Mass Extinctions Part 1: This can't be good for the trout population

Cover Image: An early mammal hides from the dinosaurs.

Picture this: you are a vaguely mammalian creature from about 65 million years ago, resting under a tree. Your family has been picked off one by one by massive, semi-feathered predators outside, and you have taken shelter with a few other survivors in a burrow deep underground. Without warning, there is at once a distant thundering sound, and a wave of heat washes over you. Over the next few years, you will contend with fire raining from the sky, massive floods, and huge plumes of smoke blotting out the sun.

In the popular imagination, this is the event that wiped out the Dinosaurs – the K-T (Cretaceous-Tertiary) mass extinction event, now more accurately known as the K-Pg extinction (Cretaceous-Paleogene). Sparked by a massive asteroid that plunged into the Earth and left behind the Chicxulub impact site in the Gulf of Mexico, many an amateur scientist and documentary-maker have been captivated by this dramatic catastrophe.

The truth is probably not quite as exciting as it seems on the surface, though it would be a stretch to call it boring. In reality, a wide variety of factors led to the dinosaurs perishing, giving way to the dominance of mammals in the modern era we call the holocene.

But to better understand how these extinction events have driven our evolutionary history and what lessons we should learn from them today, we must go on a tour of the planet’s past to visit each of the five mass extinctions commonly recognised by scientists – and the 6th one raging today.

What is a mass extinction? Every once in a while, a single event spanning a relatively short period (usually 2-3 million years) occurs where the extinction rate becomes much higher than the background extinction rate. Typically, when 75% of existing biodiversity has been wiped out, this event will be considered a major mass extinction.

This is the mass extinctions series, and in each article we’ll be travelling through time to discover more and more about the past, before finally wrapping up with what all this means now.

The great Oxygen catastrophe

A brief note: even before the five great extinctions, there was a devastating event buried far in the evolutionary record that wiped out so much life, we see very little evidence of what existed before it. In fact, scientists have reason to believe this was the deadliest single extinction event ever to occur, but because only unicellular species were affected, it doesn’t receive nearly as much attention.

Back when life was still crawling out of the primordial soup, most cells that arose were anaerobic, meaning they could make energy without oxygen, and found the oxygen we take for granted today a potent toxin. Thankfully, at that point in time, the oxygen content of the planet was so low that it hardly mattered.

All this changed when cyanobacteria evolved into being, discovering a method to transform light energy into chemical energy while producing oxygen as a waste product – photosynthesis.

All at once, the oxygen content in the ocean (where all life still resided) shot up, and the poor anaerobic cells living there perished, for the most part. This was exacerbated by the cyanobacteria taking up carbon dioxide from the atmosphere, driving a snowball Earth effect (the opposite of the greenhouse effect) that froze the surface, likely creating an ice age and perhaps even endangering themselves. Of course, there are still some deep-sea microorganisms today capable of using gases like methane from structures like hydrothermal vents to synthesise energy via a process called methanogenesis, but these ancient hangovers seem to be the exception and not the norm.

Ordovician

We now move into a period where multicellular life flourished, especially in the Cambrian explosion followed by the Great Ordovician Biodiversification Event (GOBE). Trilobites, horseshoe crabs and jellyfish all arose at around this time, and these icons of ancient life were survivors of the first real mass extinction that multicellular life had to face. The late Ordovician mass extinction occurred in two stages, or “pulses”, and in total wiped out half or more of the extant marine genera, and caused the disappearance of a staggering 85% of marine species at the time. Most of the Cambrian-era species perished in this event, and the survivors never quite recovered. The question remains: how?

The Ordovician extinction is one where so-called conspiracy theories abound, perhaps due to the veil of time obfuscating the factors that caused it. Although it is generally recognised that wildly varying oxygen levels, the rise in levels of toxic metals and sulfides, and shifts in the planet’s temperature resulted in this massacre of aquatic life, the details of why exactly this occurred are debated.

The causes of the two “pulses” are also hard to pin down: LOMEI-1, the first pulse, was characterised by the massive Hirnantian glaciation, while LOMEI-2 was instead a period of warming, receding ice and anoxia (shortage of oxygen) together with euxinia (anoxia with elevated levels of sulfides). With such different events, how could one coherent theory tie everything together?

Now, if you asked my favourite pet theory? (Or if I may be so self-absorbed to propose it?)

The “gamma-ray burst” hypothesis proposes that a nearby star in its death throes unleashed a massive burst of high-energy gamma radiation in our direction. Such an event would have resulted in the destruction of a vast amount of ozone in our atmosphere – that very same ozone that protects us from the sun’s UV radiation.

It is commonly recognised that plankton (microscopic marine animals and plants) were among the most heavily impacted groups in the first pulse of the Ordovician extinction, as well as those organisms that dwelt in shallow waters. This would indeed be consistent with a sudden exposure to UV radiation, which would not have reached too far into the ocean and would have damaged organisms near the surface instead. Furthermore, there is evidence that glaciers rapidly expanded due to cooling from dark nitrogen dioxide gas blocking the sun, nitrogen in the atmosphere having combined with oxygen formed from destroyed ozone, and a sudden loss of carbon from the surface associated with nitric acid rain (ouch!)

At present, the main issue with this hypothesis is just that there is no real proof that it did actually happen. No signs point towards any nearby star having gone supernova at that point in time, at least not definitively enough to say it could have hit Earth with a gamma ray burst. As much as a “fun” theory this hypothesis is, it’s hard to say it’s anything more than that without the smoking gun.

Another explanation is that anoxia came first, with lowered oxygen levels making it easier for toxic metals in the seabed to dissolve in the water and kill off organisms lower down on the food chain. This would have resulted in massive ripple effects up the food chain, devastating the ecosystem.

The last theory we’ll be looking at is the volcanism hypothesis.

An interesting fact to note was that the rise in extinctions actually began several million years before glaciation caused the first pulse of extinctions, an increased rate of species disappearance associated with volcanic activity in a Large Igneous Province (LIP), which could be the Alborz LIP currently located in Iran.

Signs consistent with volcanic origins behind the extinction’s first pulse include the wave of anoxia, increased mercury concentrations, volcanic phosphorus and sulfur triggering eunixia in the second pulse, and the former triggering loss of phosphorus from marine sediments that could have decreased the amount of dissolved oxygen.

However, other studies failed to find conclusive evidence of increased mercury levels, and mercury sulfides (the primary indicator of large-scale volcanism in the Ordovician) showed no signs of anomaly. As such, what really drove the Ordovician extinction beyond what we already know about the Hirnantian glaciation is a fuzzy affair that will be hotly debated for years to come.

Enjoyed this article? Come back soon for more as we explore one of the most terrifying events in the planet’s biological history in part 2, The Great Dying.

References:

1. Aiyer, Kartik, PhD. 2022. “The Great Oxidation Event: How cyanobacteria changed life.” ASM.Org. February 18, 2022. https://asm.org/articles/2022/february/the-great-oxidation-event-how-cyanobacteria-change

2. Ball, Philip. 2003. “Gamma-ray Burst Linked to Mass Extinction.” Nature, September. https://doi.org/10.1038/news030922-7

3. Harper, David a T. 2023. “Late Ordovician Mass Extinction: Earth, Fire and Ice.” National Science Review 11 (1). https://doi.org/10.1093/nsr/nwad319

4. Kozik, Nevin P., Seth A. Young, Sean M. Newby, Mu Liu, Daizhao Chen, Emma U. Hammarlund, David P. G. Bond, Theodore R. Them, and Jeremy D. Owens. 2022. “Rapid Marine Oxygen Variability: Driver of the Late Ordovician Mass Extinction.” Science Advances 8 (46). https://doi.org/10.1126/sciadv.abn8345

5. Lyu, Zhe, Nana Shao, Taiwo Akinyemi, and William B. Whitman. 2018. “Methanogenesis.” Current Biology 28 (13): R727–32. https://doi.org/10.1016/j.cub.2018.05.021

6. Zhang, Zhutong, Chuan Yang, Diana Sahy, Ren-Bin Zhan, Rong-Chang Wu, Yang Li, Yiying Deng, et al. 2025. “Tempo of the Late Ordovician Mass Extinction Controlled by the Rate of Climate Change.” Science Advances 11 (22). https://doi.org/10.1126/sciadv.adv6788

Image Credit: Vuong, Z. (2018, May 23). Why you should care about this 130-million-year-old fossil. University of Southern California - USC Today. Retrieved December 16, 2025, from https://today.usc.edu/why-you-should-care-about-this-130-million-year-old-fossil/