Since the Cambrian explosion 538.8 million years ago – a time when many of the animal phyla we’re familiar with today were established – five major mass extinction events have whittled down the biodiversity of all creatures great and small.
American researchers have found evidence that one occurred earlier than expected, approximately 550 million years ago in a period called the Ediacaran.
Even though the oceans contained a few familiar species like jellyfish and sponges at one time, much of the life that existed during this period in biological history was unknown to us. Many of the animals were soft-bodied.Some of them looked more like plant leaves stuck in place. Some had some sort of shell.
Scott Evans, a Virginia Tech paleobiologist, and his colleagues collected data on rare fossils from the squishier types of animals around the globe dated to the Ediacaran. These sudden shifts in biodiversity, previously not detected before, were not simply due to sampling biases.
Researchers have suspected that soft-bodied animals are less likely to fossilize in later stages of the Ediacaran because they don’t often fossilize as quickly as hard, more mineralized parts of anatomy. They are the result of an inability to preserve them.
The global fossil record however, suggests otherwise.
The team discovered that biodiversity increased between the Ediacaran’s earlier and middle stages, also known as the Avalon (575-560 million years ago), and White Sea stages (560-555 million years ago).
“We found significant differences in the life mode, feeding habit, ecological Tier, and maximum body sizes between the Avalon assemblages and the White Sea assemblies,” said the team. WritesIn their paper.
In between these two periods, smaller mobile animals were seen that ate on the Microbial matsThese animals dominated the seafloors. Many of the animals had been previously trapped-in-place (sessile) feeders.
The White Sea was the final stage of the Ediacaran, and it did not see any changes in feeding habits between then and now. This was the Nama (550-539 million years ago). A staggering 80 percent of species appeared to have disappeared between these two stages.
Research in the past suggested that this decline could have been caused by mobile animals that left trace fossils or burrowed, which profoundly altered and replaced sessile filter feeders. The new evidence supports this hypothesis.
The losses were similar for all types of life habits and feeding methods. Only 14 genera are still found in the Nama from 70 groups that existed at the White Sea stage. There would have been a temporal overlap between the old and new species if there had been more recently evolved species. The team argues that this was not observed. Biobiotic replacement.
Evans and co-workers stated, “The decline of diversity in these assemblages is indicative that an extinction episode has occurred with a percentage loss comparable to that experienced during the ‘Big 5’ mass extinctions.” Write.
Many White Sea animals survived the extinction and remained in Nama periods. These large organisms, which were frond-like and had high volumes to their surface area, were large. This could indicate that these animals were adapting to the loss of oceanic oxygen.
The team said that high-surface taxa with large surface areas would have been better adapted to low oxygen environments if they had maximized the relative numbers of cells in direct seawater contact. This explains.
Geochemical evidence supports this theory, and there is some recent geochemical evidence. 2018 studySigns of ocean anoxia covering more than 20% of the seafloor at end of Ediacaran.
“Our data support an association between Ediacaran biotumption and environmental change similar to major mass extinctions in geologic records.” The team finishes.
It is a story that has become all too familiar.
This research was published in PNAS.
