Looking back through time at the evolution of horse feet

From the unicorn and Pegasus of hallowed antiquity, to Ponyta, Epona and My Little Pony of modern popular culture, horses have long been a source of fascination and intrigue for mankind. In reality, the story of horse (Equidae) evolution rivals the imaginations of any wise scholar or creative video-game designer. It is a story over 56 million years in the making: from humble, diminutive beginnings in the Eocene forests of North America and Eurasia, through a renaissance in the Miocene (c. 25 to 5 million years ago), and finally a steady decline in the bitter chill of the Pleistocene. Throughout their evolution, horses have undergone a host of morphological changes, such as evolving high-crowned teeth, bulbous noses, very large and very small body size, and numerous changes to their locomotor apparatus (e.g. their limbs). It was the variation in morphology and function of their limbs which formed the focus of my doctoral project, which was ironically primarily performed on tapirs – a group of peculiar, forest-dwelling relatives of the horses still inhabiting the rainforests of Central and South America and South-East Asia.

The morphological variation in tapir forelimbs

Throughout my FWO-funded PhD, I aimed to quantify tapir limb morphology and utilise my understanding of tapir bone shape to establish new views on the locomotor apparatus of horses. In particular, I wanted to look at the forelimb transition in horses from four-toed ancestors to one-toed modern horses. Yes, modern horses, zebras and donkeys only have one toe on each of their limbs! Tapirs themselves have four toes on their forelimbs, like the ancestors of modern horses; thus, morphologically speaking, they were an ideal model system to investigate. My doctoral project cast light on the morphological variation in tapir forelimbs, which to date includes several cross-family comparisons; one looking at forelimb muscles and one testing previously presumed locomotor analogy. The comparative aspect of my investigation culminated in a recent published study which delved into the variation and adaptability of the distal limb of equids, specifically the forelimb fetlock joint.

My project has been made possible by the use of advanced 3D laser surface scanning hardware. Along with my supervisors Sandra Nauwelaerts and Peter Aerts at the FunMorph Lab of the Universiteit Antwerpen, I was able to capture the surface geometry of multiple limb bones from collections worldwide with comparative ease and rapidity, and then quantify the 3D shape by applying a series of discrete, homologous points (‘landmarks’) to the bone scans. The configurations of ‘landmarks’ for different scans and species were then compared to explore patterns of shape variation, in addition to comparing shape with other biologically informative information for the species, such as body size, dietary proxies and inter-species relationships (phylogenetics).

The forelimb fetlock

My ultimate aim was to use tapirs to understand changes in horse limbs, and my investigation into the forelimb fetlock joint of horses enabled me to do this. Yes, the tapirs ended up being removed from the final version – which made the article much clearer – but the origin of the study was rooted in my understanding of tapir morphology. Looking at the four-toed forefoot of tapirs, the fetlock joint (analogous to our knuckle) does not allow much rotation; this is partly due to the presence of a fatty foot-pad on the bottom of tapir feet, but is also attributable to the shape of the joint surfaces between the hand bones (metacarpals) and the finger bones (phalanges). When I used the landmark analysis to investigate the shape of the fetlock joint surface of tapirs and horses, I found that extinct horses from the Eocene (c. 40 million years ago) showed very similar fetlock morphologies to tapirs. Not only that, but the evolution from four-toes to one-toe in horses could be traced using only the shape of the fetlock.

Moreover, differences observed between horse species living only a few million years ago and modern species were as great as the differences between modern horses and tapirs! My demonstration that the fetlock shape is highly variable across the different horse species also flagged up some very specific morphologies, which suggest that horses evolving in South America (which are now extinct) may have adopted a very specific style of walking to move in an energy-efficient manner across the high slopes of the Andes Mountains. You can find out more on the morphology of the equine fetlock here.

The work I was able to do as part of my PhD would not have been possible without my FWO PhD Fellowship, for which I am continually very grateful. Given the opportunity I greatly enjoyed my experience working on tapirs and horses, and I will endeavour to continue pushing the boundaries of our understanding of functional morphology in these two beautifully bizarre groups of mammals.