Untangling the Devil’s Corkscrew

Imagine you’re a paleontologist in the late
1800s, working among the sandy rocks of Nebraska. You’re searching for fossils in the Harrison
Formation — a big swath of siltstone, sandstone, and volcanic ash dating back to the Miocene
Epoch some 22 million years ago. And suddenly you find a huge coil of hardened
sand stuck deep in the earth, about three and a half meters tall. Soon after, you and your colleagues start
finding more of these things like hundreds of them, clustered close together but almost
never overlapping. They’re just giant, perfect corkscrews in
the sand. It turns out that local ranchers have known
about these weird spirals for a long time, and they call them Devil’s Corkscrews, a
name that you and your fellow scientists later change to the latin name, Daemonelix. But … what the heck are these things? It’s clear these corkscrews were created
by some form of life, but what? The secret to untangling the mystery of the
Devil’s Corkscrew is in the often-overlooked fact that bones aren’t the only things that
can fossilize. Tooth marks, footprints, skin impressions,
and even dung can all be preserved. These are known as trace fossils – indications
of life from the distant past. But just like in any good mystery novel, sometimes
the clues take time to come together. Devil’s corkscrews were first described
in 1891 from the badlands of Nebraska, by geologist Erwin Hinckley Barbour. He wrote at the time: “Their forms are magnificent;
their symmetry perfect; their organization beyond my comprehension.” Barbour initially thought that the spirals
were the remains of vines, roots, or other plant matter that had gotten all tangled up,
maybe around a tree trunk that later rotted away. Then he wondered whether they were sea sponges
of some sort. But he also noticed that the corkscrews had
this weird feature at the bottom, a little flare that stuck out like a hockey stick. He thought this could’ve been a rhizome
– a special type of root that grows sideways, which some modern plants use to reproduce,
sort of like the creeping vines that come out of a strawberry plant. Meanwhile, other paleontologists thought that
the corkscrews might have been the remains of some sort of marine plant that spiraled
deep into to the seafloor to stay rooted. But what eventually solved the mystery was
what was inside the corkscrews, something that plants and sponges have never had: bones. Specifically, the bones of a small, extinct
beaver known as Palaeocastor. Turns out, Palaeocastor bones were found in
many of the corkscrews, with one specimen even found with its head, as Barbour wrote,
“appearing to peek out of the entrance.” Now, modern, aquatic beavers are known to
sometimes burrow into riverbanks, but they don’t make anything that looks like this. Plus, the beavers’ remains were mostly found
in pieces, not intact, so Barbour and other scientists at the time decided that the beavers
must have been pulled into the burrows by some other mystery predator. It wasn’t until 1977, almost a hundred years
after the first Devil’s Corkscrews were reported, that two scientists found their
‘smoking gun’. They studied the distinctive scrapes on the
edges of these burrows, and found that they were the exact size and shape of Palaeocastor’s
teeth. Add to that the fact that some baby beavers
were found in the tunnels, and suddenly the idea that beavers had made these big helical
burrows didn’t seem so far-fetched. So, Devil’s Corkscrews turned out to be
trace fossils made by extinct, burrowing, non-aquatic beavers. But, why did they make burrows that were so
… complicated? I mean, digging isn’t easy, especially if
you’re like Palaeocastor and you’re adapted to dig with your teeth. So why not dig a simple tunnel that’s just
deep and narrow enough to hide in? Well, there are a couple of possible reasons. One is based on the fact that the beavers
seemed to make many burrows in the same area. Maybe it was easier to fit more animals in
a tight space by using coils, sort of like building a duplex instead of a single house. Plus, an animal can’t dig a hole straight
down, or it would get stuck at the bottom. But making a coil would allow Palaeocastor
to dig deep while maintaining a nice, easy incline – like a ramp. But one thing we know this spiral design was
really bad at was keeping out predators. We know this because most of the beaver bones
that have been found in the burrows are only partial skeletons: namely, heads and feet,
which are the parts that predators tend to leave behind. And some burrows have even been found to hold
the remains of one of Palaeocastor’s possible predators, an ancient weasel-like mammal called
Zodiolestes. But there’s one more possible benefit to
living in a spiral burrow, one that was particularly important in the Miocene: air conditioning. Burrows in general tend to have pretty even
year-round temperatures, because they use the insulating properties of soil. This makes them a little cooler in summer,
and a little warmer in winter. But a spiral burrow takes the design a step
further, by helping to block airflow. Air can come in and out of a straight burrow
pretty easily – but with each twist, a spiral helps block air from moving deeper by slowing
and eventually stopping any wind. And 22 million years ago, this would’ve
come in handy. The climate of Nebraska back then was drier
and warmer than today, with hotter summers and cold winters. And Palaeocastor lived in grasslands that
grew on sand dunes, which occasionally could have generated big sand storms. So living underground in a spiral would have
offered a safe haven from the rough climate of Miocene America. But, not forever. Burrowing beavers like Palaeocastor thrived
for about 10 million years, as grasses spread across the continent. But eventually, they started to disappear. It could be that the changing climate made
the soil of the ancient Great Plains harder for them to dig through. Predators also could have been a problem. As the grasslands grew, so did the number
of carnivores, and maybe they hunted Palaeocastor to extinction. We don’t really know for sure. But long after Palaeocastor went extinct,
its burrows lingered. In time, sediment filled them, preserving
the remains of the beavers inside, and eventually solidified. As time passed, the softer sand around the
burrows began to erode, but not the hardened spirals. So when Barbour and other paleontologists
began to explore the area, thousands of millennia later, they found themselves surrounded by
a Miocene ghost town. This is why trace fossils are so important
to paleontologists today. Their strange shapes can inspire awe, as well
as centuries of discussion and research — a pretty lofty achievement for what began
as a simple hole in the ground. Now, do you have a favorite detective story
from the world of natural history? let us know in the comments! And of course, be sure to go to youtube.com/eons
and subscribe. And if you have other burning questions about
how the world works, you gotta check out Reactions, a show that explains everything
in the world around you, with the help of chemistry. Trust me; you’ll like it!

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