The 550-million-year-old Quaestio simpsonorum is the first fossil to show a definitive left-right asymmetry, an important sign of evolutionary development in the history of animals.
In 1947, geologist Reginald C. Sprigg announced the discovery of large fossils from the Ediacara Hills in Australia. Dating of the rock formations revealed that the Ediacara fauna was more than 550 million years old—quite a sensation at the time as nobody expected multicellular life to appear so early in Earth’s history.
“As the oldest fossil animals, the Ediacara biota, can tell us a great deal about early developmental processes,” explains lead author Scott Evans, assistant professor of geology in the Florida State University Department of Earth, Ocean and Atmospheric Science.
Scientists have long argued if the Ediacara fauna were true animals and not some sort of large-bodied amoeba, lichens (!), or a completely extinct branch of life-forms. In 2018, the discovery of chemical traces in Dickinsonia, one of the weirdest members of the Ediacara fauna, compatible with chemicals as found in modern animals, suggested that the Ediacara fossils belong to the animal kingdom.
The international research team lead by Evans adds further evidence to support this theory.
“The animal is a little smaller than the size of your palm and has a question-mark shape in the middle of its body that distinguishes between the left and right side,” explains Evans. A bilateral structure, showing an asymmetry in the position of organs, is an important step towards more complex organisms including modern animals. No other fossils from the Ediacara period have shown this type of organization so definitively, with most showing a simple radial symmetry (like modern jellyfish).
Quaestio still shares some anatomical details with other members of the Ediacara fauna. Like other Ediacaran fossils, Quaestio doesn’t seem to posses any orifices. It likely feed on algae and microbial mats by simply resting on the ground and absorbing nutrients through its skin. This could explain another observation made by the researchers.
“One of the most exciting moments when excavating the bed where we found many Quaestio was when we flipped over a rock, brushed it off, and spotted what was obviously a trace fossil behind a Quaestio specimen—a clear sign that the organism was motile; it could move,” says coauthor Ian Hughes, a Harvard University organismic and evolutionary biology graduate student.
The fossil was found in the Nilpena Ediacara National Park, a protected area located in the northern Flinders Ranges in the state of South Australia. Though scientist have been excavating at this location for decades, Quaestio was only recently discovered at one of the newest excavation sites in the park in a collaborative effort with volunteers at the South Australia Museum. The research team hopes to continue reexamining sites throughout the park’s nearly 150,000 acres.
“We’re still finding new things every time we dig,” Hughes concludes. “Even though these were some of the first animal ecosystems in the world, they were already very diverse. We see an explosion of life really early on in the history of animal evolution.”
The full study, “A new motile animal with implications for the evolution of axial polarity from the Ediacaran of South Australia,” was published in the journal Evolution & Development and can be found here.
