Prehistoric Earth: Dinosaurs, Pangaea & Mass Extinction (#95)

Today I’m covering a topic I’m honestly surprised I haven’t covered yet in almost 100 episodes, and this time, we’re going further back in history than we’ve ever gone before - we’re going prehistoric! I’m going to be talking about dinosaurs, how the Earth was before we had continents, and the mass extinction event that caused the dinosaurs to die out.

As with many of my episodes, I found myself falling down many rabbit holes during my research, so there are a few small tangents - but I hope you find it all interesting and learn some new things you didn’t know before.

Dinosaurs and Their Discovery

Dinosaurs arose during the Middle to Late Triassic Period of the Mesozoic Era, around 230 million years ago. They were members of a subclass of reptiles called the archosaurs (“ruling reptiles”), a group that also includes birds and crocodiles.

Scientists first began studying dinosaurs during the 1820s, when they discovered the bones of a large land reptile they dubbed a Megalosaurus (“big lizard”) buried in the English countryside. In 1842, Sir Richard Owen, Britain’s leading paleontologist, first coined the term “dinosaur”. Owen had examined bones from three different creatures–Megalosaurus, Iguanadon (“iguana tooth”) and Hylaeosaurus (“woodland lizard”).

They each had a few things in common:

• They lived on land
• They were larger than any living reptile
• They walked with their legs directly beneath their bodies instead of out to the sides
• They had three more vertebrae in their hips than other known reptiles.

Using this information, Owen determined that the three formed a special group of reptiles, which he named Dinosauria. The word comes from the ancient Greek word deinos (“terrible”) and sauros (“lizard” or “reptile”) (so dinosaur literally means “terrible lizard”).

While on the subject of this, it’s interesting to see some of the other common roots round in dinosaur names and it can be funny translating them into English. Some examples are:

• “raptor” = “robber”
• “stego” = roof” (so Stegosaurus means roof-lizard)
• “tri” = “three” and “ops” = face (so Triceratops means three-horned face)
• “tyranno” = “tyrant” (so Tyrannosaurus means tyrant-lizard)
• “rex” = “king” (so Tyrannosaurus rex comes together to mean king tyrant-lizard)

Safe to say, they definitely sound better with their original Latin names.

Since the 1800s, dinosaur fossils have been found all over the world and studied by paleontologists to find out more about the many different types of these creatures that existed. Scientists have traditionally divided the dinosaur group into two orders: the Ornithischia, or “bird-hipped”, and Saurischia, or “lizard-hipped”.

From there, dinosaurs have been broken down into numerous genera (e.g. Tyrannosaurus or Triceratops) and each genus into one or more species. Some dinosaurs were bipedal, which means they walked on two legs, some were quadrupedal (walked on four legs), and some were able to switch between the two. Some dinosaurs were covered with a type of body armor, and some probably had feathers, like their modern bird relatives. Some moved quickly, while others were lumbering and slow. Most dinosaurs were herbivores, or plant-eaters, but some were carnivorous and hunted or scavenged other dinosaurs in order to survive.

There are so many cool things I could say about dinosaurs, so rather than bombard you with in-depth knowledge, I thought I’d pick a few of the best facts. Some of the things mentioned here, I’ll come back to later on.

• Modern humans have only existed for a few hundred thousand years, and human civilization only got started around 10,000 years ago, which is like the blink of an eye on a Jurassic timescale. In comparison to our measly lifespan, dinosaurs roamed the earth for 165 million years, meaning they may have been the most successful vertebrate animals ever to colonise Earth.
• Just to further demonstrate how long dinosaurs existed, the age of the dinosaurs, of the Mesozoic era, is split into three periods: the Triassic period (237–201 million years ago), the Jurassic period (201–145 million years ago), and Cretaceous period (145–66 million years ago). These periods saw the development of dinosaurs, marine reptiles, fish, mammals, flying animals including pterosaurs and birds, and a huge range of plant life. The largest dinosaurs didn’t emerge until the Cretaceous period, which started over 100 million years after the start of the "age of dinosaurs”.
• Contrary to what many people think, and what we see in films where loads of species of dinosaurs seem to coexist, not all dinosaurs lived during the same geological period. Stegosaurus, for example, lived during the Late Jurassic Period, about 150 million years ago. Tyrannosaurus rex lived during the Late Cretaceous Period, about 72 million years ago. Stegosaurus were extinct for 66 million years before Tyrannosaurus walked on Earth.
• There’s a common idea that dinosaurs were really dumb, and while it's true that some plant-eating dinosaurs like the Stegosaurus had such tiny brains compared to the rest of their bodies, meat-eating dinosaurs including the T. rex possessed slightly more brainpower. They often required better-than-average sight, smell, agility, and coordination to reliably hunt down prey. (They still weren’t geniuses though - even the smartest dinosaurs were only on an intellectual par with modern ostriches.)
• Some things you class as dinosaurs might not actually be dinosaurs. The word "dinosaur" applies only to land-dwelling reptiles possessing a specific hip and leg structure, among other anatomical characteristics. As large and impressive as some genera were, flying pterosaurs and swimming plesiosaurs weren't dinosaurs at all—and some of them weren't even all that closely related to dinosaurs, except for the fact that they're also classified as reptiles. And the Dimetrodon—which is often described as a dinosaur—was actually an entirely different kind of reptile that flourished tens of millions of years before the first dinosaurs evolved.

Pangaea

At the time the dinosaurs arose during the Triassic area, all of the Earth’s continents were connected together in one land mass, now known as Pangaea, and surrounded by one enormous global ocean called Panthalassa. There were no polar ice caps, and the climate at the equator was hot and dry, punctuated by violent monsoons. Some estimates put the average air temperature across most of the continent at well above 100 degrees Fahrenheit (38 celsius). Conditions were wetter in the north (the part of Pangaea corresponding to modern-day Eurasia) and the south (Australia and Antarctica).

Pangaea’s existence was first proposed in 1912 by German meteorologist Alfred Wegener as a part of his theory of continental drift. Its name is derived from the Greek pangaia, meaning “all the Earth.”

Pangaea’s conceptualisation began with Wegener’s work in 1910. He became impressed with the similarity in the coastlines of eastern South America and western Africa and speculated that those lands had once been joined together. He began to toy with the idea that in the late Paleozoic Era (which ended about 252 million years ago) all the present-day continents had formed a single large mass, or supercontinent, which subsequently broke apart.

Other scientists had proposed that such a continent existed but had explained the separation of the modern world’s continents as resulting from the subsidence, or sinking, of large portions of the supercontinent to form the Atlantic and Indian oceans. Wegener instead proposed that portions of Pangaea had slowly moved thousands of miles apart over long periods of time. For this movement, he proposed the term “continental displacement”, which then gave rise to the term continental drift, in 1912.

The breakup of Pangaea is now explained in terms of plate tectonics rather than Wegener’s concept of continental drift, which simply stated that Earth’s continents were once joined together and lasted like this for most of geologic time. The idea of plate tectonics states that Earth’s outer shell, or lithosphere, consists of large rigid plates that move apart at oceanic ridges, and then either come together or slip past one another. The pattern of seafloor spreading indicates that Pangaea didn’t break apart all at once but rather fragmented in distinct stages.

The first oceans formed from the breakup, around 180 million years ago, were the central Atlantic Ocean between northwestern Africa and North America, and the southwestern Indian Ocean between Africa and Antarctica. The South Atlantic Ocean opened about 140 million years ago as Africa separated from South America. About the same time, India separated from Antarctica and Australia, forming the central Indian Ocean. Finally, about 80 million years ago, North America separated from Europe, Australia began to drift away from Antarctica, and India broke away from Madagascar. India eventually collided with Eurasia approximately 50 million years ago, forming the Himalayas.

Some of the earliest dinosaurs existed on Pangaea, so dinosaurs probably saw a heck of a lot of change over their 150-million year lifespan. Unfortunately, the biggest change in the lifespan of the dinosaurs would be the mass extinction event that would wipe them all out.

Extinction of the Dinosaurs

Dinosaurs mysteriously disappeared at the end of the Cretaceous Period, around 65 million years ago. Many other types of animals, as well as many species of plants, died out around the same time as the dinosaurs, and numerous competing theories exist as to what caused this mass extinction.

This event was called the Cretaceous-Tertiary extinction, or the K-T event for short, or it’s also called K–Pg extinction or Cretaceous–Paleogene extinction. This global extinction event was responsible for eliminating approximately 80% of all species of animals at or very close to the boundary between the Cretaceous and Paleogene periods, about 66 million years ago. The name K-T comes from the German word Kreide, meaning “chalk” (which references the chalky sediment of the Cretaceous Period), and the word Tertiary, which was traditionally used to describe that period.

Considering how much we all hear or learn about this mass extinction that killed the dinosaurs, you may be surprised to learn that the K–T extinction ranks only third in severity of the five major extinction episodes that happened during Earth’s lifespan. So in typical Let’s Learn About fashion, it’s time for a quick tangent about these major 5 extinction events. In an episode a while ago we talked about how incredible Earth is and how lucky we are to be here, and these extinction events prove that even further.

The 5 Major Extinction Events

Each of these events varied in size and cause, but all of them completely devastated the biodiversity found on Earth. A “mass extinction” can be defined as a time period in which a large percentage of all known living species go extinct. There are several causes for mass extinctions, such as climate change, geological catastrophes (e.g. numerous volcanic eruptions), or meteor strikes onto Earth’s surface.

How does mass extinction contribute to evolution?

A quick note that I found interesting was how these mass extinction events can actually aid evolution. Since so many species die off during these catastrophic extinction events, there is more room for the surviving species to spread out. There is less competition for food, resources, shelter, and even mates, allowing the “leftover” species from the mass extinction event to thrive and reproduce rapidly. As populations separate and move away over time, they adapt to new environmental conditions and are eventually reproductively isolated from their original populations. At that point, they can be considered a brand new species. So as new species continually evolve and adapt to their ever-changing environments, the stronger the new ecosystem can become.

Let’s have a look at the 5 major mass extinction events that have happened on Earth, and please bear with me because there are some long words I need to try and remember how to pronounce.

1. The Ordovician Mass Extinction

The first known major mass extinction event occurred during the Ordovician Period of the Paleozoic Era (about 440 million years ago). At this time in the history of Earth, life was in its early stages. The first known life forms appeared about 3.6 billion years ago, but by the Ordovician Period, larger aquatic life forms had come into existence. It’s thought there were also even some land species at this time.

The cause of this mass extinction event is thought to be the shift in the continents and the drastic climate change that followed. It happened in two waves. The first wave was an ice age that encompassed the entire Earth. Sea levels lowered and many land species couldn’t adapt fast enough to survive the harsh, cold climates.

The second wave was when the ice age finally ended—and it was not good news. The episode ended so suddenly that the ocean levels rose too quickly to hold enough oxygen to maintain the species that had survived the first wave. Again, species were too slow to adapt before extinction took them out completely. It was then up to the few surviving aquatic autotrophs to increase the oxygen levels so new species could evolve. (Autotrophs are organisms that can produce their own food using light, water, carbon dioxide, or other chemicals. Kelp is an example of an autotroph.)

2. The Devonian Mass Extinction

The second major mass extinction happened during the Devonian Period of the Paleozoic Era (about 375 million years ago). This mass extinction event actually followed the previous one relatively quickly. Just as the climate stabilised and species adapted to new environments and life on Earth began to flourish again, almost 80% of all living species—both in the water and on land—were wiped out.

There are several hypotheses as to why this occurred. The first wave, which dealt a major blow to aquatic life, may have actually been caused by the quick colonisation of land—many aquatic plants adapted to live on land, leaving fewer autotrophs to create oxygen for all of the sea life. This led to mass death in the oceans.

The plants' quick move to land also had a major effect on the carbon dioxide available in the atmosphere. By removing so much of the greenhouse gas so quickly, temperatures plummeted. Land species had trouble adapting to these changes in climate and went extinct as a result.

The second wave of the Devonian mass extinction is more of a mystery. It could have included mass volcanic eruptions and some meteor strikes, but the exact cause is still considered unknown.

3. The Permian Mass Extinction

The third major mass extinction was during the last period of the Paleozoic Era, called the Permian Period (about 250 million years ago). This is the largest of all known mass extinctions with a massive 96% of all species on Earth completely lost, and because of that, this event has been titled “The Great Dying.” Aquatic and terrestrial life forms perished relatively quickly as the event took place.

It is still a big mystery what set off this huge mass extinction event, and several hypotheses have been thrown around by scientists. Some believe there may have been a chain of events that led to so many species disappearing; this could have been massive volcanic activity paired with asteroid impacts that sent deadly methane and basalt into the air and across the surface of the Earth. These could have caused a decrease in oxygen that suffocated life and brought about a quick change in the climate.

Newer research points to a particular microbe that flourishes when methane is high. These microbes may have “taken over” and choked out life in the oceans as well. (If anyone listening has read Project Hail Mary by Andy Weir, this may sound familiar. No spoilers.)

Whatever the cause, this biggest of the major mass extinctions brought an end to the Paleozoic Era and a whole new area began: the Mesozoic Era.

4. The Triassic-Jurassic Mass Extinction

The fourth major mass extinction was actually a combination of many, smaller extinction events that happened over the last 18 million years of the Triassic Period during the Mesozoic Era (about 200 million years ago). Over this really long time span, about half of all known species on Earth at the time perished. The causes of these individual small extinctions can, for the most part, be attributed to volcanic activity with basalt flooding. The gases spewed into the atmosphere from the volcanoes also created climate change issues that changed sea levels and possibly even pH levels in the oceans.

5. The K-T Mass Extinction

The fifth major mass extinction event, as I said earlier, is perhaps the best-known, despite it not being the biggest. It is also arguably the most recent. The Cretaceous-Tertiary Mass Extinction happened about 65 million years ago, and became the dividing line between the final period of the Mesozoic Era—the Cretaceous Period—and the Tertiary Period of the Cenozoic Era. Up to 75% of all known living species died during this mass extinction event.

...Okay, Back to the Death of the Dinosaurs

And that brings us nicely back to the main topic of this episode.

For many years, paleontologists believed the K-T extinction event was caused by climate and geological changes that interrupted the dinosaurs’ food supply. One early theory was that small mammals ate dinosaur eggs, reducing the dinosaur population until it became unsustainable. Another theory was that dinosaurs’ bodies became too big to be operated by their small brains. Some scientists believed a great plague decimated the dinosaur population and then spread to the animals that feasted on their carcasses. Starvation was another possibility: large dinosaurs required vast amounts of food and could have stripped bare all the vegetation in their habitat.

But many of these theories are easily dismissed. If dinosaurs’ brains were too small to be adaptive, they would not have flourished for 160 million years. Also, plants do not have brains nor do they suffer from the same diseases as animals, so their simultaneous extinction makes these theories less plausible.

Then, for many years, climate change was the most credible explanation for the dinosaurs’ demise. Dinosaurs thrived in the planet’s consistently humid, tropical climate. But in the late Mesozoic Era, evidence shows that the planet slowly became cooler. Lower temperatures caused ice to form over the North and South poles and the oceans became colder.

Because the dinosaurs were cold-blooded, meaning they obtained body heat from the sun and the air, they would not have been able to survive in significantly colder climates. However, some species of cold-blooded animals, such as crocodiles, did manage to survive. Also, climate change would have taken tens of thousands of years, giving the dinosaurs sufficient time to adapt, so that theory has been largely disregarded too.

In 1956, Russian astronomer Joseph Shklovsky became the first scientist to consider that the extinction was due to a single catastrophic event. He theorised that a supernova (which is the explosion of a dying star - throwback to episode 84 to learn more about this!) showered the earth in radiation that could have killed the dinosaurs. Once again, the problem with the theory was explaining why dinosaurs died out and other species did not. Also, scientists said that such an event would have left evidence on the surface of the earth – there would have been tiny amounts of radiation dating back to the Cretaceous Period, but none was found.

Enter Luis Alvarez, a Nobel Prize-winning physicist, inventor and pioneer in the field of radiation and nuclear research. He and his son, geologist Walter Alvarez, were conducting research in Italy when they discovered a centimeter-thick layer of iridium-enriched clay in Earth’s geology that dated back to the precise time the dinosaurs died. Iridium is rare on earth, but more common in space.

The father-and-son duo published their findings in 1981, theorising that the thin layer of iridium was deposited following the impact of a large meteor, comet or asteroid with the earth, and that this impact could have caused the extinction of the dinosaurs. At the time, the Alvarez theory was so far removed from prevailing hypotheses that it was ridiculed. Slowly, though, other scientists began finding iridium evidence at various places around the globe that corroborated the Alvarez theory. The problem was, there was no obvious impact site to back this theory up.

However, in 1991, a massive meteor crater 110 miles in diameter was discovered on the edge of the Yucatán Peninsula, extending into the Gulf of Mexico. Scientists believe the meteor that formed this crater was roughly 6 miles in diameter, struck the earth at 40,000 miles per hour, and released 2 million times more energy than the most powerful nuclear bomb ever detonated. The heat would have boiled the earth’s surface, ignited wildfires worldwide and plunged the planet into darkness as debris clouded the atmosphere. Miles-high tsunamis would have washed over the continents, drowning many forms of life. Shock waves would have triggered earthquakes and volcanic eruptions.

We may think the extinction of the dinosaurs was like one big explosion that wiped them all out in one fell swoop—at least that’s how I’ve always imagined it. When that meteor impacted the Yucatán Peninsula, the result wasn't a huge fireball that instantly incinerated all of the dinosaurs on Earth. The process of extinction actually dragged on for hundreds, and possibly thousands, of years, as plunging global temperatures, lack of sunlight, and the resulting lack of vegetation profoundly altered the food chain from the bottom up.

After the planet was plunged into darkness because of the debris clouding the atmosphere, the resulting freezing temperatures would have killed plants, leaving herbivores with nothing to eat. Many dinosaurs would have died within weeks. The carnivores who feasted on the herbivores would have died a month or two later.

Overall, the loss of biodiversity would have been huge. Only small scavenging mammals that could burrow into the ground and eat whatever remained would have survived. The iridium layer plus the discovery of this impact crater were evidence enough to convince many scientists that the Alvarez theory was credible, and it explained much of what previous theories couldn’t.

Even so, agreement over dinosaur extinction is far from unanimous, and fossils continue to be found that add to the body of knowledge about how the dinosaurs lived and died. Only recently have birds been identified as descendants of the dinosaurs, and theories regarding dinosaur intelligence and behaviour continue to change. Even long-established truths such as dinosaurs’ cold-bloodedness are open for debate.

The climate change theory still holds sway over some scientists, who refute the idea that the meteor impact was the sole cause of the extinction. Evidence from the 65-million-year-old lava flows in India hint that a giant, gaseous volcanic plume might have initiated global climate change that threatened the dinosaurs. New information about them is constantly being uncovered.

So who knows what the future holds? Maybe one day, new discoveries will be made that change everything we thought we knew about our prehistoric past!