When Lizards Took Over the World

When Lizards Took Over the World

In a tropical forest in what’s now Brazil, a greenish lizard perched high in the branches of a tree.

It was pretty big, more than a meter long from its nose to the tip of its striped tail.

Now, I know what you’re thinking: you’ve seen me do this before.

This is where I introduce you to some totally bizarre extinct animal, right?

Well, I’m just full of surprises.

‘Cause this time, I’m just talking about an ordinary modern green iguana.

Ok but why?

Because lizards belong to the most diverse group of terrestrial vertebrates on the planet- a group known as the squamates, which includes both lizards and snakes!

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There are nearly 11,000 species of squamates-- more diverse than amphibians, mammals, even birds!

And lizards, specifically, live everywhere but the coldest environments and they range from tiny, insect-eating geckos to giant Komodo dragons!

When Lizards Took Over the World

So today, lizards are incredibly widespread and diverse.

But!

It took them a long time to get to where they are now.

Because, they used to face some pretty stiff competition from a group of lizard look-alikes - basically, lizard impostors that went around filling a lot of niches that modern lizards occupy today.

Which just seems... Suspicious.

To help knock out this competition, it took a lot of major changes -- like the break-up of Pangaea, global changes in climate, and a mass extinction event -- in order to set the stage for lizards to take over the world.

The oldest fossil of a true lizard that we know of dates back to the Mid-Triassic Period, around 242 million years ago.

And it took a while for us to figure out what exactly it was.

It was discovered nearly twenty years ago in the northern Italian Alps and was named Megachirella.

But paleontologists weren’t sure whether it was a lizard or not, and after it was described in 2002, it was just filed away in a museum collection.

But recently, an ambitious grad student revisited the specimen and used CT scans to discover that it actually had the telltale features of a true lizard.

For example, lizards have a single opening in the skull behind each eye, even though their ancestors started out with two.

Plus they have external ears and a moveable bone in the upper jaw that allows for a wider gape.

The skull of Megachirella had nearly all of these features.

So, because of this fossil, we know that lizards were around during the Triassic, at least in some places.

But it looks like they weren’t really widespread.

Instead, there were lots of things that looked like lizards, even though they weren’t.

These lizard look-alikes are known as sphenodontians.

Now, both lizards and sphenodontians had emerged from common reptile ancestors around 259 million years ago. And from the outside, sphenodontians looked a lot like lizards. They had scales, round torsos, and long tails.

When Lizards Took Over the World

They also shared some kinda lizardy adaptations, like skin that sheds, feet and ankles that are specialized for running over rough ground, and a hooked foot bone to help them climb.

But on the inside, there were some important differences.

Sphenodontians had two openings in the skull behind their eyes, no external ears, and a fixed jaw bone.

They also had two parallel rows of teeth fused to the upper jaw, so it’s easy for researchers to tell they weren’t actually lizards, but a closely related group.

Fossils of these lizard-impostors have been found in lots of places like Europe, South Africa, Brazil, and the U.S., which tells us that they were already widespread during the Triassic Period. And they had lots of different kinds of lifestyles, too.

In Germany, researchers found a sphenodontian with delicate jaws and teeth for eating prey like insects.

And in Argentina, there was another that had wider teeth for grinding plants; plus, it was one of the largest sphenodontians ever known.

But, the shape of the world that these animals lived in was changing. Like literally.

During the Triassic Period, the supercontinent of Pangea was just beginning to drift apart into a northern and southern continent.

The climate of Pangea’s interior had been hot and dry - and some sphenodontians apparently tolerated these conditions pretty well.

But while they were diversifying and spreading all over the place by the late Triassic, the lizards were still there in the background, keeping a low profile. And by low I mean almost invisible. After Megachirella, there’s a gap of 75 million years in the fossil record.

When Lizards Took Over the World

The next oldest lizard fossils date to 167 million years ago in the Jurassic Period and come from the British Isles.

They include a variety of lizards, like members of the groups that today include skinks and maybe geckos.

Other than those, there are just a handful of Jurassic fossils of lizards, from places like Morocco, Kyrgyzstan, Siberia, and the US. But around the same time, the sphenodontians were just getting more diverse and widespread.

More than 60 species lived during the Jurassic on four continents, with some, like Clevosaurus spreading to the UK, Brazil, North America, Africa, and China. And these Jurassic lizard-wannabes continued to act a lot like lizards do today: some with sharp teeth for eating insects, some with wide teeth for grinding and shredding leaves, and even some living in marine environments as fish-eaters. But, things were about to change for both of these groups.

It had been nearly 100 million years since that first lizard, Megachirella, originally showed up, but their evolutionary strategy of just sticking around was about to pay off during the Cretaceous Period.

One of the first groups of lizards to spread around the world had relatively short limbs and bony plates on their bodies.

By the early Cretaceous, fossils from this group show up in Europe, Africa, Central, and East Asia, and Brazil. Then, by the middle of the Cretaceous, there were hundreds of species that belonged to the modern families of lizards - iguanas, geckos, skinks, varanids, and helodermatids or beaded lizards.

Some specialized features had appeared, too -- like the adhesive toe pads of tree-climbing geckos.

And the first of the teiioids -- which include whiptail lizards and tegus -- showed up.

And during the Late Cretaceous, between about 86 million and 66 million years ago, lizards really took off.

When Lizards Took Over the World

The group is known as the Iguanians which includes iguanas, chameleons, and many others spread around the world.

And there was also an explosion in the diversity of the mosasaurs.

And yes!

These reptiles were true lizards, too.

Isn't that weird?

I was today years old when I learned that!

Now, as the Cretaceous lizards caught their stride, the sphenodontians, after dominating the planet for 100 million years, started to dwindle.

As Pangea continued to break apart, sphenodontians became extinct in the northern hemisphere, and their southern range started to shrink.

By the end of the Cretaceous, the only place where they really remained abundant was Patagonia.

So, paleontologists think this means that the lizards were gradually outcompeting the sphenodontians, moving into their ecological niches.

As lizards expanded in diversity and range, those same things contracted among the sphenodontians.

Ok but, why?

Well, one possible explanation is that lizards were better suited to the changing climate.

As the continents formerly known as Pangea split apart, more land was exposed to more humid, coastal conditions and inland seas. And the result was more wet habitats.

Lizards seem to have adapted better to these new environments, because the most abundant lizard fossils are from wetter places in the northern hemisphere.

Eventually, those other lizards wanna be found themselves confined almost entirely to Patagonia, which was one of the drier refuges on the southern continent. But the final, almost-fatal blow for the sphenodontians was the end-Cretaceous mass extinction event 66 million years ago, the one that wiped out the non-avian dinosaurs.

Perhaps you’ve heard of it!

Only one genus in South America is known to have made it through the extinction event, but it would ultimately end up becoming extinct.

But we know that a few other groups must have held on, too...somewhere...because sphenodontians would show up again once more, in New Zealand much later.

When Lizards Took Over the World

Meanwhile, the vast majority of lizard families survived the end-Cretaceous extinction, and we’re still not sure why.

But with the sphenodontians almost totally gone, there was an explosion in lizard diversity, with the number of species increasing exponentially -- a phenomenon that may be tied to different populations becoming isolated as the continents kept spreading apart.

So, during the Cretaceous Period, lizards had looked pretty similar everywhere. But after that, they became more continent specific - like the dozens of unique species that are only found in South America today.

As for the sphenodontians, only one species is still around: the tuatara of New Zealand. Its ancestors -- whatever they were -- survived the end-Cretaceous extinction, making it the lone survivor of that family of lizard-like non-lizards that originated more than a quarter of a billion years ago.

So, from their slow start in the Triassic to the start of their rise in the Cretaceous, and their eventual explosion ever since, lizards have really played the long game.

And while they’re part of the group of the most diverse and widespread reptiles on the planet today, it’s worth pointing out that it took major global changes to make that happen.

The time when the lizards took over the world reminds us that there’s always another group waiting to step in when the conditions are just right.

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All Articles are based on publicly available information.