The Late Ordovician marks a significant chapter in Earth’s deep history, spanning from approximately 485.4 to 443.8 million years ago. This period, the second of six in the Paleozoic Era, was a time of dynamic shifts in both geology and biology. It is widely recognized for a profound biological upheaval that reshaped marine ecosystems globally.
Earth During the Late Ordovician
During the Late Ordovician, the Earth’s landmasses were configured differently than today. The supercontinent Gondwana, comprising much of what is now South America, Africa, Australia, Antarctica, and India, was positioned over the South Pole and largely submerged underwater. North America, then part of the continent Laurentia, was almost entirely covered by shallow inland seas, providing extensive habitats for diverse life.
The Ordovician climate began warm but gradually cooled as the period progressed. Its warm, shallow seas teemed with diverse marine invertebrates. Organisms such as trilobites, brachiopods, graptolites, bryozoans, and early jawless fish thrived. Trilobites and brachiopods, in particular, reached their peak diversity during this time, adapting to various ecological niches within these vast epicontinental seas.
The First Global Mass Extinction
The Late Ordovician is most notably defined by the Late Ordovician Mass Extinction (LOME), the first of Earth’s “big five” major extinction events. This event eliminated a substantial portion of marine life, with estimates suggesting the loss of 49-60% of marine genera and nearly 85% of marine species. This makes it the second-largest known extinction event in terms of generic loss, surpassed only by the Permian-Triassic mass extinction.
The LOME unfolded in two distinct pulses, affecting different groups of animals. The first pulse, occurring at the boundary between the Katian and Hirnantian stages, heavily impacted many families of brachiopods, bryozoans, trilobites, conodonts, and graptolites. The second pulse, later in the Hirnantian, was geographically more widespread but less severe, yet it finished off many taxa that had survived or even diversified after the initial pulse, including the cold-adapted Hirnantia brachiopod fauna.
The Drivers of Catastrophe
The Late Ordovician Mass Extinction is primarily attributed to intense global cooling and glaciation, known as the Hirnantian glaciation. This glaciation, centered on Gondwana as it drifted over the South Pole, caused a significant fall in global sea levels, dropping by at least 50 to 100 meters. This dramatic sea-level decline destroyed vast shallow marine habitats, which were home to many endemic species.
The expansion of ice sheets led to a shift from a greenhouse to an icehouse climate, causing ocean surface waters to cool. This rapid cooling was particularly detrimental to warm-adapted organisms that populated the tropical shallow seas. Beyond glaciation, other contributing factors included widespread ocean anoxia, or a lack of oxygen, in deeper waters, which may have been exacerbated by changes in ocean circulation. Glaciation is a leading cause, but some research suggests that volcanism, leading to warming and anoxia, could have also played a role in triggering the extinction pulses.
Decoding Ancient Clues
Scientists reconstruct the events of the Late Ordovician by examining geological and paleontological evidence. Fossil assemblages provide insight into the types of organisms that lived and how their diversity changed over time. For example, the presence and absence of specific brachiopod and trilobite species in successive rock layers help pinpoint extinction intervals.
Sedimentary rock layers offer clues about past environments. Glacial deposits, such as diamictites found in regions like Argentina and South Africa, indicate the presence and extent of ancient ice sheets. Additionally, black shales can signal periods of anoxia in ancient oceans. Isotopic analysis provides proxy data for ancient climate and ocean chemistry, revealing shifts in temperature and ocean oxygen levels during the extinction events.