Long before the dawn of humans, dinosaurs, insects or even trees, a cascade of unfortunate events threatened to end life on earth.
During the Ordovician period, about 485 to 444 million years agoon the diversity of marine life has exploded. Trilobites and molluscs crawled along the ocean floor, plankton-like filters drifted to all depths, and corals and algae bloomed. Jawless fish, perhaps our oldest ancestors, drifted in shallow lagoons and deltas. Life may also have taken its first steps on land during this period. Some researchers speculate that Ordovician green algae they may have migrated to the coast with the help of mycorrhizal fungi.
However, sometime around 445 million years ago, 85 percent of species are extinct during the relatively short interval of 1.4 million years. This unprecedented extinction is now known as Earth’s first mass extinction, the Late Ordovician Mass Extinction, or simply the LOME. Many researchers have devoted time or even careers to uncovering the underlying forces behind extinction. But pieces of the puzzle are still missing.
“As you can imagine, trying to infer exactly what happened in the environment 445 million years ago is a pretty imprecise process,” says paleobiologist Charles Mitchell. “But we can make out some things quite clearly.”
What caused the Ordovician extinction
Around the time of the disappearance, Earth’s climate has undergone a number of significant changes. A period of warming and sea level rise was followed by an ice age. Glaciers encapsulated much of the ancient supercontinent Gondwana, landmass that gave rise to parts of every major modern continent. Eventually, the ice age gave way to warming again.
These climate changes disrupted the ways in which nutrients such as oxygen, carbon and nitrogen passed through the ocean at that time.
“When you go from greenhouse conditions to ice conditions, there will be big changes in ocean circulation patterns,” says Mitchell.
A famous theory postulates that an initial wave of extinction occurred when the Ice Age began. Organisms at the bottom of the food chain, algae and cyanobacteria, may have adapted slowly to a colder climate. The same theory links the second wave of extinction to the end of the Ice Age. Increased temperatures may have caused a global “algal bloom,” similar to the blooms caused by nutrient-rich sewage in lakes and rivers today.
This rapid spread of cyanobacteria may have caused the deoxygenation of the ocean that scientists have observed in the geological record.
A second theory that has gained some popularity, connects the two extinction waves with the warm one periods that end the Ice Age. In a 2020 papertwo geologists argue that a major volcanic eruption may also have been a contributing factor.
“Rather than being the odd Big Five extinction of cooling origin, the LOME is similar to the others in that it was caused by volcanism, warming, and anoxia,” they wrote.
While scientists will hotly debate the causes for decades to come, the results of the extinction are clearer. All the major groups of Ordovician organisms were affected – trilobites, brachiopods and bryozoans died out in large proportions. But while the mass extinctions that followed picked broad categories of winners or losers, some species from almost every major group or organism survived the LOME. During the Silurian Period, which followed the Ordovician, these survivors repopulated the oceans.
Mitchell has focused much of his work on a group of filter feeders that were hit particularly hard by the extinction: graptolites. These tubular organisms were abundant in the Ordovician oceans.
“They were planktonic, so they were directly harvesting algae, which is at the bottom of the food chain,” Mitchell says. “That’s why they’re kind of like a canary in a coal mine.”
Looking through thousands of graptolite fossils, Mitchell and his colleagues noticed something curious. The creatures died slowly, long before the sharp decline associated with the mass extinction.
“Graptolites started disappearing significantly before the big pulse,” says Mitchell. “This means that whatever caused the turnover must have been a longer-term event.”
In other words, slow and gradual change eventually gave way to rapid decline. Mitchell sees a parallel here with current human-induced changes in global biodiversity. Over the past century, vertebrate species have become extinct a rate 100 times higher than the pre-industrial average. This the percentage is expected to increase as global temperatures rise.
“Things seem to happen predictably and then you fall off a cliff,” says Mitchell. “At the moment we are still in the gradual change phase. We can’t kid ourselves that this is manageable.”