by Since the Cambrian explosion 538.8 million years ago – a time when many of the animal lineages we know today became established – five major mass extinction events have reduced the biodiversity of all creatures, large and small.
Researchers from the US have uncovered evidence that such occurred earlier, around 550 million years ago, during a period known as the Ediacaran.
Although the oceans were filled with a few familiar animals like sponges and jellyfish, most life during this early period of biological history would seem foreign to us now. Many of the animals had soft bodies. Some looked more like plant leaves stuck in place. Others had some form of shell.
Virginia Tech Paleobiologist Scott Evans and his colleagues compiled data on rare fossils of the toughest animal species from around the world dating to the Ediacaran. They found that the sudden changes in biodiversity previously detected were not simple sampling.
Because softer body parts usually don’t fossilize as easily as harder, more mutated bits of anatomy, researchers usually suspect a relative absence of soft-bodied animals in the later Ediacaran stages they are simply the result of a failure to maintain them.
But the global fossil record suggests otherwise.
The team found that there was an overall increase in biodiversity between the early and middle Ediacaran stages known as the Avalon (575 to 560 million years ago) and the White Sea stages (560 to 550 million years ago).
“We find significant differences in feeding mode, life habit, ecological level and maximum body size between the Avalon and White Sea assemblages,” the team writes in their paper.
Between these two time periods, more smaller mobile animals appeared that fed on the microbial mats that dominated the sea floors. Previously many of the animals were glued in place (sessile) feeder filters.
Feeding patterns did not change in this way between the White Sea and the last stage, known as the Nama (550 to 539 million years ago). Rather, an astonishing 80 percent of species seemed to disappear between these two Ediacaran stages.
Previous research has suggested that this decline may have been the result of mobile animals that dragged or left trace fossils, which profoundly changed the environment and slowly replaced the ambivalent filter feeders. This new evidence suggests that was not the case.
All types of diets and life habits showed similar losses, with only 14 genera still occurring in Nama out of 70 known groups from the earlier White Sea stage. If more new species had come to dominate, there would also be temporal overlap between the new and old species. This was not observed, the team argues, ruling out biotic replacement.
“The decline in diversity among these assemblages is indicative of an extinction event, with the rate of genera being lost comparable to that experienced by marine invertebrates during the ‘Big 5’ mass extinctions,” Evans and colleagues write.
Many of the White Sea animals that survived the extinction event and remained in the Nama period were large, leaf-like organisms with a high surface-to-volume ratio. This could be a sign that these animals adapted to cope with the ocean’s oxygen depletion.
“By maximizing the relative proportions of cells in direct contact with seawater, high-surface area taxa would be 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 finding signs of widespread ocean anoxia that covered more than 20 percent of the sea floor at the end of the Ediacaran.
“Thus, our data support a link between the Ediacaran biotic turnover and environmental change, similar to other major mass extinctions in the geologic record,” the team concludes.
It has become a well-known story.
This research was published in PNAS.
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