Great Ordovician Biodiversification Event: Life's Second Boom

The Great Ordovician Biodiversification Event (often shortened to GOBE) was a prolonged surge in the variety of marine life during the Ordovician Period, roughly 485 to 445 million years ago. Counts of fossil genera suggest that marine diversity rose about three- to fourfold, making it one of the largest increases in the history of life. It established the ecosystems of brachiopods, crinoids, bryozoans and corals that dominated the seas for the rest of the Paleozoic Era.

Background: after the Cambrian

The Cambrian Explosion produced most of the major animal groups, including arthropods, molluscs, echinoderms and the first chordates. Yet Cambrian seas were, by later standards, relatively low in diversity. Many groups were represented by a few lineages, and communities were simple.

The Ordovician world looked very different from today’s. Much of the land lay in the great southern continent of Gondwana, while Laurentia, Baltica, Siberia and smaller fragments were scattered across the tropics and mid-latitudes. Sea levels were among the highest of the last 500 million years, flooding continents with warm, shallow seas. Land had no plants or animals beyond microbial crusts and, from around 470 million years ago, the first simple plants.

What happened

The diversification began in the Early Ordovician and peaked in the Middle and early Late Ordovician, particularly during the Darriwilian Age (about 467 to 458 million years ago). It was not a single burst: different groups and different regions diversified at different times.

Filter feeders flourished. Brachiopods, shelled animals superficially like clams, became the most common animals on many seabeds. Crinoids (sea lilies) and bryozoans (colonial “moss animals”) spread widely, building tiered communities that reached higher into the water.

Reefs changed too. Earlier Cambrian reefs had been built mainly by microbes and sponge-like archaeocyaths. In the Ordovician, tabulate and rugose corals, stromatoporoid sponges and bryozoans began to construct reefs, creating new habitats.

In open water, the “plankton revolution” saw graptolites, acritarchs (microscopic plankton) and radiolarians diversify, providing food for larger animals. Nautiloid cephalopods, with long straight shells, became top predators, and some grew very large. Trilobites also produced many new forms.

Vertebrates appeared in greater numbers as well. Armoured jawless fish such as Sacabambaspis and Arandaspis are known from Ordovician rocks, while tiny eel-like conodonts were abundant enough that their tooth-like elements are used to date rocks.

Exceptional fossil sites

The Fezouata Biota of Morocco, about 480 million years old, preserves soft-bodied Early Ordovician animals, including the giant filter-feeding radiodont Aegirocassis, estimated at around 2 metres long. It showed that some Cambrian-style animals survived longer than once thought.

Later Ordovician sites, such as those in North America, Scandinavia and the Baltic region, record the shelly communities of brachiopods, bryozoans and crinoids in vast numbers.

Possible causes

Scientists continue to debate what drove the event. Most now see it as the result of several factors acting together.

• Geography and sea level. Scattered continents and high seas created many separate shallow-water habitats, encouraging the evolution of distinct regional faunas.
• Climate. Analyses of oxygen isotopes in conodont elements, published by Julie Trotter and colleagues in 2008, suggest the oceans cooled from very hot Early Ordovician temperatures to levels similar to the modern tropics, which may have allowed more life to flourish.
• Nutrients and oxygen. Volcanic activity and mountain-building increased the supply of nutrients to the oceans, fuelling plankton. A rise in atmospheric and ocean oxygen is also proposed.
• Meteorites. About 466 million years ago an asteroid broke up in the asteroid belt, and fossil meteorites in Swedish limestones record an increase in material reaching Earth. Some researchers have linked this to ecological disturbance and diversification, although the connection is disputed.

Measuring the boom

The event was first clearly documented through large compilations of fossil data, notably by the American palaeontologist Jack Sepkoski in the late 1970s and 1980s. His analyses identified three great “evolutionary faunas”: a Cambrian fauna, a Paleozoic fauna that arose in the Ordovician, and a Modern fauna. The Ordovician radiation marks the rise of the second.

Figures vary between studies because they depend on how fossils are counted and how sampling biases are corrected.

A patchwork, not a single burst

Early studies treated the GOBE as one global event. More detailed work since the 2000s has shown that it was a patchwork of radiations that happened at different times in different groups and on different continents. Some groups, such as certain trilobites and brachiopods, began diversifying in the late Cambrian, while others peaked only in the Late Ordovician.

Regional differences were large. The fossil records of Baltica, Laurentia, South China and Gondwana each show their own timing. Researchers also stress that apparent diversity depends on how well rocks of a given age are preserved and sampled, so part of the pattern may reflect where geologists have looked.

For these reasons many palaeontologists now see the Cambrian Explosion and the Ordovician radiation as two phases of a longer process of animal diversification, rather than two completely separate events. In this view, the Cambrian established the basic body plans of animals, while the Ordovician filled the oceans with variations on those plans.

Changing ecosystems

The GOBE was not just about more species; it changed how ecosystems worked. Animals occupied more levels of the sea floor and water column. On the bottom, filter feeders formed layered communities, with low-lying brachiopods, taller bryozoans and crinoids rising higher above the sediment to reach different currents.

Burrowing became deeper and more intense, mixing sediments more thoroughly than before. Predators diversified as well: nautiloids hunted in open water, while starfish and other echinoderms developed new ways of feeding. Boring into shells became more common, suggesting an arms race between predators and prey.

In the open ocean, the spread of plankton supported larger populations of animals feeding on them. Graptolites, colonial animals that drifted in the water, evolved so rapidly and spread so widely that geologists use them to divide Ordovician and Silurian time into short zones, some lasting only one to two million years.

Timeline

• c. 485 million years ago: The Ordovician Period begins.
• c. 480 million years ago: The Fezouata Biota lives in what is now Morocco.
• c. 470 million years ago: The earliest spores of land plants appear in the fossil record.
• c. 467-458 million years ago: Darriwilian Age; diversification peaks in many groups.
• c. 466 million years ago: An asteroid breakup increases meteorite falls on Earth.
• c. 445-443 million years ago: The Late Ordovician mass extinction ends the event.
• 2004: Webby and colleagues publish The Great Ordovician Biodiversification Event.

The end of the boom

The Ordovician ended in crisis. A glaciation centred on Gondwana triggered the Late Ordovician mass extinction, which wiped out a large share of marine species. However, the basic structure of the Paleozoic fauna survived and recovered in the Silurian.

Why it matters

The Great Ordovician Biodiversification Event turned the experimental animal life of the Cambrian into rich, complex ecosystems. Many of the groups that diversified then, including brachiopods, bryozoans and crinoids, shaped seafloor communities for some 200 million years, and their fossils make up much of the limestone found across Europe and North America.