When I look out my front window to the Wasatch Front, I can’t help but feel that there’s something missing. It’s not something seen, but something that comes from knowing what used to be here. The absence takes the shape of big, shaggy elephants called American mastodon. This was their home until not so very long ago.
From a collection of bones described in 1981 – the first such find reported in all of Utah – paleontologists know that the “bubby toothed” proboscideans lived in the Salt Lake Valley during the last Ice Age. That’s practically yesterday. When I write about non-avian dinosaurs or other ancient creatures, I can’t really get my head around just how long a span of time separates me from them. It’s easy to rattle off dates in millions of years. But American mastodon may have trod through the ground that makes up my front yard close to the boundary of when prehistory became history. It seems close enough to almost touch them, but extinction keeps them just as far away from me as any other vanished species.
Why Mammut americanum and many of its Ice Age neighbors – the giant ground sloths, sabercats, and others – died out is a mystery, and a contentious one at that. A slew of possible culprits have been implicated, including climate change, the impact of a comet, disease, and hungry, hungry humans. Some – such as the comet and hypervirulent disease – have been discarded, but even left with climate change and hunting as frontrunners, uncovering the truth about the disappearance of North America’s great megafauna is a fraught undertaking. Tracking the comings and goings of species around us is difficult enough. Replaying prehistory is even more challenging, especially when extinction cannot be boiled down to a single phenomenon such as warming temperatures or the invention of the atlatl.
If we’re going to understand what happened to the American mastodon and its megafaunal ilk, we need a more refined view of when they lived, where they lived, and how their habitats changed through time. Paleontologists are still piecing together this essential picture of Pleistocene life, and one of the latest attempts to do so was published last week in PNAS. Focusing on American mastodon bones from the Arctic, the scientists found that the great beasts had already vanished from the chilly north by the time humans arrived.
The study was spurred by an anomaly. Despite living during the Ice Age, American mastodons were not cold-weather elephants. They preferred warmer, wetter climes – usually forests dense with conifers and lowland swamps. But radiocarbon dates from rare American mastodon bones found in Alaska and the Yukon suggested that the animals lived there during the last gasp of glaciation between 18,000 and 10,000 years ago. Were the mastodons really living among the open, cold, dry steppe that covered the Yukon then, or were the dates wrong?
Grant Zazula of the Yukon Paleontology Progam joined 14 other researchers to come up with new 53 new radiocarbon dates for 36 American mastodon fossils, including those dated previously. It turned out that the earlier dates were wrong. Contamination – either naturally-occurring or from museum preservation practices – had given erroneously young dates. The mastodons weren’t trundling across the frosty steppe, after all.
The dates Zazula and his colleagues came up with are all over and around 50,000 years ago. This is the limit of radiocarbon dating. What that means is that most, if not all, the American mastodons in their sample perished before 50,000 years ago. And while the difference between 18,000 years ago and more than 50,000 years ago might not seem like much, it made all the difference to the American mastodons.
The Ice Age was not consistently icy. The great glaciers of the north waxes and waned through time, and the last time ice started to take over North America was around 75,000 years ago. The relatively warm forests fell away to make way for chilly, open steppe. And when this happened, Zazula and coauthors write, the American mastodons were extirpated from the Arctic. There was no longer anywhere for them to live, and so their range contracted to those populations living in more southern forests. The new radiocarbon dates – indicating ages over 50,000 years – are in accord with the shifting forests. And it wasn’t just the mastodons that were affected this way. The bones of Jefferson’s giant ground sloth and the giant beaver Castoroides show the same pattern.
This means that the American mastodons had already vanished from the Arctic by the time humans arrived in prehistoric Alaska around 12,500 years ago. “Over-chill” had already made the Arctic inhospitable to the beasts. Humans likely played some role in the ultimate extinction of the mastodon, but humans had to spread southward before meeting the mastodon.
Whatever happened, though, we now live in a world that could still be home to American mastodon. The world may have changed too much for the woolly mammoth, but it’s not very difficult to picture mastodon among the forests and swamps of North America today. In fact, they were making a comeback just before they were snuffed out. In the eastern part of the continent, Zazula and colleagues write, American mastodons were starting to follow forests north as the last great ice sheet receded. And that’s when their time on Earth closed, leaving me to only imagine how wonderful they would have been in life.
Miller, W. 1987. Mammut americanum, Utah’s first record of the American mastodonMammut americanum, Utah’s first record of the American mastodon. Journal of Paleontology. 61 (1): 168-183
Zazula, G., MacPhee, R., Metcalfe, J., Reyes, A., Brock, F., Druckenmiller, P., Groves, P., Harington, C., Hodgins, G., Kunz, M., Longstaffe, F., Mann, D., McDonald, H., Nalawade-Chavan, S., Southon, J. 2014. American mastodon extirpation in the Arctic and Subarctic predates human colonization and terminal Pleistocene climate change. PNAS. doi: 10.1073/pnas.1416072111