(Intro)

Blake: Hi.  I'm not Hank.  I'm sorry, but I am Blake de Pastino, editor-in-chief of SciShow and co-host and co-creator of the news series, PBS Eons.  Eons is all about the wonders of natural history and how we can use science to understand the deep past.  This is something that we have explored a lot on SciShow, too, especially the life and times of our dearly departed friends, the non-avian dinosaurs.  So do you wanna see some of our greatest hits about dinosaurs?  Of course you do!  So let's start with a throwback to an early SciShow episode, where Hank explains what a dinosaur really is and why a lot of animals that you might think are dinosaurs actually aren't.

Hank: You are probably wrong about what is and is not a dinosaur, but fear not -- I am going to fix you. For instance, pterodactyls? Not dinosaurs. Hummingbirds? Dinosaurs. Also, Pluto? Still not a planet.

The official definition of a dinosaur is land-dwelling diapsid reptile descended from archosaurs. Diapsids are reptiles that have or used to have two specialized openings in their skulls that allow for a bigger, stronger bite, and archosaurs are a kind of diapsid that were defined by a whole bunch of shared characteristics, including two additional holes in the skull right behind the eyes.

Point is, the pterodactyl was both of those things, but pterodactyls hung out mostly in the air, not on the ground, and so they weren't land-dwelling. Plus, when they were on the ground, pterodactyls walked with a kind of semi-upright stance that differed from the straighter stances of real dinosaurs. Paleontologists, turns out, are a bit picky.

Another problem with pterodactyls being dinosaurs is that pterodactyls aren't really even a thing. The term "pterodactyl", Greek for "wing-finger", was coined by French naturalist and zoologist Georges Cuvier, around the turn of the 19th century. People had started digging up all sorts of weird beaky/wingy fossils, and Cuvier was like, "Oh, those! They're, uh, they're pterodactyls. Please place them in the pterodactyl bucket in the corner." But eventually paleontologists renamed the entire group of flying reptiles "pterosaurs", or "winged lizards", and their many diverse species all got their own scientific names.

As far as modern paleontologists can tell, during the Jurassic and Cretaceous periods these carnivorous lizards ruled the skies, and some of them were frickin' huge. The Quetzalcoatlus, the biggest scientists have ever found evidence of, had a 12 meter wingspan. It's probably the biggest thing that ever flew. Ever.

So obviously pterosaurs share a lot of characteristics with birds: the ability to fly, big brains relative to their body size, lightweight bones... but birds did not evolve from pterosaurs. Pterosaurs were wiped out by whatever it was that offed most of the dinos. Instead, birds evolved from little two-legged land-based dinosaurs that survived the giant cataclysmic supercrap that befell the Earth 65 million years ago.

The fact that pterosaurs and birds have similar adaptations for flight is a nice example of what we call convergent evolution, two unrelated animals evolving similar features to tackle a specific problem -- in this case, flight.

Now, I know what you're thinking. I just said that the official definition of a dinosaur includes being both land-dwelling and a reptile, so how the frick could a bird be considered a dinosaur? Well, the rules of taxonomy are such that direct descendants of a single common ancestor must be included in the same group, so birds are considered a sub-group of dinosaurs, which means that dinosaurs are in fact not extinct. It also means that I can eat one right now. [chomps chicken thigh] I'm eating a dinosaur!

Blake: So that's what dinosaurs were, and it's why we refer to the extinct ones as non-avian dinosaurs.  Okay, but then what did these extinct creatures actually look like?  Did they really have feathers and how do we know?  Well, here's Hank again to tell us all about it.

Hank: If you've ever heard or seen anything about dinosaurs lately except for a certain popular movie franchise, then by now you probably know that at least some dinosaurs had feathers. That's been an established scientific fact for a long time, nearly 20 years, and we've had some idea that they probably had feathers for much longer than that.

Paleontologists are still trying to figure out exactly what kinds of dinosaurs had feathers and what they looked like, but at least one entire class of dinosaurs seems to have had them - including Velociraptors, and probably Tyrannosaurus rex. As early as the 1800's, experts in animal anatomy were noticing the similarity between dinosaurs and birds.

Thomas Henry Huxley, a scientist who was one of the earliest advocates for evolution, pointed out that the dinosaur Compsognathus looked a lot like Archaeopteryx, which was at the time was thought of as the first bird. And part of why Archaeopteryx was considered to be a bird was because it was fossilized with an unmistakable impression of feathers.

Feathers are made of protein, which tends to break down before it can fossilize. That means that even dinosaurs that did have feathers aren't usually discovered with them intact. But lately we've been turning up all kinds of feathery fossils, especially in China, where the geology is just right for preserving feathers. You can see the impressions of them preserved around the dinosaur or in an imprint of its skin. And over the past two decades, paleontologists have been working on studying them.

The first feathered fossil to make a big impact was discovered in 1996. It was of Sinosauropteryx, a type of dinosaur called a theropod that was two-legged, carnivorous, and eventually gave rise to birds, and the fossil clearly had a halo of fluff around it. The fluff was made of simple feathers. A bird's more complex flight feathers are rigid and have a central spine called a vane. But this fluff, which paleontologists for real call dinofuzz, has a simpler structure, so Sinosauropteryx couldn't fly. Instead, it probably used its feathers to attract mates or keep warm or both.

But even though we don't know exactly how Sinosauropteryx used its feathers, we actually do know what color they were. Sometimes you can see the structure of fossilized feathers in a microscope - including the tiny structures responsible for color. Colors like blue or yellow don't last for millions of years as far as we know, but red and black do because the cellular structures that hold red and black pigments have distinctive shapes, and in really good fossils those shapes can be preserved. Using an electron microscopes, scientists can tell Archaeopteryx had black and white feathers, and that Sinosauropteryx had reddish and white stripes. Since Sinosauropteryx, there's been a never-ending parade or fluffy dinosaurs, some of them have simple dinofuzz, but others have feathers that start to look a lot like the flight feathers of birds.

There have been so many, in fact, that it seems like all theropod dinosaurs had some form of proto-feathery fluff. Even some dinosaurs unrelated to the theropods had very simple protein fuzz which could mean that the common ancestor of all dinosaurs had it too. The theropod group includes two of pop culture's favorites - the Velociraptors and Tyrannosaurus rex. Now I'm not here to burst anybody's bubble or you know, criticize any movie makers, but Velociraptors definitely had feathers.

In the summer of 2015, paleontologists discovered a dinosaur very closely related to Velociraptors but better preserved than any Velociraptor found so far with big wing-like feathers on its arms. Because it's so closely related to the Velociraptor, they imagine it was very similar. They say it paints a picture of the Velociraptor as a fluffy, feathered poodle from hell. We've yet to find a Velociraptor fossil with its feathers intact but between the presence of feathers on its relatives, and the points on its bones for feathers to attach, it almost certainly had them.

As for whether the T. rex had feathers, it's a little less clear. Some Tyrannosaurus earlier on the evolutionary tree definitely had dinofuzz, but the T. rex has something in common with elephants and rhinos - it was huge. And even though the ancestors of elephants and rhinos had fur, modern-day versions don't have fur anymore because they're so big, they don't need it to keep warm. That logic might also apply to something the size of T. rex. It could've lost the feathers its ancestors had since it was so big, it didn't need the fuzz to keep warm.

In fact, in 2012, scientists turned up a dinosaur called Yutyrannus. It was closely related to Tyrannosaurus, was really big, and had simple feathers, so T. rex might have had them too. But until we find a T. rex with impressions of its skin, we can't be sure. But we definitely do have to change our picture of the dinosaur age to include some fluffy hell poodles.

Blake: Okay, so T-Rex probably had feathers.  Am I the only one who thinks that's adorable?  Don't you just want to snuggle with one?  Okay, we have never done an episode about dinosaur snuggling, but we have explored whether dinosaurs were actually cold-blooded, which would be good to know before you cuddle with one.  So here's Michael to talk about dinosaurs and warm-bloodedness.  

Michael: If you're a dinosaur fan, you might have noticed that the way we portray them has changed over the years. I'm not just talking about the feathers. We used to think of dinosaurs as sluggish, cold-blooded reptilian monsters, and at some point, they became the swift, active hunters of Jurassic Park, more like warm-blooded creatures. But dinosaurs might not have been either cold-blooded or warm-blooded. Instead, they were probably a little bit of both.

Ectothermic, or cold-blooded organisms like reptiles and fish, rely on their environment to warm their bodies, which helps them conserve energy. Endothermic, or warm-blooded creatures, like birds and mammals, hold their body temperatures steady. In your case, that's a toasty 37 degrees Celsius. There are advantages to being warm-blooded, since you can stay active even when it's cold out, but it also takes a lot of energy to maintain that high temperature, which means you need plenty of food.

For decades, paleontologists have been debating which category the dinosaurs fall into. Since warm-blooded birds are descended from dinosaurs, there must have been a transition to endothermy somewhere in the dinosaur family tree, but according to a study published in 2014 in the journal Science, that might have happened gradually, with dinosaur metabolism sort of in-between endotherms and ecotherms.

The key was in their bones. When bones grow, they lay down growth rings just like trees. By looking at these rings, scientists can calculate how fast the animal grew, even if it's been dead a hundred million years, and they can use growth ring data to estimate how an animal's metabolism worked. Animals with faster metabolisms use more energy, so they eat more food and grow faster.

So, the team looked at the growth rings in dinosaur fossils and compared them to those of modern animals, both endotherms and ectotherms. They pulled published data on hundreds of vertebrates and dozens of dinosaurs and crunched the numbers using statistical analysis. They found that the dinosaurs had a metabolic rate somewhere between reptiles and birds, meaning they weren't endotherms or ectotherms, they were mesotherms, or middle-blooded.

Mesothermy is an entirely different type of temperature regulation, in which animals generate their own heat, like endotherms, but don't hold their bodies to a specific temperature, like ectotherms. They end up with a body temperature that's a few degrees higher than their surroundings, but they're still dependent on their environment to some extent. Like modern methoserms, such as the great white shark or the echidna, dinosaurs might have relied on their muscles to produce heat, and their large size to retain it, without sticking to a particular temperature. Evolutionarily, it would make sense, because the dinosaurs would get to conserve a bit of energy while still being more active than their cold blooded competition.

However, some paleontologists had problems with this study. For one thing, not everyone agrees on how to interpret the growth ring data. Using other statistical methods, it might actually support much faster growth rates, giving us warm-blooded dinosaurs. For another, not all dinosaurs were the same, maybe the more bird-like theropod had faster metabolic rates than the more lizardy sauropods.

Lumping them all into one statistical group like the researchers did would hide these potential differences, but we might soon get a much more definitive answer, because there's a totally new way to study dinosaur fossils that's giving us a close look at the red blood cells. That could help solve the dinosaur metabolism problem, because the size of red blood cells is related to metabolic rate. Typically, the bigger the red blood cells, the warm-blooded the animal.

For a long time, paleontologists thought that the soft stuff, like skin and feathers and blood and tissue were rarely preserved in fossils. But living bones have lots of soft tissue inside them, and when a group of British researchers pulled apart 75 million year old bones to look for some of that soft tissue, they found some, including red blood cells. It wasn't perfectly intact, but it was enough to make out what the tissue structure would have looked like when the dinosaur was alive, and the researchers found soft tissue in nearly all the bones they searched, even though they weren't all that well-preserved or anything, which means that lots of dinosaur fossils might still have squishy organic bits of dinosaur in them. So future studies that look at soft tissue and more bones should be able to tell us if dinosaurs were really cold-blooded, warm-blooded, or something in between.

Blake: Okay, so it looks like snuggling might not have been an option, but if you're anything like me, you probably think that dinosaurs are still fun to play with.  Raawr, raaawr, ahh, mommy!  So here's a question for you.  Are plastic dinosaurs made from the remains of real dinosaurs?  Olivia has the answer.

Olivia: Here's a fun connection that people on the internet like to make: plastic is made of oil, and oil is made of dinosaurs, so when you're playing with plastic dinosaur toys you're basically playing with the remains of real dinosaurs. It's fun to think about but it's not that simple.

For one thing, the oil and natural gas that we pump out of the ground are not the chemical leftovers of dinosaurs. Instead, they're made from the ancient remains of a whole bunch of much smaller creatures. The internet might want you to think that oil equals dinosaur corpses, but the fact is that the petroleum we use to make plastics actually comes from ancient ocean floors, where there weren't any dinosaurs.

But those ocean waters did and still do support a mind-bogglingly huge mass of tiny microorganisms, plants, and animals. And not all that biomass gets eaten by bigger creatures. Some of it dies and trickles down through the ocean as so-called marine snow, a sort of perpetual rain of organic matter that falls to the seafloor. If enough of this material builds up faster than it decays, then you wind up with a layer of organic goo at the bottom of the ocean. After a while, the goo can get buried under sand and other sediment and then time, pressure, and heat turn that organic layer into oil and natural gas.

Changes in sea level and the movement of the Earth's crust sometimes force those old ocean floors to the surface, which is why some oil and gas reserves are found on dry land, but others are still underwater like in the Gulf of Mexico.

And there's one more twist to the plastic equals oil equals dinosaur story: not only is oil not made from dinosaurs, but plastic isn't always made from oil. Frequently it's made from natural gas. The main precursor to plastic is a byproduct of petroleum refining called hydrocarbon gas liquids. They're made of relatively light carbon compounds that can be condensed out of natural gas by cooling it. These compounds are used in the chemical reactions that we use to make plastic. And in the United States they're the source of most of the plastic that's made.

So in the end oil and natural gas come from the same place, but it's not some dinosaur graveyard. It's ancient ocean floors, were countless billions of tiny animals, plants, and microorganisms wound up so that your kid could play with their plastic t-rex. Whether you choose to explain that to them is totally up to you.  

Blake: Speaking of dead dinosaurs, we have found thousands of dinosaur fossils over the past few hundred years, but have we found them all?  Will we ever reach peak dinosaur?  Here's Michael to explain.

Michael: It goes without saying that many of our most precious resources are also the most limited. 

Like, there’s only a certain amount of land that we can build on, there’s only so much gold that we can mine... and there’s always the possibility of a bacon shortage.

And for some scientists, there’s another commodity that’s in increasingly short supply.

Dinosaurs.

Over the last couple of centuries, we’ve found thousands of dinosaur bones. But only a limited number of species ever existed, right?  

So some paleontologists have been wondering how many species of dinosaurs are actually left for us to discover, and how many fossils of them are out there?

Might sound like a funny thing for scientists to ponder, but it’s really an extension of a larger question that they’ve been wrestling with, which is: How many different kinds of dinosaurs were there in the first place?

Scientists who study living organisms deal with these questions all the time -- like, how many different types of plants and animals are living in this forest glade? Or how many microbes are in this petri dish?

Since they can’t possibly count every single organism, they just count a small sample. Then, they use statistics and probabilities to come up with a mathematical model of what that whole population might look like.

And in 2006, a team of biologists and statisticians used these methods to estimate Earth’s population of extinct dinosaurs.

Specifically, they wanted to figure out how many types of dinosaurs there once were, as well as how long it would take us to find fossils of each.

Scientists usually count dinosaurs by genus -- that’s the taxonomic rank just above “species” -- because nine times out of ten, there’s only one species of dinosaur per genus. 

So the team started cataloging how many dinosaur genera had already been discovered. 

Then they focused on how many of those genera were really common, and how many were really rare.

In many cases, for example, a dinosaur genus might consist of just one known specimen, like the cute and creepy Segisaurus, which used to scamper around what is now Arizona.

But other genera were really common -- like, you can hardly swing a pick-axe in parts of America’s Northern Plains without hitting a fossil of Edmontosaurus.

So, based on the abundance, or scarcity, of known organisms, scientists estimate the diversity of unknown ones as well. Or at least, they try to.

And the results so far suggest that -- while there are a lot of dinosaurs still out there for us to find -- we may have less than 200 years of good dinosaur-hunting left.

As of 2006, the study concluded, we had discovered just 29 percent of the 1,850 dinosaur genera they think are out there, waiting to be found.

So, how soon until we find the very last known genus of dinosaur?

Well, because of better techniques being used in paleontology, we’re discovering new dinosaurs faster than ever before.

The very first dinosaur fossil was identified in 1824. For the next hundred and fifty years, paleontologists only discovered an average of one new genus of dinosaur every year.

But now, we’re racking up an average of 15 new genera every year. 

And as a result, according to scientists’ calculations, somewhere between the years 2037 and 2056, we’ll have found 50% of the dinosaur genera that ever existed.

You might think of this as “peak dinosaur” -- at that point, there would be more known dinosaurs than unknown ones.

After “peak dinosaur,” there will be fewer genera left to discover, and the remaining ones will probably be scarcer. So the number of finds per year will start to decline. 

But for a while, we’ll still be finding them often enough to keep dinosaur-hunters busy.

Between 2069 and 2102, according to these projections, we’ll have found 75% of the dinosaur genera.

By the mid-22nd century, 90%.

And by the year 2200, there will be only a few genera left, and they’ll be harder to find than ever. 

At that point, even though there will probably still be some surprise discoveries of new dinosaurs every so often, those finds will be extremely rare.

Basically, the golden age of dinosaur discovery will be behind us.

However, these are just statistical estimates; it’s impossible to know when you’ve ever found the last of anything.

Plus, this timeline also only applies to dinosaurs -- those reptilian land-dwelling diapsids -- it  doesn’t include all the other ancient forms of life: pterosaurs, mosasaurs, plesiosaurs, fish, mammals, invertebrates, and plants!

So there will still be plenty of fossils for us to find, for a very long time.

And don’t forget, we’ll still have the dinosaurs’ relatives -- birds -- to keep us company.

Blake: So there are still a lot of dinosaurs left for us to find, which, I guess means that I can keep my job, and if you want to learn more about dinosaurs, the origins of life, and the whole history of the natural world, then join Hank, paleontologist Kallie Moore, and me over at YouTube.com/Eons.  We'll see you there.

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