Scientists have discovered evidence of the first mass extinction of animals on Earth

(ORDO NEWS) — Since the Cambrian Explosion 538.8 million years ago – the time when many of the animal phyla we know today – arose, five major mass extinctions have reduced the biodiversity of all creatures large and small.

US researchers have found evidence that one of these events happened earlier, about 550 million years ago, during a period known as the Ediacaran.

Although the oceans abounded with a few familiar animals such as sponges and jellyfish, most of the species of life in this early period of biological history would now seem alien to us. Many animals were soft-bodied. Some looked more like plant branches attached to each other. Others had a shell.

Virginia Tech paleo biologist Scott Evans and colleagues have collected data on rare fossils of smaller animal species from around the world dating back to the Ediacaran period. They found that the sudden shifts in biodiversity that had been detected earlier were not simple sampling error.

Since soft body parts usually do not fossilize as easily as harder, mineralized parts of the anatomy, researchers have generally suspected that the relative absence of soft-bodied animals in the later stages of the Ediacaran is simply the result of their failure to survive.

However, the global fossil record suggests otherwise.

The team found that between the early and middle stages of the Ediacaran, known as the Avalonian (575-560 million years ago) and the White Sea (560-550 million years ago), there was an overall increase in biodiversity.

“We found significant differences in feeding habits, living habits, ecological layer and maximum body size between the Avalonian and White Sea aggregations,” the group writes in their paper.

Between these two time periods, more small, mobile animals appeared that fed on the microbial mats that dominated the seafloor. Previously, many of the animals were sessile filter feeders.

Between the White Sea and the last stage, known as the Nama (550-539 million years ago), eating habits did not change in this way. In contrast, between these two Ediacaran stages, 80 percent of the species disappeared.

Past research has shown that this decline may have been the result of mobile animals that burrowed into the ground or left fossil tracks, which profoundly altered the environment and gradually replaced sedentary filter feeders. New evidence suggests that this was not the case.

All types of diets and living habits experienced similar losses: out of 70 known groups belonging to an earlier stage of the White Sea, only 14 genera survived in Nama.

If new species were replaced by more newly evolved ones, then there would also be a temporary overlap between new and old species. This has not been observed, the team argues, ruling out the possibility of a biotic replacement.

“The decline in diversity between these communities is indicative of an extinction event, with a percentage of extinct genera comparable to that experienced by marine invertebrates during the Big Five mass extinctions,” Evans and colleagues write.

Many of the White Sea animals that survived the extinction and survived into the Nam period were large, front-like organisms with a high surface area to volume ratio. This may be a sign that these animals were adapting to a decrease in the amount of oxygen in the ocean.

“By maximizing the relative proportion of cells in direct contact with seawater, taxa with high surface area would be comparatively better equipped to survive in low oxygen environments,” the team explains.

Recent geochemical evidence also supports this idea: a 2018 study found evidence of extensive ocean anoxia that covered more than 20 percent of the seafloor at the end of the Ediacaran.

“Thus, our data support a link between the Ediacaran biotic turnover and ecological change, similar to other major mass extinctions in the geological record,” the team concludes.

It’s becoming an all too familiar story.”

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