Comparative Distal Limb Anatomy Reveals a Primitive Trait in 2 Breeds of Equus caballus

The 55-million-year history of equine phylogeny has been well documented from the skeletal record, however not the soft tissue structures that are now vestigial in modern horse. A recent study reported 2 ligamentous structures resembling functional 3^rd and 4^th interosseous muscles were evident in Dutch Konik horses. The current study investigates this finding and compares it to members of the genus Equus to identify either a breed anomaly or functional primitive trait. Distal limbs (n=571) were dissected from 4 species of Equus; E. caballus, E. asinus, E. przewalskii and E. burchelli beohmi. Breed representatives of E.caballus (n=18) included the primitive Dutch Konik. The 2^nd and or 4^th interosseous muscle was evident in all 4 species, but only 2 breeds of E.caballus expressed this trait - the Dutch Konik and Bosnian Mountain Horse. These 2 breeds were the only close descendants of the extinct Tarpan (Equus ferus ferus) represented in this study. In conclusion, the 2^nd and 4^th interosseous muscles originated from the distal nodule of respective splint bones and inserted into the corresponding branches of the 3^rd interosseous muscle proximal to the sesamoids. Suggesting a functional role in medial and lateral joint stability and a primitive trait in modern equids.