FOSSILS ET AL.
The Late Ordovician Mass Extinction
A Biodiversity Diversion
About 488 million years ago, the Cambrian explosion was a thing, and the Ordovician period (488 to 444 million years ago) commenced. Animal life established itself as a mainstay while the supercontinent Gondwana moseyed towards the South Pole, containing the majority of Earth’s landmass in the Southern hemisphere.
This is a story of mass extinction. It explores a possible explanation for the tragedy that unfolded on Earth.
Setting the Stage
Laurentia, which would become North America someday, straddled the equator with Siberia and Baltica (western Europe) nearby to the east. Laurentia is mostly covered in water by a shallow sea. A volcanic episode prepares for a large orogeny, the development of the Taconic mountain range in southeastern Laurentia, starting 480 million years ago.
As Gondwana becomes an immense version of Antarctica, the northern continents coastal margins are teeming with marine life, courtesy of the large areas of the shallow sea floor and being situated in the tropics. The gigantic southern supercontinent holds Africa, Australia, Antarctica, South America, India, and Madagascar.
Around 800 million years ago, the supercontinent Rodinia began to break apart. Mid-ocean ridges formed as magma underneath the basaltic oceanic crust rose from the vast Panthalassa Ocean, pushing the continents further away from each other. Due to the immense length of this mid-ocean ridge, the tectonic activities, and the ongoing orogenies, the rise in the seafloor contributed to transgression, flooding the continents with displaced ocean water.
More shallow sea water means more life! And it proliferates. As mentioned, the Cambrian explosion occurred in the Cambrian period before the Ordovician. Animal phyla and families exploded in numbers.
The Great Ordovician Biodiversity Event
In this period, life diversifies immensely. It’s known as the Great Ordovician Biodiversity Event (GOBE). Animal phyla diversify in the beginning to middle Ordovician, in the marine continental platforms and open sea. Animals first walked on land as arthropods and plants started to colonize damp areas on land, likely next to rivers, streams, lakes, ponds, and coastlines along the oceans and seas.
Here is the list of some phyla and classes whose members diversified:
- Algae
- Poriferans (Sponges)
- Cnidarians (coral, jellyfishes)
- Bryozoans
- Brachiopods (kind of like clams)
- Mollusks (clams, snails, octopus)
- Annelids (segmented worms)
- Arthropods (crustaceans)
- Echinoderms (starfish)
- Graptolites
- Conodonts (vertebrate ancestors)
It is a great time, indeed. The climate is great. Nice tropical weather along the equator. There are hurricanes happening along the eastern side of Laurentia. But all in all life is good.
The Beginning of the End: Carbon Cycle
All good things must come to an end. In the early Ordovician, volcanic activity increased in various regions. Land accumulated by the actions of suturing and collisions of island arcs and continental margins. Tectonic activities were on the rise, helping to facilitate this activity.
The great Taconic orogeny is a product of this geologic action. Towards the middle Ordovician, the increased amount of rock causes an interesting situation. Carbon dioxide rides along raindrops and erodes the rock, forming carbonic acid.
This acid releases calcium, magnesium, potassium, and sodium ions transported by rivers into the ocean. Calcium ions combine with bicarbonate ions to form calcium carbonate. Shelly creatures use this to build their shells. Lots of shelly creatures do this.
When these organisms die, they sink to the bottom, forming layers of shells and sediments. It lithified into limestone, sequestering the carbon. And where does this carbon come from? It dissolves into the ocean from the carbon dioxide in the atmosphere. There is a lot of this gas, and the sea creatures are doing a fine job sucking it out.
Other carbon sinks develop when plants colonize the land, absorbing more carbon as they consume carbon dioxide for photosynthesis. This is happening mid to late Ordovician.
Eventually, the volcanoes simmer down, and carbon dioxide is sucked out of the air to unprecedented lower levels. Ice caps start to cover Gondwana, heralding the round of ice ages that cool the Earth. Sea levels fall, exposing shallow seas to the atmosphere.
And Along comes Extinction
Extreme habitat loss occurs in about ten million years. The copious amounts of shallow seas diminish, forcing the outs of many species, genera, and families. The tropics become refuges for the polar creatures. The interior seaways disappear, not leaving much for the refugees.
The extinction is widespread. A loss of 85% of species, 57% of genera, and 27% of families halt the rate of biodiversity. It ends the GOBE. To make matters worse, following the initial extinction event triggered by the ice, the climate subsequently warms. And it warms too much.
Sea levels rise higher than in those good times before the ice ages. The ocean waters warm, stagnating in the shallow portions. It’s hard for Life to get a break. The bilateral ocean-dwellers are hit the hardest. That would be clams, other bivalves, trilobites, and just about all reef-forming communities — brachiopods, echinoderms, ostracods, and agnostids (a sort of floating trilobite). Some never to be seen again.
Life Goes On
Despite the diminishing of Life, the end result does not eliminate entire families to the point other families can take over and fill in the empty niches. There were no dominant life forms before the mass extinctions, nor were there any after.
In other mass extinctions, some dominant groups go extinct, replaced by another group. A great example is looking at the Cretaceous-Paleogene mass extinction. The dinosaurs go bye-bye, and the mammals quickly evolve, taking advantage of the various ecological niches vacated by the loss.
In the late Ordovician, the life-enriching oceans look very similar to the Silurian marine life, one to ten million years into it. Biodiversity has slowed. The marine life remaining has settled in, and the families mentioned earlier are established.
It’s as if the peak of biodiversity occurs simultaneously with the extinctions. It won’t be until the Age of the Dinosaurs when the marine biodiversity starts to increase again.
Ecological recovery, in this sense, means more families, but the Paleozoic never sees that great burst of biodiversity trademarking most of the Ordovician.
When you enjoy shellfish such as clams, oysters, crab, lobster, and so forth, you can thank the GOBE. Fortunately, despite the significant impact of the extinctions, the considerable diversity allowed the survivors to thrive.
Sources
- Davis, Richard Arnold and Meyer, David Lachlan. A Sea Without Fish: Life in the Ordovician Sea of the Cincinnati Region. Bloomington & Indianapolis: Indiana University Press, 2009.
- Contributing Scientists and Illustrators: Peter Andrews, John Barber, Michael Benton, Marianne Collins, Christine Janis, Ely Kish, Akio Morishima, J. John Sepkoski Jr., Christopher Stringer, Jean-Paul Tibbles, Steve Cox. Stephen J. Gould, Contributing Editor. The Book of Life. W. W. Norton & Company: New York and London, 1993.
- Finney, Stanley C and Berry, William N. B. editors. The Ordovician Earth System. Special Paper 466. The Geological Society of America: Boulder, 2010.
