Eocene (Pt 6): Gophers, Camels, and Lady Gaga

Leptomeryx

World climate fluctuated during the Eocene epoch, but it was consistently warmer than it is today, including the lengthy Climatic Optimum, the hottest the world has been since the extinction of the non-avian dinosaurs. This naturally affected not just the weather, but where different types of vegetation were found. Much of North America was covered with subtropical forests, with at least some palm trees reaching as far north as Alaska, and temperate woodlands on the northern Canadian islands.

The animals you would have encountered in these woods would also have been unfamiliar, with most of the mammals not fitting within any family we would recognise today. Nonetheless, there were some exceptions.

Rodents were already in existence at the beginning of the Eocene, but became more varied as the epoch wore on. The first beavers appeared just before it ended, in the form of Agnotocastor from Canada. We don't know too much about this early form, other than it had yet to lose the third premolar tooth, which no living beaver possesses. However, the aquatic and tree-gnawing habits we associate with beavers today did not arise until much later, and we can imagine that it probably looked more like a muskrat than anything else alive today.

At the opposite end of the continent, the epoch also saw the appearance of the first gophers, in the form of Gregorymys from southern Mexico. While this is merely placed within the gopher family, not the subfamily to which all the living forms belong, some of its remains were discovered near fossilised burrows, suggesting that this habit, at least, was ancient.

Similarly, Cedromus, from Wyoming, may be one of the earliest known squirrels. The skull of one fossil was complete enough to allow a scan of its interior, revealing the size and shape of the brain. This showed the brain was, relative to body size, near the lower end for modern squirrel species, but not below it, while regions thought to be linked to visual orientation and agility were enlarged. Most likely, therefore, it was at least partially arboreal, like many of its modern relatives.

Other families present in Eocene North America failed to make it to the present day, or even out of the epoch. The sciuravids prospered during some of the hottest phases of the Eocene, disappearing as the climate settled down again in the Late part of the epoch. They seem to have been fairly diverse, with some species having been described as having a similar lifestyle to modern mice (a group that did not appear until much later), and others as being more arboreal, although there is debate as to how many species should really be included within the family, rather than placed elsewhere.

The name "sciuravid" suggests some relationship to squirrels, but this is misleading, since they appear, at least partly on the basis of their jaw muscles, to be closer to mice. On the other hand, what was probably the most successful group of Eocene rodents in North America, the ischyromyids, probably was over on the squirrel side of the great rodent family tree.

Ischyromyids are also known from other continents, are one of the few rodent groups to have originated before the start of the Eocene, and survived until about halfway through the following, Oligocene, epoch. They are, however, particularly well-known from North America, with a variety of different forms, united by typically having a long, but heavily built, snout, and a lower jaw that has the same general pattern as squirrels and their relatives.

They were, however, quite diverse. Ischyromys itself, for example, along with its relative Lophiparamys, is thought to have been arboreal, clambering about in trees and perhaps feeding on their leaves as well as more obvious food such as nuts. Paramys, on the other hand, although it may have been able to climb if it had to, was more likely to have been a scratch-digger, hiding in burrows. One estimate places the largest species of Paramys at up to 4 kg (8 lb 13 oz.) in body weight, which would also tend to count against tree-dwelling without more obvious adaptations, such as those in primates.

The cylindrodontids were another group of primitive rodents commonly found in North America, although it's possible they originally appeared in Asia. Relatively short-snouted, they ranged in size from mice to ground squirrels, and have been found in areas that were once subtropical forests, although, with few remains other than skulls, their exact lifestyle remains unclear. Examples include Tuscahomys from Wyoming and Mississippi.

There were, of course, many larger animals living alongside the rodents, and some of them also represent the start of lineages with which we are familiar today. Perchoerus is better known from the following, Oligocene epoch, but the oldest species lived in what is now the northwestern US during the last few million years of the Eocene. Long thought to be a peccary, it is now regarded as a close relative of the last common ancestor of the peccaries and pigs, placing it near the very base of the suine family tree. It was, however, far smaller than any modern example of its kind, similar to a modern house cat, and probably would not have looked much like a pig in life.

Given their present-day distribution, it is perhaps surprising that camels first evolved in North America. It was only much later that they spread through Asia to North Africa, and separately to South America, where they gave rise to the llamas. 

Quite what the oldest known fossil camel is is complicated by the common issue in palaeontology of deciding where to draw the line between "camel" and "not-quite-yet-a-camel". Taking a broad view, then Protylopus is the likely best bet. This lived during the Early Eocene, indicating an unusually early branch within the cloven-footed mammals in general. It would, however, not have looked much like a modern camel (or llama). For one thing, it lived in damp subtropical forests eating soft leaves, so it was hardly desert-adapted. Furthermore, it had four toes on each foot, probably with hooves rather than foot pads. It also only stood about 30 cm (12 inches) tall at the shoulder...

Animals that are less ambiguously camels came along not much later, during the Middle Eocene. Poebrodon is one of the earliest examples, living from Wyoming to southern California during the Middle Eocene. While still not desert-dwelling, the group had begun to diversify by the end of the epoch, with the Late Eocene Stevenscamelus from Texas being the earliest known "narrow-snouted camel", a group of browsers on leafy bushes that survived in North America until well into the Miocene.

Other cloven-footed animals of the time belonged to no family that we would recognise today. Bathygenys, which lived from Texas to Montana in the Late Eocene, was an oreodont, with four hooves on each foot. This was a group that would become much more successful in the following two epochs, reaching relatively large size and adapting to an increasingly dry and prairie-like habitat as the world's climate changed.

Early entelodont "hell-pigs" crossed over from Asia in the Late Eocene, probably in the form of Brachyhyops, which lived from Texas to Saskatchewan, but is also known from older deposits in Mongolia. Anthracotheres, which somewhat resembled hippos, also originated in Asia before crossing over the Bering land bridge to North America. The earliest-known example on the continent is Heptacodon, whose oldest definitive fossils hail from Texas, but which also reached as far north as Oregon, and possibly Canada. Compared with others of its kind, it had an unusually short, broad snout and large canine teeth with edges that flared out to the sides.

The protoceratids of later epochs are best known for the unusual property of having three horns, at least in the males; two in the usual place and a third one near the tip of the snout. Eocene forms, however, such as Leptoreodon and Pseudoprotoceras, were hornless, although they would otherwise have resembled small deer. The family was unique to North America, and may be most closely related to either the camels or the true ruminants, although they do not fall directly into either group.

The hypertragulids may have actually been true ruminants, although they are sufficiently primitive that it's difficult to tell. It's perhaps more likely that they are outside the living groups, although likely more related to ruminants than to camels. The group is currently considered to be unique to North America, although close relatives also lived in Asia, where they likely first originated. 

An unusual feature of the hypertragulids is that the front feet still had five toes, although one was already vestigial; the hind feet had the usual pattern we see in cloven-footed mammals today. They were also among the first cloven-footed animals to develop the "selenodont" pattern of having crescent-shaped cutting surfaces on their molar teeth, improving their ability to chew vegetation, which may have been a factor in the subsequent expansion of the larger group. Examples include Hypertragulus itself, which is found from Oregon to Mexico, and Hypisodus from Montana and Colorado. Both were small animals, resembling today's muntjacs, and likely had a similar browsing diet.

Leptomeryx was similar, although it lived slightly later in the epoch than the earliest hypertragulids. Estimated to have weighed around 3 kg (6 lb 20 oz.) it, too, had four functional toes on the front feet and two on the hind ones. A significant difference is that, whereas the hypertragulids had enlarged canine teeth, similar to those of modern musk deer, they were reduced or absent altogether in Leptomeryx and its relatives. They probably lived in open woodlands or on the edge of denser forests.

Gagadon, which was only the size of a rabbit, is among the most primitive of all cloven-footed mammals in North America. It lived in Wyoming during the Early Eocene, and belongs to an obscure family that may or may not be related to the distant ancestors of camels, but still retain the full set of mammalian teeth, including the upper incisors that are replaced by a toothless chopping board in almost everything else. They were likely fast-running animals, hiding in forest undergrowth and feeding on soft leaves. 

It is, however, perhaps more famous for its full scientific name, Gagadon minimonstrum, which translates, very roughly, as "Lady Gaga's little monster". Because modern palaeontologists are getting young enough not to name everything after The Beatles or Motorhead...

[Photo by "ghedoghedo" from Wikimedia Commons.]