- The early years of the career of geologist William Lee Stokes were a fossil hunter's dream.
He spent the summers from 1939 to 1941 digging up incredible skeletons of Diplodocus and Allosaurus in Utah, establishing what is now known as the famous Cleveland-Lloyd Dinosaur Quarry.
You might think it'd be pretty hard to top a discovery like that, but he would go on to make a find that would kick off decades of debate among paleontologists.
In 1952, Stokes was in Arizona studying uranium deposits when he stumbled upon something unexpected.
Pterosaur fossils.
And not just any pterosaur fossils, but footprints.
He'd found one of the first pterosaur trackways ever to be discovered.
He named this discovery Pteraichnus.
And today, almost 70 years later, pterosaur tracks have been identified in the fossil record from the Jurassic and Cretaceous periods at over 50 sites around the world.
These trackways give us a unique glimpse into part of pterosaur life that most people probably don't think about that often, because if you know one thing about pterosaurs, it's that their flyers.
And while pterosaurs may be well known for their domination of the skies in the Mesozoic era, they didn't live their entire lives in the air.
So how did we figure this out?
And what were they like when they finally came down?
(soft music) Long before paleontologists discovered the tracks of pterosaurs, they discovered their bones.
Today, we know that pterosaurs are a type of Archosaur, which means they're part of the larger group that includes dinosaurs, birds and crocodilians, but this wasn't always so certain.
Back in the late 1700s and early 1800s, paleontologists had only identified a handful of pterosaur fossils, so they didn't have much material to study.
This resulted in some creative interpretations of what kind of creature these bones came from.
The first person to describe a pterosaur specimen was an Italian historian named Cosimo Alessandro Collini.
And he thought that the fossils belonged to some sort of marine animal, which, yes, does seem pretty silly now, but this was something entirely new at the time.
Using the only fossil of a pterosaur that had been discovered at that point, Collini made the best interpretation he could based on the limited information that he had.
All science, including paleontology, has to start somewhere.
It's a process that self-corrects over time as more data is collected.
By the early 1800s, paleontologists had realized that the limbs of pterosaurs were actually wings, but beyond that, they weren't much closer than Collini to really understanding these animals.
Several of them decided that the wings were similar enough to those of living bats that they must have belonged to some extinct species of bat.
And with bats on the brain, these researchers thought that if it looked like a bat, it must have also moved like most bats.
This led them to assume that pterosaurs would've been skilled flyers but slow walkers that would've had to drag themselves along the ground on their bellies to get around on land.
Despite eventually being correctly identified as flying reptiles rather than bats, this image of pterosaurs as clumsy walkers stuck around well into the late 1900s.
And it only changed with the discovery of a lot more pterosaur fossils.
In 1983, a paleontologist named Kevin Padian used this new wealth of pterosaur fossil material to conduct a more in-depth study of their anatomy.
He suggested that the pelvis and hind limbs of pterosaurs fit together like those of birds instead of like bats.
This would mean that pterosaurs would've been bipedal and walked on their toes with their legs directly under their body, just like we see in birds.
And Padian even took it one step further.
He proposed that not only could pterosaurs walk on two legs, they could do it well.
This was a really surprising idea at the time, one that spurred decades of debate over the true terrestrial capabilities of the pterosaurs.
For years, paleontologists fought back and forth over whether pterosaurs were more bat-like or birdlike, unable to reach a consensus.
The thing is though, pterosaurs aren't bats or birds.
Each of these groups evolved the power of flight from very different sets of anatomical building blocks, so there's no reason to think that they would all end up looking the same or flying exactly the same way.
pterosaurs have their own unique anatomy that isn't quite like anything alive today, so how do we start to understand their behavior?
Well, if bones aren't enough to figure it out, another way is to look for the actual traces of behavior they left behind.
We needed tracks, which presented a whole other set of questions to answer.
First being, how do you know when you've found a pterosaur track?
Identifying exactly what animal made fossilized tracks isn't easy, especially for animals that have gone extinct and don't have modern relatives.
And it's just about impossible to be sure exactly which species made a particular set of tracks unless they literally died in them.
So fossilized tracks actually aren't given the same name as the animal that made them.
Instead they're assigned their own unique ichnotaxon, which is just a scientific name for a specific trace fossil.
So when Stokes named those footprints he found Pteraichnus, he wasn't assigning the tracks to a new species of pterosaur, he was just broadly identifying that the tracks were made by some kind of pterosaur.
But even general identifications like this can be hard to make when it comes to trackways.
Like the arguments surrounding the bones of pterosaurs themselves, Pteraichnus fossils also have a long history of debate when it comes to who the tracks were really made by.
Stokes published his paper identifying the tracks as belonging to a pterosaur in 1957 based on the unique anatomy of the hand and wrist shown in the impressions.
You see, pterosaur wings have an elongated fourth digit that would've supported most of the wing membrane.
And they also have a unique bone in their wrist called the pteroid that would've supported the portion of the membrane between the wrist and the shoulder on the front edge of the wing.
Stokes noticed several curved grooves preserved alongside the tracks, and he believed that these grooves were likely made by either that fourth digit or the pteroid dragging along on the ground.
And these distinctive hand prints were paired with matching footprints, indicating to Stokes that pterosaurs walked on all fours.
But in 1984, however, Kevin Padian and one of his colleagues disagreed.
Remember, just a year earlier, Padian had published that paper, suggesting that pterosaurs would've walked on two legs like birds.
So as far as they were concerned, the tracks Stokes had found couldn't possibly have been made by a pterosaur because, based on Padian's research, pterosaurs walked on two legs, not four.
They argued that the tracks were made by a crocodilian instead.
They gave a side by side comparison of Pteraichnus to modern caiman tracks, and while they may appear similar at first glance, this was far from the final word on the subject.
Because over the next 10 years, we found more and more Pteraichnus fossils.
So in 1995, a group of paleontologists decided to use all this new data to reevaluate the origin of these footprints yet again.
And they totally rejected the idea that the tracks could have been made by a crocodilian for a number of reasons.
Their most incriminating piece of evidence was that crocs are known to drag their tails, which creates an impression on the ground.
But none of the Pteraichnus tracks had ever been found with a tail drag impression.
That, combined with the hand anatomy shown in the print, which featured three short digits followed by an elongated fourth digit, helped them to confidently determine that the tracks had been made by a pterosaur.
After almost 40 years of debate, this paper was just the first of many to confirm that Stokes had been on the right track all along.
And once Pteraichnus fossils were correctly identified as pterosaur tracks, this gave us a whole new body of information to draw on for research on pterosaur terrestrial behavior.
Like in 1996, another paleontologist analyzed pterosaur body fossils alongside Pteraichnus trackways to determine that pterosaurs probably had a flatfooted semi-erect posture while they walked on all fours.
Although details of their terrestrial behavior are still debated, most paleontologists agree on this general semi-upright stance.
We even know how one pterosaur landed.
Footprints published in 2009, suggest that it probably slowed its descent using its wings and landed lightly on its feet, dragging its toes and hopping forward slightly before putting its hands down.
And even more recent discoveries have helped flesh out the idea that these animals were just as busy on the ground as they were in the air.
For example, in 2012, an in-depth study of the skull of a pterosaur that lived in what's now the UK during the Cretaceous, revealed that it probably used its jaws to shear meat off of scavenged carcasses, including dead dinosaurs.
And in 2017, a group of paleontologists reported on a pterosaur nesting ground discovered in China.
They found hundreds of fossilized eggs, some of which even contained preserved pterosaur embryos.
The nests were found in several distinct layers on top of one another, which supports the idea that not only did some pterosaurs nest in colonies, but they've returned to the same nesting sites year after year.
These studies have shown us that pterosaurs didn't struggle on land like early researchers once thought, instead they used that time on the ground for things like eating and nesting, pretty significant parts of their lives.
It's hard to think of pterosaurs without picturing them soaring through the air, but their fossils have shown us that there was a whole other side of their lives beyond flying.
And despite their surface level similarities to modern day flyers, unlocking the secrets to their terrestrial behavior wasn't just as simple as saying they were just like a bat or just like a bird.
Instead, pterosaurs were a unique evolutionary experiment, one that developed their own way of balancing life in the air with life on land.
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