About a year ago Dr. Bakker wrote a post about the different extinction periods that killed off the dinosaurs, mammoths, and saber-tooths. Recently, one of our readers asked some very thoughtful questions. R. Richards asks,  “How long would it take for this lethal concoction of plagues to do significant damage to a variety of species? Even with multiple pathogens and mutation allowing cross species infections it would have to take quite some time for this scenario to run its course among all of the different fauna.” (To see the full comment, click here.)

Dr. Bakker has taken this opportunity to write a follow-up blog, and hopefully answer any more questions that our faithful readers might have.

Thanks for the nuanced and provocative thoughts about dino-plagues and dino-extinctions.

I’ll discuss the time-line of extinctions first. Then, in a follow-up blog, I’ll talk about body size and rebound after an extinction.

The question here is: When dino-style extinctions happen, do the die-offs occur everywhere in the world at the same time?

photo credit: Javier Paredes

The most recent extinction event is the Ice Age die-offs, and these extinctions do offer help in interpreting the terminal Cretaceous disaster among dinosaurs. The “Asteroid Theory” predicts that extinction should happen almost instantly – in a matter of weeks or months after the impact. We have an excellent fossil record for what happened when a land bridge permitted exchange between North and South America just as the Ice Age began. South America had been an isolated island continent that evolved its own unique large herbivores and carnivores, very different from what North America had. If the Immigrant Plague Theory works, we should see episodes of extinction that coincide with episodes of mingling of faunas, North and South.

We do. The first pulse of die-off happened when North American mammal carnivores began to invade South America, towards the end of the Pliocene about 3 million years ago. The South American native carnivores suffered right away. The giant carnivorous ground birds died out almost completely and the giant marsupial predators disappeared. The invasions were not all one sided. Some South American herbivorous maxi-fauna did invade North America – giant ground sloths came up in great abundance. And one species of killer ground bird made it to Florida.

photo credit: BryanKemp

A second pulse happened as mastodons, deer and other Northern large herbivores invaded South America. Several native South American herbivore clans disappeared.

The third round of extinctions happened 15,000 to 10,000 years ago, when all the native South American big mammals went extinct. Humans entered South America about this time. At the same event all the biggest invaders too died out – gone were mastodons and saber-tooth cats that had come across the Panama land bridge from the North. In North America, the saber-cats, mastodons and mammoths died out along with the ground-sloths that had invaded from the south.

The entire Ice Age extinctions, world wide, took over three million years. They did not happen all at once, everywhere.

photo credit: gwenturnerjuarez

The final dino-die offs were also complicated. You see the beginning of the crash in North American diversity about 72 million years ago, in the Horseshoe Canyon Fauna. Only a very few big dinosaurs have large numbers. In the Lancian Fauna, 67 to 65.5 million years ago, we still have some dinosaurs but only two herbivores are common: Triceratops and Edmontosaurus. Then, at 65.4 million years ago, all the remaining big dinos go extinct.

These pulses of dino die-offs probably coincide with pulses of faunal interchange among the continents.

Conclusion: the Cretaceous dino extinctions were complicated in time and space. They did not happen suddenly all over the globe.