Traces of the tapeworm-like creature Plexus ricei as seen in the fossil record. Individuals were likely 2 to 31 inches (5 to 80 cm) long.
Credit: Droser Lab, UC Riverside
A puzzling fossil find from the Ediacaran period, an era that occurred more than 500 million years ago, has scientists curious about how "bilateral" creatures such as humans evolved.
The Plexus ricei organism resembled a tapeworm or flatworm. Mysteriously, it appears to have had "bilateral", or left-right, symmetry before anything else living 540 million to 575 million years ago.
"Plexus was unlike any other fossil that we know from the Precambrian," said study researcher Mary Droser, a paleobiologist at the University of California at Riverside, in a statement. The Precambrian was the period before abundant animal life appeared on our planet, and represents latter part is called the Ediacaran period.
"It was bilaterally symmetrical at a time when bilaterians — all animals other than corals and sponges — were just appearing on this planet." [In Photos: Spooky Deep-Sea Creatures]
The tubular creature was about 5 to 80 centimeters (2 to 31 inches) in length and 5 to 20 millimeters (0.2 inches to 0.78 inches) wide, and it lived on the seafloor. That wasn't unusual, as all life on Earth lived in the oceans during this time. The earliest evidence of a bilaterally symmetrical organism comes from about 585 million years ago. That slug-like animal, less than a half-inch long, left itsy-bitsy tracks found fossilized in Uruguay.
"Ediacaran fossils are extremely perplexing: They don’t look like any animal that is alive today, and their interrelationships are very poorly understood," Lucas Joel, a former graduate student at UC Riverside who led the research, said in a statement.
The Ediacaran period also lacked bioturbation, Joel said, or churned-up seafloors from marine organisms snuffling for food.
It was only in the Cambrian period — the explosion of life that started around 540 million years ago — that organisms began churning up the seafloor. By contrast, large algae mats carpeted the Ediacaran oceans, a rare occurrence on Earth today.
Because the seafloor remained undisturbed, organisms that died and drifted to the ocean floor were preserved when sediments accumulated over their bodies, creating a mold.
"What this means is that the fossils we see in the field are not the exact fossils of the original organism, but instead molds and casts of its body," Joel said. That makes it sometimes difficult to tell if a fossil represents an organism like Plexus or simply an empty burrow created by a long-ago creature worming its way through the sand.
Joel and his colleagues discovered that Plexus ricei was not, in fact, a trace fossil (fossil of an organism's path), but rather was an unknown organism. The species gets its name from the Latin word "plexus," which means braided, and from Dennis Rice, a field assistant at the South Australian Museum who excavated many specimens of Plexus.
Droser said researchers need to confirm that Plexus was truly bilateral, but the scientists suspect that this tapeworm-like creature is a distant ancestor of other bilateral organisms — including humans.
The findings were published in March in the Journal of Paleontology.