An early bang in the evolutionary history of animals is emerging: around 550 million years ago, a lack of oxygen could have led to the first mass extinction, according to a study. Comparative studies of fossils found before and after the Ediacarian caesura suggest that about 80 percent of the animals disappeared at that time. It seems that those whose physical characteristics indicate a comparatively high oxygen requirement were particularly affected. According to the researchers, various processes could have led to a lack of availability of the elixir of life.
It is considered to be the era in which the first complex animal creatures arose: the Ediacarian covers the period from 635 million years ago to the beginning of the Cambrian - 539 million years ago. It is the earliest age from which fossils of animals that can be seen with the naked eye are known. These were sometimes bizarre-looking aquatic organisms that differed significantly from modern-day creatures. Traces of this Ediacarian fauna can be found, among other things, in deposits that can be assigned to two periods: "White Sea", 560 to 550 million years ago, and "Nama", 550 to 539 million years ago. It is striking that fewer species were discovered in the Nama deposits than in the White Sea sites.
mass extinction confirmed
What caused the difference, however, remained unclear: A mass extinction seemed to be imminent, but it is possible that the older deposits simply had better preservation conditions. "It was already suggested that an extinction had occurred at this time, but this remained speculative. Therefore, we gathered all available information to verify this statement," says first author Scott Evans of Virginia Tech in Blacksburg. As part of their study, Evans and his colleagues systematically examined and compared the fossil traces in the Nama and White Sea deposits. Her focus was on clues to possible differences in fossilization conditions and what characterized the extant creatures.
As the researchers report, it was fundamentally clear from their results that there were hardly any differences in the formation of the two deposits with the Ediacarian fossils. The two ecosystems were probably also fundamentally similar. The scientists therefore assume that the lower biodiversity in the Nama Formation is not due to a distortion, but actually to a decline in the animal world of the time. Specifically, they found that of the 70 genera found in the White Sea deposits, there are only 14 in the Nama Formation, indicating a relatively rapid loss of 80 percent of the species. This indicates an extent that is comparable to the mass extinction events of later times.
Indications of oxygen deficiency
As the team further reports, they found an interesting pattern when comparing the traits of the animals: the survivors of the caesura had a comparatively large surface area to volume ratio, which presumably enabled them to take up more oxygen from the water. “Those whose physiques indicate they depended on significant amounts of oxygen appear to have been particularly hard hit by the extinction event. This suggests that this extinction event was due to environmental changes, like the others in geological history," says Evans.
But what could have caused the decrease in oxygen supply? "We can't say for sure. Volcanic eruptions, tectonic plate movements, an asteroid impact and other events or their combinations are possible,” says Evans. Thus, different scenarios could have led to the lower availability of oxygen reflected in the differences between Ediacarian animals before and after 550 million years ago. "Basically, however, it is clear that abiotic influences have had significant effects on the diversity patterns during the more than 570 million year history of animals on our planet," the authors write.
As the scientists conclude, the Ediacarian mass extinction may also have been of fundamental importance for the further development of life. Because the turning points in the course of the earth's history are known as important steps in the evolution of life on our planet. "Ultimately, the extinction wave 550 million years ago may have paved the way for the evolution of animals as we know them today," says Evans.
Source: Virginia Tech, technical article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2207475119