Mass Extinctions

Many of us know of the asteroid that wiped out the dinosaurs approximately sixty-five million years ago. However, there have been at least four other mass extinction events that drastically changed life on Earth in the last five hundred million years

The extinction of species is not rare, as we know very well. However, we also know that more and more species are going extinct than usual. The background extinction rate refers to how many species are dying per a set number of years. Specifically, the background rate is how many species in ten thousand go extinct in one hundred years. If one species goes extinct on average, the background extinction rate is 1E/MSY, one extinction per ten thousand species per hundred years. Scientists can calculate the expected background rate by examining fossils or by reviewing the average lifespans of a species. In fact, the background rate can be different for different families or more varied. For example, it could take vertebrates around one thousand years for two species to go extinct, whereas two mammal species could go extinct in an estimated three thousand years. 

The extinction rate also depends on how conservative you want to be with your estimates. Amphibians could take anywhere between two thousand and ten thousand years for two species to go extinct, whereas reptiles may only have a difference of about a thousand years. Of course, the background extinction rate changes over time, but there are five outstanding instances when the extinction rate was way more than it would have been regularly.

Name	#mya*	%lost	Significance/About
End-Ordovician	443mya	85%	The first known mass extinction. An ice age starts and ends.
End-Devonian	375mya	80%	Plants cause problems. Earth cools dramatically
End-Permian	250mya	96%	The worst extinction event known. VOLCANOES!
End-Triassic	201mya	80%	Pangea is breaking apart. More volcanoes.
End-Cretaceus	65mya	75%	End of dinosaurs. The most recent extinction event. Meteor!

*mya = million years ago

Figure 1: Acidaspis buchi: (photo by Utrup, J., 2013)

The End-Ordovician extinction event, sometimes called the Ordovician-Silurian extinction, began about 443 million years ago in the Paleozoic Era. There were two major extinction events that are considered the same. The first half of the extinction occurred at the beginning of an ice age, and the second half at the end. The two major changes in climate created situations that species were unable to adapt to. During this time, about 85% of all marine species went extinct. However, members of the five major families at the time managed to survive. These families were the trilobites (see figure 1), brachiopods, corals, crinoids, and graptolites. 

Figure 2: Habrostroma centrotum: (photo by Stock, C. W., 2007)

The second major extinction event, also of the Paleozoic Era, is known as the Late or End Devonian extinctions. The Devonian extinction began 375 million years ago. In the course of three events, the Givetian, End-Frasnian, and Famennian extinctions, approximately 75-80% of species were wiped out. These three extinctions mostly affected tropical areas, though the Frasnian one in particular, and were likely caused by a large cooling event, possibly from a lack of CO2 in the atmosphere. In the opposite of what’s happening to the planet now, large plants and trees were evolving and removing more carbon dioxide from the atmosphere than usual, making it harder for the planet to retain heat. Very few trilobites survived this period, and corals took a great hit as well. Stromatoporoids (see figure 2) went entirely extinct during the final event of this extinction.

Figure 3: Early Permian Horseshoe Crab; Adelophthalmus luceroensis: (photo by Barkley, J., 2019)

The Great Dying, or the End Permian extinction, about 250 million years ago, is the worst extinction event that we know of, killing off about 96% of all living species at the time. After over 200 million years, trilobites finally went extinct, corals took another huge hit, and reef production in shallow areas stopped for 14 million years. Vertebrates, which had just begun evolving, lost about 70% of their species. During two events, the Capitanian extinction and the End-Permian extinction, the Great Dying was probably caused by a massive volcanic eruption in what is now Siberia. While there was plenty of lava for sure, the real killer was the large quantities of carbon dioxide that poured into the air. The Earth heated significantly, and the ocean absorbed more carbon dioxide than the life inside it could handle, creating a sea of toxicity, possibly increasing ocean temperatures by up to 14.5°F. 

The next major extinction event is known as the End Triassic extinction and occurred 201 million years ago during the Mesozoic Era. Not nearly as serious as the End Permian extinction, but still devastating, the End Triassic extinction rendered about 80% of all species extinct. Only one event is associated with the mass extinction. At the end of the Triassic period, the continent Pangea began to break up, creating a huge rift between what is now North America and Africa. As the continent split, the Atlantic Ocean started to form with giant volcanic activity below its surface. Similar to the end-Permian extinction, massive amounts of carbon dioxide were released into the atmosphere, the Earth heated, and the oceans became acidic. Yet again, corals took great losses, and phytosaurs (crocodilian creatures; see figure 4) went extinct.

Figure 4: Phytosaur 3D Model (By Jun3dstudio)

The final major extinction event is probably the most famous. While not killing as many species as the End Permian extinction, the End Cretaceous extinction event at the end of the Mesozoic Era, 65 million years ago, killed all dinosaurs except for birds.

Figure 5: Ichthyosaurus Fossil (Photo by Joaquín Corbalán)

A massive asteroid as big as six miles in diameter crashed into what is now Yucatán, Mexico. Dust would have cloaked the planet, causing a significant reduction in sunlight, killing off plants and plankton, and eventually non-photosynthesizing 

species due to the lack of food. Not only that, but dust falling from the sky would have been hot and could have caused abnormal amounts of wildfires. If that wasn’t enough, volcanic activities could have released greenhouse gases and caused even more climate change. Up to 75% of all species were eliminated in this event, most notably, the dinosaurs. While the End Permian extinction was indeed the most devastating, the End Cretaceous was by far the most dramatic planet-wide.

Many people speculate that we are facing a sixth mass extinction event right now. We are doing the same things to the climate that the volcanoes have done in the past. 70% of land surfaces have been transformed, it should have taken 2,000-10,000 years for as many vertebrates to go extinct as they have in just the past 100, agriculture has caused soil degradation and deforestation, species that never should have been in various areas have been unnaturally introduced to species who haven’t evolved to handle them, and some species have been killed off simply because we targeted them, either for food or out of fear. The outstanding evidence that we are in a mass extinction event, though, is indeed the fact that species are going extinct 100-1,000 times faster than they should be, given their background extinction rates. If this is the sixth mass extinction event, then it’d be the first one caused by a single species.

In the past 500 million years, the planet and the life that lives on it have faced five mass extinctions, and possibly now a sixth. From overbearing plants to space rocks, life on Earth has been through a lot and has managed to survive despite it. Each species is finely tuned to its environment, and when the environment changes, they either adapt or another species takes its place. So while Jurassic Park was cool and all, maybe we should focus more on the species we are causing to go extinct instead of the ones that died long ago.

Sources/more reading:

Sam Noble Museum (Specifics on all five mass extinctions)

Berkeley: University of California (General information on mass extinctions)

Thoughtco (Quick info on all five mass extinctions)

American Museum of Natural History (Even quicker info)

Natural History Museum (About what mass extinctions are)

Our World in Data (Overview of the five mass extinctions)

Population Education (On background extinction rate)

Yale Peabody Museum (Fossil photos –with details)

Canva - generic photos