Why Aren’t Pterosaurs Dinosaurs?

By Jude Coleman

If your favorite dinosaur is a pterodactyl, bad news: It wasn’t a dinosaur. It was a flying reptile. 

Because dinosaurs are also reptiles, it can be tempting to assume all large extinct reptiles were dinos. But dinosaurs were in their own taxonomic group of animals that evolved from their ancestors separately from flying reptiles. That means Pterodactylus — a genus in a larger group of animals called pterosaurs — are in an entirely different group of reptiles. This is similar to the common misconception surrounding “marine dinosaurs,” which didn’t exist, said Randy Irmis, the Natural History Museum of Utah’s Curator of Paleontology. Instead, there were many species of marine reptiles, which are lineages that evolved separately from dinosaurs.

Pterosaurs evolved during the Late Triassic Period, around 230 million years ago, and lived until the end-Cretaceous extinction 66 million years ago. “We see a large diversity of different types of pterosaurs, and there's a lot of variation in their body size, skull shape, and their teeth,” Irmis said. There were some the size of modern-day sparrows, whereas others were nearly the height of a giraffe. Because of the variety in their size and bodies, pterosaurs would have had a range of lifestyles. It’s likely that small pterosaurs ate insects, while larger ones may have dined on fish or even small dinosaurs. Researchers have even found species like Pterodaustro that had numerous long skinny teeth analogous to whale baleen, indicating that they might have filtered food from the water. But because paleontologists only have fossils to go off of, there are still many unknowns about pterosaurs’ habits and biology.

One thing evident from their fossils is that to be able to fly, pterosaurs had light, hollow bones similar to birds. As a result, their skeletons were very fragile, which means geologic forces were often too rough to preserve the bones as fossils. In Utah, many fossils are preserved in ancient rivers, streams and floodplains. This moving water easily broke the delicate pterosaur bones, Irmis said. Few pterosaur fossils hail from Utah as a result. “The bad news is that these fluvial environments don't do a good job of preserving pterosaur bones, but they are great places to preserve pterosaur footprints,” he said.

As part of the Natural History Museum of Utah’s DinoFest: Prehistoric Skies, in the paleontology collections visitors will be able to see fossilized Jurassic pterosaur tracks unearthed from central Utah’s San Rafael Swell — some of the first pterosaur tracks ever discovered. Other DinoFest displays featuring ancient aerial wonders are open January 31 through February 1.

Dinosaurs, Birds, And Pterosaurs

Paleontologists know that pterosaurs are a distinct group from dinosaurs because the fossil record shows they evolved from different species. Dinosaurs — ornithischians and saurischians — evolved from a common ancestor that pterosaurs don’t share. There’s also anatomical differences that put pterosaurs squarely in the “not-dino” category. Dinosaurs have a hole in their hip socket, for example, while pterosaurs’ hip sockets are a bony depression. And inner ear structures detectable with CT scans of fossil skulls are also distinct: in dinosaurs, the semi-circular canals are short with a wide diameter, whereas in pterosaurs the canals are thin, tall and hoop-like.

Diagram of homologous bones and analogous flying adaptions in the forelimbs of three groups of flying vertebrates both recent and fossilized: pterosaur (Pterosauria), bat (Chiroptera), bird (Aves)

Just because pterosaurs have wings and can fly doesn’t make them closely related to birds, either. For starters, birds are dinosaurs, evolving from small meat-eating (theropod) dinosaurs during the Jurassic Period — much later than flying reptiles. “[Pterosaurs] are a completely different reptile lineage from birds,” Irmis said. “Also, the structure of the wing is completely different.” Pterosaurs wings had a thin skin membrane instead of feathers. A defining feature is four fingers:  three that protrude from the wing as claws and a fourth extending all the way down the length of the leading edge of the wing. Birds have three fused fingers (the spindly end of a tasty chicken wing!), which stop at the front of the wing. Their fingers are also completely internal within the wing structure — which is why you’ve never seen a bird with tiny hands.

After non-flying dinosaurs and pterosaurs went extinct, flight evolved independently again about 55 million years ago: bats. It’s thought that bats arose from small, forest-dwelling mammals, making them yet another flying vertebrate animal distinct from pterosaurs and birds.

Why Didn’t Pterosaurs Survive?

At the end of the Cretaceous Period, pterosaurs met their demise like many animals, and the only dinosaurs to continue on were birds. So, if birds survived, why didn’t pterosaurs?

One reason could be that there wasn’t enough food to support their large body size, Irmis said. By the time the Cretaceous extinction came around, the only pterosaurs on the scene were large, such as the species Quetzalcoatlus. Unlike reptiles today, pterosaurs were also warm-blooded. That, combined with their large size, meant they would have needed a lot of food. But Late Cretaceous pterosaurs likely ate a specialized diet, evidenced by their long toothless beaks. “A reasonable hypothesis is that it's the combination of being large-bodied and an ecological specialist that doomed pterosaurs at the end,” Irmis speculated. In contrast, small, generalist birds — similar in size to sparrows or chickadees — would have needed far less food and been able to better make do with what was available.

Although birds survived to carry on the dinosaur legacy, nothing on Earth comes close to a pterosaur today; there are no flying reptiles. While there are some “flying” lizards, they don’t truly fly. Instead, they glide through the air on thin flaps of skin.