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Sunday 6/6/21 - Your Dinosaurs Are Not Dinosaurs

Links to previous posts in this series:

Your Dinosaurs Are Wrong

Your Pterosaurs Are Wrong

Dimetrodon, Mark Witton

You know how when you're really into something, like a fandom or maybe a hobby, and you're like "I know outsiders to this thing won't know everything, but there's surely some information on my thing that's common knowledge", and then you're wrong? That's me after every time I make a Dinosaur post.

In the previous two blogposts I made about extinct giant reptiles, I explained the inconsistencies between scientific knowledge and common knowledge on how Dinosaurs and Pterosaurs actually looked. But it seems there's also a gap in the public's knowledge of what actually constitutes a Dinosaur.

The Mesozoic Period was an age between 252-66 million years ago, often called the Age of the Reptiles, and although Dinosaurs were definitely the dominant clade of animals on land then, other reptiles and reptile-adjacent animals were successful too. Today I'll enlighten readers on what non-dinosaur animal groups were successful during that age, and what they were actually related to.

Pterosaurs

Cladogram of the evolutionary relationship of Archosaurs. Crocodilians diverged first, then Pterosaurs, before Dinosaurs.

I covered a lot about pterosaurs in a previous post, but just so it's covered in this post, let's talk about them again. Since writing that last post, I heard a good way to describe the relationship between Pterosaurs and Dinosaurs. With a lot of the animal groups covered in this post, it's like "No this is definitely not a dinosaur", but with Pterosaurs, it's more like "They're technically not a dinosaur", in the same way a rabbit is technically not a rodent. They're closely related sister groups, but they diverged long enough ago that scientists agree they're separate.

Pterodactylus antiquus, Mark Witton

As an aside, something I wanted to discuss in the Pterosaur post, is that a lot of people mistakenly use the name "Pterodactyl" to refer to all pterosaurs. There was no such animal named that. There was a species called Pterodactylus, but it is probably a smaller and less impressive Pterosaur than what you have in your head.

Synapsids

Top: Gorgonopsid, Jonathan Kuo

Bottom Left: Dicynodont, Gabriel Ugueto

Bottom Right: Dimetrodon, Gabriel Ugueto

In the early Mesozoic, when dinosaur's ancestors were all tiny and had yet to take over the world, a group of animals called Synapsids were the dominant force. Mammals eventually branched off from this group, so they're sometimes called stem-mammals or mammal-like reptiles. Dimetrodon, arguably the most commonly dinosaur-labelled non-dinosaur, was in this group. Others include the beak faced Dicynodonts, and the sabre toothed Gorgonopsids, which all look almost dinosaurian but they're not.

Pseudosuchia

A collection of Pseudosuchians, ColinM_Art on twitter

People often refer to Crocodilians as living dinosaurs (even though birds are the closest living relatives). Although it is true that the crocodile/alligator shape has persisted for millions of years, ancient relatives (collectively referred to as Pseudosuchia) experimented with lots of different forms. The very ferocious Postosuchus was an apex predator in the Triassic, and the suspiciously dinosaur-looking Desmatosuchus was an armoured herbivore, and many other Triassic relatives became bipedal predators like Poposaurus. Later in the Mesozoic, a lot of these more specialised crocodile relatives were replaced by more successful dinosaur counterparts, but the big aquatic ambush predator type still persists to this day.

Plesiosaurs

Top: Thalassiodracon, Mark Witton

Bottom: Liopleurodon, Kuzim Art

I sometimes see books or publications discuss "Aquatic Dinosaurs" or "Sea Dinosaurs", and most often they are talking about Plesiosaurs. Plesiosaurs were a member of flippered, streamline shaped reptiles with necks of varying length that specialised in hunting sea life. Scientists aren't certain of their origins, but most refer to them as Pantestudines, sharing a common ancestor with turtles and tortoises. Elasmosaurus and Liopleurodon are some of the more popular members of the group.

Icthyosaurs

Top: Ichthyosaurus larkini

Bottom Left: Shonisaurus

Bottom Right: Cartorhynchus

(All 3 Arts done by Juilo Larceda)

Although less resembling a dinosaur shape, and therefore less often mistaken for one, Ichthyosaurs were another marine reptiles group that lived alongside them. Although early relatives were more reptile shaped, later ichthyosaurs developed a shark/dolphin body plan, and were the most aquatically agile marine reptiles. They could range in size from the dolphin sized Ophthalmosaurus to the whale sized Shonisaurus. They diverged from other reptiles groups very early, but some scientists think they share a common ancestor with lizards.

Mososaurs

Top: Mososaurus, Mark Witton

Bottom Left: Tylosaurus, Gabriel Ugueto

Bottom Right: Xenodens, Andrey Atuchin

Mososaurs evolved much later than most of these other non-dinosaurs. They superficially resembled short-necked Plesiosaurs, but would have likely used their tail more than their flippers for propulsion. Mososaurs evolved from aquatic monitor lizards in the late Cretaceous, and quickly reached sizes bigger than any other lizard would ever reach. Mososaurus, which featured (much bigger than it was in life) in Jurassic World, is one of the best known species.

Thanks for Reading

Tumblr only allows so many pictures in one post, but I've covered most of the main groups of animals people mistakenly call dinosaurs. Just as a final titbit, someone recently told me that they thought the "-saur" or "-saurus" ending on many ancient animals automatically means dinosaur, but it is just Greek for reptile, and scientists slap it on the end of many scientific names, many of which aren't even reptiles, just may look like them.

As an example, Basilosaurus was actually found to be a Whale after its fossils were better studied. But it kept its name.

Source: The7thSea, DeviantArt

If there's more you'd like me to cover on the topic of dinosaurs, any of the other cool animals I've discussed here, or maybe other animals you're unsure are dinosaurs or not, leave a comment or send us a message.

Cheers for reading, and hope you learned something today.