A study of the first animal communities has concluded that what some paleontologists have considered the first mass extinction was something very different, with animals specializing and forming more complex relationships.
Some 540 million years ago animals suddenly and dramatically diversified, not only in species but in basic architecture, a process known as the Cambrian Explosion. Although the underlying structures of modern lifeforms were set at the time, these were not the first animals. Those, as far as we can tell, appeared in the late Ediacaran Era (572-541 million years ago). The Ediacaran had its own, smaller, explosion, where life also diversified. Then, however, the diversity dropped.
Some scientists have concluded that this fall in diversity represented a mass extinction, earlier than the five confirmed ones in the history of life on Earth, and the one taking place today. However, in PLOS Biology three Cambridge University scientists have challenged this view, after conducting the first investigation of the community structures of fossil deposits from three parts of the late Ediacaran, known as assemblages.
Although individual species were becoming less diverse in the late Ediacaran, the authors argue, the community structures were increasingly complex. This indicates more specialized animals that interacted in more involved ways, just as modern animals have evolved extraordinary interactions between species, such as symbiosis. These are the hallmarks of ecological succession, not a mass extinction. Some animals, unable to cope with the new, more tangled world died out, but life as a whole was richer, not poorer.
“We found that the factors behind that [Cambrian] explosion, namely community complexity and niche adaptation, actually started during the Ediacaran, much earlier than previously thought," said lead author Dr Emily Mitchell in a statement. “The Ediacaran was the fuse that lit the Cambrian explosion.”
The first Ediacaran assemblage, known as Avalon, was relatively simple. During the White Sea assemblage, 558-550 million years ago, a much wider range of animal shapes appeared. Movement became the norm among animals, not the exception, and feeding strategies such as grazing appeared.
Life came together in dense communities that varied significantly depending on the environment. For example, lifeforms adapted to the depth of water they were living in so that shallow communities looked very different from those deeper down.
However, in the time of the Nama assemblage (549-543 million years ago), there was a decline in biodiversity, despite the first appearance of biomineralizing species, increasing the prospects of their preservation as fossils. The correlation between species and depth is much stronger in Nama deposits than in the earlier assemblages.
During mass extinctions, it is usually the generalists, not the specialists that survive. Yet when species declined in the Ediacaran, the opposite was the case. Changes observed over the course of the Nama period were small, but in the opposite direction from what is expected of an ecosystem recovering from an external shock, such as an asteroid strike or volcanic eruption. Putting these observations together, the authors are confident the late Ediacaran changes were driven by internal competition, rather than an external shock.