Mechanics of evolutionary digit reduction in fossil horses (Equidae)

Abstract

Digit reduction is a major trend that characterizes horse evolution, but its causes and consequences have rarely been quantitatively tested. Using beam analysis on fossilized centre metapodials, we tested how locomotor bone stresses changed with digit reduction and increasing body size across the horse lineage. Internal bone geometry was captured from 13 fossil horse genera that covered the breadth of the equid phylogeny and the spectrum of digit reduction and body sizes, from to To account for the load-bearing role of side digits, a novel, continuous measure of digit reduction was also established-toe reduction index (TRI). Our results show that without accounting for side digits, three-toed horses as late as would have experienced physiologically untenable bone stresses. Conversely, when side digits are modelled as load-bearing, species at the base of the horse radiation through probably maintained a similar safety factor to fracture stress. We conclude that the centre metapodial compensated for evolutionary digit reduction and body mass increases by becoming more resistant to bending through substantial positive allometry in internal geometry. These results lend support to two historical hypotheses: that increasing body mass selected for a single, robust metapodial rather than several smaller ones; and that, as horse limbs became elongated, the cost of inertia from the side toes outweighed their utility for stabilization or load-bearing.

What Is Equus? Reconciling Taxonomy and Phylogenetic Analyses

“…Potential morphological adaptations for this mode of life comprise modifications of the locomotory and digestive systems. Possible locomotor adaptations to living in open habitats include the elongation of distal autopodial bones as well as the development of monodactyly and the reduction or loss of metapodials II and IV (Simpson, 1951;Shotwell, 1961;Janis and Wilhelm, 1993); however, we note that alternative explanations for digit reduction have been proposed (Thomason, 1986;Biewener, 1998;McHorse et al, 2017). The enhanced development of the stay-apparatus, which allows the individual to conserve energy while standing, is also potentially an adaptation to living in open habitats MacFadden, 1992, 1996).…”

A Novel Non-Invasive Selection Criterion for the Preservation of Primitive Dutch Konik Horses

“…Consistent with our findings, a 1975 anatomic text describes the 2nd and 4th Metacarpals in the Domesticate horse as variable, but generally equal in length, and located between two-thirds to three-quarters along the length of MC3 [ 21 ]. These references to the length of splint bones still exist for the Domesticate horse in scientific literature with the further adage that they are vestigial [ 23 , 24 ]. In contrast to this, the Dutch Konik horses examined in this study showed large variation in length between the 2nd and 4th Metacarpals, with the 2nd always longer than the 4th.…”

A Novel Non-Invasive Selection Criterion for the Preservation of Primitive Dutch Konik Horses

“…It would appear that for the modern Domesticate horse, a reduction of the medial and lateral polydactyl digits to "Splint bones" together with the elongation of MC3 and MT3 for speed outweighed the demand for stabilisation or loadbearing [23]. This decreased the energetic cost of locomotion by lessening distal limb mass [23]. By comparison, it seems that increased stabilization and/or loadbearing was of greater evolutionary advantage to the ancestors of the Dutch Konik.…”