The Island of the Last Surviving Mammoths

- In 1942, the Cleveland Museum of Natural History organized a fossil-hunting expedition through southeastern Montana.

There, the crew prospected the Hell Creek Formation, a world famous deposit dating back to the very late Cretaceous Period, one that's known for fossils of Tyrannosaurus rex.

And on that trip, they found a fossil that sure looked like a T.rex.

Clearly it belonged to a member of the same family.

Whatever this thing was, it lived at the same time and in the same place as T.rex did, but at just 57 centimeters long, the skull was less than half the size of the biggest known Tyrannosaurus heads.

Today, some paleontologists call this fossil the Cleveland Skull, and it wasn't the last odd find from Montana.

In the early 2000s, scientists found another Tyrannosaurus-like fossil, which they nicknamed Jane.

This one was a partial skeleton, except Jane didn't look like a typical T.rex either.

Its limbs were proportionately longer, and like the Cleveland Skull, it had narrower teeth than a full-size T.rex.

And it was only about 6.5 meters long, much smaller than a 12.3-meter adult T.rex, which makes Jane a very unusual creature.

See, in some parts of the dinosaur fossil record, you've got your big carnivores and your small carnivores, without much in between.

Call it the carnivore gap.

The remains of medium-sized predatory dinosaurs, animals weighing between 100 and 1,000 kilograms, are actually pretty rare in places where giant predators like T.rex existed, which is weird because that's just not how ecosystems work today.

For example, if South Africa's Kruger National Park had the same carnivore gap as some dinosaur communities did, then every meat-eating mammal species at the park would either be lion sized or no bigger than a 4-kilogram bat-eared fox.

So what's going on here?

How can we explain this dino-sized gap?

Well, some experts think that predators like T.rex went through some growing pains And if that's true, maybe the medium-sized dinos we've been looking for were hiding in plain sight all along.

(bright music) Now, at most dig sites around the world, small to mid-size dinosaurs, in general, aren't very common.

That goes for carnivores, herbivores, and omnivores alike.

One contributing factor might be the fossilization process itself because preservation issues can create biases in the fossil record.

If you're a small animal with little bones, your skeleton is more likely to get broken up, eroded away, or picked apart by scavengers.

That means big animals can be overrepresented at fossil sites.

Another problem is rock availability.

We don't have many fossil outcrops from certain points in geologic time, so we know less about those dinosaurs from those times.

But it looks like there's more to this dinosaur-sized mystery.

In 2012, two paleontologists analyzed the published size data for over 2,400 extinct animals, including 329 species of non-avian dinosaurs.

And they compared this to the available info on living animals.

Within all but one of the major animal groups these authors looked at, they found that small species are way more diverse than large ones, which makes sense.

Small animals tend to need less food and less space so it's easier for big populations of them to coexist with each other.

But it looks like the non-avian dinosaurs may have bucked this trend.

The researchers found that small-bodied non-avian dinosaurs are vastly outnumbered by the big ones in the fossil record.

And if we blamed all this on preservation bias, well, we'd be missing a whole lot of dinosaurs.

For example, let's assume the non-avian dinosaurs had the same ratio of big species to small ones as the extinct land mammals of the Cenozoic Era did.

If that were the case, then by the calculation in that study, about 90% of all extinct dinosaurs weighing under 60 kilograms that ever existed are completely absent from the global fossil record, which is highly improbable, according to the authors.

It's possible that non-avian dinos as a group just naturally skewed larger than land mammals and other animals do.

That said, we do still find the bones and skeletons from some small dinosaurs.

At the end of the Cretaceous Period when T.rex stalked North America, we know there were some little carnivores like Acheroraptor and Saurornitholestes running around under foot.

Both probably weighed less than 25 kilograms.

But again, medium-sized predatory dinosaur bones are very uncommon in rocks where Tyrannosaurus fossils are found, and that's where Jane and the Cleveland Skull come in.

Many experts think that they were both juvenile T.rex.

Jane's probably a juvenile something at least.

One study from 2020 on the microstructure of Jane's femur and tibia found that it was only about 13 years old when it died.

And other studies estimate that T.rex could potentially live into their late 20s.

And Jane was a lot lighter than the oldest, heaviest T-Rex specimens on record.

Weighing around 660 to 954 kilograms, it definitely qualified as a medium-sized theropod, and so did the smaller Cleveland Skull's owner.

Now, those two animals and a few other similar fossils that have turned up over the years look very different from huge T.rex specimens paleontologists are used to finding, so much so that some authors think they represent an entirely different species.

Supporters of this idea say that Jane and her mid-sized peers were a kind of dwarf Tyrannosaur that's been called Nanotyrannus lancensis.

This would be really significant because if Nanotyrannus existed, it would help plug the carnivore gap that we see in late-Cretaceous North America.

The smaller animal would've likely filled a different ecological niche than T.rex, its contemporary cousin.

However, many recent studies don't think Nanotyrannus is a separate genus or even a valid dinosaur.

Experts on this side of the debate tend to identify Jane and the Cleveland Skull as young Tyrannosaurus rex.

And if that's the case, then T.rex dramatically changed shape with age.

Proportionately, Jane and Jane-like skeletons have longer legs than a full-sized T.rex.

And anatomical research published in 2020 found that Jane would've been a better sprinter and more agile overall than a grown Tyrannosaurus.

But the authors of the same paper also found mature T.rex were better marathon walkers.

They could travel more efficiently over great distances.

Also, with narrower jaws and blade-shaped teeth, Jane probably couldn't bite it as hard as the big guys did, and mathematical models back this up.

Paleontologists have calculated that while Jane's maximum bite force was around 2,400 to 3,850 newtons, a full-sized T.rex could exert a bone-splintering 64,000-plus newtons when it closed its jaws, yikes.

So if Jane and the Cleveland Skull were juvenile T.rex, then the teenagers in this species were better sprinters and more agile than the grownups, but their bites were weaker.

The adults may have been slower and less agile, but least they had bone-crushing jaws to make up for it.

Take all this information together and it starts to look like young and old T.rex were built for two very different lifestyles.

It's possible that the speedier juveniles hunted small herbivores, like the roughly 2-meter Leptoceratops or the 4.5-meter Pachycephalosaurus, while bite marks on fossils tell us that adults ate big game, such as triceratops.

So it looks like we've got a scenario where T.rex adults filled the role of giant predator in late-Cretaceous North America, and their teenage offspring acted as mid-size predators.

And maybe that helps explain the carnivore gap in that environment.

Maybe the reason we don't really find adult meat-eating dinosaurs of medium size in Tyrannosaurus' ancient habitat is because those animals would've faced stiff competition from juvenile T.rex.

So ecologically, one species did double duty.

Basically, a single population of T.rex would've occupied two different niches at the exact same time, all thanks to physical differences between the age groups.

Ecologists call this ontogenetic niche shifting.

Komodo dragons do the same thing today.

Young Komodos, weighing under 20 kilograms, prefer smaller prey than adults do, and they actively wander around foraging for food.

Whereas grown dragons are sit-and-wait carnivores who prefer to hang out and let food come to them.

Their niche changes with age.

And just like dinosaurs, Komodos lay eggs.

Egg laying explains the huge size difference between most colossal dinosaurs and their hatchlings.

Physically, eggs can only get so large.

Bigger eggs need thicker shells for protection but if the shell gets too thick, the embryo inside won't get enough oxygen.

The biggest non-avian dinosaur eggs we've ever found are around 61 centimeters long.

But the vast majority are lot smaller, with most dino eggs probably weighing under 10 kilograms in life.

And here's the thing.

Remember how I mentioned the mammals of Kruger National Park way back at the beginning?

If we compared giant dinos hatching out of very small eggs to a mammal, like an African elephant, we see much greater size changes in dinosaurs from newborns to adults than we see in most mammals.

Because of that, hatchlings and juveniles of some extra large dinosaurs couldn't have eaten the same things their parents did.

And these dinos would've changed in size, shape, and diet a lot more throughout their lives than mammals do today, passing through more different niches along the way.

Now, this didn't just affect Tyrannosaurus rex.

The hadrosaur Edmontosaurus got dramatically larger with age, and so did Camarasaurus, an herbivore that lived in the Jurassic Period.

Juvenile Camarasaurus also had proportionately shorter necks than grownups did, which could point to differences in lifestyle and behavior between the age groups.

Bottom line, growing up is hard.

Physics put huge constraints on baby dinosaurs from T.rex to Camarasaurus to Edmontosaurus, so as some of these little newborns matured and new dietary options became available, they changed proportions and took on different niches.

And by recognizing this, by thinking about these dinosaurs as living animals that actually grew up, we may have finally solved the mystery of the missing medium-sized dinosaurs.

(gentle music)