The fossil record is full of startling bonebeds - places that preserve the deaths of anywhere from dozens to thousands of extinct animals.

Think sites like the La Brea Tar Pits, Ashfall Fossil Beds, or the Cleveland-Lloyd Dinosaur Quarry.

Some of them accumulated their bones over millennia, while others formed in months or even days.

And while places like these can inspire captivating and dramatic ideas about what the past was like, there’s one mass death site from Ecuador that paints a much less pretty picture.

I’m talking about Tanque Loma, where at least 22 giant ground sloths in the genus Eremotherium met their end.

And today we've brought together all three Eons hosts, including our newest host Michelle Barboza-Ramirez, to tell you...it was not an awe-inspiring one.

Because, of the five hypotheses the researchers proposed for what killed the sloths, the best supported one right now is that they died surrounded by their own poop.

Tanque Loma is a site in southwest Ecuador dating from the Late Pleistocene, between about 23,500 and 17,000 years ago.

And it is full of bones - literally thousands of them, most of which belong to that one species of giant ground sloth.

This species of Eremotherium lived from the Late Pleistocene to the Early Holocene and its fossils have been found from southern Brazil to the southeastern United States.

But what was it about Tanque Loma that was so deadly for them?

Now, this is actually a more complicated question than it might seem.

To figure it out, paleontologists have to ask a bunch of other questions first.

Like, were all the sloths the same age when they died?

Did they all die at the same time?

How long did it take for the bonebed to form?

And is there any evidence that points to a specific outside cause of death?

To try to answer these questions and ultimately solve the mystery of Tanque Loma, a group of researchers tested five hypotheses in a paper published in 2020.

And the first one was that the sloths were mired or trapped at the site.

Now, bone beds like these can be formed by tar pits that continuously trap unlucky animals over time.

But, while the sediments at Tanque Loma are saturated with natural asphalt, it doesn’t seem to have been a tar-pit-style trap.

These kinds of deposits have a couple of distinctive features that Tanque Loma just doesn’t.

For one thing, there are almost no carnivores or birds preserved at Tanque Loma, while these tend to be the most common fossils at places like the La Brea Tar Pits.

And for another, the proportion of juvenile sloths compared to adults doesn’t match the age distributions we see at real tar pits.

At La Brea, for example, up to 75% of bones can come from juveniles, while at Tanque Loma, it’s less than 50%.

There also aren’t very many limb bones found at Tanque Loma, unlike other trap deposits.

Plus, the way the site is laid out is just wrong.

Tar pit deposits tend to be isolated, deep, and cone-shaped, but the fossils at Tanque Loma occur in a single, relatively flat layer.

So the researchers think the asphalt actually seeped in after the bonebed was laid down.

And the bonebed was formed in a marshy, aquatic environment - one that dried out from time to time, based on the plant remains that have been found there.

After ruling out death by tar pit, the paleontologists considered whether a predator might’ve been responsible.

And it seems pretty unlikely.

Adults of this species of ground sloth weighed something like 3500 kilograms - they were way too big for any of the carnivores around at the time to want to take them on, even as part of a pack.

There also weren’t any clear tooth marks found on any of the bones.

And as far as human predators go, the sloth bones are older than the oldest evidence of people in South America and there aren’t any archaeological materials at the site, either.

So, what about a natural disasters?

Was Tanque Loma the sloth version of Pompeii?

It looks like a no here, too.

There’s not much debris mixed in with the sloth bones and there’s no charcoal or ash, so floods, fires, and volcanic eruptions are off the table.

That leaves two possible explanations.

First, that Tanque Loma was a site visited regularly by the sloths over an extended period of time and some individuals happened to die of natural causes there.

This is sometimes called an attritional assemblage and usually includes more juveniles and older adults - picture something like lions preying on wildebeest calves at a waterhole.

And second, that a terrible event happened at the site within a relatively short period of time - one with a biological cause, like disease or drought.

This would leave behind what’s called a catastrophic assemblage.

Instead of just taking out the young and the old, this event would’ve also affected prime-aged adults.

This kind of assemblage gives paleontologists a better snapshot of what the demographics of a group of sloths would’ve looked like in life.

Now, the sloth fossils from Tanque Loma do include a lot of adults, so the researchers think it’s probably a catastrophic assemblage… Though they can’t definitively rule out the possibility that something might’ve happened while the site was forming that just made it look like that.

Like, if there just weren’t very many juveniles in the group at the time.

BUT, they’ve come up with a pretty compelling explanation for why it was probably a biological catastrophe.

And it’s based on living hippos.

See, hippos like to hang out in water - it keeps them cool and protects them from things like the sun and biting insects.

And they congregate in relatively large numbers at places like watering holes, especially during the dry season.

But when those waterholes start to dry up...well, they become more hippo poop than water.

And this is generally bad news.

Between the water drying up and the poop contaminating what remains, there’s not much left for the hippos to drink or hang out in.

The drying affects the surrounding plants, too - leaving the hippos without food.

Which, okay, isn’t a direct result of the poop, but if the hippos weren’t so tied to their now-poopy water source, maybe they would’ve survived...

Unless the poop-contaminated pool made them sick.

Eventually, what ends up happening is the hippos die in large numbers from drought, starvation, or disease, or some combination of the three.

This can happen quickly too.

An example from the mid-1970s found that one herd lost around 100 individuals in the span of a week!

And when death comes...well, they’re surrounded by their own poop.

Now, giant ground sloths aren’t hippos - obviously - but researchers think that it’s totally plausible that they might’ve wallowed in pools, because lots of other big herbivores that live in hot, dry environments do this.

It’s even been suggested that some giant sloths were basically hairless, in which case, hiding from the sun and bugs in a cool pond sounds even better.

I mean, that sounds like a good plan to me...just no poop, please.

And, because, as adults, these sloths could be larger than hippos, they were probably capable of producing A LOT of poop.

Along with using our understanding of living hippos to support the death-by-poop hypothesis, there’s also some good geological evidence for this scenario.

Like, the layers of sediment at Tanque Loma suggest that the site itself did dry out periodically.

There’s a layer of a kind of soil that forms under waterlogged conditions under the bones and a layer of soil that forms on dry surfaces above the bones.

And we can see from pollen and isotopes found in ocean cores that the region of Ecuador where Tanque Loma is located dried out during the time period when the bonebed was forming.

There are also marks on some of the bones that point to them being trampled on the bottom of the watering hole by large wading animals, before being totally covered by mud -- like they would’ve been if this waterhole slowly dried out over a period of weeks or months.

And other than those trample-marks, the bones don’t look like they were transported from anywhere else and they aren’t very weathered, either.

This means the sloths probably died in that location and that their bones were likely covered - either by sediment or by water - pretty quickly.

And then there’s all the plant material found around the sloth bones - lots of it.

The little blunt-ended bits of twigs are all roughly the same size - and that size matches up pretty well with what you’d expect the teeth of these sloths to produce by chewing.

So researchers think that the plant material is either the fossil poop or the gut contents of these sloths.

Which means…they might’ve died the way those hippos did, by continuing to contaminate the waterhole while it was drying out, leading them to eventually die of drought-related starvation or disease.

And, yes!

That is gross and weird!

But it also tells us some interesting things about the behavior and ecology of these giant sloths - which is awesome, because there’s really nothing like them alive today to compare them to.

It provides more support for the idea that they wallowed, for one thing.

Living sloths are surprisingly good swimmers and there were even large, semiaquatic sloths in the past.

So giant ground sloths enjoying a dip from time to time wouldn’t be that strange, from an evolutionary perspective.

And for another thing, the range of ages represented by the bones, along with just how many sloths were found at the site suggests that this species was probably social!

They might’ve hung out together in multigenerational groups, with adults caring for the juveniles.

And this is pretty unlike any living sloth.

Both two-toed and three-toed sloths today are solitary animals.

Young stay with their mother for up to a year, depending on the species, but after that, sloths go solo.

Or hang solo, as the case may be.

So if giant ground sloths were social and lived in groups, that expands our understanding of what it means to be a sloth!

Either way, Tanque Loma is an incredible — not to mention gross and fascinating — mystery.

And it's a good reminder that what starts as an investigation into a mass death in the fossil record can open a window into the past, allowing paleontologists to better understand