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The research offers new insights into the early evolution of animals.

Researchers led by Shuhai Xiao of Virginia Tech discovered a 550-million-year-old marine sponge fossil, shedding light on a 160-million-year gap in the fossil record. This fossil, which suggests that early fungi did not have mineral skeletons, provides new insights into the evolution of one of the earliest animals and influences how paleontologists search for ancient fungi.

At first glance, the simple sea sponge is not a mysterious creature. No brain. No instinct. We have no problem dating it to 700 million years ago. Yet conclusive sponge fossils only date back to about 540 million years, leaving a gap of 160 million years in the fossil record.

In an article published June 5 in the journal Naturegeobiologist Shuhai Xiao of Virginia Tech and colleagues reported a marine sponge 550 million years old from the “lost years” and suggested that the earliest marine sponges had not yet developed mineral skeletons, offering new parameters for the search for the missing fossils.

The mystery of the extinct sea sponges centers on a paradox. Molecular clock estimates, which involve measuring the number of genetic mutations that accumulate over time, show that fungi must have evolved about 700 million years ago. And yet no convincing mushroom fossils have been found in rocks this old.

For years, this conundrum has been the subject of debate between zoologists and paleontologists.

This latest discovery fills out 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’s groundbreaking discovery

Xiao, who was recently inducted into the National Academy of Sciences, first saw the fossil five years ago when a collaborator sent him a photo of a specimen dug up along the Yangtze River in China. “I had never seen anything like this before,” said Xiao, a faculty member in the College of Science. “Almost immediately I knew it was something new.”

Xiao and collaborators at the University of Cambridge and the Nanjing Institute of Geology and Paleontology began ruling out the possibilities one by one: no sea jet, no sea anemone, no coral. Could it be an elusive ancient sea sponge, they wondered?

Virginia Tech geobiologist Shuhai Xiao and colleagues report a 550-million-year-old marine sponge fossil, filling a gap in the evolutionary family tree of one of the earliest animals. Photo by Spencer Coppage for Virginia Tech. Credit: Spencer Coppage for Virginia Tech

In an earlier study published in 2019, Xiao and his team suggested that early sponges left no fossils because they did not develop the ability to generate the rigid needle-like structures known as spicules that characterize marine sponges today.

Members of Xiao’s team traced the evolution of fungi through the fossil record. As they went further back in time, the sponge spicules became increasingly organic in composition and less mineralized.

“If you extrapolate back, then maybe the first ones were soft-bodied creatures with all-organic skeletons and no minerals at all,” Xiao said. “If that were true, they wouldn’t survive fossilization except in very special circumstances where rapid fossilization outpaced degradation.”

Later in 2019, Xiao’s international research group discovered a sponge fossil preserved in just such a circumstance: a thin layer of marine carbonate rock known to preserve an abundance of soft-bodied animals, including some of the earliest moving animals.

“Most often this type of fossil is lost in the fossil record,” Xiao said. “New discovery offers window into early animals before they developed hard parts.”

A new fossil discovery and its implications

The surface of the new fossil sponge 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 most closely related to a certain kinds from a glass sponge,” said Xiaoping Wang, a postdoctoral fellow at the Nanjing Institute of Geology and Paleontology and the University of Cambridge.

Another unexpected aspect of the new fossil sponge is its size. “When I was looking for fossils of early fungi, I expected them to be very small,” said Alex Liu, a researcher at the University of Cambridge. “The new fossil is about 15 inches long with a relatively complex, conical body plan that challenged many of our expectations for the appearance of early fungi.”

While the fossils fill in some of the missing years, they also provide researchers with important guidance on how to look for these fossils—which will hopefully expand understanding of early animal evolution further back in time.

“The discovery suggests that perhaps the first mushrooms were spongy but not glassy,” Xiao said. “Now we know we have to broaden our horizons when looking for early mushrooms.”

Reference: “Late Ediacaran crown-group sponge fauna” by Xiaopeng Wang, Alexander G. Liu, Zhe Chen, Chengxi Wu, Yarong Liu, Bin Wan, Ke Pang, Chuanming Zhou, Xunlai Yuan, and Shuhai Xiao, June 5, 2024. , Nature.
DOI: 10.1038/s41586-024-07520-y