Kangaroos too heavy to hop,  and apes too big to climb, and lemurs too tasty to ignore.

These are some of the biggest  animals of their kinds. While being big has some  advantages, it also sometimes limits what an individual can physically do, and even makes them more vulnerable to threats.

These are 6 stories about  triple-XL extinct animals and the lives they lived, and sometimes lost, because of their impressive bulk. [♪ INTRO] Let’s start by taking a  trip down under to check out the giant extinct relative of modern kangaroos. Procoptodon goliah is a giant species of kangaroo that lived in Australia during the last Ice Age. And unlike most kangaroos we see today, these Ice Age-era kangaroos  probably couldn't even hop!

Modern kangaroos are already pretty large. The biggest species are red kangaroos, which can be as heavy as 92  kilograms, or just over 200 pounds. Procoptodon goliah wasn’t  much taller than your average red kangaroo male, but what made  them unusual was their mass.

At 200-240 kg, this kangaroo was three times as heavy as an average red kangaroo. And with all that extra weight packed onto an only slightly larger frame,  these were some chunky boys. So chunky that they probably couldn’t  generate enough force to leap, at least not without risking  snapping their own tendons.

So instead of hopping, we think  these guys walked upright like us, based on their wide hips and knees,  strong ankles, and big ol’ butts, which are all things that humans have too.   One way to know for certain  would be to find high quality Procoptodon tracks, but until that  day, I will be imagining these thick, giant kangaroos walking  around kinda like humans do, because it’s quite hilarious. Speaking of fossil footprints, let’s talk about an animal we know about  mostly from just its tracks. Trackways are ichnofossils,  a type of fossil that records an animal’s behaviors, but  doesn’t preserve any body parts.

And while trackways tell us a lot about an animal’s size, movement, and social behaviour, we can’t always match a  track to a specific animal. So researchers will give these  tracks their own special names, just like fossils get. For instance, this kind of trackway  is called Diplichnites cuithensis.

And when Canadian geologist J. W.  Dawson encountered the marks on a trip to Nova Scotia some  time around 1860, he was stumped. He thought that the animal that left them could’ve been anything from a frog to  a worm to a weird salamander.

But it turns out he was way off. These tracks actually belong to Arthropleura, a group of giant millipedes that lived between 345 and 290 million years ago. At 2 and a half meters long  and 55 centimeters wide, the largest species of Arthropleura was the biggest arthropod ever to live on land.

And for a while, researchers  thought that the main reason they got so big was that the amount of oxygen in the atmosphere increased around  the time they first showed up, but newer evidence doesn’t  really support that idea. It turns out that the oxygen back then was about the same concentration that it is now. Which is fine by me, I don’t mind some cool bugs.

As long as there’s like, a  thick layer of glass between us. Instead, we think that there just  weren’t enough land predators to chow down on something this  big and this exoskeleton-y, so getting big and beefy was good  protection against being eaten. Whatever the reason they got so big, we know that they got to be super-sized because of the spacing of their little footprints, and from a few preserved bits of its exoskeleton that researchers have been able to find.

I’m pretty excited about these bugs, but I’m glad we don’t have to touch them. Now we’re going to hop in the water and swim ahead a few hundred million years, where we’ll find our next giant: the biggest salmon to ever swim… And it has tusks. Yes, tusks, like an elephant or  walrus.

But this time it’s on a fish. Oncorhynchus rastrosus was  “only” 2.4 to 2.7 metres long, and while there’s plenty of  other fishes that were bigger, this one is the largest salmon we’ve ever found. Which is a bummer, because can you imagine the nigiri we could have made out of this guy?!

Plus, how many other fish do you know with tusks? Paleontologists in the ‘70s  originally thought that their oversized teeth pointed  downward, like a sabertooth cat. But in 2024, researchers announced  that they’d found new fossils of this guy that showed that  instead of pointing down, like this, their teeth pointed out to the side, like this.

Meaning they weren’t sabers at all, but tusks. And while we know what direction  they went in, that leaves the question of why a salmon would  need tusks in the first place. The researchers don’t think  these tusks were used to hunt, since not only is stabbing  with the side of your face a bit of a challenge, but also  these guys were filter-feeders.

You don’t need to stab your  food if it’s itty bitty. We also know that both male  and female salmon had tusks, and that they got bigger as the fish got older. So maybe the tusks were important  for migration or breeding.

Or the salmon could have used  them like pickaxes to dig nests, or as anchors against strong river currents. And there’s always the possibility  that they were just for show. Whatever the case, it made for a  fish with an absolutely wild smile.

Our next animal’s teeth also  confused paleontologists, but for a different reason. Like the tusked salmon,  these extinct rodents aren’t huge in the grand scheme of things. But compared to their modern  relatives, they are ginormous.

Paenemarmota is an extinct group of squirrels that used to live across the central  USA and in parts of Mexico. And the biggest ones were the size  of corgis, so like 14 to 16 kg, which is ridiculous for a squirrel. Like, just imagine looking up  into a tree and seeing a squirrel, except it’s the size of a corgi.

And we know they were this size because of, and in some ways in spite of, their teeth. See, paleontologists love teeth. Teeth can tell us what an  animal ate, where it lived, and how old it was.

But like any good poker player,  fossils can lie through their teeth. And Paenemarmota is a good example of that. See, some earlier teeth-based estimates of their size suggested that these  squirrels were more like 30 kg.

So instead of a corgi, picture a Siberian Husky. Why the big discrepancy?  Weirdly, it’s all about timing. In the spring and the summer,  squirrels run around all day looking for mates and making babies.

But then the leaves change,  and colder weather sets in. The squirrel stuffs itself  with food, hunkers down, and spends the winter months snuggled up in a den. A squirrel’s body weight  fluctuates wildly as it cycles between hot squirrel summer  and cozy squirrel winter.

So while a squirrel may have teeth that are the size of a 30 kg animal’s,  that squirrel probably isn’t 30 kg for most of its life, or maybe even ever. And because squirrels are rodents, some of their teeth are always growing, which also means you have to choose to use teeth that stay the same size through the animal’s life if you want to get a good estimate. So then researchers combined a whole bunch of body size estimates using lots of body parts, from different types of teeth to  the length of their leg bones.

All in all, the estimate that  lined up with the most measurements was somewhere in the ballpark of 16 kg. Which is how the world’s biggest squirrel went from this to this. Still  pretty wild, in my book.

Our next fossil is also known mostly from its massive teeth that also tell  us the story of its downfall. Gigantopithecus blacki is a fossil  great ape from the Pleistocene, and is related to modern orangutans. And Gigantopithecus was truly  the greatest of the great apes.

These guys weighed in between 200 to 300 kg, which is twice the size of a gorilla. They were so big that the adults likely couldn’t have climbed in trees at all. But they weren’t alone in the woods.

Gigantopithecus also had a  cousin living in the same area called Pongo weidenreichi,  which is another great ape even more closely related to  orangutans than Gigantopithecus was, and was a lot closer in size  to modern orangutans too. And when Gigantopithecus went extinct, P. weidenreichi went on  living. For a while, anyway.

So the question was, why would only one of the resident apes in town go extinct? In 2024, researchers published a study looking at over 20 cave sites containing  Gigantopithecus fossils, where they analyzed how old the fossils were, what kind of pollen was around, how dry it was, and how all those factors changed through time, but especially towards  Gigantopithecus’s final days. Right around the time that  Gigantopithecus disappeared, between 295,000 to 215,000 years  ago, the summers got hotter, winters got colder, and the environment dried out.

Large swatches of forest were replaced by ferns and other ground plants, and  since the fruits in those trees had been Gigantopithecus’ main food source, that wasn’t great news for them. As trees got fewer and farther between, that also means that the lowest-hanging fruits were way harder to find, too. So even if Gigantopithecus could find trees, all their favorite fruits would  be further up in the canopy, where only climbing animals could get them.

And like we said, Gigantopithecus couldn’t climb, so that meant their favorite  snacks were just out of reach. But you know who could climb  those trees? P. weidenreichi.

We can tell based on marks on  their teeth that as the trees disappeared, Gigantopithecus  started eating different foods. See, earlier Gigantopithecus individuals  preferred fruits, but towards the end, they were mainly eating  fibrous and abrasive foods. This is often a sign that an animal  is eating a fallback food, or something that isn’t their favorite  thing but gets them through a crisis.

Like a squished granola bar  at the bottom of your bag. But nobody can survive on  squished granola bars forever. Not to mention that bigger bodies  just need more fuel in general.

So food scarcity is a much  bigger problem for giant animals than it is for more travel-sized ones. Without enough of the fruits they  really needed, they died out, leaving Pongo weidenreichi all on its own. The bigger they are, the  harder they fall, I guess.

This last animal is the only  one on our list that was around at the same time as humans were. And that didn’t work out so great for them. Archaeoindris was an early,  supersized lemur relative.

Lemurs are only found only in Madagascar, and all the modern ones are  small, like 20 pounds or less. But Archaeoindris, the biggest  of the lemurs, is another story. This absolute unit weighed over 200 kg, so the size of a female gorilla.

And they were still kicking  around on Madagascar until 350 BC. So if lemurs could be so  massive even that recently, why are all of today’s lemurs so tiny? Well it turns out that we  … uh… ate all the big ones.

See, Madagascar was one of the last places to be settled by humans,  who trickled in from places like East Africa and Borneo  just a few thousand years ago. The arrival of humans on the  island also lines up with the disappearance of other  large animals in Madagascar, like elephant birds and hippos. There were even a few other  groups of oversized lemurs alive at this time, like  Palaeopropithecus and Pachylemur, which aren’t as big as Archaeoindris, but they’re still waaaay bigger  than their living relatives.

And we know that early  inhabitants definitely encountered these lemurs because of how we find their bones. Many of the Palaeopropithecus and Pachylemur bones we’ve found look pretty rough. They’ve got sharp cuts around their joints, and they’re dented and cracked  in a fairly uniform way.

Which is all the damage you’d expect when a human with a stone tool is making  that animal into dinner. While we aren’t certain that  Archaeoindris was also on the menu, we do know that humans got to  Madagascar before they went extinct, and that humans did some damage to the overall lemur population once we got there. So that may be why Madagascar's  lemur are so small: Because a few thousand years ago, being big meant being hunted by the newest,  scariest predator on the island.

Even though these prehistoric  giants are no longer around, it’s still cool to imagine  what they might have been like and how awesome it would have been to see what they were like in real life. It kinda makes you wish you could travel back to when they were alive just  to get a glimpse of them. Well, except for the giant millipede.

Maybe a glimpse from like  behind several feet of glass. [♪ OUTRO]