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!
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