Most people are familiar with the Cretacious-Tertiary mass extinction about 65 million years ago, which killed off the last of the dinosaurs. Well, except for birds, which are totally still dinosaurs.
Some people know about the Permian mass extinction, which left only about four percent of species alive. That’s also the one that finished off the trilobites, which is a bummer because they seem pretty cool.
…and the eight-foot-long millipede known as Arthropleura.
So there’s that.
First off, it’s helpful to know how geologists and paleontologists mark time in a history-of-life-on-Earth sense. They first divide the history of Earth, starting when it formed about 4.6 billion years ago, into eons. The first three eons aren’t really important right now, so just know that we are living in the Phanerozoic eon, which began about 541 million years ago, about the time the first animals began to appear in the fossil record. (In case you’re curious, the previous eon, known as the Proterozoic, began about 2.5 billion years ago at the time scientists believe complex life first developed.)
The current eon is subdivided into three eras:
- Paleozoic, from 541 to 252 million years ago;
- Mesozoic, from 252 to 65 million years ago; and
- Cenozoic, from 65 million years ago to the present.
- Eon: Phanerozoic, starting 541 million years ago
- Era: Cenozoic, starting 65 million years ago
- Period: Quaternary, starting 2.6 million years ago
- Epoch: Holocene, starting about 11,700 years ago
- Age: We don’t have a designated age within the Holocene epoch, although some scientists have proposed declaring a new epoch, the Anthropocene, beginning with the Industrial Revolution or earlier.
Three mass extinction events occurred during the Paleozoic era, including the one that marked the end of that era and the start of the Mesozoic. The other two occurred during the Mesozoic, and the last one marked the dividing line between that era and the current one. (You can probably see a pattern there.) We are arguably in the middle of another mass extinction, known as the Holocene Extinction, thanks at least in part to humans meddling with everything, but that is a topic for another day.
Two things to keep in mind:
- The sheer scale of time involved here. The entire Phanerozoic era, which basically constitutes the entire history of multicellular life on Earth, only account for about 11.8% of the Earth’s history. To use the example of the clock, where Earth’s history is reduced to 24 hours, that’s only about two hours, forty-nine minutes, and twenty-one seconds. Humanity’s 6,000 years of recorded history amounts to just over one-tenth of one second, which is a span of time that means nothing to anyone who isn’t a high-frequency trader.
- Plate tectonics. Since the continents are always moving, the map of the Earth doesn’t look like anything we are used to for most of these events.
The very first mass extinction event probably didn’t look very dramatic, and is not counted among the five. Known as the Great Oxygenation Event, it occurred about 2.3 billion years ago when the production of oxygen by microorganisms reached a tipping point. This likely killed off most of the anaerobic microbes at the time, and led to the dominance of oxygen-breathing or -exhaling organisms (i.e. most living things today.) I say it “probably didn’t look very dramatic” because the only living things on Earth at the time, as far as we know, were microscopic, so there were no tableaux of dinosaurs gasping for breath like you see in artist’s renderings. They also didn’t leave behind much in the way of fossils, being microscopic and all.
Now then, let’s move on to the major mass extinctions. The occasion for the beginning of the Phanerozoic eon, the Paleozoic era, and the Cambrian period was a sudden (by which I mean over the course of about 20 million years) appearance of complex animal life known as the Cambrian explosion. This lasted from about 541 to about 521 million years ago. This is the time period when the trilobites first showed up, and in my imagination they swam happily around the primordial oceans for millions of years, bringing peace and joy to all the other animals. They had about 74 million years to do so before the first big extinction.
1. The Ordovician-Silurian extinction (O-S) events took place over several million years, beginning about 447 million years ago. I say “events” because it was apparently two separate events about four million years apart. Over sixty percent of marine animal life died out, making it the second-biggest extinction event in the oceans after the Permian event. I’m not sure how much life, if any, existed on land at this point. The continent Gondwana was moving south, which had an impact on climate and sea level that may have caused the extinction:
When Gondwana passed over the north [sic] pole in the Ordovician, global climatic cooling occured to such a degree that there was global large-scale continental resulting in widespread glaciation. This glaciation event also caused a lowering of sea level worldwide as large amounts of water became tied up in ice sheets. A combination of this lowering of sea-level, reducing ecospace on continental shelves, in conjunction with the cooling caused by the glaciation itself are likely driving agents for the Ordovician mass extinction.
Note the abrupt glaciation of the southern continent in the animation above.
2. The Late Devonian (LD) extinction began about 375 million years ago, and lasted for an uncertain period of time. Estimates of the duration of the extinction event range from hundreds of thousands to tens of millions of years. Unlike other extinction events, it did not mark the end of a geological period or era. Approximately nineteen percent of all biological families, and fifty percent of all genera, died out in this event.
By the late Devonian period, plant and animal life had established itself on land, but it appears that the extinction almost solely affected marine life, to the tune of about seventy percent. The cause remains uncertain, with theories including global cooling again, oceanic volcanism, a meteor impact, or a combination of factors. The animation shows sudden greening of the continents, suggesting some form of climate change.
3. The Permian-Triassic (P-T) extinction, which happened about 252 million years ago, is the biggest one in history. It marked the end of not only the Permian period, but the Paleozoic era. Ninety percent of all species on Earth, possibly more, went extinct, both in the oceans and on land. The species that survived, such as the homely Lystrosaurus, would go on to achieve a certain sort of greatness.
By this time, all of the world’s landmasses had combined to form the supercontinent Pangea. Glaciation is once again a possible cause for the extinction, but the sheer scale of it has led scientists to look for other, more dramatic factors. This is where the Siberian Traps come in. This is a region in (obviously) Siberia composed of volcanic rock formed from massive eruptions that occurred (you guessed it) about 252 million years ago. This would have released not only ash, but a wide range of nasty gases and other substances into the atmosphere. The volcanoes might not have been enough to cause such extensive destruction—to the extent that the Permian extinction event is also known as the “Great Dying.” Additional suspects include anoxia in the world’s ocean (one continent means one ocean) and a microbe that filled the atmosphere with methane. (Please try not to think of it as microbes that won’t stop farting. History deserves better than that. The animation depicts widespread volcanism, with nary a farting microorganism in sight.)
4. With the beginning of the Mesozoic era came the first of the dinosaurs. Our pal Lystrosaurus became much more widespread at the beginning of the Triassic period. He was part of the group of animals called synapsids, which included “mammal-like reptiles” like Dimetrodon and the actual mammals. Most synapsids died out in the Permian extinction, and had to wait almost 200 million years to get their chance again. Eventually, Lystrosaurus and his kin had to make way for the archosaurs, which were better adapted to the conditions of the time. Some of the archosaurs evolved into dinosaurs, while others still exist today as birds and crocodilians.
The fourth mass extinction event, known as the Triassic-Jurassic (T-J) extinction, marked the beginning of a period with a familiar name. This occurred about 201 million years ago. About half of all known species died out. All of the remaining non-dinosaur archosaur species, except for the crocodilians, went extinct. Most of the famous dinosaur species had not appeared yet, such as Tyrannosaurus rex and other dinosaur predators. The extinction of the other archosaurs, large amphibian species, and other species probably paved the way for dinosaur evolution.
The leading contender for a cause of the Triassic-Jurassic extinction involves the breakup of Pangea into two smaller supercontinents, Laurasia and Gondwanaland. This caused massive volcanic eruptions in the Central Atlantic Magmatic Province, which might have played a significant role in the extinction event. Pangea’s separation created the Atlantic Ocean, which is still expanding today along the Mid-Atlantic Ridge. (Note the “ring of fire” effect in the Atlantic in the animation.)
Once the Jurassic period was underway, dinosaurs had the world largely to themselves for around 136 million years, until…
5. The Cretaceous extinction event occurred about 65 million years ago, and spelled the end for the dinosaurs. It also marked the end of the Cretaceous period and the Mesozoic era, and the beginning of the Cenozoic era and the Paleogene (or Tertiary) period. For this reason, it is often called the “K-Pg” or “K-T” event.
About seventy-five percent of all species alive at the time went extinct. This included all of the dinosaurs, since the birds had already formed their own evolutionary line. During the period that dinosaurs “ruled” the Earth, fish evolved to a form that would be familiar to us today, and flowering plants began to flourish. Plate tectonics were moving the continents into a position that would look at least a bit familiar to us today.
As for the cause of the extinction, one theory involves volcanic eruptions in the Deccan Traps, in what is now India. The theory that has gained a large amount of acceptance, however, is that an asteroid struck Earth 65 million years ago, causing global devastation. The discovery of the Chicxulub crater off the coast of the Yucatan Peninsula has offered substantial support to this idea, although other possibilities, or a combination thereof, remain.
I must admit, though, the asteroid impact theory is much more marketable. I kind of get the impression that the whole point of the animation was to get to the big explosion.
Just as a fun exercise, let’s see how much time passed between each event:
- From the O-S to the LD extinction: 72 million years.
- From the LD to the P-T extinction: 123 million years.
- From the P-T to the T-J extinction: 51 million years.
- From the T-J to the K-T extinction: 136 million years.
Wow, dinosaurs had a good run there. If we use the 24-hour clock analogy, and we count the time from the T-J to the K-T events, dinosaurs had 42 minutes and 34 seconds. Even the most generous interpretation of the evidence of the origin of Homo sapiens, at 200,000 years, only gets us 3.76 seconds out of the day.
Still, as far as we know, in all that time dinosaurs never learned how to make chocolate. They definitely never learned how to make animated GIFs of Olivia Munn.
If I haven’t already made that a running gag, I’m doing it now.
Photo credits: “The 5 mass extinctions” posted by Waffurur, via Imgur; “Kainops invius lateral and ventral” by Moussa Direct Ltd. (Moussa Direct Ltd. image archive) [CC BY-SA 3.0], via Wikimedia Commons; “Mixopterus BW” by Nobu Tamura (http://spinops.blogspot.com) (Own work) [GFDL or CC BY 3.0], via Wikimedia Commons; “Giardino dei semplici, mostra dinosauri, arthropleura armata” by Sailko [GFDL or CC BY-SA 3.0], via Wikimedia Commons; “Extinction intensity” by Smith609 [GFDL or CC BY-SA 3.0], via Wikimedia Commons; “Lystr georg1DB” by Dmitry Bogdanov [GFDL or CC BY-SA 3.0], via Wikimedia Commons; “Chicxulub impact – artist impression” uploaded by Danielm at Dutch Wikipedia [Public domain], via Wikimedia Commons; Olivia Munn GIF © HBO (I assume), via BuzzFeed.