The Cambrian Period, famous for the “Cambrian Explosion,” saw an unprecedented burst of evolutionary innovation when most animal body plans first appeared. This era of rapid diversification did not proceed unchecked, as the world’s ecosystems faced a prolonged biological crisis toward the period’s end. A mass extinction is a sharp, widespread decrease in the diversity of life where the rate of species loss far exceeds the typical background extinction rate. The events that curtailed the Cambrian Explosion were not a single catastrophe but a series of severe biological turnovers that fundamentally reshaped the trajectory of life on Earth.
Timing and Scope of the Extinction Pulses
The biological crisis of the Cambrian was not one singular, sudden event, but a sequence of distinct extinction pulses spanning millions of years, collectively known as the Late Cambrian Extinction events. These events are often recognized as biomere extinctions—sudden, non-evolutionary faunal turnovers primarily recorded in shallow marine strata. The overall crisis is spread across the later stages of the period, from the late early Cambrian into the terminal Cambrian.
The earliest and most severe pulse was the End-Botomian extinction (around 513 to 509 million years ago), which decimated early ecosystems. Estimates suggest this event eliminated between 50% and 80% of all marine genera living at the time. Later, the Steptoean and Dresbachian pulses continued to reorganize the shallow marine realm, each wiping out numerous trilobite families across the continental shelves. This pulsed pattern suggests a recurring environmental stressor rather than a single, isolated impact event.
Life Forms Most Affected
The most recognizable casualties of these successive events were the iconic organisms that had dominated the Cambrian seas. Trilobites, the segmented marine arthropods, were particularly hard-hit, suffering massive losses during each extinction pulse. Shallow-water trilobite faunas, adapted to the warm, sunlit continental shelves, were repeatedly decimated, though deeper-water groups often survived. The End-Botomian event saw the complete extinction of the two most geographically widespread trilobite groups, the olenellids and the redlichiids.
Another group that suffered a catastrophic decline was the Archaeocyathids, an extinct group of sessile, reef-building sponges. These organisms constructed Earth’s first metazoan reefs, but their complex, porous structures were vulnerable to environmental change. Nearly 90% of Archaeocyathid genera disappeared during the End-Botomian pulse, effectively ending their role as the primary reef architects. Other groups, including some species of brachiopods and small shelly fossils, also experienced significant losses, confirming the widespread nature of the marine ecosystem collapse.
Leading Theories for the Cause
The primary scientific hypotheses for the Cambrian extinctions revolve around catastrophic shifts in ocean chemistry and global climate. One leading theory points to the widespread depletion of oxygen in bottom waters, known as Oceanic Anoxia Events (OAEs). During these periods, oxygen-poor water masses from the deep ocean expanded onto the continental shelves, creating toxic, sulfidic conditions that were lethal to most marine life.
Another major hypothesis involves significant climate change, likely driven by rapid cooling events. Evidence suggests a phase of global cooling, potentially involving glaciation, which rapidly dropped sea levels as water became locked in ice sheets. This sea-level fall destroyed vast shallow-water habitats, and the cooling itself would have been lethal to many tropical organisms, with studies indicating a temperature drop of about 5°C in surface waters. Some researchers propose that large-scale volcanic eruptions, such as those that formed the Kalkarindji Large Igneous Province, could have been the ultimate trigger, injecting massive amounts of greenhouse gases that initially caused warming, followed by ocean acidification and a disruption of the global carbon cycle.
Geological Evidence of the Event
Scientists confirm these extinction pulses and their potential causes through two primary lines of evidence preserved in the rock record. The most direct evidence comes from fossil record gaps, where the strata show the sudden, simultaneous disappearance of index fossils, particularly entire families of trilobites, across multiple continents. This abrupt truncation of the fossil record, followed by the appearance of a new, low-diversity fauna, defines the boundaries of these extinction pulses.
Geochemical analysis provides a powerful second line of evidence through Carbon Isotope Excursions (\(delta^{13}C\)) in marine carbonate rocks. These excursions are significant shifts in the ratio of the stable carbon isotopes \(^{13}C\) and \(^{12}C\), which record major disturbances in the global carbon cycle. For instance, the End-Botomian extinction correlates with negative \(delta^{13}C\) excursions, suggesting a massive release of light, carbon-12 rich organic matter, often linked to anoxia or methane release. Conversely, the later Steptoean event is associated with a distinct positive \(delta^{13}C\) excursion (the SPICE event), which indicates a major burial of organic carbon, coinciding with environmental stress and biotic turnover.
Recovery and Ecological Restructuring
Following the successive Cambrian crises, the surviving groups underwent a period of rapid diversification, a process known as Ecological Restructuring. The extinction events had cleared many niches, especially in shallow marine environments, creating opportunity for new forms to evolve and proliferate. The trilobites that survived, often those adapted to cooler, deeper waters, migrated back onto the continental shelves and gave rise to new, more robust families.
The long-term consequence of this restructuring was the fundamental alteration of marine ecosystems, leading to the rise of the Paleozoic Fauna. While the archaic Archaeocyathids were gone, new reef-builders like corals and calcifying sponges eventually took their place in the subsequent Ordovician Period. Other groups, such as articulate brachiopods, bryozoans, and cephalopods, diversified substantially, establishing the complex, tiered ecological structures that characterized the seas for the next hundreds of millions of years.