When it comes to the major mass extinctions on Earth, the first of the Big Five is considered to be the Late Ordovician, around 445 million years ago. But, evidence has been mounting for a few years that a major dying of animals took place about 100 million years before that. Some 550 million years ago, a sudden decrease in global oxygen availability led to the death of 80 percent of all mammals.

Evidence for this Ediacaran Period extinction comes from two crucial fossil records. One dates from between 560 and 550 million years ago, and it is known as the White Sea assemblage. The more recent one dates between 550 and 539 million years ago and is called the Nama assemblage. The earlier fossils show 70 genera of macroscopic animals, but only 14 were present in the subsequent fossils.

It is possible that the difference is due to sampling or preservation biases between the two assemblages, but the team found no evidence of this. They did find that among these marine animals, those that had a larger surface area relative to their volume were more likely to have survived. That’s an indication that those that could get more oxygen were the winners.

"Our study shows that, as with all other mass extinctions in Earth's past, this new, first mass extinction of animals was caused by major climate change—another in a long list of cautionary tales demonstrating the dangers of our current climate crisis for animal life," said Scott Evans, from Virginia Tech, in a statement.

It is unclear what the exact cause of this mass extinction was and, if it truly was due to a depletion of oxygen, how it happened. It could have been volcanos, tectonic plates, or even a space rock. Mass extinctions have been a staple of the evolution of life on Earth and have shaped the type of organisms that survive today.  

The work was conducted during the COVID-19 pandemic by creating a database of available records of the time period. Fossils of these animals are imprinted in ancient stones, and they show these multicellular being quite, in the researchers' words, "weird".

"These organisms occur so early in the evolutionary history of animals that in many cases they appear to be experimenting with different ways to build large, sometimes mobile, multicellular bodies," Evans said. "There are lots of ways to recreate how they look, but the take-home is that before this extinction the fossils we find don't often fit nicely into the ways we classify animals today. Essentially, this extinction may have helped pave the way for the evolution of animals as we know them."

The study was published in Proceedings of the National Academy of Science.