Setting the Scene

Laurentia, the precursor to what we now know as North America, lay across the equator, flanked by Siberia and Baltica (the western part of Europe) to the east. Most of Laurentia was enveloped by a shallow sea. A volcanic period initiated the formation of the Taconic mountain range in southeastern Laurentia, beginning around 480 million years ago.

As Gondwana took shape, resembling a colossal version of Antarctica, the coastal regions of the northern continents flourished with marine life, thanks to the expansive shallow sea floor located in tropical regions. This massive southern supercontinent comprised Africa, Australia, Antarctica, South America, India, and Madagascar.

Approximately 800 million years prior, the supercontinent Rodinia began to fragment, leading to the formation of mid-ocean ridges. As magma from the depths of the Panthalassa Ocean surged beneath the oceanic crust, it pushed the continents apart. The length of these mid-ocean ridges, along with tectonic activities and ongoing orogenies, caused an uplift in the seafloor, resulting in transgressions that flooded land areas with ocean water.

More shallow seas equate to more life! The Cambrian explosion set the stage for an incredible increase in animal phyla and families.

The Great Ordovician Biodiversity Event

During this era, life experienced remarkable diversification, referred to as the Great Ordovician Biodiversity Event (GOBE). Marine continental platforms and open seas witnessed an explosion in animal phyla. For the first time, arthropods ventured onto land, while plants began colonizing moist habitats near rivers, streams, lakes, and coastlines.

Here’s a glimpse of some phyla and classes that saw significant diversification during this time:

• Algae
• Poriferans (sponges)
• Cnidarians (corals, jellyfish)
• Bryozoans
• Brachiopods (similar to clams)
• Mollusks (clams, snails, octopuses)
• Annelids (segmented worms)
• Arthropods (crustaceans)
• Echinoderms (starfish)
• Graptolites
• Conodonts (ancestors of vertebrates)

The climate was favorable, characterized by warm tropical conditions along the equator, albeit with hurricanes affecting the eastern regions of Laurentia. Overall, life flourished.

The Decline: A Shift in the Carbon Cycle

However, all good things eventually face an end. As volcanic activity surged in various areas, land accumulation from the collision of island arcs and continental margins increased. Tectonic movements contributed significantly to this geological activity.

The Taconic orogeny was a direct outcome of these processes. By the mid-Ordovician, the heightened rock presence led to an intriguing scenario: carbon dioxide combined with rainwater to erode rocks, forming carbonic acid.

This acid released ions such as calcium, magnesium, potassium, and sodium into rivers, which eventually flowed into the ocean. In the ocean, calcium ions reacted with bicarbonate ions to produce calcium carbonate, enabling shelly organisms to create their shells. Upon their demise, these organisms sank, forming layers of sediments that lithified into limestone, thus sequestering carbon.

This carbon originated from atmospheric carbon dioxide, which the abundant sea life effectively absorbed. Additional carbon sinks emerged as plants started to colonize land, further consuming carbon dioxide during photosynthesis. This process continued through the mid to late Ordovician.

Eventually, volcanic activity decreased, leading to a significant drop in atmospheric carbon dioxide levels. Ice caps began to form over Gondwana, signaling the onset of ice ages that cooled the planet and reduced sea levels, exposing shallow seas to the air.

And Then Came Extinction

Extreme habitat loss unfolded over a span of about ten million years. The vast shallow seas shrank, resulting in the extinction of numerous species, genera, and families. Tropical regions became refuges for polar species as interior seaways vanished, offering little solace for these refugees.

The extinction event was extensive, with an estimated 85% of species, 57% of genera, and 27% of families disappearing, effectively halting biodiversity and marking the end of the GOBE. Subsequently, as the climate warmed excessively after the initial extinction event, sea levels rose beyond their previous heights, and the warming ocean waters stagnated in shallow areas.

This tumultuous shift severely impacted bilateral ocean dwellers, including clams, bivalves, trilobites, and various reef-forming communities like brachiopods, echinoderms, ostracods, and agnostids—many of which never returned.

Resilience of Life

Despite the severe loss of biodiversity, the aftermath did not entirely obliterate all life forms, allowing others to fill the ecological gaps. Unlike previous mass extinctions, where dominant species were replaced by emerging groups, the late Ordovician saw a persistence of life forms.

In contrast to the Cretaceous-Paleogene extinction, which saw dinosaurs vanish and mammals rapidly evolve to occupy available niches, the late Ordovician marine environments bore a striking resemblance to Silurian life, persisting for about one to ten million years post-extinction.

Biodiversity stagnated, with surviving marine life establishing themselves in familiar niches. It seemed that the peak of biodiversity coincided with the mass extinction, and it would not be until the Age of Dinosaurs that marine biodiversity began to rise again.

In this context, ecological recovery led to the emergence of more families, but the Paleozoic era never witnessed the same explosive biodiversity that characterized the Ordovician.

When enjoying shellfish like clams, oysters, and lobsters, one can appreciate the legacy of the GOBE. Fortunately, despite the dramatic effects of the extinctions, the remaining 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.