Mastodons suffered total extinction by around 10,000 years ago, and many think that early humans who crossed the Beringia Land Bridge from Asia to the Americas 14,000 years ago were to blame. But according to a reexamination of mastodon bones, these giants vanished from arctic and subarctic latitudes more than 50,000 years earlier that we thought—long before humans colonized the continent. The findings were published in Proceedings of the National Academy of Sciences this week.
Between 125,000 and 10,000 years ago, the American mastodon (Mammut americanum) spread across many parts of North America, from the tropics of Honduras to the arctic coast of Alaska. Radiocarbon dating previously placed the disappearance of mastodons from northwestern North America at around 18,000 years ago. But that chronology doesn’t jive with evidence that mastodons preferred woody forests and lowland swamps over the sparse tundra. Unlike mammoths who grazed on grasses across the tree-less ice caps, mastodons liked leafy and woody foods. But 18,000 years ago would have been the height of the last ice age—could they still be living that far north?
A team led by Grant Zazula of the Yukon Palaeontology Program decided to reexamine the ages of 36 mastodon teeth and bones from the Beringia region of eastern Alaska and western Yukon. “Mastodon teeth were effective at stripping and crushing twigs, leaves, and stems from shrubs and trees,” Zazula says in a news release. “So it would seem unlikely that they were able to survive in the ice-covered regions of Alaska and Yukon during the last full-glacial period.” (A molar is pictured to the right.)
The team used two different types of radiocarbon methods—collagen ultrafiltration and single amino acid dating—designed to target material from bone collagen, and not from modern contamination. The mastodon specimens consistently date to more than 50,000 years old—or beyond the limits of radiocarbon dating. Varnish and glues from museum preservation techniques may have led to the younger radiocarbon dates.
That means, the arctic and subarctic were only temporary homes to mastodons during a warm interval when the area wasn’t covered by massive ice sheets. Other megafaunal mammals, such as Jefferson’s ground sloth (front center and right), flat-headed peccary (front left), and western camel (rear left) also extended their habitat into northern latitudes during the interglacial period around 125,000 years ago.
"The residency of mastodons in the north did not last long," Zazula says. They became locally extinct in the arctic and subarctic around 75 millennia ago at the end of the interglacial period. "The return to cold, dry glacial conditions along with the advance of continental glaciers around 75,000 years ago effectively wiped out their habitats.”
The populations were displaced to areas much further south at temperate latitudes. "We suspect that once the northern group died off, the species was already heading toward trouble," Zazula tells the Los Angeles Times. "What ultimately pushed them over the edge, though—hunters picking off the last of them or climate change at the end of the ice age being just too much for them—is an unanswered question.”
Images: Courtesy of George Teichmann