Meet the Pterosaurs
Pterosaurs, colloquially known as “pterodactyls,” were flying reptiles that lived during the Mesozoic Era, soaring in the skies above dinosaurs. Despite coexisting with dinosaurs, pterosaurs themselves were not dinosaurs (an important point that we’ll discuss in a minute). Pterosaurs were warm-blooded, flying reptiles and the earliest known flying vertebrates, first appearing around 228 million years ago in the Late Triassic Period, not long after we see the earliest dinosaurs. They went extinct at the end of the Cretaceous Period 66 million years ago, along with non-avian dinosaurs and marine reptiles. All things considered, they were a very successful group, ruling the skies for over 150 million years.
There are three groups of vertebrates that are known to have developed flight: birds, bats, and pterosaurs. Each of these groups has figured out a different way to achieve powered flight. Pterosaur wings were formed by a membrane supported by a hugely expanded fourth finger, as seen below.
The membrane stretched from the tip of this elongated finger and attached to the hind limb. Where exactly it attached to the hind limb isn’t known for every pterosaur, and the position seems to have varied between different groups. Pterosaurs had an additional membrane in front of their arms supported by a bone unique to pterosaurs called a pteroid, which provided a stiff, leading edge for flight.
Pterosaurs were very diverse in both size and ecology. Some had wingspans of around one foot and could have nestled in the palm of your hand. The largest had wingspans approaching 40 feet and stood as tall as giraffes! There was a tremendous amount of variation in their beaks, crests, teeth, necks, tails, and wings. For example, pterosaurs like Anurognathus had small, pin-like teeth for catching insects. Others, such as Rhamphorhynchus or Scaphognathus, had long, slender teeth for spearing fish and small mammals. Dsungaripterus possessed short, rounded teeth, perfect for crushing hard-shelled animals like clams and crabs. Species like the famous Pteranodon had long, toothless beaks and swallowed prey whole, while Tapejara used its short, toothless beak to eat fruits and seeds. Pterodaustro filter fed with bristle-like teeth. The largest pterosaurs of all, the azdarchids, likely even preyed on small dinosaurs!
Wait, pterosaurs aren’t dinosaurs?
Despite being commonly referred to as “flying dinosaurs,” pterosaurs were in no way dinosaurs. In a previous post, I discussed certain features dinosaurs have that other animals, including pterosaurs, don't- such as a unique hole in the hip and legs held directly underneath their body. Though pterosaurs and dinosaurs are different groups, they are still related to one another. Let's take a look at exactly how they're related.
Here we have a phylogenetic tree, which shows how different organisms are related to each other. Although they have a fancy name, they're basically family trees, only upside down compared to the way you’re used to reading them. In these trees, everything towards the bottom is older (like grandparents and great-grandparents).
Both pterosaurs and dinosaurs are reptiles, and lie within a group of reptiles called archosaurs. The group Archosauria can be divided into two major sub-groups. To keep things simple, we can call these two divisions “crocs” (which includes modern crocodilans and their extinct relatives) and “not-crocs” (archosaurs that are not closely related to crocodiles).
We can ignore the crocodilian side of the tree today. Within the “not-crocs” branch there is a group called Avemetatarsalia, which basically means that the animals in this group have bird-like foot bones.
We can further split this group into "pterosaurs" and "dinosaur-like animals."
As you can see, while pterosaurs and dinosaurs do share a common ancestor, they are not in the same group! Notice on the right side of this tree there is a node called "Dinosauria." As you might expect, this group contains everything we call dinosaurs, and pterosaurs are not inside that bracket.
Thus, clarifying that pterosaurs are not dinosaurs is not just splitting hairs or being pedantic. It's like saying humans are a type of marsupial- it's not a technicality; it simply isn't true.
Many pterosaurs had very large heads relative to their bodies. Next time you’re in a museum with a pterosaur displayed, pay attention to how small the torso really is in relation to the size of the head and wings.
The bodies of pterosaurs were covered in fur-like filaments called pycnofibres. These pycnofibres looked like fur at a glance, but were more similar in structure to feathers. This is why modern pterosaur reconstructions are shaggy rather than scaly. A 2019 paper even described a pterosaur fossil found with branching pycnofibers which bore a remarkable resemblance to feathers!
The membranous wing of a pterosaur may bring another, living winged animal to mind- a bat. However, while bat wings are supported by a membrane, there are some important differences. The wing membrane of a bat is extended between five widely-spread fingers (yes, that means bats are flying around with jazz hands!). But pterosaurs, remember, have a membrane supported by a single finger.
The structure of the wing membrane itself is also quite different between bats and pterosaurs. Bat’s membranes are little more than some blood vessels between a couple layers of elastic skin. Pterosaur wing membranes were thicker, stronger, and more complex, being composed of layers of skin, sheets of muscle, blood vessels, and unique structures called actinofibers. Actinofibers were hair-thin fibers made of keratin (the same material that makes up hair and fingernails) that ran along the length of the wing. This would have given the wing extra strength and durability.
Speaking of pterosaur wings, there’s been a lot of research into how pterosaurs took off from the ground. It was originally thought by some paleontologists that, like birds, pterosaurs were bipedal and took off by simply jumping off the ground with their hind limbs and flapping. However, we now know that pterosaurs were not bipedal, and instead performed a “quadrupedal launch” to get off the ground. That is, their front limbs were doing all the heavy lifting.
Here’s a video of quadrupedal launching in action:
Studying pterosaurs isn’t as easy as studying larger animals like dinosaurs because most pterosaur fossils look like roadkill, crushed flat by the weight of overlying sediment. The walls of pterosaur bones are extraordinarily thin and don’t survive the fossilization process very well, making their fossils very rare compared to the dinosaurs they lived alongside. It's no surprise that there are just under 200 species of pterosaur known compared to over 1,000 species of dinosaur.
Given the right circumstances, however, pterosaurs can be preserved with an incredible amount of detail. Certain rocks preserve fossils extremely well, revealing structures like wing membranes, tail vanes, soft tissue crests, and even eggs with baby pterosaurs inside!
One of the most intriguing unsolved puzzles in pterosaur research is figuring out what sort of animal they evolved from. Their origins are still shrouded in mystery. For some time, it was thought that a reptile from the Triassic Period known as Scleromochlus was near base of the pterosaur family tree. However, a paper published in 2020 revealed this animal wasn't quite what we thought it was, putting us back to square one in regards to pterosaur origins.
There’s still a lot we don’t know about pterosaurs. Where did they come from? What colors were their pycnofibres? How did they interact with other species in their ecosystem? Did flightless pterosaurs exist? How did they breed? How many eggs could different species lay? To answer these questions, more fossils need to be found, prepared, and described. There’s always more research to do!