Pterosaurs are not dinosaurs!

This post isn't the kind of post I normally do, but it stems from a conversation I had with someone at Science Borealis when they shared my Canadian pterosaur post and incorrectly called them dinosaurs. Being a pterosaur palaeontologist, this is something that I deal with constantly, the misconception that pterosaurs are 'flying dinosaurs'. I am going to try to explain why this is scientifically inaccurate.

The first thing to understand is that both the terms "dinosaur" and "pterosaur" are scientific terms with specific definitions and meanings, just like a mammal, reptile, or fish. All of these are scientific names that are used within the common tongue, but hold specific scientific definitions. Dinosaur stems from Dinosauria, the name of the group that includes all dinosaurs, while pterosaur represents a member of the Pterosauria or Pterosauromorpha. There is a tendency in popular culture to call any large extinct animal, particularly if it lived during the Mesozoic, a dinosaur. Unfortunately, this is incorrect. Pterosaurs were not dinosaurs, marine reptiles (like plesiosaurs, ichthyosaurs, and mosasaurs) were not dinosaurs, and neither was Dimetrodon, that weird sail-backed reptile from the Permian. In fact it's more closely related to you and I than it is to dinosaurs as it is a mammal-like reptile. To learn more about that, you can go to a blog I previously wrote for Jurassic Forest called Mesozoic Musings.

So how are pterosaurs and dinosaurs related, if at all? 

Pterosaurs and dinosaurs are closely related, meaning they share a number of features, but are still distinct groups, or clades as we call them in biology. They both belong to a group called the Archosauria, which includes crocodilians, dinosaurs (including birds, as they evolved from dinosaurs and therefore are dinosaurs by definition), and pterosaurs. Archosaurs share a number of characteristics including an antorbital fenestra (a hole in the skull in front of the eye) and teeth set in sockets. However, early in archosaurian evolution there was a split between crocodilians and their close relatives (the crurotarsans or pseudosuchians) and birds and their closest relatives, including dinosaurs and pterosaurs (known as the avemetatarsalians). Avemetatarsalia is a mouthful, but it's pretty easy to break down. Basically this group is united by a bird-like ankle, among other features. Within this group is another group called the Ornithodira, which means bird-neck, again uniting the group with features of the neck that are bird-like, and includes both pterosaurs and dinosaurs.

So now we know that pterosaurs and dinosaurs are united by a number of features including (but not limited to) an antorbital fenestra in the skull, teeth set in sockets and a bird-like structure of both the ankle and neck.

Cladogram from Nesbitt (2011) showing the relationships in the Archosauria, including the Avemetatarsalia, Ornithodira, Pterosauromorpha (including pterosaurs and their close relatives) and the Dinosauromorpha.

What features separate pterosaurs and dinosaurs?

There are a large number of features that distinguish each group, and they are very different anatomically, but I will only mention some of the major ones, specifically features that pterosaurs have and dinosaurs do not. Pterosaurs are highly modified for flight, and right now, we don't fully understand how they evolved. Several of the features that distinguish them from dinosaurs and other animals are related to this. The two most obvious features include:

1.  An elongated 4th digit (finger) to which a flight membrane attached. Pterosaurs have lost their fifth digit (their pinky finger), but have an extremely long 4th finger. Imagine you had no pinky, but a ring finger that was longer than the rest of your arm. These is a feature unique to pterosaurs, and found in all pterosaurs. 

2. Possession of a pteroid bone. In the pterosaur wrist, an additional bone is present called the pteroid.  This bone points most likely antero-medially (forward and into the middle in flight) and likely controlled the position of the wing membrane in between the wrist and the body. This bone is not found in any other animal.

Drawing of the wing of the pterosaur "Santanadactylus pricei" showing the elongated 4th finger and pteroid bone. Image from Witton (2013), redrawn from Wellnhofer (1991).

There are a number of other anatomical features that separate pterosaurs from dinosaurs that are unrelated to the wing, including several features in the skull (e.g. their skull is very long with respect to their vertebral column), vertebrae (e.g. their neck vertebrae are long compared to other vertebrae), and legs. In total, there are at least 13 characters that unite the Pterosauromorpha, that are not found in combination or at all in dinosaurs. 

Hopefully this has shown why palaeontologists cringe whenever someone calls a pterosaur a flying dinosaur. To quote Brian Switek's article on the same topic, "calling a pterosaur a dinosaur is an error of the same order of magnitude as saying that our species is a marsupial". So next time you talk about a pterosaur, or write an article about one, please don't call them a dinosaur! 

References

Nesbitt, SJ (2011) The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pages.

Wellnhofer, P (1991) Weitere Pterosaurierfunde aus der Santana-FOrmation (Apt) der Chapada do Araripe, Brasilien. Palaeontographica 215: 43-101.

Witton MP (2013) Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press, Princeton, USA. 304 pages.