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 how the animals that died there actually lived. Want more science content? Then you’ll want to check out PBS’ new
show Animal IQ. Hosted by Trace Dominguez and Dr. Natalia
Borrego, Animal IQ features deep dives on animal minds to find out just how smart the
animal kingdom really is. We know that humans are clever, but can you
find your friends in a crowd as well as a baby penguin? Drive a car as well as this rat? Sense Earth’s magnetic field like a fox? Head on over to PBS Terra to find out, and
be sure to tell them that Eons sent you. Thanks a sloth to this month’s Eontologists:
Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert! Become an Eonite at patreon.com/eons and you
can get fun perks like submitting a joke for us to read, like this one from Carl S Which dinosaur has the softest skin? The aloe-saurus. And as always thanks for joining me in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more creature
features.
What a gravestone