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The Rise and Fall of Ordovician Life

February 28, 2022

Biosphere, Earth Science, EarthSphere Blog, Environment, Forgotten Origins, Geosphere, Hydrosphere, Repost, Science

The Cambrian Party Comes to an End

Published in The EarthSphere Blog. Feature Image: Wide Shallow Seas by WM House (©ArcheanArt)

Prologue

The underpinnings of life, as we understand it today, emerged during the Cambrian Period during an episode of hyper evolution called the Cambrian Explosion. Our series, Forgotten Origins, continues as the next major geological saga begins. Life expands down new pathways in the Ordovician.

The Party is Over

Like all good parties, the Cambrian Period came to an end. After a roaring and fun-filled 56 million years, the seeds of our modern biosphere were planted, and it was time for a change. Out with the old and in with the new; the Ordovician Period started 485.4 million years ago. The Period takes its name from the Ordovices, a Celtic tribe known for its resistance to the Roman invasion and occupation of the British Isles.

The first insights into this Period are credited to Charles Lapworth, who, in 1879, delineated a distinct set of fossilized fauna that differed from the preceding Cambrian Period and appeared separate from the younger Silurian fossils. Prior to that recognition, geologists were prone to follow their fundamentally argumentative nature and bicker about where the Cambrian ended, and the Silurian began. However, the Ordovician was not formally recognized as a distinct Period until 1960, when the International Geological Congress adopted it as the second Period of the Paleozoic Era.

The termination of the Cambrian party was not without drama, and it ended in a mass extinction event. College parties are famous for ending when the vital resource of alcohol becomes scarce, having been consumed voraciously for too long. The Cambrian also ended with the scarcity of a vital resource as oxygen levels collapsed, wiping out many species. Some researchers also believe rising levels of toxic hydrogen sulfide also accompanied the persistent anoxia of late Cambrian oceans.

Life clung on, though, and once it got back on its feet, a diverse group of marine invertebrates appeared, including graptolites, brachiopods, conodonts, cephalopods, corals, crinoids, and gastropods. Additionally, our old friends, the trilobites, regrouped and continued to flourish. Life thrived in the shallow Ordovician seas, and the first plants invaded the land.

Wide Shallow Seas

As in the Cambrian, shallow oceans were still the focus for life during the Ordovician. A combination of sunlight and oxygen was needed to keep evolution moving forward, and shallow, widespread oceans accommodated this need.

Temperatures steadily rose throughout the Cambrian and into the Ordovician. By the early to mid-Ordovician, Earth was a steamy, moist, hot planet with global atmospheric temperatures averaging about 49 Celsius (120 degrees F) and ocean temperatures reaching 43 Celsius at the equator. Remember, today we worry about an average global temperature of 15 Celsius. It was only towards the end of the Ordovician Period, about 443 million years ago, when Earth began to cool.

We don’t need to engage in deep thinking to envision the Ordovician as a water-world. There were no massive ice sheets, so all the planet’s water was in Earth’s oceans, filling them to the brim. Not only was there more water in the oceans than today, but the water took up more space.

In a 2019 report, NOAA tracked a global sea-level rise of 9 centimeters over a 25-year period. Thermal expansion from increasing ocean temperatures accounted for 40% of the rise, and the remaining 60% was new melt-water from ice caps. Other research indicates seas may rise as much as 2.3 meters for each 1 degree C increase in average ocean temperature. The math is staggering. The lack of ice sheets placed Ordovician seas 70 meters higher than today, and the thermal expansion added an additional 20 to 60 meters. Ordovician seas could have been over 400 feet higher than today.

Large portions of North America were underwater, but these conditions provided opportunities for evolution to press forward. The massive areas of shallow marine habitat gave newly evolved species room to expand and dominate their ecological niches.

Drifting Continents

Earth was also geologically reorganizing during the Ordovician. Tectonic plates were on the move. Gondwana edged towards the South Pole, and Laurentia stabilized near the equator. Two other large landmasses, Baltica and Avalonia, slowly converged on Laurentia. They eventually collided in an event known as the Caledonian orogeny — a mountain-building event spanning the Ordovician and early Silurian.

The movement of Gondwana to the South Pole was one of the triggers eventually leading to the demise of the Ordovician Period. Ironically, one of the hottest periods in Earth’s history ended in the cold. A South Pole covered by land provided the substrate for glaciation and the formation of massive ice sheets.

The success of life in the Ordovician was driven by the availability of vast flooded continents, which formed the shallow marine habitats for a dizzying array of new and successful species. They thrived in warm oceans under the light of an ancient sun. Ice was the enemy of these successful ecosystems. Ice cooled the planet and sucked valuable water from the seas, locking it away on land.

The assault was a twofold attack on the biosphere’s critical habitats, wide shallow oceans. As water progressively accumulated in glacial ice sheets, sea levels dropped, and the atmosphere cooled. The cooling atmosphere sucked heat from the oceans, and simple physics worked against life since the cooler water occupied less space and furthered hastened falling sea levels.

As water drained off the drowned continents and created dry land again, the habitats for Ordovician life slowly disappeared. With shrinking resources, species competition increased, creating winners and losers. Some species went extinct so others could survive.

But another unsuspecting player entered into the fray, helping to usher in the well-known Ordovician-Silurian mass extinction — plants invaded dry land, changing the dynamics of chemical weathering. (to be continued…)