What is - and is NOT - a Dinosaur?

Often, creatures that are commonly called "dinosaurs" are not actually dinosaurs, but some other type of prehistoric animal. Today, we want to answer some questions, such as:

What is a dinosaur? What is NOT a dinosaur?

Are Plesiousaurs dinosaurs?

Are Pterosaurs dinosaurs?

Are mosasaurs dinosaurs? 

Is Dimetrodon a dinosaur?

These are common questions that come up when discussing animals of ancient times, so we thought we'd help get to the bottom of these and other questions.

Let’s play a game. Take a look at the following pictures, and try to answer the question, “Which one shows a dinosaur?”

Which did you choose? You may have thought to yourself, “Easy: the dinosaur is obviously the scaly reptilian looking creature with the sharp teeth. That other picture is just a bird.” Well, if you did indeed think that, we’re about to blow your mind…

The photo on the left does indeed show a bird, a Raven in fact, but SURPRISE! Birds are dinosaurs. That’s right, though it was once thought that all dinosaurs went extinct at the end of the Cretaceous Period around 66 million years ago, the truth is that extinction only applied to MOST dinosaurs. A small group of theropod dinosaurs survived, and you now probably see at least one every day without even thinking about it. Over many millions of years, their scales turned into feathers, which eventually became specialized for flight. They also lost the need for teeth, instead evolving hard toothless beaks. But take a look at any bird’s feet, and you’ll see they’ve still got some scaly, reptilian traits left over from their dino days.

What about the critter on the right? That photo shows Dimetrodon, a distinctive sail-backed animal that is often incorrectly considered a dinosaur, although the truth is it’s more closely related to humans than it is to T-rex. Dimetrodon was a synapsid, often described as a “mammal-like reptile” although that’s not entirely accurate. The truth is, Dimetrodon and its relatives are not quite mammals, but also not quite reptiles, occupying a space in between. It’s also worth noting that Dimetrodon lived during the Permian Period, and died out a good 30 to 40 million years before the first dinosaurs would appear in the Triassic Period.

Now let’s look at another set of photos. Which of the pictures below do you think shows a dinosaur?

This is a tough one…but it’s a bit of a trick question. Neither of those is a dinosaur! The flying reptile is Quetzalcoatlus, a pterosaur, while the creature with the flippers is Tylosaurus, a sea-dwelling mosasaur. Pterosaurs were members of a group called “Archosaurs” which also includes dinosaurs and crocodiles, so they’re a distant relative of dinosaurs. Mosasaurs, meanwhile, were actually giant lizards, close relatives of the monitor lizards of today, including Komodo Dragons. A good basic rule is: If it flew in the air (and didn’t have feathers) or lived entirely in the sea, it’s not a dinosaur. Speaking of which…

That goes for these guys above too. Like mosasaurs, Elasmosaurus on the right and Kronosaurus on the left both had large flippers instead of hands and feet, designed for a life swimming through the seas. However, these two are pliosaurs, a group of ocean-dwelling reptiles that were neither giant lizards nor dinosaurs. Pliosaurs came in two main groups: the long-necked Plesiosauromorphs, like Elasmosaurus, and the Pliosauromorphs like Kronosaurus, which had much shorter necks and larger heads.

Let’s try two more:

So, if you made it this far into this blog, you’re probably looking at the two photos above and thinking “Those look like dinosaurs, but Safari Ltd is probably trying to trick me again.” Well, you’ve found us out! The two reptiles pictured above are, once again, not dinosaurs. So what exactly are Postosuchus (left) and Kaprosuchus (right), if not dinos? After all, Postosuchus lived during the Triassic Period and Kaprosuchus lived during the Cretaceous Period, both well within the time of the dinosaurs. Both also have fairly erect stances, with their legs held directly below them like dinosaurs, rather than splayed out to the side like most other reptiles. But still, these creatures are not dinosaurs. So what are they?

These two are actually prehistoric relatives of crocodiles. The older of the two, Postosuchus, lived 220 million years ago and belonged to a group of animals called Rauisuchids. Kaprosuchus, meanwhile, lived around 95 million years ago and was more closely related to modern crocodiles. Though only its skull is known, lots of features about it suggest an animal that lived mostly or entirely on land and was an active predator, hunting down its prey. This is very much unlike today's alligators and crocodiles, who live mostly in or near water and are strictly ambush predators, lying in wait for their prey and attacking when it gets close enough to strike. Based on the theory that Kaprosuchus was active and land-based, it probably had longer, more supportive legs than the short, splayed legs found in living crocodilians.

So if reptiles like Rauisuchids could hold their legs directly under their body like dinosaurs, what’s so different about them? The answer is a bit complicated, but mainly deals with bone structure. The joint that connects a dinosaur’s hip to its leg is on the side of the bone, while in Rauisuchids, the joint plugs straight up into the hip bone, which overhangs the leg. It may not seem like a huge difference, but it shows how these bones developed and points to these reptiles being more closely related to crocodiles than dinosaurs.

While it all might seem quite confusing, the main takeaway is this – there were many more awesome animals stomping around in our prehistoric past than just dinosaurs. While dinos are pretty cool, without a doubt, they represent just one of the many groups of amazing creatures of long ago.