Sunday, 30 July 2017

Miocene (Pt 2): Before There Were Mice

After a brief cold snap at the very dawn of the epoch, the world of the Miocene warmed rapidly. Europe became, if not truly tropical, at least subtropical, with the interior covered by great forests of oak, laurel, and cinnamon, with magnolia and figs joining pine trees in the highlands. Along the coasts, the hot, damp, climate encouraged the growth of mangrove swamps and palm trees as warm sea currents flowed in from the Indian Ocean - still connected to the Mediterranean at this time. With no ice caps at the North Pole, and relatively few at the South, sea levels were much higher, and parts of continental Europe may, in those days, have still been islands. Certainly, Aqutaine in south-western France and the lower Rhône valley in the south-east were shallow bays stretching some way inland, as was what is now the Tagus valley in Spain and Portugal.

This rich and verdant landscape was home to a wide range of animals, many of them survivors of even earlier times. Many of these, such as the tapirs, didn't survive long in Europe, but a great many did, with musk deer, pigs, and rhinos dominating the herbivorous fauna, and animals less familiar to modern eyes taking the lead among the large carnivores.

But then, as now, the great majority of mammal species were small. While the sight of Diaceratherium rhinos wallowing in the lush swamps of the Swiss shoreline is the sort of thing that would draw the immediate attention of a time-travelling tourist, there was also plenty going on underfoot. Yet the two most common groups of small non-flying mammals that we have in Europe today - the mice and the voles - did not yet exist. So what was there?

Well, at the end of the previous epoch, four families of rodents had been dominant in Europe: hamsters, dormice, squirrels, and a group called the eomyids, gliding animals that looked rather like flying squirrels, but were, in fact, more closely related to gophers. While all of these survived through the dawn of the Miocene, the ones that seem to have prospered the most were the dormice, with numerous new species evolving, and many of them surviving for millions of years to come. The other three groups all took a hit, with many of the older forms dying out,and the hamsters, in particular, almost seeming to disappear for a while.

However, around 19 million years ago, a wave of new hamster species entered the continent from the east, so that suddenly, they rivalled the previous dominance of the dormice. Perhaps coincidentally, this happened around the time that Arabia first collided with Turkey, closing off the eastern Mediterranean from the warm waters of the Indian Ocean. That turned out to be only a temporary closure, and the straits would open again shortly after, only to re-close for good around 14 million years ago. That final change heralded a cooler, drier time in Europe, thinning out the forests, and forcing herbivores to rely on tougher, less verdant fare. The eomyids, as gliding animals, suffered significantly as a result, while ground squirrels came to outnumber their tree-dwelling kin, and the dormice also declined; the hamsters don't seem to have been as bothered.

One last burst of a moister climate around 12 million years ago gave a boost of recovery to the tree squirrels and eomyids, and also brought a number of new beaver species to the continent, some of them remarkably like the modern sort. Dormice also entered the trees, with some close relatives of the modern Japanese species going as far as to develop gliding membranes like those of flying squirrels.

But it wasn't just rodents that had a different mix this far back in time. At the dawn of the Miocene, the most common cloven-hoofed animals in Europe appear to have been relatives of modern musk deer, although exactly how closely related isn't absolutely clear. A lot may depend on where we choose to draw the line between the musk deer family as we know it today, and the earlier palaeomerycid family, which likely includes the common ancestors of musk deer and true deer, but which continued to live alongside them for millions of years. What we can say is that, to the untutored eye, such early animals as Oriomeryx and Dremotherium would have looked much like modern musk deer, being relatively small deer-like animals with large canine teeth, but no antlers.

Slightly less common, but still reasonable widespread, were the pigs. Most of the early forms, such as Hyotherium, were moderately sized animals with teeth adapted to feeding on an omnivorous diet with a high proportion of soft vegetation, but one example, Lorancahyus, had rather odd, tubular, teeth that suggest a diet of much harder food items, possibly including a lot of soil. Arriving in Europe slightly later, the listriodontine pigs, some of which seem to have had horn-like bumps on their foreheads, had unusually large and broad incisor teeth, suitable for clipping leaves in the style of some antelopes.

Europe at the dawn of the Miocene
Various other kinds of pig followed, often with differences that seem minor to a non-specialist, with the most significant, appearing around the end of the Middle Miocene, being Propotamochoerus. About 20% heavier than even the largest wild boar today, this may have been the first "modern" pig, closely related to all living species, and likely living a very similar lifestyle, wallowing in the mud along lake shores and snuffling in the soil for edible roots.

Other European arrivals very early in the Miocene include short-necked giraffes such as Teruelia and Lorancameryx, which were both primitive enough to still have canine teeth in their upper jaw. The latter had leg bones with what appear to be pathological thickening, as if from some strange bone disease; it has been suggested that, in their case, this be an unusual adaptation to having to carry extra body at certain times of the year. Tragulids, small and hornless antelope-like creatures that still live today in some tropical forests, were also found in Europe at this time. Both they and the giraffes seem to have originated in Asia - the oldest known giraffe, Progiraffa, lived in Miocene Pakistan - and used the newly formed Turkey/Arabia land bridge as a means to enter Africa. Later European "giraffes" retained the short necks and legs, but sometimes had unusual distinguishing features, such as the four (rather than two) horn-like protrusions seen on the head of Giraffokeryx.

This mixture of relatively primitive forms was to change around 19 million years ago with the arrival of the first European deer. As with musk deer, where we draw the dividing line between true deer and their ancestors is something that can be debated; some definitions put the first deer towards the end of the preceding, Oligocene epoch, and others early in the Miocene. Either way, they probably first appeared in Asia, and the oldest forms that have antlers certainly do date from the Miocene.

The first deer to enter Europe, such as Procervulus, were small animals, with the males still bearing the canine tusks that the majority of modern deer have lost in favour of their antlers. The antlers themselves were also primitive, consisting of a straight bony core permanently attached to the skull that formed about two-thirds of the length, with the remainder branching into two and only occasionally being shed, if at all. An evolutionary tendency towards shedding antlers, possibly in response to a more variable climate, has been noted in later species of the genus, and, in any event, the contemporary Acteocemas does have a primitive burr similar to that found at the base of modern, annually shed, antlers and so presumably shed and replaced at least the top third of the structure at least some of the time.

These early forms disappeared around the Middle Miocene, to be replaced by deer that more closely resemble the sorts we recognise today. Initially, the antlers of animals such as Heteroprox and Dicrocerus were still of the same general shape as those of the earlier species, although, in Dicrocerus at least, females had antlers, as well as males. While we know that these antlers could be, and were, shed and replaced, it is only with Euprox, first appearing around 16 million years ago that true, modern antlers, appear, and must surely have been shed on an annual basis. It appears to have lived in relatively damp and verdant forests.

At about the same time as the deer reached Europe, so did Eotragus, the earliest known antelope, which had originated not much earlier, somewhere around Pakistan. This was a very small antelope, by modern standards, with short, straight horns and teeth adapted for eating soft plants, and possibly the occasional insect. It survived for a long time, and spread from Thailand to Africa, but, by the Middle Miocene, it was joined by many other, rather more advanced forms.

Examples include Miotragocerus that lived in humid climates, and the unusually large and cow-like Austroportax, both of which are probably primitive bovines, perhaps with a particularly close relationship with the modern nilgai. Tethytragus and Gentrytragus had backward curving horns, and were about the size of a large gazelle; they seem to have lived in open, or even hilly, terrain and could probably eat a wide range of plant types. It is possible, but by no means certain, that they may be related to the distant ancestors of, among other things, goats and sheep.

Other antelopes living in or near Europe at the time include the grazing, twisted-horned Hypsodontus, which also lived in both China and Africa, and gazelles that are virtually indistinguishable from the modern species.

And all of this profusion takes us merely to the end of the Middle Miocene, as Europe is poised to enter a long period of cooler, drier, climates. But, before we turn to see what happened to the deer, antelopes, and all the rest in those rather more testing times, we should take a look back at those rhinos on the coast of Switzerland; they didn't last long, as it happens, but many of their relatives did...

[Photo by "Ghedoghedo", from Wikimedia Commons. Map adapted from Akhmetiev et al. 2005.]


  1. Gliding has been evolved numerous times in several mammalian clades, and often there are closely related and similar gliding and non-gliding species. Do we know that Eomyids were gliding, or only that some Eomyids glided?

    1. We have actual soft tissue impressions from eomyid fossils in which the patagium is visible, so there's no real doubt that they could glide - one of four times gliding evolved in rodents alone. But, as to your main point, the great majority of eomyid species are known only from their teeth, and they were around for millions and millions of years... so we don't know, but, personally, I'd be surprised if it was quite as universal as I may have implied in the post.