Ancient sea sponge discovery fills 160-million-year gap in evolution

Virginia Tech geobiologist Shuhai Xiao holds the 550 million-year-old sea sponge fossil. Credit: Spencer Coppage/Virginia Tech.

Researchers have uncovered a 550-million-year-old sea sponge fossil, solving a long-standing mystery in the history of one of the earliest animals.

Sea sponges might seem simple and not very mysterious.

They don’t have brains or guts, and scientists have estimated they existed 700 million years ago.

However, until now, the oldest known sponge fossils were only about 540 million years old, leaving a puzzling 160 million-year gap in the fossil record.

In a study published in the journal Nature, Virginia Tech geobiologist Shuhai Xiao and his team describe finding a sea sponge from this “missing” period.

They suggest that the earliest sea sponges hadn’t developed hard skeletons yet, which is why their fossils were so hard to find.

The mystery revolves around a paradox. Using a method called molecular clock estimates, scientists have calculated that sponges must have evolved around 700 million years ago.

But no sponge fossils from that time had been found, leading to debates among scientists.

This new discovery helps complete the evolutionary family tree of early animals, answering questions that go back to Charles Darwin. Xiao first saw the fossil five years ago when a colleague sent him a photo of a specimen found along the Yangtze River in China.

“I had never seen anything like it before,” Xiao recalls. He and his team, including researchers from the University of Cambridge and the Nanjing Institute of Geology and Paleontology, began to investigate.

They ruled out other possibilities like sea squirts, sea anemones, and corals. They wondered if it could be an ancient sea sponge. In 2019, Xiao’s team suggested that early sponges left no fossils because they hadn’t developed hard, needle-like structures called spicules. These spicules are characteristic of modern sea sponges.

The researchers traced sponge evolution through the fossil record. They found that as they looked at older fossils, the spicules became more organic and less mineralized. They theorized that the earliest sponges were entirely soft-bodied with no minerals, making them unlikely to fossilize except under special conditions.

Later in 2019, Xiao’s team found a sponge fossil preserved in such a condition—a thin bed of marine rocks that preserved many soft-bodied animals. This fossil showed an intricate pattern of tiny boxes, suggesting it was related to a type of glass sponge.

Surprisingly, the new fossil was much larger than expected. “When searching for early sponges, I expected them to be very small,” said Alex Liu from the University of Cambridge. “This fossil is about 15 inches long with a complex, conical shape, challenging our expectations.”

This discovery not only fills in the missing years but also provides clues on where to find more early sponge fossils. Xiao concludes, “The discovery indicates that perhaps the first sponges were soft and not glassy. We now know to broaden our search for early sponges.”

This study was funded by the US Army Research Office and the National Institute of Health, with support from Virginia Tech and international collaborators.