Sunday 14 May 2023

Oligocene (Pt 2): Europe's Big Break

The dawn of the Oligocene is marked by a sudden cooling of the Earth's climate, of which the most obvious consequence was the creation of the Antarctic ice sheets. These locked up so much water that sea levels dropped worldwide, reshaping coastlines. Nowhere were the consequences of this more apparent than Europe, despite its great distance from Antarctica.

Prior to the Oligocene, it would have been possible for a hypothetical traveller to sail from what is now the eastern Mediterranean, through the Paratethys Sea (now the Black and Caspian Seas) due north and into the Arctic Ocean. The body of water that made this possible, the Turgai Strait, was already becoming shallower and narrower as the Oligocene approached, and the sudden dip in sea level finished it off altogether, closing off the sea route that had once run along the eastern flank of the Ural Mountains. 

In other words, it became possible, perhaps for the first time since the Jurassic, to walk from Asia into Europe; the continent of Eurasia, as we know it today, had been born. The result was a flood of Asian animals into Europe, wiping out many of the indigenous, apparently less adaptable, animals. 

In the 19th century, the existence of the Oligocene had been defined on the basis of a change in the fossil sea shells at the beginning of the epoch. In 1910, Swiss palaeontologist Hans Georg Stehlin identified a remarkably sudden change in the mammalian fauna of the Paris Basin at around the same time. He named it the Grande Coupure, and the name has stuck, even in English. 

It was a continent-wide event, and recent evidence from Spain and England narrow its timing to very close to the start of the Oligocene, just as sea levels were dropping and the Turgai Strait closed. This is, however, unlikely to be the only cause; at the very least there would have been a "double whammy" as the closure of the Strait was itself caused by a sudden drop in worldwide temperature. That similar, if less dramatic, changes occurred at the same time in Asia and elsewhere across the world supports this idea, and a finger has also been pointed at a couple of major asteroid impacts happening at around the same time - one in Siberia, and the other at the mouth of Chesapeake Bay.

Indeed, while the Grande Coupure proper was a European phenomenon, it was simply the most dramatic element of a global extinction event - the single largest such event to have occurred between the KPg impact that wiped out the non-avian dinosaurs and the arrival of the human species.

In Europe, it has been estimated that as many as 60% of the mammalian groups that existed before this "big break" survived into the Oligocene. Primates took an especially big hit, vanishing from the continent altogether, and not returning until the Miocene. The colder climate likely plays a role here, as dense forest was replaced by more open woodland or scrub across much of the continent. But many other animals were also affected, with ancient groups being replaced, in some cases by those we would be more familiar with today.

Early Oligocene Europe
(click to enlarge)
This is by no means restricted to the big dramatic animals that a time traveller would likely notice straight away. For example, the comparatively primitive shrew Quercysorex is one such new arrival, being widespread across Europe and surviving into the Miocene; Belgicasorex had sharper teeth but seems to have had a more limited range. Tetracus, originally discovered in France, but now also known from other parts of western Europe, was a small member of the hedgehog family, but perhaps not literally a hedgehog, likely being more related to today's moonrats. 

Shrews and hedgehogs replaced earlier groups of insect-eating mammal that no longer exist today, but others did survive, and even prosper, through the transition. The earliest-known mole, Eotalpa, had been in Europe for some time, and seems to have survived into the Oligocene without much obvious change. It was probably already capable of digging, if far less effectively than modern species; perhaps it sheltered in burrows but did not spend its entire life there. Amphiperatherium, which was either a very primitive opossum-like marsupial, or a close relative of the first true marsupials, seems to have been entirely unconcerned by the Grande Coupure, and eventually struggled on into the Miocene, becoming the last non-placental mammal native to Europe.

While we're on the subject of small mammals that eat insects, it's also worth mentioning Palaeonycteris, a fossil horseshoe bat and the earliest-known member of its family. A rare example of an early fossil bat, it seems to have been unique to Europe, with no other related fossils known until the Miocene.

The Grande Coupure saw a significant change in the types of rodents that lived in Europe. Many of those that vanished were, like the disappearing primates, tree-dwelling leaf-eating animals presumably hampered by the replacement of dense forests with more open woodland. No fewer than five different rodent families came in from Asia to fill the gap, making their European debut in the aftermath of the Coupure.

Perhaps the most significant of these were the squirrels, in the form of Paleosciurus. Known from Spain almost immediately after the Coupure, they are the oldest known fossils that can be definitively described as "squirrels". Since it seems unlikely that the family would coincidentally appear at this exact point, especially given how quickly thereafter they managed to reach North America, they probably first appeared in Asia and crossed the former Turgai Strait when it closed - but they can't have been living in Asia for long, and we have no fossils to confirm the theory. Judging from their limb proportions, they spent most of their time on the ground, but they could probably climb trees if they had to.

A similar story applies to beavers, with the first beaver in Europe, Propaleocastor, also appearing in the Oligocene and surviving well into the following, Miocene epoch. It was about half the size of living beavers, but already had the sorts of adaptations we would expect to see in a river-dwelling animal. In its case, the earliest fossils of the genus come from China, illustrating a likely Asian origin, although the beaver family itself is older still.

Sciurodon and Plesispermophilus also headed across from Asia, although, in their case, their ancestry may lie even further away, in North America. They belonged to a group called the aplodontids, of which only one species survives today, the so-called "mountain beaver" or sewellel of the western US, but which was once much more diverse. Although not the oldest known members of their family, they are among the most primitive, and, judging from their teeth, mainly fed on soft leaves.

Not all of the new rodent groups arriving in Europe are still alive, however. Eomys represents a family of unusually small tree-dwelling rodents that survived and prospered for millions of years before finally dying out during the Ice Ages. They too, seem to arrived as soon as the land bridge with Asia formed, rapidly reaching as far as southern England. Significantly, one fossil was preserved so well that we could see the outline of the animal's soft tissue, strongly suggesting that it had gliding membranes like those of a flying squirrel - long before the modern sort evolved.

By far the most successful of the new rodent families in Europe, however, were the cricetids, or hamster family. Pseudocricetodon and Atavocricetodon were the first to arrive, but they soon gave rise to many new species, grouped into at least eight different genera. These early examples probably didn't look much like modern hamsters, being smaller and more generalised animals, but they certainly seem to have been numerous, and they or their close relatives are likely the ancestors of the voles - which today are far more successful than the hamsters proper.

Not all of the older groups of rodents died out as these new arrivals muscled their way in. Dormice had been in Europe for some time, and not only survived the upheaval in the form of Gliravus, but diversified to create some new species and genera that lasted throughout the Oligocene. An ancient group known as the theridomorphs were less lucky, but, unlike other "primitive" groups, a few of their species did make it through the break. One of them, Pseudoltinomys, is known from a skeleton complete enough for us to know that it looked like a rat-sized gerbil, hopping along on its hind legs, probably in semiarid woodlands.

If there had been rodents in Europe during the preceding epoch, the same was not true of the lagomorphs. They were already entrenched in Asia, however, and joined their gnawing kin in heading over to the newly accessible continent. Ephemerolagus is the first known European lagomorph, being first identified from a fossil site in France, but it was soon joined by others such as the widespread Asian genus Shamolagus. While they are sometimes described as early rabbits, their exact placement remains uncertain and they may either be related to the short-eared pikas or represent an early branch with no living descendants.

Of course, all of these small animals were accompanied by much larger ones, including some surprisingly fearsome-looking herbivores. Which is where I will turn next...

[Drawing by Nobu Tamura, from Wikimedia Commons. Map adapted from Popov et al. 2002.]

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