The Evolution of a Single Toe in Horses: Causes, Consequences, and the Way Forward

Abstract

Horses are a classic example of macroevolution in three major traits—large body size, tall-crowned teeth (hypsodonty), and a single toe (monodactyly)—but how and why monodactyly evolved is still poorly understood. Existing hypotheses usually connect digit reduction in horses to the spread and eventual dominance of open-habitat grasslands, which took over from forests during the Cenozoic; digit reduction has been argued to be an adaptation for speed, locomotor economy, stability, and/or increased body size. In this review, we assess the evidence for these (not necessarily mutually exclusive) hypotheses from a variety of related fields, including paleoecology, phylogenetic comparative methods, and biomechanics. Convergent evolution of digit reduction, including in litopterns and artiodactyls, is also considered. We find it unlikely that a single evolutionary driver was responsible for the evolution of monodactyly, because changes in body size, foot posture, habitat, and substrate are frequently found to influence one another (and to connect to broader potential drivers, such as changing climate). We conclude with suggestions for future research to help untangle the complex dynamics of this remarkable morphological change in extinct horses. A path forward should combine regional paleoecology studies, quantitative biomechanical work, and make use of convergence and modern analogs to estimate the relative contributions of potential evolutionary drivers for digit reduction.

Mammalian forelimb evolution is driven by uneven proximal-to-distal morphological diversity

“…Digitigrady is observed in many carnivorans providing limb elongation and thus increasing stride length ( Polly, 2007 ; Wang, 1993 ). Extant horses exhibit one of the most dramatic modifications of the third metapodial and phalanges among all unguligrade taxa: the limb is uniquely supported by the third toe, which is considerably enlarged and elongated, whilst the lateral fingers are markedly reduced ( McHorse et al, 2019 ). One recent study suggested that the evolutionary transitions in foot and hand postures are associated with strong selection for rapid changes in increasing body size ( Kubo et al, 2019 ).…”

A first glimpse at the influence of body mass in the morphological integration of the limb long bones: an investigation in modern rhinoceroses

“…In particular, the authors observed a frequent co-occurrence of discrete traits between the radius-ulna and the femur among all rhinos, which seems to be consistent with our results indicating a strong covariation between the forelimb zeugopodium and the hind limb stylopodium. As the postcranial body plan appears to have been implemented early in Rhinocerotoidea evolutionary history (Lord et al, 2019) and may be less variable than in phylogenetically close taxa such as equids (McHorse et al, 2019), this may imply strong inherited developmental constraints within this group canalizing the shape covariation (Hallgrímsson et al, 2002) even between non-homologous bones.…”

Biogeography a key influence on distal forelimb variation in horses through the Cenozoic

“…The transition of the equid forelimb from a tetradactyl (four-toed) condition to the modern monodactyl (one-toed) horse (Equus spp.) is a well-known example of morphological macroevolution [6]. The sister family to equids within Equoidea (the palaeotheres; Palaeotheriidae) also underwent digit reduction from tetradactyly to tridactyly (three-toes) during the Eocene and early Oligocene [7].…”

“…Hypsodont dentition counteracted the higher percentage of grit and phytoliths in equid diets as they exploited more open (grassland) habitats [12]. Increases in hypsodonty were concurrent with increases in body size for many equid clades [10,13], with increasing reliance on the central digit for locomotion as the side digits became more reduced [6]. By contrast, palaeotheres evolved large sizes in the Eocene (e.g.…”

“…In this study, I investigate the morphology of the central forelimb metapodial joint surface, the metacarpophalangeal joint (MCP joint), across Equoidea. The forelimb was chosen as, unlike the hind limb, it undergoes full digit reduction from four functional digits to one during equid evolution [6]. Using a geometric morphometric approach to quantify MCP joint shape, I combine traditional and novel alignment protocols to assess the magnitude of divergence in each phenotype from an ancestral morphology, and examine shape changes with respect to intrinsic biological traits (mass/tooth crown height) and extrinsic environmental drivers.…”