485 million years ago the Earth was a very different place. The Earth's climate was mostly warm and wet, with sea levels nearly 2,000 feet above those of today. The area north of the tropics was almost entirely ocean, and most of the world's land was collected into a southern supercontinent called Gondwana. The time from about 485 million years ago and 440 million years ago is known as the Ordovician Period.
The Ordovician period is best known for its diversity of marine life such as graptolites, trilobites, brachiopods, and the early vertebrates known as conodonts. A marine community during this time consisted of the above animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods. Life was thriving in the oceans. Then 440 million years ago the planet was suddenly thrown into a 20-million-year ice age, marking the end of the Ordovician Period and much life on the planet.
Reconstruction of a seafloor during the Ordovician period.
Mass extinction events are named for the geographical periods during which they occur. The first mass extinction event is known the End Ordovician or the Ordovician-Silurian extinction, as the end of the Ordovician period marks the beginning of the Silurian period. The extinction event at the end of the Ordovician Period was the world’s first known mass-extinction. It was the second most severe of the great extinctions and probably happened in two phases. As most life was in the sea at this time, many sea creatures suffered substantial losses. These included nautiloids, the cousins of today's squid and cuttlefish, which were the top marine predators of their time. Brachiopods, corals, bryozoans, echinoderms, graptolites and eel-like creatures call conodonts were lost or severely impacted. Over 90 percent of trilobite species disappeared. When all was said and done, 26 percent of all marine families, 60 percent of all genera, and 85 percent of all species were lost. The species that did survive did so with greatly reduced numbers.
As mentioned earlier, at the end of the Ordovician Period, the world entered a very sudden era of global cooling that resulted in an intense ice age. Global cooling spelled disaster for a planet of warm-adapted species that had nowhere to migrate. It is thought that this climate change event was partially brought about by the positioning of the supercontinent Gondwana over the south pole and partially because of another culprit that we’ll get to in a moment.
Earth during the Ordovician period about 450 mya.
Gondwana was the result of a plate tectonic version of a continental logjam. What we call Africa, South America, Australia and Antarctica today, were packed together to form one giant land mass called Gondwana. Gondwana gradually shifted towards the South Pole where much of it was eventually submerged underwater. The positioning of Gondwana enabled large ice sheets to form in the Late Ordovician causing sea levels to fall dramatically, draining inland seas perhaps by as much as 300 feet. This particularly affected the corals and bryozoans that were living in shallow inland seas, which were drained of water as sea levels fell.
Then after about 1 million years (a very brief amount of time from a geological standpoint), the glacial conditions that had just begun ended very rapidly. This delivered a second lethal blow to much of the marine life that had just managed to survive the first climate shift. Sea levels rose back toward their previous heights. The newly malted glacial water was low in oxygen, flooding shallow marine habitats and stagnating deep ocean waters which disrupted ocean currents and weather patterns. Earth’s geological forces were causing abrupt climate changes after a long period of relative stability. But geological forces were not the exclusive reason for global cooling and other climate change events. Nor are humans the first life form on Earth to drastically alter the planet’s climate.
Just before the global cooling and extinction events, the atmosphere had much higher levels of carbon dioxide (CO2). Something caused the CO2 levels to plummet. CO2 is a greenhouse gas that traps heat in the atmosphere. Its disappearance can’t be accounted for by the positioning of Gondwana over the south pole. Scientists would have to get clues through fossil records. What those records indicate are that near the end of the Ordovician life began to move out of the water. Fossils of microscopic spores indicate that simple plants first arrived on land at the end of the Ordovician period. Plants similar to liverworts and mosses began to spread across Gondwana.
Reconstruction of an Ordovician plant community
Before plants arrival on the surface, only water dwelling algae enjoyed the ability to use sunlight and CO2 to supply their nutrients by a process we all know as photosynthesis. During photosynthesis carbon atoms are clipped off of the two oxygen atoms they are bonded with to form carbon dioxide (CO2). The carbon is used by the plant to create glucose while the oxygen atom is ultimately returned to the atmosphere.
Using a climate model, Liam Dolan, a study researcher and professor of botany at Oxford University, determined that if plants like the moss inhabited 15 percent of the currently vegetated land between 475 million and 460 million years ago, atmospheric carbon dioxide would drop enough to cause global cooling and trigger the spread of glaciers. Decreased amounts of atmospheric CO2 contributed to a significant reduction in ocean acidity as atmospheric carbon is largely responsible for surface ocean carbon levels.
Plant life also enhances a process called silicate weathering, which sucks CO2 out of the atmosphere and ultimately tucks away the carbon at the bottom of the oceans while returning the oxygen atoms back into the atmosphere.
Silicate weathering as part of the longterm carbon cycle
Here's how it works: CO2 in the atmosphere forms carbonic acid. It falls as acid rain, dissolving rocks to form bicarbonate. The bicarbonate eventually washes into the ocean where it gradually forms limestone. The process works like a pump that sends CO2 to the ocean floor where it's locked away and prevented from being released back into the atmosphere. To get the nutrients they need, plants secrete acids that slowly dissolve rocks causing the release of minerals such as phosphorous. This extra phosphorus is added to the silicate weathering process and could have caused the water’s oxygen levels to drop.
The loss of 85 percent of the species on the planet meant that there were many evolutionary gaps to be filled. Surviving species would go on to fill the planet with an astounding variety of new species. The following Silurian period saw the wide and rapid spread of fish, including the first known freshwater fish and jawed fish. Other marine fossils commonly found show some species such as trilobites, graptolites, conodonts, and corals reemerged, evolved and thrived. One of the most notable biological events in the Silurian period was the evolution of vascular plants, which have been the basis of plant life on land since their appearance. Life would go on smoothly for almost 100 million years until the Late Devonian mass extinction would press reset on Earth’s biodiversity once again... which will be the subject of next week's post.
Cheers!
- Greg
Notes & Work Cited
All dates and figures are approximation and estimations based off the available data and scientific analysis.
Kolbert, Elizabeth. The Sixth Extinction: An Unnatural History. Henry Holt, 2014. Print.