Since the Cambrian explosion 538.8 million years ago, when many of today’s known animal phyla emerged, 5 mass extinctions have reduced the biodiversity of all living things.
Researchers in the United States have found evidence that one of these occurred earlier, about 550 million years ago, during a period known as the Ediacaran period.
Although the oceans were once teeming with some familiar animals such as sponges and jellyfish, much of life in this early period of biological history now seems strange to us. Many of the animals were flabby. Some looked like leaves stuck in place. Others had some chance.
Scott Evans, a paleontologist at Virginia Tech, and colleagues have collected data on rare Ediacaran fossils of sponge species from around the world. They found that the sudden shifts in biodiversity that had been detected earlier were not just patterns.
Since soft body parts tend to decompose more easily compared to the more robust and 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 a failure to preserve them.
But the global fossil record suggests otherwise.
The team found a general increase in biodiversity between the early and middle Ediacaran stage known as Avalon (575–560 million years ago) and the White Sea stage (560–550 million years ago).
“We found significant differences in dietary habits, lifestyle, ecological class, and maximum body size between the Avalon and White Sea populations,” the team wrote in their paper.
Between these two time periods, a greater number of smaller, mobile animals appeared that fed on the microbial mats that dominated the seafloor.
Feeding conditions did not change in this way between the Mediterranean and the last stage known as Nama (550–539 million years ago). Instead, 80 percent of the amazing species disappear between these two Ediacaran phases.
Previous research has suggested that this decline may be the result of agile animals that have dug up or left behind archaeological fossils, greatly altering the environment. This new data suggests that this was not the case.
All dietary and lifestyle habits have suffered a similar loss: only 14 genera still belong to the Nama of the 70 known groups from the previous Mediterranean stage. If more new evolved species were captured, there would also be a temporal overlap between new and old species. The team claimed that this was not observed and ruled out a vital replacement.
“The decrease in diversity among these populations is indicative of extinction, as the percentage of lost genera is comparable to the percentage experienced by marine invertebrates during the mass extinction,” Evans and colleagues wrote.
Many of the White Sea animals that survived the Nama extinction were large creatures with a high surface area to volume ratio. This may be a sign that these animals have adapted to the low oxygen content of the ocean.
“By maximizing the proportion of cells in direct contact with seawater, relatively high surface area ratings could be better adapted to survive in a low oxygen environment,” the team explains.
There is also recent geochemical evidence to support this idea, as a 2018 study found signs of hypoxia in the vast oceans that covered more than 20% of the seafloor at the end of the Ediacaran period.
The team concluded: “Our data support a link between the turnover of the Ediacaran biota and environmental change, similar to other major mass extinctions in the geological record.”
This study was published in PNAS.
Source: Science Alert.