Fetlock Morphology in Equoids (Mammalia: Perissodactyla) and its Relationship to Mass, Diet and Extrinsic Evolutionary Drivers

The fetlock joint of equoids (Perissodactyla: Equoidea) is a highly specialized joint with multiple functions, including shock absorption, distal limb stability, and facilitating efficient elastic recoil during the swing phase of locomotion to enable sustained, rapid running. Here, we investigate how this joint has changed in shape (and potential function) from extinct tetradactyl equoids (equids + palaeotheres) to modern horses in a quantitative framework, comparing patterns of shape change with internal (mass / diet) and external (geological / climatic) ecological drivers. We used landmark based geometric morphometrics to quantify fetlock shape in Equoidea using landmarks applied to the distal metacarpal. 268 metacarpals were laser scanned and landmarked. A hypothetical ancestor was calculated from basal-most metacarpal shapes using a time-calibrated phylogeny. Divergence from the ancestral shape was calculated using ordinary sum of square distances (OSS) from geometric morphometric analysis, averaged per species (n=57). First time derivatives for body mass, hypsodonty and OSS were taken at 1 Ma intervals to study trait shifts. When examined through time, positive correlations between shape divergence and bodymass / hypsodonty occurred at < 1 Ma, with negative correlations at -5Ma. Small-bodied palaeotheres drive a divergence in fetlock shape in the late Eocene, with North American anchitheres also inducing a morphological shift away from the ancestral morphology in the late Oligocene. Increases in divergence were observed at the 'Grande Coupure' and Mid Miocene Climatic Optimum, with decreases at the Vallesian turnover and Quaternary Glaciation. Overall, we demonstrate that fetlock morphology in equoids is linked with fluctuations in body mass, diet regime, and with key turnover events. Localized morphological evolution also plays a key role, with morphological peaks and troughs correlating with origination and extinction of specialized endemics (e.g., Plagiolophus, Nannippus).