The Pleistocene Epoch starting ~ two million years ago and ending 10,000 years ago, was punctuated by roughly thirty alternating glacial and interglacial intervals. Glacials were ice-ages that saw the formation of large continental ice sheets. Interglacials were warmer periods with conditions like today's. The last glacial maximum occurred 18,000 years ago. At that time:
Continental ice sheets require both cold summers and significant precipitation. Although cold enough to support continental ice, Beringia was too dry for them to form. Beringia was bordered by:
Within these boundaries was the mammoth-steppe a unique plant community, distinct from that of North America south of the ice and with no modern analog. Whereas modern central Alaska is dominated by moss and herb covered tundra or stunted taiga forest, the mammoth-steppe covering Beringia was dense grassland, vaguely similar to the prairies of great plains but colder and dotted with the shrub-like dwarf trees (e. willows) that we see today. We see evidence for it in the fossil record of land animals, pollen, and insects.
The mammoth-steppe environment owed its existence to a complex interaction of climate, physical geography, and plant and animal behavior.
Why was the Pleistocene different?
So, why don't grasses take over today?
The result was a positive feedback loop in which:
So, what happened?
What removed the grazers?
The major grazers:
|Steppe bison (Bison priscus): Ecologically similar to African cape buffalo. We have much information thanks to the discovery of Blue Babe - a well-preserved mummified carcass. Although smaller and behaviorally much different, both the American and Eurasian bison are directly descended from the steppe bison.|
|Wooly mammoth (Mammuthus primigenius): The last elephant to enter North America (America south of the ice was already occupied by the columbian mammoth, the mastodon, and Cuvieroneus). The wooly mammoth was specifically cold adapted with dense fur and small ears. Its tusks were adapted for removing snow from forage. What most illustrations don't show is that its trunk ended in a broad flange, presumably for scooping up snow.|
|Yesterday's camel (Camelops hesternus): Camels evolved in North America but, ironically survive today only where they dispersed to South America and Eurasia. Camelops, one of the last North American camels was common both in Beringia and south of the ice.|
|Horse (Equus lambei): These were probably ecologically similar to Przewalski's horse and the wild asses of central Eurasia.|
|Saiga (Saiga tatarica): Eurasian immigrants, the saiga are cold-adapted antelopes. Extingished in Beringia 11,000 years ago, they persist in central Eurasia.|
|Helmeted muskox (Bootherium bombifrons): Somewhat taller, slenderer, and less cold-adapted than the modern tundra muskox.|
Also present were wapiti and moose similar to modern ones
Their major predators:
|Timber wolves (Canis lupus): Recent immigrants from Eurasia, it has recently been shown that the timber wolves of Beringia evolved into a robust bone-crushing form (reminiscent of spotted hyaenas.) In this, they resembled the even more robust indigenous American dire wolf, that hunted south of the ice at the same time.|
|Lions (Panthera indet.): Lions were definitely common in Beringia. Depending on your authority, these were either a beringian extension of the prehistoric range of eurasian lions (Panthera leo) or the northernmost representatives of the endemic American lion, Panthera atrox (pictured to the right behind a modern Panthera leo.) Possibly both were present at different times. Either way, they were specialists in group hunting of large mammals in open grasslands like the modern Serengeti or Pleistocene Beringia. (But, shhhhh! Some researchers think that P atrox was a tiger.)|
|Scimitar-toothed cat (Homotherium serum): A lion-sized solitary predator. With no living analog, it is difficult to say exactly how sabre-cats used their long canines. Homotherium ranged across Eurasia and North America south of the ice, as well as Beringia. Note: Smilodon, the endemic American sabre-cat did not inhabit Beringia.|
|Short-faced bear (Arctodus simus): This large, long-legged bear was the top predator of both the beringian mammoth-steppe and North America south of the ice. It was ecologically distinct from other bears being a hypercarnivore and built for speed and endurance over long distances. Surely capable of attacking large prey that it encountered, it probably also stole prey from lions and smaller predators. It is shown with three minor components of the Beringian fauna, the ground-sloth Megalonyx jeffersoni, the giant beaver Castoroides ohioensis, and Homo sapiens. The bear, beaver, and ground-sloth were also common south of the ice, whereas humans, if present at all, were quite rare.|
Beringia was a staging-ground for the invasion of North America by several species at the end of the Pleistocene, around 11,000 years ago, when the Laurentide and Cordilleran ice separated, creating a corridor through Yukon and Alberta into Montana. Immigrants included: