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!
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!
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 awhile 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.
They also shared some kinda lizard-y 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 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.
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.
The group known as the Iguanians -- which includes iguanas, chameleons, and many others
-- spread around the world.
And there was also an explosion in 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 lizard wanna bes 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.
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.
Now before you go, you should check out an amazing new show called Overview on Terra.
Hosted by my friend Joe Hanson from It’s Okay to be Smart,
Overview uses drone footage to tell mesmerizing stories about nature, engineering and more from a 10,000 foot view (literally).
It’s gorgeous, informative and soothing stuff.
Check out the link in our description and tell them Eons sent you!
Ok who's ready for some puns?
Brace yourself, Blake...
I-guana thank this month’s Eontologists: Sean Dennis, Jake Hart, Annie & Eric Higgins,
John Davison Ng, and Patrick Seifert!
Become an Eonite by supporting us at patreon.com/eons.
And remember Eonites get perks like submitting a joke for us to read!
Like this one from Nora, you ready?
Why was the sedimentary rock so cheap?
Because it was on shale.
Where was I?
I can't even.
And as always thank you for joining me in the Konstantin Haase studio.
If you like what we do here, be sure to subscribe at youtube.com/eons. Thank you!