dinogorgon skull - National Geographic Creative

Congratulations, you are part of the 1 percent. That’s the 1 percent of Earth’s species that haven’t gone extinct yet: For the past 3.5 billion years or so, about 99 percent of the roughly 4 billion species that once evolved are no longer around.

Many evolutionary family trees got the ax, so to speak, during a mass extinction. These events are defined as the loss of at least 75 percent of species in the geological blink of an eye—which can range from thousands to millions of years. Researchers have enough fossil records from just over half a billion years ago to identify five such mass extinction events, and many scientists believe we are in the middle of a sixth.

The Great Extinction is the result of a perfect storm of multiple disasters, such as ocean acidification combined with a spike in land temperatures. While the catalysts for these events are sometimes unclear, large-scale volcanic activity spread across an entire region is a usual suspect. Theories that asteroid impacts initiate mass extinctions remain largely speculative: only one cosmic scale has been conclusively linked to mass extinctions.

Each mass extinction ends a geologic period—that’s why researchers call them by names like the end of the Cretaceous. But not all news is bad: mass extinctions collapse ecological hierarchies, and in this vacuum, surviving species often thrive, exploding in diversity and range.

1. Late Ordovician: The 1-2 Strike

Trilobites (foreground) began more than 520 million years ago, but faced their first decline during the end-Ordovician mass extinction. (Credit: Esteban De Armas/Alamy Stock Photo)

When: About 443 million years ago

Why: In the first pulse of a double shock, the ice sheets advanced, radically altering ocean currents and creating harsh climates in the equatorial and mid-latitudes. Many of the species that survived this first shock adapted to their new world, only to succumb to the second shock: sudden melting, another change in climate, and another change in ocean circulation.

How bad: About 86 percent of species and 57 percent of genera—the next higher taxonomic division that may be a better indicator of biodiversity loss—disappeared.

What died: Among the animals that didn’t make it were most species of trilobites, many corals, and a few brachiopods, a hard-shelled marine invertebrate that today is often confused with a clam.

What thrives: Marine sponges did well after the end of the Ordovician, a pattern that repeated itself in subsequent mass extinctions. And 2017 Current Biology study suggests these humble marine animals can help restore entire ecosystems. Sponges stabilize the sediment, creating a favorable environment for brachiopods and other suspension feeders.

Read more: The end-Ordovician mass extinction wiped out 85 percent of life

2. Late Devonian: The Long Road to Oblivion

The placodermal lineage of ferocious-looking armored fish, such as Dinichthys herzeri, ended during the end-Devonian mass extinction, a long downward spiral in biodiversity. (Credit: History of Science Images/Alamy Stock Photo)

When: 359 million to 380 million years ago

Why: Until the deadline mass extinction may imply an instantaneous global catastrophe, these events may take millions of years. The Late Devonian, for example, consisted of a series of pulses in climate change over 20 million years that led to periodic and sudden declines in biodiversity, including the Hangenberg Crisis, which some researchers consider a separate mass extinction. The changes, likely the result of significant volcanic activity in Siberia, reduced oxygen levels in the oceans and caused other environmental changes.

How bad: About 75 percent of species and 35 percent of genera have disappeared.

What died: Unfortunately, these great-looking armored fish, known as placoderms, were exterminated. Many species of coral and more trilobites also bit the dust.

What thrives: The little guys did best, especially vertebrates under a meter (about 3.3 feet) long. Among the survivors are tetrapods, four-legged animals that transitioned from sea to land and would eventually evolve into reptiles, amphibians, and mammals.

Read more: Marine life died out in the Late Devonian mass extinction

3. End of Permian: The Big

Widespread volcanic activity in Siberia probably caused the largest mass extinction documented in the fossil record, the end of the Permian. Oceans have acidified and massive wildfires have consumed entire ecosystems. (Credit: Julius Csotonyi)

When: 251 million years ago

Why: Volcanic activity in Siberia is believed to be the main culprit for this most powerful mass extinction. Regional eruptions spewed out toxic gas and acidified the oceans. The destruction may have even torn the ozone layer, allowing in deadly ultraviolet radiation. A 2014 study suggests that the apocalypse took place over about 50,000 years.

How bad: About 96 percent of species and 56 percent of genera are extinct.

What died: So many forests were destroyed, apparently during wildfires, that there was nothing left to burn afterwards; researchers have identified a “coal gap” in the geologic record. Among the animals that met their demise: many amphibians and several synapsids, a diverse group of animals that included several apex predators of the time.

What thrived: Several species of fungi weathered the event well, probably because they fed on all the decaying organisms that didn’t. The loss of many synapsids from the top of the food chain may have cleared the way for the earliest dinosaurs—and the first mammals to descend from a low synapsid branch that survived.

Read more: The Late Permian Mass Extinction Explained

4. End Triassic: History almost repeats itself

Keichousaurus hui was a common marine reptile during most of the Triassic period, but did not live until the Jurassic that followed. (Credit: O. Louis Mazzatenta/National Geographic Creative)

When: 201 million years ago

Why: The volcanoes were at it again, but this time we can’t blame Siberia. Instead, massive eruptions at a hot spot in the center of what would eventually be the Atlantic Ocean created a new bout of climate disaster. And it sounded like an echo from the end of Perm.

How bad: About 80 percent of species and 47 percent of genera have disappeared.

What died: This was not a good time to be a conodont. Already in decline, the last of these ribbon-like fish, as well as many reptiles, were among the hardest hit.

What thrives: Dinosaurs really hit their stride after the end of the Triassic, exploding in diversity, as did their archosaur relatives, the crocodylomorphs, whose descendants include modern crocodiles.

Read more: The Late Triassic Mass Extinction and Why Dinosaurs Survived

5. End of Chalk: Really made an impact

The mass extinction event at the end of the Cretaceous wiped out almost all dinosaurs, including the iconic T. rex. Only one line, now known as birds, survived. (Credit: Mark Garlick/Science Source)

When: 65.5 million years ago

Why: The cause of the late Cretaceous extinction remains hotly debated. No one disputes that a piece of space rock crashed into the planet near Mexico’s Yucatan Peninsula at that time. But researchers disagree on whether the asteroid impact caused or merely contributed to the deaths. Large-scale volcanic activity in the Deccan Traps of India was already underway before the impact and a Scientific progress a study published in February suggested that both the asteroid impact and Deccan Traps activity coincided with a general upsurge in volcanism across the planet. Whatever the catalyst, the spike in carbon dioxide and drop in ocean oxygen levels appears to have been followed by rapid post-asteroid cooling.

How bad: About 76 percent of species and 40 percent of genera have disappeared.

What died: The sun finally set on the Long Day of the Dinosaurs, although many other organisms were also wiped out, such as ammonites, molluscs with distinctive spiral shells.

What thrives: Mammals quickly adapted to exploit the newly freed ecological niches, as did the only surviving branch of the dinosaurs, better known today as birds.

Read more: The End of the Dinosaurs: The End-Cretaceous Mass Extinction

6. Sixth Extinction: Present and Future?

In the 1990s, paleontologist and noted conservationist Richard Leakey warned that human activity was causing a “sixth extinction.” In the decades since Leakey’s observation, with reams of new supporting evidence, many more researchers have bought into the idea.

In time and all over the planet, extinction of one or another individual species is always happening. Known as the “background frequency” and documented both historically and in the fossil record, these extinctions are like low-volume static noise compared to the sudden crash of cymbals in a mass extinction. Determining extinction rates as they develop is difficult, but in 2015 Scientific progress a study using a range of conservative estimates put the current rate at up to 100 times the normal background level.

Human activities are to blame, including population growth, increased consumption of resources, and climate change caused by the burning of fossil fuels and the release of greenhouse gases.

In the diary PNAS in 2017, scientists concluded that a focus on species extinction may not actually represent the severity of what one team called “biological extinction.” After all, the global extinction of a species is just the final nail in the coffin.

The downward spiral begins with the destruction and fragmentation of habitats and the introduction of invasive species and pathogens. Killing individual members of a species through overhunting or poaching also takes its toll. Just ask any rhino if you can find one. All these activities can lead over time to local extinctions known as extirpations. Even before the global extinction of a species occurs, these extinctions reduce biodiversity and can destabilize ecosystems, leading to more extinctions.

As Leakey noted in his seminal 1995 book on the subject: “Homo sapiens may not only be the agent of the sixth extinction, but also risk being one of its victims.

Source link

Leave a Reply

Your email address will not be published. Required fields are marked *