At first glance, sponges seem too simple to be mysterious.
They have no brains or guts, and scientists have long thought they originated about 700 million years ago. However, clear fossil evidence only goes back about 540 million years, leaving a puzzling gap in the record of 160 million years.
Fossils of the “lost years”
In a study published in a journal natureVirginia Tech geobiologist Shuhai Xiao and his colleagues describe a 550-million-year-old sponge fossil that falls squarely within this missing interval. The research team also proposes an important explanation for this gap. The earliest sponges may not have had mineral skeletons, making them much less likely to fossilize.
This idea helps resolve a long-standing contradiction in evolutionary science.
The mystery of lost sponge fossils
Using molecular clock estimates that track the accumulation of genetic mutations over time, scientists suggest that sponges first evolved about 700 million years ago. However, no convincing sponge fossils have been obtained from rocks from that era.
This disconnect has sparked a long-standing debate between zoologists and paleontologists.
New discoveries can help bridge that gap. This adds an important piece to the evolutionary history of Earth’s oldest animals and provides an explanation for why finding old fossils has been so difficult. It also connects to the question first posed by Darwin about when early animal life appeared.
Amazing discoveries along the Yangtze River
Xiao first encountered this fossil about five years ago, when a collaborator sent him a photo of a specimen excavated along the Yangtze River in China.
“I’ve never seen anything like this before,” said Hsiao, a faculty member in the School of Science. “Almost immediately I realized it was something new.”
Xiao, in collaboration with researchers from the University of Cambridge and the Nanjing Institute of Geology and Paleontology, began testing various possibilities. The fossil did not match any known features such as sea squirts, sea anemones, or corals. That leaves one intriguing possibility: an ancient sponge.
Why early cavernous bodies rarely fossilize
In an earlier study published in 2019, Hsiao and his team suggested that the first sponges may not have produced the hard, needle-like structures called spicules that are characteristic of modern sponges.
By examining the fossil record, researchers found that the needle-like crystals in cancellous bone became more mineralized over time. The deeper we looked, the more organic and less mineral-based structures appeared.
“If we extrapolate the other way around, the first living organisms were probably soft-bodied creatures with completely organic skeletons and no minerals,” Hsiao said. “If this were true, they would not survive fossilization except in very special circumstances where rapid fossilization outpaces decomposition.”
In late 2019, a research team identified such a rare case. They found the spongy fossils preserved in thin layers of marine carbonate rock known for trapping soft-bodied creatures, including some of the earliest animals that could move.
“Most of the time, these kinds of fossils are lost from the fossil record,” Hsiao says. “New findings provide clues about early animals, before hard parts developed.”
Unique patterns and unexpected sizes
The newly described fossil stands out for its detailed surface pattern. It is covered with a regular box-shaped grid, each of which is subdivided into smaller repeating units.
“This particular pattern suggests that fossilized sponges are most closely related to certain species of glass sponges,” said Xiaopeng Wang, a postdoctoral researcher at the Nanjing Institute of Geology and Paleontology and the University of Cambridge.
Its size also surprised researchers.
“When I was looking for early sponge fossils, I expected them to be very small,” said co-researcher Alex Liu from the University of Cambridge. “The new fossil is about 15 inches long and has a relatively complex conical body shape, challenging many of our expectations for the appearance of early sponges.”
Rethinking the search for early animal life
This discovery not only helps fill in some missing parts of the fossil record, but also changes the way scientists search for early life.
If the first sponges were soft and lacked mineral skeletons, many might have disappeared without a trace. This means researchers need to look beyond traditional fossil clues and focus on the rare conditions that may preserve delicate organisms.
“This finding indicates that the first corpora were probably spongy, but not glassy,” Hsiao said. “It turns out we need to broaden our horizons when looking for early sponges.”
