Late Ordovician mass extinction facts for kids
The Late Ordovician mass extinction (LOME) was one of the biggest mass extinction events in Earth's history. It happened about 443 million years ago. This event is often called the second-largest extinction ever. It caused 49–60% of all ocean animal groups (called genera) to disappear. Nearly 85% of all ocean species vanished.
Only the Permian-Triassic mass extinction was worse for losing so many different types of life. The Late Ordovician extinction strongly affected many animal groups. These included brachiopods, bryozoans, conodonts, trilobites, echinoderms, corals, bivalves, and graptolites. Even with so many losses, the basic ways ecosystems worked didn't change much. Life slowly recovered over the next 5 million years.
Scientists usually say this extinction happened in two main parts, or "pulses." The first pulse (LOMEI-1) started when the Earth got very cold. This was at the start of the Hirnantian stage of the Late Ordovician Period. A huge ice age spread over the supercontinent Gondwana. This changed Earth from a warm "greenhouse" climate to a cold "icehouse" climate.
The cooling and falling sea levels destroyed many animal homes. This was especially true for animals living on the continental shelves. Many unique animals that couldn't handle cold temperatures died out. During this time, there were also big changes in chemicals found in rocks, like carbon and oxygen. Some ocean life did adapt to the cold. A new group of cold-water animals, called the "Hirnantia fauna," appeared.
The second extinction pulse (LOMEI-2) happened later in the Hirnantian. The ice age ended quickly, and warm conditions returned. This second pulse was linked to widespread anoxia. This means the ocean water had very little oxygen. It also had euxinia, which means toxic sulfide chemicals were present. These conditions lasted into the next period, the Rhuddanian stage of the Silurian Period.
Some scientists think there might have been a third pulse of extinction. This would have been in the early Rhuddanian. It's suggested by more low-oxygen conditions. However, others believe this was just a longer part of the second pulse.
How Life Was Affected
Changes in Ocean Life
The Late Ordovician mass extinction happened after a time called the Great Ordovician Biodiversification Event (GOBE). This was one of the biggest increases in different types of life in Earth's history. At the time of the extinction, most complex living things were in the sea. There was only rare evidence of small early land plants on land.
Around 100 families of ocean animals disappeared during the extinction. This was about 49% of all genera. Brachiopods and bryozoans were hit very hard. Many trilobite, conodont, and graptolite families also vanished.
The extinction had two main pulses:
- The first pulse was at the start of the Hirnantian stage.
- The second pulse happened later in the Hirnantian stage.
Each pulse affected different animal groups. After each pulse, some new types of animals appeared. Studies show that the loss of different types of life was mainly due to a sharp rise in extinctions. It wasn't because fewer new species were forming.
After so many animals died, the communities in the Silurian Period were simpler. Animals could live in more places. However, in South China, warm-water communities with complex food webs did well right after the extinction. Animals that used to live only in certain areas were replaced by animals found all over the world. This pattern lasted through most of the Silurian.
The Late Ordovician extinction didn't cause as many long-term changes as other big extinctions. For example, the Permian-Triassic and Cretaceous–Paleogene extinctions had bigger impacts. Life also recovered much faster after the Late Ordovician event. Still, many types of animals disappeared quickly. This changed which groups were common and how many there were.
How Different Ocean Animals Changed
Brachiopods: These clam-like animals were strongly affected. Some ancient types never recovered. Other types, like the rhynchonelliform brachiopods, reacted differently. Some declined, while others like the Pentamerida and Spiriferida were less affected. They even grew in numbers after the extinction.
The first pulse mainly affected deep-water species and those living in warm, shallow seas. A group called the Foliomena fauna, which lived in low-oxygen deep waters, completely died out. During the ice age, a cold-water brachiopod group, the Hirnantia fauna, appeared. But this group died out in the second extinction pulse. After the extinction, brachiopods that survived were often found in only one area. But they spread out more as life recovered.
Trilobites: These ancient arthropods were hit hard by both extinction pulses. About 70% of their groups (genera) and 50% of their families died out. Deep-water species and those with larvae or adults that floated in the water were especially affected. One entire group, the Agnostida, disappeared. A cold-water trilobite group, the Mucronaspis fauna, appeared and then died out at the same time as the Hirnantia brachiopods. After the extinction, trilobite groups that survived the event became dominant.
Bryozoans: Over a third of bryozoan groups died out. But most families survived and recovered in the Silurian. Some types, like cryptostomes and trepostomes, never regained their old diversity. Bryozoan extinctions started in coastal areas. Their loss of different types of life seemed to be a long process.
Crinoids: About 70% of crinoid groups died out. Most extinctions happened in the first pulse. However, they recovered quickly in warm areas. They reached their old diversity levels early in the Silurian. Many other echinoderms, like cystoids and edrioasteroids, became very rare.
Sponges: Sponges were not much affected by the extinction. They even thrived and became common in ocean ecosystems right after the event. They were able to live in low-oxygen areas. Sponges may have helped other animal groups recover. They did this by stabilizing the seafloor, which allowed other animals to settle there again.
What Caused the Extinction?
The Great Ice Age
The first pulse of the Late Ordovician Extinction is usually blamed on a huge ice age. This ice age was very sudden and severe. It happened in the Hirnantian stage. The supercontinent Gondwana was located at the South Pole then. This ice age was one of the worst in the Paleozoic Era. Before this, Earth had been a warm "greenhouse" planet.
Scientists debate what caused this ice age. One idea is that early land plants and tiny ocean plants grew a lot. They might have used up a lot of carbon dioxide from the air. Carbon dioxide is a greenhouse gas that traps heat. Less carbon dioxide would make the Earth cooler. Volcanoes might also have played a role. They can release cooling sulfur particles into the air. Or they can create rocks that absorb carbon dioxide.
Two main changes from the ice age caused much of the extinction:
- Global cooling: Animals were used to a very warm Earth. Most shallow ocean habitats were in warm, tropical areas. The sudden cold was very bad for them.
- Falling sea levels: Water got locked up in the huge ice caps. This made sea levels drop. Vast shallow seas disappeared. This destroyed the homes of many unique animal communities. Falling sea levels might have made the Earth even colder. As shallow seas went away, less carbon dioxide was removed from the air by ocean processes. This led to more cooling.
Ice caps formed on Gondwana as it moved over the South Pole. Rocks in North Africa and South America show signs of this ice. When glaciers grow, they lock up ocean water. When they melt, they release it. This caused sea levels to drop and rise many times. The vast shallow Ordovician seas disappeared, then returned, then disappeared again. Each change wiped out more life. Rocks in North Africa show five pulses of glaciation.
The cold also changed how ocean currents worked. This brought nutrients from deep waters to the surface. Animals that could handle these new conditions survived. They filled the empty spaces left by the extinct animals.
Low Oxygen and Toxic Water
Another big factor in the Late Ordovician extinction was anoxia. This means the ocean water had no dissolved oxygen. Anoxia is bad for most life. It also helps toxic metals and chemicals form. One common poison is hydrogen sulfide. When oxygen is low and sulfide is high, it's called euxinia. Anoxia is thought to be the main cause of the second extinction pulse. It's also linked to many other mass extinctions.
Early Low Oxygen Ideas
Some scientists think anoxia also played a role in the first extinction pulse. This idea is debated. In the early Hirnantian, shallow ocean sediments show changes in sulfur. This suggests that deep-sea anoxia might have been common. However, other tests show that most waters were oxygenated then. Black shales, which show low oxygen, were rare globally.
Glaciation could indirectly cause anoxia. If sea levels drop, organic matter from land flows into deeper ocean basins. This matter would release nutrients like phosphate. More phosphate in the water leads to too much growth of tiny organisms. When they die, they use up oxygen, causing anoxia. This would affect deep-water animals, which fits the first pulse.
However, many studies show that waters were well-oxygenated during the glaciation. Ocean models suggest that glaciation would have increased oxygen in most areas. Also, the sulfur changes could be explained by tiny microbes that use sulfur, not necessarily by widespread low oxygen.
Late Hirnantian Low Oxygen
The late Hirnantian saw a huge increase in black shales. This happened as the ice age ended. Black shales appeared everywhere, at all depths. This shows a global anoxic event, called the Hirnantian ocean anoxic event (HOAE). Other chemical signs also point to widespread low oxygen. In some areas, the water first had iron, then became toxic with sulfide.
This global anoxia might have lasted over 3 million years. It continued through the entire Rhuddanian stage of the Silurian period. This would make it one of the longest low-oxygen events in Earth's history.
The cause of this event is not certain. Like most global low-oxygen events, it could be due to more nutrients in the ocean. This would cause huge blooms of algae or microbes. When they die, they use up oxygen. Cyanobacteria are likely culprits. They can create their own nitrogen, which gives them an advantage.
Melting glaciers could have caused this. They might have exposed more land, sending more nutrients into the ocean. This would be a steady source of food for ocean life.
The second extinction pulse affected all regions and ocean environments. Many animals that survived the first pulse died in the second. This included the Hirnantia brachiopod fauna and Mucronaspis trilobite fauna. Other groups, like graptolites, were less affected. The slow recovery after the second pulse might be linked to how long the low-oxygen event lasted.
Early Rhuddanian Low Oxygen
Black shales continued to be common into the earliest Rhuddanian. This means low oxygen lasted into the Llandovery. Many organisms became much smaller, a phenomenon called the Lilliput effect. Many old animal groups from the Ordovician disappeared. This points to a third extinction period. It was linked to low-oxygen conditions spreading into shallower waters. This was likely due to a period of global warming.
Metal Poisoning
Toxic metals on the ocean floor might have dissolved into the water when oxygen levels dropped. More nutrients in the oceans and slower ocean currents could also have played a role. High levels of mercury have been found in some rocks from this time.
These toxic metals might have killed off tiny organisms at the bottom of the food chain. This would cause a decline in their numbers. Then, animals higher up the food chain would starve.
Gamma-Ray Burst Idea
A less common idea suggests that the first extinction pulse was caused by a gamma-ray burst. This would come from a huge star explosion (a hypernova) in a nearby part of the Milky Way galaxy. If it happened within 6,000 light-years of Earth, a short burst could have destroyed half of Earth's ozone layer. The ozone layer protects us from harmful extreme ultraviolet radiation.
Under this idea, animals living near the surface, like tiny floating organisms, would be hit hardest by UV radiation. This matches observations that these organisms suffered greatly. Also, shallow-water species were more likely to die out than deep-water species.
A gamma-ray burst could also explain the rapid growth of glaciers. High-energy rays would break down ozone, which is a greenhouse gas. This would create nitrogen dioxide, a dark gas that cools the planet. It would also explain changes in carbon isotopes. This suggests more carbon was removed from the atmosphere. This would happen if nitrogen dioxide reacted with water to form nitric acid, which would then help plants grow and absorb carbon dioxide. While this idea fits some patterns, there's no clear proof that such a burst ever happened.
Volcanoes and Weathering
The late Ordovician glaciation happened after a drop in carbon dioxide in the air. This drop is linked to a burst of volcanic activity. New volcanic rocks absorb carbon dioxide as they break down. Volcanic activity might also be linked to higher mercury levels in rocks.
Throughout the Late Ordovician, gases from volcanoes were balanced by the breakdown of rocks from the rising Appalachians and Caledonides. This process also removed carbon dioxide from the air. In the Hirnantian stage, volcanic activity slowed down. But the rock breakdown continued. This caused a fast and big drop in carbon dioxide, leading to the quick ice age.
More recently, a study in 2020 suggested that the first extinction pulse was caused by volcanoes. This would have led to global warming and low oxygen, not cooling. This early extinction phase is linked to large volcanic areas and a warming period called the Boda event.
Volcanoes could also explain low oxygen during the first pulse. A small amount of phosphorus from volcanoes might have started a chain reaction. This would keep low oxygen levels going. Also, the breakdown of nutrient-rich volcanic rocks could have reduced oxygen even more.
However, other studies disagree with the volcano idea. They say volcanic activity was low in the Ordovician. They also point to a lack of mercury changes in some rocks. Some mercury changes that were blamed on volcanoes might actually be from mercury-rich minerals.
Asteroid Impact Idea
A small number of scientists suggest that a large asteroid impact might have started the Hirnantian glaciation and the first extinction pulse. They point to a huge impact structure in Australia, which is dated to around the start of the extinction.
Images for kids
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An example of pyrite.
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Sulfate-reducing microorganisms can cause changes in sulfur without low oxygen.
