A small pit with long, thin grooves radiating away from it like spokes on a wheel, this faint fossil may not take your breath away. But it should. It represents one of the most monumental moments in the history of life: the evolution of a skeleton.
Called Coronacollina acula, the organism that left this impression lived on the seafloor between 560 million and 550 million years ago. Bearing a passing resemblance to primitive sponges, its thimble-shaped body left its mark in the rock as a depression a few millimeters to 2 centimeters deep.
What really made Coronacollina special, though, are its needle-like spicules, which cast the long, thin grooves in the rock. The animal most likely used these spicules to hold itself in place — something no known creature had ever done before.
Each animal sported at least four of these novel support struts, each one between 20 and 40 centimeters long. One end of each spicule attached to the thimble-shaped body; the other end was embedded in the seafloor.
In life, Coronacollina acula probably looked something like this:
A team of paleontologists from the University of California, Riverside, who describe their results in the April issue of the journal Geology, originally discovered these inventive creatures in the Ediacara Hills of South Australia.
Named after the Ediacara Hills are a famous fossil trove of unusual animals called the Ediacaran fauna, which scientists had long assumed were all soft-bodied. The discovery of Coronacollina acula among them turns that assumption on its head.
Until now the understanding had been that hard parts, which allowed animals to grow larger and protect against predators, did not show up until long after Coronacollina’s reign. The rise of animals with skeletons was thought to have coincided instead with a rapid diversification of life that took place during a later period of time known as the Cambrian, which lasted from 542 million to 488 million years ago.
The early debut of Coronacollina acula not only signals that the origin of skeletons well preceded the Cambrian explosion but also anchors Ediacaran animals within the evolutionary lineage of animals as we know them, explains UCR paleontologist Mary Droser, whose research team made the discovery.
“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,” Droser said in a press release. “Our discovery shows that they did not.”
Fossil impression of Coronacollina acula, the oldest know animal with a skeleton. Courtesy James G. Gehling of the South Australian Museum.
A reconstruction of how Coronacollina would have appeared in life, using its spicules as support struts to keep it stationary on the ocean floor. (Daniel Garson)