Geobiologist's team discovers 'extinct' sea sponges

At first glance, there is nothing mysterious about the simple sea sponge. No brain. No guts. No problem dating it back 700 million years. Yet compelling sponge fossils only date back about 540 million years, leaving a 160 million-year gap in the fossil record.

In an article published on June 5 in the journal NatureVirginia Tech geobiologist Shuhai Xiao and colleagues report a 550-million-year-old sea sponge from the “lost years” and propose that early sea sponges had not yet developed mineral skeletons, thus offering new parameters in search of missing fossils.

The mystery of extinct sea sponges is based on a paradox.

Molecular clock estimates, which measure the number of genetic mutations that accumulate over time, indicate that sponges must have evolved around 700 million years ago. And yet no convincing sponge fossils had been found in rocks this old.

For years, this enigma has been the subject of debate among zoologists and paleontologists.

This latest discovery completes the evolutionary family tree of one of the earliest animals, explaining its apparent absence in older rocks and connecting the dots to Darwin's questions about when it evolved.

Xiao, who was recently inducted into the National Academy of Sciences, first discovered the fossil five years ago, when a collaborator texted him a photo of a specimen discovered along the Yangtze River in China.

“I’ve never seen anything like this before,” said Xiao, a faculty member in the College of Science. “Almost immediately I realized this was something new.”

Xiao and his collaborators at the University of Cambridge and the Nanjing Institute of Geology and Paleontology began ruling out possibilities one by one: not an ascidian, not a sea anemone, not a coral. They wondered if it was an ancient, elusive sea sponge?

In an earlier study published in 2019, Xiao and his team suggested that early sponges left no fossils because they had not evolved the ability to generate the hard, needle-like structures, called spicules, that characterize sponges. sea ​​sponges today.

Members of Xiao's team traced the evolution of sponges through the fossil record. As they moved back in time, the sponge spicules were increasingly organic and less mineralized.

“If you extrapolate back, then maybe the first ones were soft-bodied creatures with an entirely organic skeleton and no minerals,” Xiao said. “If this were true, they would not survive fossilization except in very special circumstances where rapid fossilization outweighed degradation.”

Later in 2019, Xiao's international research group discovered a sponge fossil preserved under such circumstances: a thin bed of marine carbonate rocks known to preserve abundant soft-bodied animals, including some of the earliest mobile animals .

“More often than not, this type of fossil would be lost in the fossil record,” Xiao said. “This new discovery opens a window into early animals before they developed hard parts.”

The surface of the new sponge fossil is dotted with a complex array of regular boxes, each divided into smaller, identical boxes.

“This specific pattern suggests that our fossilized sea sponge is very closely related to a certain species of glass sponge,” said Xiaopeng Wang, a postdoctoral researcher at the Nanjing Institute of Geology and Paleontology and the University of Cambridge. .

Another unexpected aspect of the new sponge fossil is its size.

“When I was looking for ancient sponge fossils, I 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 about when the first sponges appeared.”

While the fossil fills in some of the missing years, it also provides researchers with important guidance on how to look for these fossils, which will hopefully lead to a deeper understanding of early animal evolution further back in time.

“The discovery indicates that early sponges may have been spongy but not glassy,” Xiao said. “We now know that we need to broaden our vision when looking for the first sponges.”