Sunday 19 February 2023

Tree-Dwelling Almost-Lemurs of the Canadian Arctic

Northern Canada, it has sometimes been noted, can get a bit cold. The northern coasts, and the islands beyond them, are covered in treeless tundra, the permanently frozen subsoil preventing anything that needs deep roots from growing there. The interiors of the more northerly islands don't even have that much, just permanent fields of ice and snow. 

The most northerly island in Canada is Ellesmere Island, whose most northerly point is not far from being the most northerly piece of solid land on the planet, only beaten by parts of Greenland. Midsummer temperatures reach a daily high of about 9°C (49°F) in midsummer, and it often snows in July. Winter temperatures regularly drop below -35°C (-31°F) on January nights. So, yeah, that's uncomfortable.

As one might expect for somewhere so desolate, there is not a wide variety of animals living there. Those that do, such as caribou, muskox, wolves, and Arctic hares are specialists in frozen environments and many travel across the sea, such as polar bears and seals. There are also, as one might expect seabirds, and perhaps more surprisingly, insects, including a couple of species of cold-adapted bumblebee.

So, not entirely barren, but hardly what you'd find in more hospitable climes. Yet we have, for over a hundred years, been digging up fossil sites on the island, and on Axel Heiberg Island just to the west, that show that it was rather different in the distant past. There were large carnivores, such as Pachyaena, and weasel-sized ones such as Miacis, and herbivores such as the miniature brontothere Eotitanops and the "Arctic tapir" Thuliadanta. These animals lived alongside several non-mammalian forms, including not only the birds and fish one might expect, but lizards, constrictor snakes, and, of all things, an alligator, Allognathosuchus

You might think that this must have been because Ellesmere Island was further south in the past than it is now... but no, the fossils are old, but they're not that old. The fossil strata in question date from the Eocene, between around 53 and 38 million years ago, when the northern continents were at least approximately in their present positions even if, for example, the North Atlantic was much narrower than it is now.

No, the difference wasn't the position of the island, but the state of the world climate. The Eocene was a much warmer time than today, and the older deposits date from just after the Paleocene-Eocene Thermal Maximum - the hottest the world has ever been since the time of the dinosaurs. Plant fossils show flooded broadleaf forest described as resembling the swamp-cypress of the southeastern US, and imply a climate not far off that of, say, North Carolina.

On the other hand, given that it was already north of the Arctic Circle, Ellesmere Island would have had "midnight sun" in the summer, and 24-hour darkness in the winter. Today, the sun doesn't rise between November and February, and, while the exact latitude was likely different in the Eocene, there was still some similarity. This would have created an environment we simply don't see on Earth today, and it's worth noting that there are some types of animal common elsewhere in North America at the time that don't seem to have been able to put up with that. There are, for instance, no cloven-footed or early horse-like animals, despite the fact that they would clearly have been able to reach the region. Nor are there any primates, a group of animals that (humans aside) pretty much are entirely tropical.

Except... well, it rather depends on what you mean by "primate".

A few years ago, some fossils were unearthed from a site near Bay Fiord on the central western coast of Ellesmere Island that looked distinctly primate-like. Last month, they received their formal description, being named as belonging to two new species of the previously known genus Ignacius. The genus was first named in 1921 for two fossils, consisting of a tooth and part of a jaw, that had been uncovered in Colorado. In 1940, it was placed in a newly created family of fossil animals, along with a few apparently close relatives. What's less clear is whether or not that family, the paromomyids, should count as primates or not.

What we can be reasonably confident of is that Ignacius and the other paromomyids are not descended from the last common ancestor of living primates. So, if we take a narrow definition of what a primate is, they wouldn't count and they're equally related to, say, lemurs and monkeys. On the other hand, they are at least in the right general part of the family tree, consistently placed in a wider group of animals called the "plesiadapiforms". Quite what those are is the real question. 

They are usually regarded as closer to primates than to anything else alive today, but whether they're close enough to be included within the primates, or should instead be considered as an informal collection of close relatives is more debatable. An earlier theory had them even less closely related to living primates, but no longer seems to be popular. Wherever they fit, from what we have of their skeletons, they were very obviously tree-dwelling and would have looked something like modern lemurs. In the case of these new specimens, this would make them the only known arboreal mammals from Ellesmere Island.

The fossils in question are estimated to be 50 million years old, putting them in the early Eocene, the warmest part of the epoch. Significantly, this is millions of years later than any other Ignacius fossil, meaning that these may have been among the last species of their kind. (Or the fossil record is incomplete which, given how far back we're talking, is entirely possible).

They consist only of a few teeth and bits of jaw so we can infer their arboreal habits only from the fact that we have more complete specimens of other species in the genus. However, even this is enough to show that they are at least half again the size of the equivalent parts of the previous largest known Ignacius species. That was about the size of a grey squirrel and itself quite a bit larger than the oldest fossils of the genus. While it's hard to tell from such small samples, the authors of the new species estimate them to have weighed between about 1.1 and 2.0 kg (2½ to 4½ lbs), similar to some modern lemurs.

While all Ignacius species have teeth that seem adapted to eating relatively hard food, rather than insect grubs or soft fruit, the new ones from the Arctic are even more so. This may be connected with their ability to survive in the region when true primates could not. For instance, it's possible that they ate plenty of nuts and seeds, which are not normally a major primate food source, but which could have been helpful during the prolonged darkness of the Arctic winter when little else would have been available.

Paromomyids as a group are also known from Europe, but all the other species of Ignacius are North American, with the apparent closest relative of the two Arctic species living in Wyoming about 5 million years earlier. This suggests that the genus colonised the islands from that direction, as other animals from the same fossil deposits seem to have done, probably because the North Atlantic had just opened up between Britain and Greenland, blocking what had previously been an available route from the east. Nonetheless, it is thought likely that the paromomyids crossed into Europe via that land bridge around 55 million years ago, which must have taken them pretty close to Ellesmere Island on the way - we just don't have any fossil record from that precise age on the island to confirm this.

That the fossils belong to two different species suggests that their ancestors not only survived on the islands but prospered enough to diversify. Since we know that many other groups of animals common in North America at the time failed to do so, that would suggest that it wasn't so much the temperature that was a problem but the long sunless winters. Yet in the perpetual darkness of an Arctic winter, forests once grew that could shelter something that looked much like a nut-eating lemur.

[Image by Nobu Tamura, from Wikimedia Commons.]


  1. Do we have any idea why, say, a horse (sensu lato) should be more troubled by midnight sun and/or all-day darkness than a tapir (sensu lato)?

    1. Not that I am aware of. But given how many of them there were relatively nearby, something must have deterred them.