Tag Archives: sloths

Episode 103: Trace Fossils

You may know what fossils are (I hope), but have you heard of trace fossils? You have now!

A giant ground sloth footprint with a human footprint inside it, made some 11,000 years ago:


A “devil’s corkscrew”:

A Paleocastor fossil found at the bottom of its fossilized burrow:



Gastroliths found with a Psittacosaurus:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

This week we’re going back in time to look at fossils, but these aren’t regular fossils. They’re called trace fossils, or ichnofossils. Instead of fossilized bones and other body parts, trace fossils are records of where organisms were and what they were doing.

Fossil footprints are one of the most common trace fossils. We have lots of dinosaur footprints, and from them we know that dinosaurs held their tails off the ground, that some dinosaurs traveled in herds with the young in the middle, and things like that. A fossil footprint is formed when an animal steps in soft mud or sand, usually near water, and the resulting footprints were covered with sediment which then dried, protecting the footprints. If the footprints continued to be protected from water and other processes that might wipe them out, over the years more and more sediment was deposited on top, eventually compacting it so that pressure and chemical reactions within the sediment turned it to stone. This is why we sometimes have two impressions of the same footprints: the actual footprints and a cast of the footprints made by the sediment that filled them initially.

The White Sands National Monument in New Mexico has so many footprints of so many animals around what was once a lake that it’s referred to as a megatrack. Seriously, we’re talking hundreds of thousands of footprints. In 2014 a team studying the tracks found a set of ancient human footprints, the first ones found in the park. But while the tracks were well preserved, the team couldn’t pinpoint how old they were. They invited other researchers to come examine the prints to help date them.

In 2016 a British paleontologist named Matthew Bennett came to examine the prints, but while he was there, he took a look at some giant ground sloth prints nearby. And when he did, he made an amazing discovery. There was a sloth footprint with a human footprint on top of it, actually within the sloth’s footprint. The sloth’s print was 20 inches long, or almost 51 cm. And after that, the next sloth footprint also had a human footprint in it. And after that another. And another. And another. Ten sloth footprints in a row had human footprints inside.

Since the tracks were made in sandy lake mud and both tracks were reasonably clear, the researchers determined that the tracks were probably left on the same day. In other words, the human was probably trailing the sloth.

But that’s not all. Bennett and the other scientists at the site followed the tracks of both sloth and human and found marks where the sloth turned around and reared on its hind legs to face the approaching human. And there are more human prints that approach at a different angle—not just human prints, but prints that suggest the human was actually tip-toeing.

The most likely explanation is that the humans were hunting the sloth, with one human getting its attention while a second crept up behind it. But we don’t know for sure. One odd thing is that the human trailing the sloth actually had to stretch to step inside each sloth print. Even small giant ground sloths were enormous, nine or ten feet long, or about three meters, with long curved claws. Ground sloths were plant-eaters that used their claws to strip leaves from branches and dig giant burrows, but the claws made formidable weapons too. It’s possible that the ancient human was just amusing himself by stepping exactly in the sloth’s prints.

Since this initial finding, researchers have found more sites where sloths appear to have turned to face an aggressor, possibly humans. The age determined for the prints, around 11,000 years old, corresponds with the time that giant ground sloths went extinct in North America. Researchers have long suspected that humans hunted them to extinction, and now we may have some direct evidence that this happened.

But fossil footprints aren’t just of big animals. Small squidgy ones leave footprints too, or trails that show where an animal traveled even if it didn’t actually have feet. For instance, 510 million years ago, during the Cambrian period, a creature lived along the shores of a shallow sea and left tracks that have been found in North America. The fossil tracks are called Climactichnites and while we don’t know what animal actually left them, paleontologists have determined that there were two species and that they were probably slug-like in appearance, possibly an early mollusk, since modern slugs and their relatives sometimes leave similar tracks. We even have some body prints of the stationary animal, and some of them were 27 inches long, or 69 cm.

Similarly, fossilized burrows are considered trace fossils. But often fossilized burrows don’t actually look like holes in the ground. Instead, the burrow has filled up with soil that then fossilizes, leaving the shape of the burrow behind in a rock that looks different from the surrounding rock. And these can be remarkably difficult to identify in some cases.

Back in 1891, a rancher in Nebraska showed a visiting geologist some weird formations he’d found. The geologist, Erwin Barbour, didn’t know what they were. He and the rancher dubbed the formations “devil’s corkscrews,” and probably had a laugh. But the formations did look like corkscrews—but they were enormous, taller than a full-grown man and always situated straight up and down. Some were as long as ten feet, or three meters.

Barbour suggested that the corkscrews were freshwater sponges, since the prevailing belief was that the area had once been a lake. Other scientists thought they might be the remains of fossilized tree or other plant roots. And a couple of people thought they might be fossilized burrows of an unknown rodent.

Those people were right, of course, but at the time, no one knew for sure. And if the corkscrews were burrows, what made them?

The mystery was solved when fossils of a beaver relative called Paleocastor was found at the bottom of one of the corkscrews. Unlike modern beavers, it wasn’t an aquatic rodent but a burrowing one, and it lived around 25 million years ago. Once the Paleocastor fossil was discovered, it was clear that the marks noted on some of the corkscrews, which had been interpreted as scratch marks from claws, were actually tooth marks. They perfectly matched Paleocastor’s teeth, which meant the beaver excavated its corkscrew-shaped burrow by chewing through the dirt instead of digging through it.

So why did Paleocastor dig burrows with such an odd shape? The answer may lie in another fossil found not in the bottom of the burrow but stuck in the corkscrews. Zodiolestes was an extinct weasel relative. Possibly it had gone down the burrow while hunting beavers, become stuck in the tight corkscrew turns in the tunnels, and died.

More recent research shows that Paleocastor burrows were frequently connected to one another with side passages, sometimes hundreds of burrows joined together like the burrows and tunnels of modern prairie dogs. This suggests that Paleocastor was a sociable animal that lived in colonies.

As it happened, Dr. Barbour had been right about one thing. The area where the devil’s corkscrews were initially found had once been a lake. His mistake was not realizing that the structures had been dug into the ground where the lake had once been.

Some of the oldest fossils known are trace fossils called stromatolites. These are stones that were formed by microbes. Early life consisted of microbial mats, colonies of microorganisms like bacteria that grow on surfaces that are either submerged or just tend to stay damp. Microbial mats are still around today, often growing in extreme environments like hot springs and hypersaline lakes. When microbial mats grow on a sea or lake floor, they tend to build upwards, forming columns or even reefs that rise out of the mud and toward the light. But while stromatolites are formed by bacteria, they’re not formed of bacteria. Instead, the stones are formed from grains of sand and other sediments that were trapped and cemented together within the mats, which forms a thin layer of limestone. The layers grow over time, giving stromatolites a banded or striped pattern. But it can be really hard to tell them apart from regular old non-stromatolite rocks that also happen to have a banded pattern. Geologists spend a lot of time studying stromatolites and suspected stromatolites to find out more about them. Microbial mats evolved almost 3.5 billion years ago and it’s possible they were around as much as 4 billion years ago. The earth is about 4.5 billion years old, if you were wondering.

But let’s return to more modern times, with animals and fish and things. Another trace fossil is one I’ve mentioned here a few times, the coprolite. A coprolite is a fossilized poop. Most of the original organic material has been mineralized, preserving it. Coprolites are valuable since paleontologists can cut them open to find out what the animal was eating, if it had intestinal parasites, and lots of other information. Coprolites are also frankly hilarious. Did you know that if you become a scientist whose area of study is coprolites, you’re called a paleoscatologist?

We’ve also talked about gastroliths before. Gastroliths are small stones swallowed by an animal to help digest its food. The stones especially help grind up plant material, which eventually causes the stones to become smooth. Lots of animals use gastroliths for digestion, including birds that eat plants, crocodiles and alligators, seals and sea lions, although they may swallow them by accident, and many dinosaurs, especially sauropods. We know sauropods swallowed stones to help in digestion, because we’ve found gastroliths associated with sauropod fossils.

Other trace fossils include marks an animal may have made during its life, like those tooth marks preserved in the devil’s corkscrews. Skin imprints, or fur or feather imprints, are also trace fossils but are incredibly rare. Sometimes a skin imprint remains in place around an animal’s fossilized body parts, which gives paleontologists incredible insight into what an animal looked like while it was alive. That’s how we know a lot of dinosaurs had feathers. Root cavities are trace fossils too, caused not by animals but by plant roots that burrow into the soil but rot away, leaving a hole that fills with dirt and later fossilizes in the shape of the original roots. There’s even a type of trace fossil called a urolite, which was caused when an animal urinated and the urine stream left marks on soft ground.

Since trace fossils are usually hard to match up with the animal that made them, trace fossils are given scientific names of their own. This allows scientists to refer to them without guessing at what made them, and it reduces confusion.

Trace fossils are remains of biological activity. But animals and plants aren’t the only things that can move soft soil. Cracks in dried-out mud are sometimes fossilized, as are ripple marks from water and little dimples made by raindrops or bubbles. Geologists use these fossilized moments in time to help determine how the rock strata have been shifted by geologic forces. They know that a rock that shows ripple marks was once flat, so if it’s been tipped up sideways or deformed into a curve, they can determine what forces were at work on the rocks over the centuries.

It’s not all that uncommon to find these non-biological traces alongside trace fossils and body fossils. I’ve seen big flat rocks that show the bottom of a shallow sea, with ripple marks, the tracks of tiny animals that trundled around looking for food in the sandy mud, and the occasional fossil like a bryozoan or fragment of shell. It’s the closest thing we have to photographs of prehistoric times.

You can find Strange Animals Podcast online at strangeanimalspodcast.com. We’re on Twitter at strangebeasties and have a facebook page at facebook.com/strangeanimalspodcast. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. We also have a Patreon if you’d like to support us that way.

Thanks for listening!

Episode 022: Megatherium

Episode 22 is all about megatherium, the giant extinct ground sloth–and a little bit about glyptodon, the giant extinct…thing.

Megatherium vs trees was basically no contest. Giant ground sloth FTW!

Giant sloth big, yeah yeah yeah, it’s not small, no no no

Glyptodon. Like a giant armadillo that can’t roll up and doesn’t need to.

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

For this week’s episode, let’s learn about some Ice Age megafauna! But first, a quick note about my schedule. I’ll talk more about this in a few weeks, but in August I’m going to be in Helsinki, Finland for WorldCon 75. I don’t have the final schedule yet, but I am going to be on a panel about podcasting. If you’re going to WorldCon too, or if you’ll be in Helsinki the week of the convention or the week after, let me know so we can meet up! I’ll also be in Oslo, Norway for part of the day on August 7. I got a cheap flight to Helsinki because it has an 11-hour layover in Oslo, but to me that’s a bonus. Oslo has birds! Birds I’ve never seen before! So if you’re going to be in Oslo on August 7 and you’d like to meet me for a birding trip and/or lunch, definitely let me know! And don’t worry, I’ll schedule episodes ahead of time so you can continue to learn about strange animals even while I’m gone.

Now, on to the megafauna. Until about five million years ago, South America was a big island continent the way Australia is today. As a result, many of the animals that evolved there at the time don’t look anything like animals in other parts of the world.

The various species of giant ground sloth, such as Megatherium and Eremotherium, were South American mammals that lived from around 30 million years ago until only about 10,000 years ago—but we’ll come back to that in a minute. Those two species were huge—as big as African elephants. It was 20 feet long and stood more than 12 feet high on its hind legs. They liked woodlands and grasslands and ate plants.

Megatherium had huge curved claws on its forefeet just like modern sloths, four claws that were a foot long each, and we know it walked on the sides of its paws as a result because we have some fossilized tracks. A ground sloth could walk on its hind legs, at least for short distances, and when feeding it spent a lot of its time reared up on its hind legs, helped to balance by its thick tail. It could reach branches some 20 feet off the ground that way. It hooked the branches down with its claws to eat the leaves.

Around 5 million years ago, South America became connected to North America by the Central American Isthmus, which is volcanic in origin. Over the millennia, peaking around 3 million years ago, North American animals migrated south, and South American animals migrated north, called the Great American interchange. A lot of South American megafauna went extinct with the increased competition for resources, but nothing bothered the giant ground sloths. One medium-sized species, named Megalonyx by Thomas Jefferson, spread throughout North America as far north as Alaska. It was “only” about 10 feet long and weighed some 800 pounds, with three claws on its forefeet.

The North American sloths died out first, around 11,000 years ago. It didn’t take long for most of the South American sloths to go extinct too, a little over 10,000 years ago. And yes, that was the same time that humans were spreading deeper into the Americas. It’s not a coincidence, although climate change after the last big ice age probably played a part too. Ground sloths had thick skin reinforced with osteoderms, knobs of bone tissue that grow in the skin like armor, so killing one would have been a lot of work for our ancestors, and was undoubtedly dangerous too.

But a whole lot of islands make up the Carribbean, and giant sloths lived on some of those islands. Many had developed in isolation long enough that they’re now considered separate species from the mainland sloths. And many of the island sloths persisted for thousands of years after their gigantic mainland cousins were long dead.

The island sloths were much smaller than Megatherium. Megalocnus only weighed about 200 pounds—a big sloth, but nothing like the five tons that Megatherium could weigh. But Megalocnus survived until some 6,000 years ago in Cuba and maybe much more recently. Another Cuban sloth lived another thousand years after that. A small ground sloth called Neocnus survived on Hispanolia until only about 4,500 years ago.

You may have heard recently about a lot of huge tunnels in Brazil. Until recently, people assumed they were natural caves. It wasn’t until the 2000s that geologists started investigating the tunnels and immediately saw that they weren’t natural at all. They were burrows, many with claw marks on the walls as though just dug, thousands of them scattered across Brazil and a few other parts of South America. Some are tall enough to stand up inside comfortably. One paleoburrow in the Amazon is a network that adds up to around 2,000 feet of tunnels, six feet tall and almost that wide. It was probably used by generations of animals, enlarged and extended as new adults dug their own burrows.

The burrows were probably dug by giant sloths. No one is sure why. Giant sloths had no predators until humans moved into the area. But it’s also possible that some or most of the burrows were dug by the extinct ancestors of armadillos, glyptodon.

The glyptodonts are related to both the giant ground sloths and modern-day armadillos. Glyptodon and its two related species, Panochthus and Doedicurus, lived in the same areas where the giant burrows have been discovered. And modern armadillos are good burrowers. But Glyptodon had even less reason to need burrows than giant ground sloths did. It was an enormous animal, 11 feet long and five feet high, weighing over two tons, with a massive domed carapace like a tortoise shell, made of rows of osteoderms. It also had osteoderms that protected its head like a cap, and rings of bony plates on the end of its thick tail that made it into a club-like weapon. Even its jaws contained osteoderm ridges, which helped grind up the plants it ate, although it also had huge grooved teeth.

In other words, glyptodon was a walking tank. Nothing much ate them until humans showed up. A full-grown glyptodon was a bonanza for humans if they could kill it. Not only did it provide a whole lot of meat, its shell could be used as shelter. Clean it out good first. At least one human burial has been found in a glyptodon shell.

Considering how amazing glyptodonts are, you’d think they’d be more well known and better studied. There’s still a whole lot we don’t know about them, including how many species there actually were and how recently they died out. There aren’t even very many reported sightings of living ones, which tends to happen with just about any extinct animal.

Giant ground sloths, on the other hand, do get reported every so often, and there are hints that giant ground sloths might have lived until much more recently than ten or eleven thousand years ago. Megatherium remains found in caves sometimes seem suspiciously fresh, although so far radiocarbon dating hasn’t given us any surprises. In 1740 the Portuguese historian Lozano mentioned an animal that sounds a little like a ground sloth, which was supposedly called the su by locals.

Some cryptozoologists believe that a legendary South American monster, the mapinguari, may have been inspired by megatherium. The mapinguari is supposed to be nine feet tall and smelly, with feet that face backwards, an extra mouth in its belly, skin that deflects arrows, and sometimes it’s said to have only one eye in the middle of its forehead. It also eats meat. That sounds a little on the far-fetched side to me, and a lot of cryptozoologists group the mapinguari with bigfoot type monsters.

There is another monster story from Patagonia that sounds a lot more sloth-like on the surface. The yemisch is supposed to be a cow-sized animal that sleeps in burrows it digs with its huge claws. It can’t be killed because arrows bounce off its hide. In fact, yemisch is supposed to mean “the one covered in little stones.”

That sounds promising, but the story comes exclusively from a man called Florentino Ameghino, who was convinced that a smaller giant ground sloth named mylodon still lived in Patagonia. The first mention anywhere of the Yemisch comes from Ameghino’s 1898 paper about mylodon, where he said the Tehuelche of Patagonia referred to it as the water tiger. It was semi-aquatic, spending much of its time in the river. It was said to drag horses into the water with its huge claws. Its feet were flat, its ears tiny, it had huge claws and fangs, and its toes were webbed for swimming. It was bigger than a puma but with shorter legs.

This doesn’t sound like a ground sloth, which were not carnivores despite their big claws. In 1900 a French naturalist, Andrew Tournouer, spotted an animal in a stream that looked a lot like Ameghino’s description of the Yemisch. Tournouer said it was definitely not a ground sloth; his guide said it was called a Hymche.

The water tiger Ameghino describes is well known in Patagonian native lore, but not under the name Yemisch. It’s possible Ameghino mangled the word Hymche. Whatever the water tiger is, though, it’s definitely not a giant ground sloth and I’m going to save it for a future episode if I can dig up more about it.

There was an aquatic giant ground sloth once, though, Thalassocnus. It grew to around five or six feet long and lived off the Pacific coast of South America, where it ate seaweed and other marine plants. Fossils document how it adapted to marine life over the generations. The earliest Thalassocnus fossils are of semi-aquatic animals that grazed in shallow water. Fossils from more recent species show increasing adaptations to deeper water, including increased weight of the skeleton to help it stay underwater instead of bobbing up to the surface. It died out around two and a half million years ago, after the Isthmus of Panama formed, probably because the new land mass caused the water temperature to cool and many of the ocean plants in its habitat went extinct.

Whether or not any giant ground sloths are still alive in the remote parts of South America, I think we can all agree that they’re not going to eat anyone. So if you see one, don’t shoot it unless it’s with a camera.

You can find Strange Animals Podcast online at strangeanimalspodcast.com. We’re on Twitter at strangebeasties and have a facebook page at facebook.com/strangeanimalspodcast. If you have questions, comments, or suggestions for future episodes, email us at strangeanimalspodcast@gmail.com. We also have a Patreon if you’d like to support us and get awesome rewards.

Thanks for listening!