A groundbreaking discovery of a 550 million-year-old sea sponge fossil offers new insights into sponge evolution and guides future fossil searches. Reconstructed life position of Helicolocellus on Ediacaran seafloor. Credit: Yuan XunlaiResearchers led by Shuhai Xiao at Virginia Tech have discovered a 550 million-year-old sea sponge fossil, shedding light on a 160 million-year gap in the fossil record.
Xiao and collaborators from the University of Cambridge and the Nanjing Institute of Geology and Paleontology began ruling out possibilities one by one: not a sea squirt, not a sea anemone, not a coral. They wondered, could it be an elusive ancient sea sponge? “If you extrapolate back, then perhaps the first ones were soft-bodied creatures with entirely organic skeletons and no minerals at all,” Xiao said. “If this was true, they wouldn’t survive fossilization except under very special circumstances where rapid fossilization outcompeted degradation.”
Another unexpected aspect of the new sponge fossil is its size. “When searching for fossils of early sponges I had expected them to be very small,” said Alex Liu, a collaborator from the University of Cambridge. “The new fossil is about 15 inches long with a relatively complex, conical body plan, which challenged many of our expectations for the appearance of early sponges.”