5 “Dinosaurs” That Weren’t Dinosaurs At All

“You may think that an animal is a dinosaur because it’s large and scaly,” paleontologist Danny Barta, then a PhD Candidate at the Richard Gilder Graduate School, American Museum of Natural History, in a 2018 video from the Museum. “But these traits evolved much earlier in evolutionary history.”

These are far from the only examples of things you may have thought were dinos, but actually aren’t. So, who else qualifies?

Pteranodons and Pterosaurs

Yes, we know – they’re one of the most famous types of prehistoric creature out there; one of the Big Four you learn of as a kid (alongside Tyrannosaurus rex, Triceratops, and Stegosaurus, of course); heck, they’re literally five out of the six main characters in Dinosaur Train. But no: Pteranodons – and their cousins, pterosaurs – are not, in fact, dinosaurs.

“Dinosaurs and pterosaurs separated from one another nearly 250 million years ago,” explained Barta. “250 million years ago is really a long time geologically,” he added, “so dinosaurs and pterosaurs, while they may be close cousins in an evolutionary sense, are really quite widely separated.”

An apt comparison might be humans and chimps. We share a common ancestor, and we’re pretty closely related – but still, we’re definitely not the same thing. 

There are measurable physiological differences separating the two clades: dinosaurs, notably, have a hole in their hip socket and a crest along their upper arm, while pterosaurs don’t; pterosaurs, conversely, have a super-long fourth finger that supports their wing – like a bat – while dinos not only lacked this but tended to lose digits the further through time they survived.

But “just because pterosaurs aren’t dinosaurs doesn’t make them any less amazing,” Barta pointed out. “They ruled the skies for over a hundred and fifty million years, and I think that’s really cool.”

Mosasaurs

Okay, come on now: this one not only has “saur” right there in the name, but it’s also a star attraction in Jurassic World (the resort) and the hero of Jurassic World (the film).

But, alas, the mosasaurs were not dinosaurs. As with plesiosaurs, the biggest giveaway is its habitat: “With the exception of some birds, for example penguins, dinosaurs lived on land, not in the sea,” points out the Natural History Museum, London. 

That’s not the only feature that disqualifies the mosasaurs from dinodom, though. Dinosaurs “had an upright stance, with legs perpendicular to their body,” the Museum explains – “this is the main feature that sets dinosaurs apart from other reptiles.” As aquatic, swimming reptiles, mosasaurs had fins, or flippers, or at best little flappy leggies that hung out on either side of their body like a crocodile’s.

Then there’s the issue of… well, issue. Dinosaurs laid eggs; mosasaurs didn’t. They were quite the mish-mash of features from a modern-day perspective, actually: they were scaly, like lizards, but most likely warm-blooded; they lived in the water, but breathed air rather than through gills – and, very unusually for a reptile, they gave birth to live young.

So, if not dinosaurs, then what? Well, one way to think about it is this: while the dinosaurs – or at least, the therapods – evolved down into birds, mosasaurs are most likely related to today’s snakes or monitor lizards.

Dimetrodons

This one isn’t so immediately recognizable, which is a shame, because it’s doing its best. The genus Dimetrodon was big – potentially over 2 meters (6.6 feet) in length, depending on species – scaly; ate “basically whatever they wanted,” Kirstin Brink, a paleontologist and paleontologist at the University of Manitoba in Canada, told Discover Magazine last year; and just look at that sail!

But Dimetrodon species were not dinosaurs – in fact, the extinction of these huge predators actually predates the dinosaurs by around 60 million years. That may be why it’s still such a mystery to modern experts: its sail, for example, is “definitely confusing,” Brink said, and even its classification is “still a bit of a mess.”

What we do know, though, is that Dimetrodon species weren’t just not-dinosaurs – they were actually more closely related to mammals than other dinosaurs. “We are actually closer related to Dimetrodon than any other reptile is,” Brink told Discover. 

Now, to be clear, the animals likely weren’t the ancestors of modern mammals: they were what’s known as synapsids – a group that today consists only of mammals, but that’s just because everything else in the family tree died out. 

Outside of its genealogy, though, perhaps you can see a few other telltale signs that Dimetrodon doesn’t belong in the dinosaur group. First up, there’s that leg position: squat and sprawling, not upright and underneath, like dinosaurs. Check the skull, and you’ll see a single hole behind the eye socket – a small clue, but an important one, since dinosaurs “had two holes behind the eye socket,” the Natural History Museum, London explains, with “large, strong jaw muscles [that] went through the holes to attach directly to the top of the skull.”

Crocodilians

They’re big, scaly, scary, and famously have been around since just about the dawn of time. So surely, crocs and alligators are dinosaurs, right?

Nope! Like Pteranodons, these reptiles branched away before the dinosaurs really became dinosaurs at all – in fact, they’re further away, genealogically speaking, than Pteranodons are from the dinosaurs (and of course, with their sprawling leg position, they don’t qualify as dinosaurs regardless of when they split off).

Now, we may think of crocs and gators as scary mofos today, but nature really didn’t mess around back in the Cretaceous period. Take Deinosuchus, for example – the name literally means “terrible crocodile”, and that’s probably because it was some 10 meters (33 feet) in length, weighed up to five tonnes, and had a bite force potentially more than six times as strong as a modern saltwater crocodile.

It’s not surprising, then, that experts think this mega-dile likely chowed down on real dinosaurs – even quite large ones, like the nine-meter-long, four-tonne Kritosaurus, whom it lived alongside in what is now Texas.

And if the crocodilians have a motto, it’s obviously “if it ain’t broke, don’t fix it”. Not only do today’s crocs look virtually identical to their ancient ancestors – albeit smaller – but they also likely hunt the same way, using “death rolls” that were perfected some 80 million years ago by great-great-grandma Deinosuchus.

Tanystropheus

Tanystropheus was a weird lookin’ creature: 6 meters (20 feet) long from nose to tip, and more than half of it was neck. 

It was, Stephan Spiekman, a paleontologist at the University of Zurich, told the New York Times in 2020, “one of the most baffling animals that ever lived.” 

“How could this animal even breathe or swallow?” he said. “And then there is the evolutionary question: Why on earth did this animal evolve this ridiculously long neck?”

Now, Tanystropheus has basically been baffling paleontologists ever since it was first described back in 1852. At first, its neck was so long as to convince its finder that it was actually two animals instead of one. Then, people thought those stretched-out cervical vertebrae must have supported a wing membrane, and classified it as a pterosaur. But even when they were getting it so wildly wrong, the experts all agreed on one thing: Tanystropheus wasn’t a dinosaur. 

So why not? After all, it’s scaly, ancient, and – at least some of the time, potentially – a land animal (most likely it was at the very least semi-aquatic, if not mostly aquatic).

On the other hand, it lacks the upright legs of a dinosaur – once again, we’ve got a waddler on our hands. They also lived a tad too early to comfortably be a dinosaur, hanging out in the middle Triassic rather than the dino heyday of the Jurassic and Cretaceous eras. 

Mostly, though, Tanystropheus is just in the wrong part of the evolutionary tree. They’re descended from a different line from dinosaurs – think humans vs apes; pteranodons vs dinosaurs; just even further back than that. 

Still, it was “quite successful in evolutionary terms, living for at least 10 million years and occurring in what is now Europe, the Middle East, China, North America, and possibly South America,” Spiekman said in a 2023 statement. 

That was despite its ridiculous neck, which Spiekman and colleagues had found was undoubtedly a weak point for the animal: an inspection of some fossils of Tanystropheus heads and necks – just the head and necks, brutally ending before the torso as if ripped in two – revealed clear bite marks at the break, suggesting a predator had gone straight for the neck in its search for a tasty meal.

Ah, well. “Evolution is a game of trade-offs,” Spiekman said. “The advantage of having a long neck clearly outweighed the risk of being targeted by a predator for a very long time.”