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Submit your own photos of misinformed Dimetrodon toys and games to !

Learn more about the Field Museum's fossil mammal and synapsid collection:

Major thanks to Kenneth D. Angielczyk, Ph.D. Associate Curator of Paleomammalogy for his generous help in the creation of this episode! You can read his paper "Dimetrodon is Not a Dinosaur" here:

Additional thanks to Bill Simpson for providing the physical specimens from the Fossil Vertebrate collection!

Producer, Writer, Creator, Host:
Emily Graslie

Producer, Editor, Camera:
Tom McNamara

Theme music:
Michael Aranda

Created By:
Hank Green

Production Assistant:
Katie Kirby
Filmed on Location and Supported by:
The Field Museum in Chicago, IL

Huge thanks to synapsids like Tony Chu, Seth Bergenholtz, Martina Šafusová, and Moyaccercchi for translating this episode!

Well I'm here with my Jurassic Park toy and we have dimetrodon. I'm just interested, I guess, in a, kind of what this trading card says.

"Running into a dimetrodon in Jurassic Park would have been an interesting experience, and a dangerous one."

That's pretty interesting, because if you were to have run into dimetrodon in Jurassic Park, I'd be really surprised. Dimetrodon wasn't a dinosaur.

How do you get a dino-strike jaw?

It certainly would have been an interesting experience to run into dimetrodon in Jurassic Park, especially considering that dimetrodon existed 60 million years before the first dinosaurs.

Inaccuracies in science fiction stories aside, it's a commonly held belief between toy manufacturers and people alike, that dimetrodon was a dinosaur. Most people don't even know that dimetrodon wasn't even a reptile.

You may be familiar with the phrase "mammal-like reptile," sometimes used to refer to these animals. But this outdated terminology is another major misnomer that makes paleontologists weep.

Dimetrodon is classified into a diverse group of animals known as the synapsids which includes all modern-day mammals in addition to the ancient relatives from which all mammalian life diverged.

Does this mean that dimetrodon was a mammal? Not quite. Synapsids are terrestrial vertebrates -- four-legged, back-boned animals that spanned back 315 million years.

But why do paleontologists get so upset when they're referred to as reptiles, or even worse, dinosaurs? What's the harm in a common misconception? 

Fundamentally, the phrase "mammal-like reptile" implies that reptiles and synapsids, and therefore mammals, share the same evolutionary line of descent. However, this isn't correct.

Reptiles and synapsids share a common ancestor, but represent separate lines of descent that diverged about 320 million years ago.

Think about it like this, you and your cousin share a common ancestor, your grandparents. But at some point that line of descent diverges from your parents to you and from either your aunt or uncle to your cousin. Since you and your cousin exist at the same time, you can't be an ancestor of your cousin, or vice versa.

This sort of relationship is how synapsids eventually diverged from reptiles. Keeping that in mind, let's look at one of these evolutionary trees. 

You can see at this point that reptiles and mammals are both amniotes. Meaning they evolved as four-legged animals that laid eggs on land, rather than in the water. From there, mammals and reptiles branched off and Reptilia diversified into everything from turtles and lizards and snakes to plesiosaurs and crocodiles and eventually even birds.

Mammals, on the other hand, went a different route as Synapsida, resulting in our buddies, like the cynodonts and gorgonopsians, like this dude, and eventually us.

So, while superficially dimetrodon may have looked more like a dinosaur or a reptile than a mammal, genetically these groups were headed down completely different paths and on separate evolutionary journeys. 

Sorry buddy, you don't get a starring role in Jurassic Park. Wah wah. 

Additionally, we aren't giving ourselves an appropriate amount of credit for our own diversity. To say that dimetrodon is a "mammal-like reptile," considering dimetrodon is a distant relative of ours, implies that we are descendants of reptiles. Which we aren't.

There are certain characteristics that help us to classify a reptile as a reptile and a mammal as a mammal. These traits are inherited by evolutionary ancestry.

You are already familiar with many of these traits. For instance, we know a mammal is a mammal when it has fur and lactates. Mammals feed their young with protein and fat-rich milk secreted from mammary glands. 

OK, maybe you knew this, but as a result, this also means that all mammals suckle. While this may not seem like a ground-breaking revelation, the act of suckling is due to a unique adaptation. 

Mammals have solid palates in their mouths in order to create the negative pressure necessary to suck. This unique fusion of maxillary bones, known as the secondary palate, is not found in other animals.

This trait has been passed down from our earliest ancestral relatives and is a characteristic that paleontologists look for when classifying early mammalian life. 

Another characteristic indicative of mammalian life is the temporal opening, which is an additional opening around the eyes, behind the orbital and zygomatic arch. This space is filled with what is called the temporalis muscle, allowing for improved chewing and biting capabilities.

So, while dimetrodon may not have had fur or mammae, they did have this key temporal opening which links us together over the past hundreds of millions of years. 

If two animals have some of these traits in common, does that mean that they're related to one another on some evolutionary level? Not quite.

It is entirely possible that two completely different animals evolved extremely similar characteristics entirely independent of one another. 

For example, the ability for both birds and bats to fly. That's an instance of convergent evolution even though there's no genetic or evolutionary link between the two.

Dimetrodon was a primitive creature. "Primitive" being a descriptive term used to indicate that a group or species of life has not accumulated as many evolutionary changes over time.

I blame the makers of Pokemon for instilling misconceived ideas that "evolution" is equal to "leveling up." 

This does not mean the primitive animal in question is somehow lesser. On the contrary, it indicates that this is an animal that has existed, sometimes unchanged, for hundreds of millions of years.

Dimetrodon and its contemporaries were still extremely well-suited for their environment. That massive sail on their back didn't come out of no where. 

If I haven't completely blown your mind yet, think about it like this: we are more closely related to dimetrodon than dimetrodon was to T. rex. 

By not acknowledging the incredible variety of synapsid life over the last hundreds of millions of years, we're cheating ourselves out of really understanding how remarkable it is that humans ended up evolving at all.

So the next time you're at your neighborhood toy store or perusing the children's section at a local book shop, take note of dimetrodon incorrectly labeled as a dinosaur, and send me a photo.

(Credit music plays)

What was a dimetroda..da..down? I'm a low to the ground terrestrial vertebrate. [laughs] That's it.

TOM: That's it!

EMILY: This one [continues laughing]

EMILY: It still has brains on it...