COMMON DESCENT

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66 million years ago, the planet experienced one of the most devastating – and famous – events in its 4.6-billion-year history. On this episode … The K-Pg Mass Extinction.

In the News:
Fish Out of Water
We often wonder what drove the first fish to leave the water. A new study shows that fish eyes evolved to see through air (crocodile-style) well before the jump to land. [Report]
The First Sharks
A 400-million-year-old fossil from Canada clearly has features of a shark, but also features of the extinct acanthodian fish – strong evidence for a transitional form early in shark evolution. [Press Release]
Really Old Croc Eggs
The world’s oldest crocodylomorph eggs! At 152 million years old, they’re quite similar to living croc eggs. And some, for unknown reasons, were found in a dinosaur nest. [Report]
Really, Really Old Plants
Microfossils from India, at 1.6 billion years old, appear to be the world’s oldest algae. If so, these are not only the oldest known plants, but the oldest known multicellular eukaryotes … whoa. [Report]

The K-Pg Extinction

At the boundary between the Cretaceous (K) and Paleogene (Pg) Periods, ecosystems all around the world suffered major losses of species in a geologically short time period. This is called a mass extinction, and the K-Pg event was one of history’s worst. It marks not only the end of the Cretaceous Period, but the end of the entire Mesozoic Era.

Just about every group of life got hit by this event. Some disappeared entirely, including pterosaurs, mosasaurs, and ammonites. Many suffered great losses but ultimately survived, such as mammals, crocodilians, fish, corals, land plants, and many more. Famously, dinosaurs were almost completely wiped out, with only a small portion of birds surviving. All told, this event has all the big signs of a worldwide ecosystem collapse.

What caused the K-Pg extinction?

Paleontologists don’t have a simple answer to this question, but we’re far from clueless. There are a few prominent suspects in this ancient Who-dun-it:

Asteroid Impact

This is the famous answer to the question of “what killed the dinosaurs?” Evidence for asteroid impact comes in the form of the K-Pg boundary layer itself, full of elements and minerals characteristic of impact, plus the 120-mile (180-km) crater buried beneath the Yucatan Peninsula in Mexico. The asteroid or comet that produced these effects is estimated to have been at least 6 miles (10 kilometers) across.

From geologic evidence and computer simulations, we know that this impact would have had a wide variety of dramatic effects, including earthquakes, tsunamis, burning debris, long-term cooling and darkening of the sky, and even longer-term warming. This could certainly have resulted in widespread ecological disturbance, especially for plants.

You can run your own asteroid-impact simulation with Purdue’s Impact Earth!

Volcanoes – the Deccan Traps

India is home to the Deccan Traps, a preposterously vast series of igneous rocks dating to the K-Pg boundary. This is a Large Igneous Province, leftover from a period of intense volcanic activity. Over several hundred thousand years, these eruptions released enough lava to cover hundreds of thousands of square miles/kilometers. In the process, this activity would have spewed out tons and tons of gases, having similar effects as the asteroid impact.

Deccan Trap volcanism started well before the K-Pg boundary and continued well after it, but the bulk of the eruptions happened right around the time of the asteroid impact and extinction. This close timing makes it very difficult to tease apart the effects of these two calamities.

Sea Level

Across the K-Pg boundary, there was an overall drop in sea level. There’s reason – and evidence – to suspect that this could have caused long-term rearrangement and disturbance of habitats. Some research has found evidence of lower diversity and ecological instability in certain aspects of Late Cretaceous ecosystems. This is unlikely to have caused the mass extinction by itself, but could certainly have left global populations vulnerable.

Many questions remain…

-Would this extinction have occurred if any one of those effects hadn’t happened, or were all three important factors?
-Did one of these effects have a great influence on extinction than the others?
-If the asteroid impact and volcanic eruptions had happened at a time when ecosystems were more stable, would the extinction have been less dramatic?
-What if the asteroid or volcanism had struck at a different part of the planet? Or under a different climate?
-Why did some life forms survive (such as birds and small mammals) while others didn’t (such as pterosaurs and small non-bird dinosaurs)? How much of this was luck vs. other factors?
We’re still working on answering these, and many more!

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 45 – The Permian Extinction
• Episode 15 – The end-Triassic Mass Extinction
• Episode 65 – The Late Devonian Extinction(s)
• Episode 85 – The Ordovician Mass Extinction
• Episode 55 – The “Sixth Extinction” (Modern Biodiversity Crisis)

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