A short ๐งต on the Beringian Buckle, or- Why Were There No American Rhinos and No Eurasian Ground Sloths?
Beringia is well-known as a sunken land bridge that intermittently connected North America to Eurasia throughout the last ice age. But many don't realize that it's mostly still above water- it is the region bound between the Lena River in Russia and the Mackenzie River in Canada.
When the land bridge existed, it served as an ice-free corridor ideal for faunal transfer between the two continents. Most famously, humans used it to enter North America, but many other genera did as well- mammoths, elk, moose, bison, horses, brown bears, and lions.
However, despite the land bridge allowing many genera to cross between America and Eurasia and vice versa, many other genera DIDN'T cross, despite their ranges butting right up against Beringia.
For example, giant ground sloths like Megalonyx ranged as far north as Alaska, but never crossed into Eurasia.
Musk-ox originally came to North America from Eurasia between 200k-90kya, but during the Last Glacial Maximum they didn't cross back over to recolonize Eurasia.
Similar to the musk-ox, camels originated in North America and colonized Eurasia millions of years ago. But High Arctic camels known from the Late Pleistocene of Canada don't seem to have made the leap to Eurasia.
The short-faced bear is another North American native that never colonized Eurasia (certainly to the benefit of prehistoric humans!)
To our knowledge, no tigers ever crossed into North America, but that may be due to misidentification of skulls. Regardless, even if there were some tigers in Beringia they never made it SOUTH of Beringia into the greater continent.
Finally, and perhaps most mysteriously, the woolly rhinoceros- Coelodonta antiquitatis- never colonized North America, despite being omnipresent along the "mammoth steppe" that stretched across all of Eurasia at the time. It went right up to the border of Beringia and no further.
So... what happened?
It's still debated, but it mostly seems to come down to Beringia's climate. The region formed a "buckle" in the otherwise contiguous mammoth steppe that acted as a biogeographic filter.
It's still debated, but it mostly seems to come down to Beringia's climate. The region formed a "buckle" in the otherwise contiguous mammoth steppe that acted as a biogeographic filter.
The land-bridge, being right on the ocean, was a moisture trap. It blocked rain from entering much of the Alaskan interior and the Yukon, allowing steppe-tundra environments to flourish there (appealing to mammoths). But it also made Beringia itself very soggy.
Quoting Elias & Crocker (2008)
"The more mesic conditions would have had a corresponding effect on the dominant vegetation, which was unsuited to the derived feeding characteristics of the grazing woolly rhinoceros."
"The more mesic conditions would have had a corresponding effect on the dominant vegetation, which was unsuited to the derived feeding characteristics of the grazing woolly rhinoceros."
"The woolly rhinoceros would probably have avoided both deep snow and sodden ground in the same way as the Pleistocene musk ox (Ovibos moschatus) would have done [...] as their wool would have ceased to have been an effective insulator against the cold if wet and waterlogged."
The authors also note that since the musk ox and woolly rhino "had a similar pelage, stance, and gait" it is likely that the waterlogged terrain of Beringia prevented BOTH species from making the crossing, each from its respective continent.
The same could apply to camels and ground sloths- the wide hooves of camels, so fit for enduring sand and snow, would have been inadequate for crossing a boggy mire. And the ground sloth's shaggy coat would have lost all insulation in the damp environment.
There also were no trees in Beringia for ground sloths to dine on.
This explanation is compelling- it is known that the insect faunas of Siberian Beringia and Alaskan Beringia were different- something was happening in the middle preventing transfer between tundra beetles.
This explanation is compelling- it is known that the insect faunas of Siberian Beringia and Alaskan Beringia were different- something was happening in the middle preventing transfer between tundra beetles.
It also seems that there were "islands" of tundra-steppe in the middle of Beringia, where the terrain was high and dry enough to permit tundra vegetation to persist- some of these isolated areas are now literal islands today, such as St. George and St. Lawrence.
It doesn't explain EVERY instance of lack of biotic exchange- I don't see why short-faced bears were unable to cross while grizzlies did. But they didn't, and this seems to be the most likely explanation for why.
Primary study used for this thread:
rhinoresourcecenter.com
rhinoresourcecenter.com
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