Royal Society Open Science · 5 authors, 6 centres
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The authors tested whether the frog of the modern horse hoof is formed from remnants of digits II and IV, as recently proposed, and found that ichnological evidence from fossil footprints of tridactyl equids contradicts this hypothesis by showing frogs were already present in three-toed ancestors.
This comparative morphological and ichnological study examined whether distal remnants of accessory digits contribute to the frog of the modern horse hoof, as proposed by Solounias et al. The authors measured ungual phalanges (n=225) and hoof prints (n=44) across tridactyl and monodactyl equids, applying ovality index analyses with Welch's t-tests and Mann-Whitney U-tests. The study found that fossil footprints of tridactyl equids such as Hipparion show frog impressions, contradicting the hypothesis that the frog arose from co-opted distal side-toe remnants. While proximal remnants of all five pentadactyl digits are retained in modern horses, confirmed by developmental evidence, no support was found for retention of distal portions. Polydactyl horses that possess frogs on both central and additional toes further support the conclusion that the frog is independent of side-digit remnants. The frog likely represents a modified remnant of the ancestral footpad and has been present for much of equid evolutionary history.
