Late Ordovician Mass Extinction: The First of the Big Five

The Late Ordovician mass extinction was the first of the “Big Five” mass extinctions in Earth’s history, occurring about 445 to 443 million years ago at the end of the Ordovician Period. An estimated 85% of marine species died out in two pulses, closely linked to a brief but severe ice age known as the Hirnantian glaciation. It cut short the remarkable diversification of Ordovician sea life, yet the main groups of Paleozoic animals survived and recovered.

Background: a thriving Ordovician world

During the Great Ordovician Biodiversification Event, marine life had diversified enormously. Shallow seas covering parts of the continents were filled with brachiopods, bryozoans, crinoids, corals, nautiloids and trilobites, while graptolites and other plankton drifted in the open ocean.

For much of the Ordovician the climate was warm and sea levels very high. The supercontinent Gondwana, which included present-day Africa, South America, Antarctica, Australia and India, lay over the South Pole, with what is now North Africa close to the pole itself.

The Hirnantian glaciation

Late in the Ordovician the climate cooled, and during the Hirnantian Age, roughly 445 to 444 million years ago, large ice sheets formed on Gondwana. Glacial deposits, scratched rock surfaces and features carved by ice are preserved across the Sahara, including Algeria, Libya and Mauritania, and in South America and the Arabian Peninsula.

Why the climate cooled is still debated. Suggested causes include falling carbon dioxide due to the weathering of newly formed mountains and volcanic rocks, the burial of organic carbon, and possibly increased weathering by early land plants. The Hirnantian is named after Cwm Hirnant, a valley near Bala in Wales.

What happened

The extinction is generally described as two pulses.

The first pulse coincided with the start of the glaciation. As ice sheets grew, sea level fell by an estimated 50 to 100 metres, draining many of the shallow seas that had supported rich communities. Tropical, warm-water species were hit hard as ocean temperatures dropped. Plankton and animals living in the water column, such as many graptolites and trilobites with swimming larvae, also suffered.

The second pulse came as the glaciation ended and the ice melted. Sea levels rose again and the cool-adapted animals that had spread during the cold interval, including the brachiopod-dominated Hirnantia fauna, declined. Many researchers link this pulse to the spread of low-oxygen or oxygen-free water across the sea floor.

The two pulses were separated by an interval estimated at a few hundred thousand years to about a million years.

Who was affected

Almost all marine groups were affected to some degree.

• Brachiopods lost a large proportion of their genera, though they remained abundant afterwards.
• Trilobites suffered heavily; many families disappeared, and the group never regained its former diversity.
• Graptolites were nearly wiped out, with only a few lineages surviving to give rise to Silurian forms.
• Conodonts, bryozoans, corals, crinoids and acritarchs (microscopic plankton) also lost many species.

Reef ecosystems were badly disrupted, although reef-building communities returned within the Silurian.

Other proposed causes

Glaciation is widely accepted as the main driver, but scientists have suggested additional or alternative factors.

• Anoxia and euxinia. Chemical evidence from rocks points to spreading oxygen-poor and, in places, sulphide-rich waters, especially during the second pulse.
• Toxic metals. A 2015 study reported malformed microfossils in rocks of this age, suggesting high levels of metals in seawater associated with low-oxygen conditions.
• Volcanism. Some researchers have linked the event to large volcanic eruptions, which could have caused both cooling and changes in ocean chemistry; the evidence is still debated.
• Gamma-ray burst. In 2004 a team proposed that radiation from a distant stellar explosion might have damaged the ozone layer. This remains speculative and unsupported by direct evidence.

By the numbers

Estimates differ between studies depending on data and methods.

Recovery and legacy

Unlike the later Permian-Triassic extinction, the Late Ordovician event did not fundamentally change the structure of marine ecosystems. The same groups that dominated before the crisis, such as brachiopods, crinoids and corals, recovered in the Silurian, though with different families and species. Recovery took perhaps around 5 million years.

The concept of the Big Five was established by David Raup and Jack Sepkoski in 1982, based on their analysis of the marine fossil record. The Late Ordovician extinction is the earliest of the five, followed by the Late Devonian extinction, the end-Permian, the end-Triassic and the end-Cretaceous events.

Evidence in the rocks

Geologists reconstruct the extinction from rock sections that span the end of the Ordovician and the start of the Silurian. Some of the most important are found in Britain, China, North America and North Africa.

Dob’s Linn, near Moffat in the Southern Uplands of Scotland, was studied by the geologist Charles Lapworth in the 1870s. Its black shales are rich in graptolites, and Lapworth used them to show how fossils could divide rocks into zones. Dob’s Linn was later chosen as the international reference point for the base of the Silurian.

Anticosti Island in Quebec preserves an unusually complete sequence of shallow-sea limestones across the boundary, recording the changes in brachiopods, corals and other bottom-dwelling animals. A section at Wangjiawan near Yichang in South China serves as the international reference point for the start of the Hirnantian Age.

Chemical signals in these rocks support the link with glaciation. A sharp positive shift in carbon isotopes, known as the Hirnantian isotope excursion, and changes in oxygen isotopes recorded in fossil shells point to cooling and changes in the global carbon cycle. In the Sahara, glacial features such as scratched rock pavements and buried glacial valleys are preserved in rocks of the same age.

Lessons for today

The Late Ordovician event is sometimes compared with modern climate change, but the comparison has limits. The Hirnantian glaciation was a cooling, not a warming, and it took place over hundreds of thousands of years in a world with very different continents and organisms.

Even so, it shows features common to many mass extinctions: rapid climate change, sea-level change and loss of oxygen in the oceans. It also suggests that tropical, specialised species and plankton can be particularly vulnerable, and that ecosystems may suffer a second blow as conditions change again. Studying how life recovered afterwards, over millions of years, also helps scientists understand how long ecological recovery from a major crisis can take.

Timeline

• c. 450 million years ago: Gondwana lies over the South Pole; climate begins to cool.
• c. 445 million years ago: The Hirnantian Age begins; ice sheets expand and sea level falls.
• c. 445 million years ago: First pulse of extinction.
• c. 444-443 million years ago: Ice sheets melt; second pulse of extinction.
• c. 443 million years ago: The Silurian Period begins.
• 1982: Raup and Sepkoski identify the Big Five mass extinctions.
• 2004: The gamma-ray burst hypothesis is proposed.

Why it matters

The Late Ordovician mass extinction shows that a relatively short episode of climate change, including cooling and sea-level fall, can devastate marine life. It is one of the few mass extinctions linked mainly to glaciation rather than warming, which makes it an important case study for understanding how ecosystems respond to rapid environmental change.