This strange sponge owned the world's first known skeleton

Illustration for article titled This strange sponge owned the worlds first known skeleton

This rather bizarre organism is Coronacollina acula, which lived on the seafloor about 550 million years ago. This sponge-like creature doesn't look like much of, well, anything, but its discovery throws a whole new light on the evolution of skeletons.


Coronacollina acula dates back to the Ediacaran Period, which spans from 630 to 542 million years ago. That in itself is significant - we tend to think of the Cambrian (542-488 million years ago) as the time when species diversified and skeletons evolved. As such, Coronacollina acula isn't just the first animal with a skeleton — it's also much earlier than we would have expected it to be.

Its name literally means "little rimmed hill with needles", and that's pretty much exactly what it was - a hard cone-shaped body with a few long, brittle spindles around it. UC Riverside geologist discovered the fossils in southern Australia. It was a tiny creature, probably no more than an inch or two tall, though its support spindles were much longer, probably between eight and fifteen inches long. Lead research Mary Droser explains just why the find is so significant in our understanding of early evolution:

"Up until the Cambrian, it was understood that animals were soft bodied and had no hard parts," said Mary Droser, a professor of geology at the University of California, Riverside, whose research team made the discovery in South Australia. "But we now have an organism with individual skeletal body parts that appears before the Cambrian. It is therefore the oldest animal with hard parts, and it has a number of them - they would have been structural supports - essentially holding it up. This is a major innovation for animals. It therefore provides a link between the two time intervals. We're calling it the 'harbinger of Cambrian constructional morphology,' which is to say it's a precursor of organisms seen in the Cambrian. This is tremendously exciting because it is the first appearance of one of the major novelties of animal evolution. The fate of the earliest Ediacaran animals has been a subject of debate, with many suggesting that they all went extinct just before the Cambrian. Our discovery shows that they did not."


What's particularly interesting is just why this species developed skeletons in the first place. As Droser points out, the evolution of skeletons is typically associated with developing a defense mechanism against predators. But it's thought that the Ediacaran period was a time before the advent of predators, which means Coronacollina acula likely evolved its skeletal structure for some other, currently unknown reason.

Via Geology. Image by Daniel Garson for Droser lab, UC Riverside.

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I was always under the impression that hard anatomical bits arose as support against gravity. Predation played a roll in their morphology certainly but I was taught and always assumed that immobile animals developed hard interiors first as a means to increase physical surface area which would be advantageous for opportunistic feeders in the collection of food.