The evolution and anatomy of the horse manus with an emphasis on digit reduction

Abstract

We revisit digit reduction in the horse and propose that all five digits are partially present in the modern adult forelimb. Osteological descriptions of selected tetradactyl, tridactyl and monodactyl equids demonstrate the evolution of the forelimb. Histological, osteological and palaeontological evidence suggest that the Equus distal forelimb is more complex than traditionally conceived. The current understanding is that the horse distal forelimb consists of one complete digit (III) and two reduced splint metacarpals (II and IV). Metacarpals II and IV each exhibit a ventral ridge, which we suggest represents the undifferentiated digits I and V. These ridges are present in the tridactyl Mesohippus, but are absent in the tetradactyl Hyracotherium. The carpal articulations of the five metacarpals match those of pentadactyl taxa. Distally, the frog, a V-shaped structure on the ventral hoof represents digits II and IV, and the wings and hoof cartilages of the distal phalanx are digits I and V. We relate this revised interpretation of the Equus forelimb to Laetoli footprints, and suggest the Hipparion side impressions are created from the hooves of I and V, rather than from II and IV. We show shades of pentadactyly within the Equus manus.

Evidence of five digits in embryonic horses and developmental stabilization of tetrapod digit number

“…the merging of DI with DII, and DIV with DV), clearly showing that cell lineages from all five metacarpals persist in the older horse FL and that DI and DV are not absent or solely carved away as previously thought, but rather they also fuse at this stage (figure 2a,b,e,f,i,j; electronic supplementary material, figures S1 and S3). This observation is in agreement with the recent palaeontological hypothesis that remnants of all five digits persist in adult Equus metacarpals [6]. More proximally, all condensations join at the carpal-metacarpal joint and are hard to distinguish (electronic supplementary material, figures S1 and S3).…”

“…Nevertheless, this limited data on horse embryology suggested that horses only ever form three digit remnants during their development and became an important cornerstone of the general view of the evolutionary developmental biology of digits [8][9][10]. By contrast, a recent palaeontology paper proposed a novel hypothesis, based on bone articulations and ridges in fossil horses and vasculature in late foetal horses, that the identities of all five ancestral digits might be preserved in the metacarpal anatomy of the modern horse adult FL [6], but direct evidence was lacking. This same study also proposed that the horse 'frog' (distal hoof cartilage) is a distal remnant of all five digit condensations, although embryological evidence was again lacking.…”

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

“…During the same epoch, the perissodactyl forebears of Equidae , the tridactyl Mesohippus and Merychippus had anatomic structures comparable to the Acrocodia indica (Malayan tapir), including interosseous muscles [38]. The three digits in the tridactyl were referred to as II, III and IV (medial, middle and lateral respectively), with III being the largest, widest and dominant during weightbearing [38,39]. Each interosseous muscle corresponded to its metacarpal or metatarsal; hence, the interosseous muscles in the Mesohippus and Merychippus would be labelled IM2, IM3 and IM4 from which they originated [12,15].…”

Comparative Distal Limb Anatomy Reveals a Primitive Trait in 2 Breeds of <em>Equus caballus</em>

“…During the same epoch, the perissodactyl forebears of Equidae, the tridactyl Mesohippus and Merychippus had anatomic structures comparable to the tapir, including interosseous muscles [36]. The 3 digits in the tridactyl were referred to as II, III and IV (medial, middle and lateral respectively), with III being the largest, widest and dominant during weightbearing [36,37]. Each interosseous muscle corresponded to its metacarpal or metatarsal, hence the interosseous muscles in the Mesohippus and Merychippus would be labelled IM2, IM3 and IM4 from which they originated [12,15].…”