Since the Cambrian explosion 538.8 million years ago, a time when many of the animal phyla we know today became established, five major mass extinction events have reduced the biodiversity of all creatures, big and small
American researchers have discovered evidence of an earlier event, about 550 million years ago during a period known as the Ediacaran.
Although the oceans were full of animals known as sponges and jellyfish, most life during this early period of biological history would seem alien to us now. Many of the animals were soft-bodied. Some looked more like plant fronds stuck in place. Others had some form of shell.
Virginia Tech paleobiologist Scott Evans and colleagues compiled data on rare fossils of the world’s softest animal types dating to the Ediacaran. They found that the sudden changes in biodiversity that had previously been detected were not simple sampling biases.
Smithsonian Institution Diorama of Ediacaran Sea Life. (Ryan Somma/Wikipedia/CCB-SA 2.0)
Because softer body parts typically don’t fossilize as easily as harder, more mineralized bits of anatomy, researchers tend to suspect that a relative absence of soft-bodied animals in the later Ediacaran stages is simply the result of a failure to be preserved.
But the world fossil record indicates otherwise.
The team found that there was an overall increase in biodiversity between the early and middle Ediacaran stages, known as the Avalon stages (575 to 560 million years ago) and the White Sea stages (560 to 550 million years).
“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 mobile, smaller animals appeared that fed on the microbial mats that dominated the seafloor. Previously, many of the animals were attached to the site (sessile).
Food 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 appeared to disappear between these two Ediacaran stages.
Previous research has suggested that this decline may have been the result of mobile animals that burrowed or left fossil tracks, which profoundly altered the environment and slowly replaced sessile filters. This new evidence suggests that was not the case.
All food and lifestyle types experienced similar losses, with only 14 genera still seen in Nama of the 70 known groups from the earlier White Sea stage. If more recently evolved species had taken over, there would also have been a temporal overlap between the new and the old species. This was not observed, the team argues, ruling out biotic substitution.
“The decline in diversity among 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,” write Evans and colleagues. leagues
Many of the White Sea animals that survived the extinction event and remained in the Nama period were large, frond-like organisms with a large surface-to-volume ratio. This could be a sign that these animals were adapting to cope with a reduction in ocean oxygen.
“By maximizing the relative proportions of cells in direct contact with seawater, taxa with a high surface area would have been comparatively better adapted to survive in low-oxygen environments,” the team explains.
There’s also recent geochemical evidence to support this idea, with a 2018 study finding signs of widespread oceanic anoxia covering more than 20 percent of the seafloor at the end of the Ediacaran.
“Thus, our data support a link between Ediacaran biotic turnover and environmental change, similar to other major mass extinctions in the geologic record,” the team concludes.
It has become an all too familiar story.
This research was published in PNAS.