MAMMAL ORIGINS | No Powerful Steed:

Ireland's First Horse

Over the next few months, I will be taking a look at the origins of a number of Ireland's modern mammals and how they have changed compared to their earliest ancestors and relatives. And we can begin this exploration by taking a look at one of Ireland's most familiar and best-loved mammals – the horse.

Today, horses are synonymous with speed, size, and power, as reflected in many common phrases like 'as strong as a horse' or 'I'm so hungry I could eat a horse', while we measure the power of vehicles in terms of 'horsepower'. The modern horse, then, is not just a byword for power, it is the very unit by which the power of other things is measured. But the members of the horse family were not always so physically imposing and impressive. This is especially true of those earliest forms, the so-called 'dawn horses', which trod the floors of ancient forests, including possibly Ireland's, on an Earth where mammals had yet to establish themselves as the true rulers on land after the great fall of the non-avian dinosaurs.

To visit this ancient Earth we must wind the clock back around 55 million years to a time in the planet's history known as the Eocene epoch (c. 56–34 million years ago).

Ireland 55 million years ago was a much warmer land than it is today. At times, average global temperatures were almost twice what they are at present, and these sweltering conditions had the greatest effect on those lands in higher latitudes as, in the absence of any polar ice caps, there was far less of a temperature gradient from equator to pole. Ireland lay in the middle of a mighty belt of dense, subtropical forest which contained a collection of trees and plants more characteristic of Vietnam or Malaysia today, while crocodiles and other reptiles ruled the steamy swamps and marshes.

Just as these forests clothing Ireland's surface were far different to those which can be found scattered in isolated patches in Ireland today, so too would have been the animals that inhabited them. Admittedly, we do not have direct fossil evidence of Ireland's mammal fauna at this time, but fortunately the fossil record for Eocene Britain is very rich and since these two lands are thought to have been joined together during this epoch we can pretty safely assume that many of the forms inhabiting Britain lived in Ireland also.

So, among the rich collection of mammals zipping along branches or leaping from tree to tree in Ireland during the early Eocene would have been ancient primates which looked somewhat like the lemurs and tarsiers of our modern world, while opossum-like relatives of the marsupials also scurried about on the forest floor. Sharing the forest floor with them was a variety of primitive hoofed mammals, some of whose lines died out in the ancient world, while others can still claim relatives living in modern Ireland.

Indeed, the very name 'Eocene' means 'dawn of the new/recent', a title which refers to the fact that it is from the beginning of this epoch onwards that the earliest members of many of the modern mammal orders begin to appear in the fossil record. And among the pioneers of these modern lines likely present in the steamy forests of early Eocene Ireland was a little creature called Pliolophus – the first known horse.

Pliolophus differed in many striking ways from Ireland's modern Equus. Today, Ireland is world famous for its racehorses, which millions of years of evolution and recent generations of careful breeding have turned into biological marvels, capable of blistering speeds for such a large animal. On the track, the heavy drum of dozens of hooves pounding the turf is as raw and powerful a sound as that of a great force of nature; like thunder made flesh. But the sound of a group of Pliolophus running across the leaf litter on the floor of an ancient forest in early Eocene Ireland would have struggled to match the mild whisper of a soft breeze.

If a jockey jumped on the back of this early horse he would surely flatten it, as it was only about the same size as the red fox that lives in Ireland today. Also, although it was quite well adapted for running, Pliolophus had relatively short legs and a strongly curved back which were more suited for leaping and cantering in dense forest than for thundering across open ground.

Unlike its modern relatives, then, Pliolophus was adapted for living in closed forest environments, nimbly bouncing its way through the palm trees and other exotic plants, stopping every now and then to munch on some leaves or a piece of fruit or to hide in the shadows from one of the strange meat-eaters prowling these primitive forests for easy prey. However, there was one feature of Pliolophus which shows that it was already clearly moving in the direction of its modern kin.

Ireland's horses today move around on hooves, which are simply enlarged fingernails at the end of a single digit, the other four digits having been lost over the course of their evolution (although recently it has been found that remnants of all ancestral five digits persist in the foot of modern horses). Indeed, this is where the hoofed mammals, or ungulates, get their name, unguis meaning 'nail' in Latin. Like all other mammals, the earliest ancestors of the horses would have been shrew-like mammals with five digits, but while mammals like ourselves have retained all five digits, others have lost some of theirs, with cats, for instance, having only four digits on their hind feet while cattle and sheep have just two digits on each foot.

Pliolophus shows that, as early as 55 million years ago, the trend towards the loss of the digits in horses had already begun, as it possessed only four toes on its front feet and three at the back. What's more, it is also clear that it carried most of its weight on the third toe.

Thus, although it would take almost another 50 million years for the line leading to our large, modern, one-toed horses, Equus, to appear, the first steps on the road to these regal beasts were clearly taken by the tiny, unassuming Pliolophus. As rightly impressed as we might be with Ireland's modern horses, then, whose star quality blazes like the powerful rays of the midday sun, they owe all they are to their earliest and far less impressive progenitors like Pliolophus – those 'horses of the dawn'.