May 31, 2015

MAMMAL ORIGINS | As Small as a Housecat:

The Earliest Relative of Ireland's Sheep and Cattle

Two Diacodexis startled by the approach of the early (and still terrestrial) whale Pakicetus

Last month we took a look at the first ancestor of Ireland's horses, so it seems only fair this month to investigate the origins of those fellow ungulates they so often share the fields of Ireland with – sheep and cattle. Like horses, cattle especially can impress with their physicality, and while not quite as fast as their speedy equine neighbours, on average they outstrip them in weight. Large bulls can be particularly formidable in form, selective breeding by humans having enhanced their muscularity to turn them into biological tanks. As with the horses, then, keeping these modern hulks in mind, it is all the more striking when we hurtle back through the past to meet the earliest members of their order, who were a world away from them both in time and appearance.

Of course, before we go back to meet the earliest members of the order Ireland's cattle and sheep belong to, we must first learn something about the order itself, which will also allow us to reveal the true evolutionary relationship between these farm animals and their fellow domesticate, the horse.

Despite their similarity in appearance, especially between the cow and the horse, the truth is that these mammals are not particularly closely related. In fact, horses and cattle belong to two separate orders: the horses to the Perissodactyla, or 'odd-toed' ungulates, and the cattle to the Artiodactyla, or 'even-toed' ungulates.

Although the horses (which include donkeys, mules, etc.) are the only representatives of the odd-toed ungulates in Ireland (not counting zoo animals), the even-toed ungulates, the artiodactyls, are far better represented and include not only the cattle and sheep but other domesticated mammals like goats and pigs, and also Ireland's wild natives such as the fallow deer and red deer as well as their more recently introduced kin, the sika deer, muntjac deer, and roe deer.

The red deer is just one among many members of the order Artiodactyla in Ireland

Indeed, the earliest member of the even-toed ungulates to appear on Earth looked somewhat like a deer, albeit a very small one with a long tail and a strongly curved back. This was Diacodexis, whose presence in Britain's fossil record from around 55 million years ago strongly indicates that it populated the hot and humid forests of early Eocene Ireland also.

Diacodexis was even more diminutive than its perissodactyl contemporary, the early horse Pliolophus, being no bigger than an average Irish housecat. However, just like Pliolophus, Diacodexis was already displaying features in its foot which are characteristic of its modern relatives.

As their names suggest, the two great modern ungulate orders derive their names from the number of toes present in their members, the odd-toed ungulates having either one or three toes, while the even-toed ungulates have either two or four. Although Diacodexis still retained five toes on its front feet, its hind feet already sported just four and most of its weight was borne by the third and fourth digits. Clearly, then, by 55 million years ago, the even-toed ungulates were already evolving towards their modern form, just as their odd-toed neighbours were.

To return to their modern representatives in Ireland, the fact that the horses and cattle belong to two separate orders which diverged over 55 million years ago reveals a great deal about their true relation. Although they may look quite similar in terms of form, the fact that their ancestors separated from each other so far in the past, when they were still most likely small, shrew-like creatures, means that their current resemblance is not due to their sharing a recent common ancestor but to what is known as convergent evolution.

Mammal evolutionary tree from Graphodatsky, et al. (2011) based on genetic evidence, which clearly depicts the ancient separation between the orders Perissodactyla and Artiodactyla (Cetartiodactyla)

Convergent evolution is where the members of two separate groups of animals evolve similar forms or features in response to similar environmental conditions rather than having inherited them from a recent common ancestor. In this case, the ancestors of both the horses and the cattle evolved longer legs and larger body sizes partly as adaptations to the development of more open landscapes, as well as the spread of grasslands in the last 20 million years or so.

The simple fact that Ireland's horses and cattle belong to two separate orders also reveals that they are related, respectively, to far more exotic mammals than they are to each other.

Irish horses, then, are more closely related to those imposing, armoured goliaths the rhinos and the stocky, betrunked tapirs than they are to cattle and sheep, as these mammals also belong to the order of odd-toed ungulates and so share a more recent common ancestor. In their turn, the cattle and sheep are more closely related to the long-necked camels and giraffes, as well as the barrel-shaped, mighty-tusked hippos, which are also even-toed ungulates.

Ireland's sheep and cattle are more closely related to the long-necked giraffes of Africa than they are to the more similar-looking horses they share Irish fields with

This may seem quite bizarre, but many of these exotic mammals actually have their origins in northern lands and their modern geographic distribution is only a faint echo of a once much more widespread range.

The rhinos, for example, like the horses, originated in Eurasia and remained an important part of the fauna across the vastness of this landmass right up until the end of the last Ice Age when the woolly rhino suffered extinction. Eurasian rhinos are now found only in southern Asia. As for African rhinos, these are descendants of immigrants from the north which crossed into this southern continent when tectonic events produced a land connection around 20 million years ago. Modern rhinos, therefore, are now found quite far from what were their original homelands.

Finally, returning to Ireland, the fact that horses and cattle belong to different orders also presents some fascinating possibilities as regards their true relation to other common mammals in Ireland. Although many genetic studies conclude that, as orders, the odd-toed ungulates and even-toed ungulates are closely related, others have concluded that either of them might be more closely related to another order, the Carnivora, which includes the dogs, cats, bears, etc. This would mean that the dog at work in Ireland's fields may be more closely related to the horses or cattle it is rounding up than either of them is to each other!

Some genetic studies conclude that dogs may be more closely related to either cattle or horses than either of these similar-looking mammals is to each other

This, again, may seem jarring at first, but we must always keep in mind that as all of these orders diverged from one another well over 55 million years ago, when their members were all still small, shrew-like creatures, and they subsequently evolved in different, specialised directions, their current similarities or dissimilarities in form are really no indicator of their true relation.

It seems clear, then, that there are wonders lying in plain sight all over the island of Ireland. Sights which many of us will have beheld a thousand times and more we now see for the first time, our eyes truly open. The familiar and seemingly humdrum picture of cattle, sheep, horses, and dogs in an Irish field suddenly explodes in colour, our new understanding exposing its underlying richness and imbuing it with new meaning as we see that it is part of a vibrant evolutionary story whose characters occupy a global stage.