In his acclaimed bestseller The Rise and Fall of the Dinosaurs, American paleontologist Steve Brusatte enchanted readers with his definitive history of the dinosaurs. He’s back with another definitive natural history book, The Rise and Reign of the Mammals.: A New History From the Shadow of the Dinosaurs to Us. He discussed the way mass extinction events took shape, killed the dinosaurs, and cleared the way for the evolution and spread of mammals.
Prior to the Late Permian, mammals had yet to make an appearance. However, there were many proto-mammals walking the earth, specifically fellow therapsids. Can you paint a picture of the creatures sharing the Earth?
In many ways, the Permian was an alien world. Individual continents were ceasing to exist, and all land was coming together to form the supercontinent of Pangea, which stretched from north pole to south pole, and was covered by vast deserts. This harsh landscape was the realm of the synapsids.
Earlier, in the Carboniferous period, the synapsid line set off on its own, separating from the reptile lineage on the family tree. What defined the synapsids was a single big hole behind the eye socket, which held large muscles that controlled the jaws, enabling stronger bites. Among these synapsids were some of the first big predators in Earth history: monsters like the sail-backed Dimetrodon and the saber-toothed gorgonopsians, and some of the first big plant-eaters in Earth history, like the pot-bellied caseids.
From the vantage point of the late Permian, it might seem like these synapsids would go on ruling the world forever, but alas, only a few would even survive.
How did the Late Permian extinction even begin?
Around 252 million years ago, give or take, the Earth’s mantle started to churn underneath what is now Siberia. Energy concentrated and flowed in gyres, deep underground. This heated the crust above and pushed currents of magma up towards the Earth’s surface, and as the magma pulsed, it burned the rocks it passed through.
As it burned, it released carbon dioxide and methane—potent greenhouse gases. And then the magma would erupt as lava, and scorch the surface of the Earth. And this happened on a huge scale, for hundreds of thousands, or even millions, of years. This was a ‘megavolcano’, a type of volcano that thankfully humans have never experienced. Fissures in the Earth, canyons in the crust, like the ground was slashed by a giant machete, with lava pouring out.
All in all, a land area much larger than modern-day Europe was covered by lava. Incinerated. But the greenhouses gases were an even worse problem. These caused a global catastrophe.
In The Rise and Reign of the Mammals, you hint at the fact that the extinction process occurred gradually over a long period of time and not in one catastrophic moment. Can you explain that?
When the dinosaurs died, the cause was an instantaneous explosion: the impact of a six-mile-wide asteroid with the Earth, on one random day—a stroke of horrific bad luck. And about 75% of all species died. But the end of the Permian was worse, and different. Maybe up to 95% of species died. But it didn’t happen quickly, or as the result of a single moment of tragedy. It was the result of hundreds of thousands, or millions, of years of magma burning through the crust and erupting through the megavolcanoes in Siberia, releasing unholy amounts of greenhouse gas, which caused extreme global warming.
Who was able to survive? How did they make it through?
If you were around at the end of the Permian, your species probably had about a 5% chance of survival. Imagine that, a gun, with 20 chambers. 19 of them hold a bullet. Take your shot. That’s your chance of survival.
Some things did survive. On land, a few types of animals with bones made it through. Some were small, agile, fast-running reptiles called dinosauromorphs: little cat-sized nobodies that would eventually give rise to the dinosaurs.
Others were a type of synapsid, called a cynodont. These cynodonts were small, about the size of a miniature dachshund at most. They were great burrowers, so they could hide easily and hunker down and sit out the worst of the chaos. They grew fast, maturing and reproducing within a year of being born. And they probably had some hair to cover their bodies and protect themselves from hot and cold spells. These cynodonts were the ancestors of mammals.
If they did not make it through the end-Permian extinction, mammals would never have the chance to eventually evolve. Which means we wouldn’t be here having this conversation.
How did this extinction event influence the future evolution of mammals?
The extinction event was a great leveller, a great filter. Most of those synapsids that thrived in the Permian—the ones with potbellies for scoffing plants and saber-tooths in their jaws—died. They couldn’t cope with the sudden climate changes. But cynodonts could. And when the volcanoes stopped, and the Earth started to heal, they found themselves in a brave new world. A largely empty world. A world bristling with opportunity, primed for the taking. It was from this ecological release, this unexpected opening of ecosystems, that the mammalian descendants of the cynodonts would soon emerge.
Your description of the effect of greenhouse gasses being released really drove home how dangerous the current state of greenhouse gasses emissions and how dangerous it is. Is it possible to inform one extinction event with another? If so, what does it tell us about the situation today?
Absolutely. There are a few reasons why paleontologists like me study fossils. One, they’re simply cool. They are relics from times long past, and many of them are big and scary and engaging, and we become fascinated with them as children. But the other main reason, the scientific reason, is that fossils are clues from prehistory. They are records of real plants and animals that had to endure real moments of climate and environmental change.
Temperatures are rising today, weather is becoming more extreme, sea levels are rising. It is worrying. But these things have happened before—for different reasons, of course: volcanoes at the end of the Permian, humans burning hydrocarbons today. But these things have indeed happened. And we can look to fossils to gauge how the Earth and its ecosystems responded. What lived, what died, how long it took to recover. The basic facts. And those facts can help guide our future. They can help us better understand and prepare for and mitigate climate and environmental changes today. Just as a historian may look to wars and famines in an attempt to learn from them so they are never repeated, paleontologists look to the fossil record.