Since the Cambrian Explosion 538.8 million years ago – a time when many of the animal phyla we know today were established – five major mass extinction events have reduced the biodiversity of all creatures, large and small.
American researchers have discovered evidence of an earlier event, around 550 million years ago, during a period known as the Ediacaran.
Although the oceans were teeming with a few familiar animals like sponges and jellyfish, most life during this early period of biological history would now seem alien to us. Many animals had soft bodies. Some looked more like plant fronds stuck in place. Others had a shell shape.
Virginia Tech paleobiologist Scott Evans and his colleagues have compiled data on rare fossils of spongier animals from around the world dated to the Ediacaran. They discovered that the sudden changes in biodiversity that had been detected before were not just sampling biases.
Since softer body parts generally do not fossilize as easily as harder, more mineralized parts of anatomy, researchers have generally suspected a relative absence of soft-bodied animals in the later stages of the Ediacaran. . are simply the result of a failure to be preserved.
But the global fossil record indicates otherwise.
The team found that there was an overall increase in biodiversity between the earlier and middle stages of the Ediacaran, known as the Avalon (575 to 560 million years ago) and the stages of the White Sea (560 to 550 million years ago).
“We find significant differences in foraging pattern, life habits, ecological level, and maximum body size between Avalon and White Sea assemblages,” the team wrote in their paper.
Between these two periods, smaller mobile animals appeared that fed on the microbial mats that dominated the seabed. Previously, many animals were stuck-in-place (sessile) filter feeders.
Feeding patterns did not change that way between the White Sea and the top floor, Nama says (550 to 539 million years ago). On the contrary, a staggering 80% of species seemed to disappear between these two Ediacaran stages.
Previous research has suggested that this decline may be the result of mobile animals digging or leaving fossil tracks, which profoundly altered the environment and slowly replaced sessile filter feeders. This new evidence suggests that was not the case.
All types of diets and lifestyles suffered similar losses, with only 14 genera still seen in the Nama out of 70 known earlier White Sea stage groups. If more newly evolved species had taken over, there would also have been temporal overlap between the new and old species. This was not observed, according to the team, ruling out biotic replacement.
“The decline in diversity between these assemblages is indicative of an extinction event, with the percentage of genera lost comparable to that experienced by marine invertebrates during the ‘Big 5’ mass extinctions,” Evans and colleagues write.
Many White Sea animals that survived the extinction event and remained during the Nama period were large frond-like organisms with a high surface area to volume ratio. This could be a sign that these animals were adapting to cope with reduced ocean oxygen.
“By maximizing the relative proportions of cells in direct contact with seawater, large-area taxa would have been comparatively better adapted to survive in low-oxygen environments,” the team explains.
There is also recent geochemical evidence to support this idea, with a 2018 study revealing signs of widespread ocean anoxia that covered more than 20% of the seafloor during the late Ediacaran.
“Thus, our data support a link between Ediacaran biotic turnover and environmental change, similar to other major mass extinctions in the geological record,” the team concludes.
It has become an all too familiar story.
This research was published in PNAS.
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