One night about 230 million years ago, a reptilian
predator stalked the desert in whatâs now Scotland. The critter could probably fit in the palm
of your hand, but it was a hunter nonetheless. It relied on its massive eyes to find prey
in the darkness, like insects and small vertebrates. Itâs known today as Scleromochlus, and it
was really well adapted to life in the desert. Its long legs and feet carried it across the
sand without sinking. And when it had eaten its fill, it curled
up with some other members of its species to sleep the day away in the shadows of the
dunes. Today, most fossils of Scleromochlus are found
curled up together, suggesting they were buried by dune collapses. And entombed with these little bug-hunters
is an evolutionary key. Because, even though it had no wings, Scleromochlus
had some features that link it with another group of mysterious reptiles: the pterosaurs. Some scientists think that the relationship
between pterosaurs and Scleromochlus is pivotal for understanding some of the greatest evolutionary
mysteries about pterosaurs:Â like, where they came from, and how they developed the
ability to fly. But, when pterosaurs first took flight, you
could say that it marked the beginning of the end for the winged reptiles. Because, strangely enough, the power of flight
-- and the changes that it brought about -- may have ultimately led to their downfall. To unravel the mystery of pterosaur origins,
we have to start by figuring out what theyâre related to. And this is quite a puzzle, because pterosaurs
had very fragile bones. This made them light in the air, but it also
meant that they didnât fossilize very well. And because they were highly specialized for
flight, they lost some of the features that could tell us who theyâre related to. Most paleontologists consider pterosaurs to
be a type of archosaur, a group that includes dinosaurs and crocodiles, among other things. And many experts also consider pterosaurs
to be closer to dinosaurs than they are to crocodiles, part of a grouping called ornithodirans
or âbird-line archosaurs.â But while pterosaurs are hard to place in
relation to other animals, itâs pretty easy to place them together as a group. All pterosaurs look like, well, pterosaurs
- with unusually large skulls, extra-long fingers, and of course, fully developed wings. One of the oldest pterosaurs currently known
is Eudimorphodon, from the late Triassic of Europe. And it belongs to one of the two main groups
of pterosaurs, a group called Rhamphorhynchoids. These pterosaurs were generally on the smaller
side, and often had longer tails, and had differentiated teeth. And they were distinct from the other group
of pterosaurs, known as the Pterodactyloids. Those pterosaurs typically had shorter tails,
longer wrist bones, big crests, and either simple teeth or no teeth at all. The Pterodactyloids would go on to include
the largest animal that ever flew, Quetzalcoatlus. But of course, the rhamphorhynchoids were
also fully equipped for flying, with complete wings and membranes that ran the length of
their legs. But for each group, the fossil record doesnât
tell us much about where they came from. Which leads us to the next big mystery surrounding
pterosaurs â how did they develop the ability to fly in the first place? Well, pterosaurs probably didnât go from
having no-wings to having full-wings in a single evolutionary leap. So when it comes to how pterosaurs got their
wings, there are three general theories. The first theory suggests that the ancestors
of pterosaurs lived in trees, and got around by jumping from branch to branch. This is called the arboreal leaping theory,
which sounds kinda fun. And it suggests that pterosaur ancestors may
have had a bit of membrane on their arms helped them glide for short distances. Then, this advantage mightâve selected for
a larger, partial wing that wouldâve let them go a little farther. And then eventually the same pressures would
lead to the membrane becoming a full wing. But other scientists think the ancestors of
pterosaurs might have been falling, rather than jumping. This theory -- known as the arboreal parachute
model -- suggests that, like some lizards today, pterosaur ancestors could have scrambled
around in trees and then fallen down to the next branch if they were threatened. This, again, wouldâve been safer with skin
connecting their arms and legs, so that advantage could have led to larger and larger membranes
until ... wings. But not all theories have the earliest pterosaurs
living in trees. Thereâs also the ground-up theory, which
suggests that they were terrestrial and bipedal. So in this model, the feeding habits of pterosaur
ancestors might have involved running and jumping to catch things like flying insects. In which case, having partial wings would
have helped them jump higher to catch their prey â and eventually, winged jumping could
have turned into flight. So which theory works best? Did pterosaurs come from bipedal, running
ancestors, or quadrupedal ancestors that lived in trees? To answer that, we need to go back to Scleromochlus. Scleromochlus wasnât a pterosaur. I mean, probably. But based on its physical features, most studies classify Scleromochlus as an early ornithodiran,
one of those âbird-line archosaursâ that include the pterosaurs. Which kinda makes sense because Scleromochlus
had a lot of traits in common with pterosaurs. It had shoulder blades that were a lot like
those of pterosaurs, with long bony projections where arm muscles attached. Now, Scleromochlus couldnât fly, but itâs
easy to see how these features could have been adapted by an early flying pterosaur. It also had four long toes, a really big but
lightly-built head, a square pelvis, and very small fibulae, or lower leg bones -- all traits
found in pterosaurs. So even if Scleromochlus itself wasnât the
pterosaur ancestor, many paleontologists think something like it was probably what ancestral
pterosaurs looked like. So the next question is: Did Scleromochlus
live in the trees, or on the ground? Well, its body gives us some important clues. Its back legs were much longer than its front
legs, and it had a long tail. It seemed to have walked its toes, rather
than on the flats of its feet. And its fingers werenât very long, so it
didnât have the grasping ability that many tree-dwelling animals have. You know what? In some ways, it actually kind of looked like
a jerboa â a little modern mammal that gets across the desert by hopping like a kangaroo. So Scleromochlus was likely terrestrial, and
either hopped or ran along the ground. And it wasnât alone. Several other early ornithodirans have also
been interpreted as small, jerboa-like animals. So the terrestrial ancestor fits perfectly
with the ground-up theory, right? Episode done, goodbye everybody! Haha, nah. Thereâs actually lots of evidence to suggest
that, even if their ancestors were bipedal, pterosaurs themselves were quadrupedal. They have shoulder joints that are oriented
for walking on all fours, and the weird tracks they left behind are all quadrupedal. So little Scleromochlus appears to be a bipedal,
ancestral cousin of a quadrupedal group. Some scientists think that when their wings
became large and heavy enough, early pterosaurs started to walk on four feet, so they didnât
fall on their faces. But many other scientists think there was
something between a bipedal ancestor and the pterosaurs â something that was arboreal,
and quadrupedal that hasnât been found in the fossil record yet So the evolution of pterosaurs is still foggy. But what we do know is that, over their 150
million years of existence, pterosaurs only became more well-adapted for flight. They probably had unidirectional air flow,
just like modern birds do, making them very efficient at breathing, even while flying. And some pterosaurs also had weird little
feathers. And even though pterosaurs are all lightly
built, the later pterosaurs were even lighter. They had more air chambers in their bones
and fewer vertebrae. All of these things, and more, helped pterosaurs
conquer the skies ⌠eventually. Fact is, for the first 100 million years or
so of their existence, pterosaurs werenât very diverse; the earliest ones were all just
little bug eaters. But around 125 million years ago, they started
to change. They became dramatically more diverse, nearly
doubling in the number of species. They grew extravagant crests and began hunting
fish and other vertebrates instead of just bugs. Familiar species like Pteranodon and the largest
creature to ever fly, Quetzalcoatlus , took over the skies. What happened to make them so diverse? A little evolutionary pressure from another
group related to those early Ornithodirans... birds. Birds began to show up in the fossil record
during the late Jurassic Period, about 160 million years ago. So birds and pterosaurs coexisted for 60 million
years. And it wasnât until AFTER birds evolved
that pterosaurs started to move out of their niche of âtiny, flying bug-eatersâ. In fact, some fossil formations, like the
Jehol group in China, preserve lots of different birds and pterosaurs together, each of which
ate slightly different things in slightly different environments. But one of the things that pterosaurs did
best was something bodybuilders know a bit about: getting big. Pterosaurs grew to sizes that birds couldnât
reach. The biggest birds of the late-Cretaceous,
like Avisaurus, had wingspans no more than 2 meters across. But pterosaurs like Quetzalcoatlus reached
at least 11 meters! These larger sizes allowed pterosaurs to conquer
ecological niches that werenât available to birds, like flying long distances over
the open ocean in search of food. So pterosaurs and birds coexisted peacefullyâŚ
until things changed. Weâve all seen this movie by now! The end of the Cretaceous Period, some 66
million years ago, was marked by the extinction event that wiped out the non-avian dinosaurs. But the animals that did survive had something
in common: they were all pretty small. Being small meant mammals and birds could
live on less food, a helpful trait in a post-impact world. But Pterosaurs had grown bigger and bigger
to find roles where they could hold their own against birds. By the late Cretaceous, they all had wingspans
between 2 and 10 meters, meaning that the smallest pterosaur at the time of the extinction
event was about the size of a bald eagle. So in the end, it may have been that their
size â impressive as it was - doomed them to extinction. Perhaps -- just perhaps -- if they'd stayed
on the ground, or remained the size of little Scleromochlus, pterosaurs would still be around,
sharing our skies today with birds. So What do you think? Did pterosaurs acquire the ability to fly
by living in the trees or on the ground? Did they jump to catch air, or did they fall? Let us know in the comments which hypothesis
you support and why! Always gotta give a shout out to this monthâs
Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng, and Steve! Go pledge your support patreon.com/eons and
become an Eonite! And also thanks for joining me in the Konstantin
Haase Studio. If you like what we do here, then subscribe
at youtube.com/eons.
Eons is such a great new resource and learning tool for not just my understanding of the history of life but also for my students. I've used several videos of theirs to illustrate content.
Those little bug eating guys are hilarious! I love them!! Theyâre so baby đł