In a tropical forest in whatâs 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 youâre thinking: youâve
seen me do this before. This is where I introduce you to some totally
bizarre extinct animal, right? Well, Iâm just full of surprises. âCause this time, Iâm 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 werenât 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 werenât really widespread. Instead, there were lots of things that looked
like lizards, even though they werenât. 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 itâs easy for researchers to tell they werenât 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 Pangeaâs 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, thereâs 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 youâve 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 weâre 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 theyâre part of the group of the
most diverse and widespread reptiles on the planet today, itâs 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 thereâs 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 Itâs Okay to be Smart, Overview uses drone footage to tell mesmerizing stories about nature, engineering and more from a 10,000 foot view (literally). Itâs 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 monthâs 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!
Who remembers Liz from The Magic School Bus?