444-Million-Year-Old Microscopic Fossils Reveal Early Seafloor Recovery After Mass Extinction
In the aftermath of a mass extinction 444 million years ago, the oceans were simplified. Many familiar forms of marine life vanished, and conditions at the seafloor became unstable. New fossil evidence reported in Nature Ecology & Evolution shows that microscopic seafloor ecosystems were already active during this recovery period, even as the end-Ordovician extinction — driven by climate cooling and oxygen-poor waters — eliminated roughly 85 percent of marine species.
Preserved in ancient mudrocks from the Cederberg Mountains of South Africa are microscopic burrows and droppings left by meiofauna, including nematodes and foraminifera, organisms small enough to live between grains of sediment. Their traces indicate that these tiny animals were feeding and recycling nutrients at the seafloor soon after the extinction, suggesting that basic ecological processes resumed before larger marine life returned.
“Although some amazing fossils have been found in the Cedarberg rocks in the past — these are from creatures that swam in the surface waters. We did not expect to find fossils of creatures living on the harsh seafloor, especially from a period immediately following a mass extinction when 85% of marine species vanished. Remarkably, these tiny creatures were able to withstand those conditions, and even thrive,” said lead author Claire Browning, in a press release.
Microscopic Fossils Reveal Seafloor Recovery
Unlike shells or skeletons, the fossils preserved in the Cederberg rocks record activity rather than anatomy. Narrow burrows, meandering trails, and clusters of microscopic droppings are preserved three-dimensionally inside the rock, capturing how organisms moved through sediment, fed, and interacted with their environment.
Micro-CT scanning made it possible to examine these traces without breaking the rock apart, revealing patterns that are not visible at the surface. The burrows recur within specific sediment layers that also preserve fossilized marine snow, organic material produced when phytoplankton blooms in surface waters collapsed and sank to the seafloor.
That close association suggests a simple but functional food web was already operating at the sediment surface. Organic matter produced in the upper ocean settled onto the seafloor, where sediment-dwelling meiofauna fed on it directly, breaking it down and recycling nutrients within the sediment. Similar responses are seen in modern oxygen-limited environments, where meiofauna intensify feeding and movement when fresh material reaches the seafloor and reduce activity as conditions deteriorate.
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Life on an Oxygen-Poor Seafloor
Geochemical evidence suggests oxygen levels were low, and at times, conditions may have been toxic to many forms of life. That makes the presence of an active sediment community especially striking.
Rather than reflecting a fully recovered ecosystem, the fossils point to a simplified but functional one. A small number of stress-tolerant organisms dominated, capable of breaking down organic matter and recycling nutrients under harsh conditions.
This stripped-down system may have helped stabilize marine environments after the extinction. By processing carbon at the seafloor, meiofauna could have influenced oxygen and nutrient availability, setting the stage for the gradual return of more complex marine communities.
Ancient Seafloor Ecosystems Worldwide
The fossils from South Africa may represent only a portion of a broader pattern. During the Ordovician, continents were arranged differently, and regions now separated by oceans were once connected.
“Geology does not respect modern borders. For example, rocks of the same age in South America were once connected to those in the Cederberg mountains and may also hold hidden evidence of marine snow, dust and meiofauna. Mapping the extent of these ecosystems will help us understand their broader role in regulating ancient oceans’ carbon and nutrient cycles,” said Browning.
If similar traces are found elsewhere, they could reveal that microscopic ecosystems played a widespread role in regulating ancient oceans and supporting recovery after one of Earth’s most severe marine crises.
Read More: Giant 115-Million-Year-Old Shark Fossil Found in Australia Rewrites Evolution Timeline
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