The first mass extinction resulted from global... cooling: The Ordovician-Silurian extinction

Despite the five — or arguably six — mass extinction events that have occurred on our planet, most of us are only familiar with the one that wiped out the dinosaurs. In this series, I investigate the causes and effects of the other great mass extinctions, and then examine recent literature to determine whether, indeed, we are in the midst of the sixth mass extinction. This is the first part in a five-part series.

The first mass extinction event occurred between approximately 447 and 444 million years ago, when life on earth had not yet left the seas. It was the second-largest mass extinction, and perhaps the best understood, despite being the oldest. This first extinction (known as the Ordovician-Silurian) occurred in two phases. The first was when glaciation began, and the second was when glaciation stopped. By some estimates, this extinction reduced Earth’s biodiversity by half.

Before the event, atmospheric carbon dioxide levels were high, meaning most life adapted to an earth warmed by the greenhouse effect. But volcanic activity was creating silicate rocks, which drew carbon dioxide from the air as they eroded. On the rapidly cooling earth, the supercontinent of Gondwana moved toward the South Pole. Meanwhile, glaciers began to form, eventually causing sea levels to drop by 100 to 150 meters. More ice meant that more light was reflected off the earth, rather than absorbed by it, which hastened global cooling (Scientists call this the “albedo” effect). Strong temperature gradients turned the previously stable ocean into a convection-driven blender. These events led to the first pulse of extinction.

Eventually, advancing glaciers covered the silicate rocks, preventing them from sequestering any more carbon dioxide. Glacier formation came to a halt, and the strong, temperature-driven mixing in the ocean stalled. This stagnation presented a new challenge to those species that had managed to adapt to the rapidly cooling world. Those that failed to adapt a second time fell victim to this second pulse of extinction. Taxonomic diversity on Earth would not recover until five to seven million years into the following epoch.

These climactic trends set the scene for the first extinction, but which factors actually killed off species? In this case, we find three culprits working together.

First, as the sea level dropped 100 to 150 meters, shallow so-called epicontinental seaways dried up. We can think of these like canals, connecting one body of saltwater to another. When these waterways disappeared, the organisms adapted to them lost their habitat.

Second, the change in temperature was lethal. Imagine an Aztec community whose environment turns into Antarctic tundra, or an Inuit community whose environment abruptly warms to a sub-tropical climate. There’s a slight possibility that we humans could adapt, thanks to our ingenuity and mainly to our mammalian ability to control our own body temperatures. But all species were cold-blooded at this time; they didn’t have any chance at all, especially those organisms that were attached to a hard surface and therefore were immobile, or sessile.

Third, the newly churning ocean re-distributed nutrients, such as oxygen, plankton, and essential minerals like phosphorus and calcium. Anyone who’s ever owned an aquarium knows that extreme shifts like these are bad news for sea life. Organisms with adaptations to specific nutrient levels were suddenly out of balance with their environment. Some species found themselves lacking nutrients, could not adapt quickly enough to withstand the shortage, and became extinct. Others had nutrients in excess. Species which could most efficiently utilize this new abundance were able to out-compete others in their communities, driving some to extinction.

Generally, communities that existed only in a single region (endemic communities) became more cosmopolitan throughout the extinction. Brachiopods and trilobites were best able to maintain diversity through the extinction, because many forms of these animals were already distributed amongst many regions. Widely distributed groups such as these usually have the advantage in most mass extinctions. In contrast, other life forms were mostly or completely lost to the world during this mass extinction event.

Unfortunately, most of the species that died out are described only by jargon-laden fossil analysis (four radiating septa, or six?) and pictured only in inscrutable black-and-white photos of strange fossilized patterns in rocks. Lone fossils of calcified shells don’t give much information about how the living animals actually fit in the context of their environment. Consequently, not much is known about their natural history. Furthermore, many animals that died out during this extinction were part of larger groups which died out in subsequent extinction events. Therefore, they’re mostly unrecognizable to us today. For example, a certain order of trilobites went extinct, but this somehow feels irrelevant, considering that all other orders in that class are extinct now, too.

But I will describe one group that was nearly lost the first mass extinction: the Lingulata. We know more about this group because a few species survived to become “living fossils.” This means that the survivors look much the same as their fossilized ancestors, who lived over 400 million years ago! I leave you with a picture of a modern Lingulata, with its odd, long appendage called a pedicle, used to burrow into the sand and mud in search of food. The ocean must have looked pretty bizarre back then.