ligament). Hence, providing a functional medial and lateral stability to the metacarpophalangeal joint (fetlock joint), which equates to one of the functions of the medial and lateral digits in the Mesohippus and Merychippus. The comparable anatomic links between species of the same family that experienced geographical isolation yet display structural conformity appears to be in response to a specific environment. Surmising this potential remnant of functional evolution is a primitive trait and not a breed anomaly