This study provides new insights into the early evolution of animals.
Researchers led by Shuhai Xiao of Virginia Tech have discovered a 550-million-year-old marine sponge fossil, shedding light on a 160-million-year gap in the fossil record. The fossil suggests that early sponges lacked mineral skeletons, providing new insights into the evolution of one of the most ancient animals and influencing the way paleontologists search for ancient sponges.
At first glance, simple sponges aren’t mysterious creatures: they have no brains or guts, and they can easily date back 700 million years. But convincing sponge fossils date back only to about 540 million years ago, a gap of 160 million years in the fossil record.
In a paper published in the journal on June 5, NatureVirginia Tech geobiologist Shuhai Xiao and colleagues report a “lost era” of sponges dating back 550 million years, proposing that the earliest sponges had not yet developed mineral skeletons and providing a new parameter for the search for lost fossils.
The mystery of the missing sponges is fraught with contradictions: Molecular clock estimates, which measure the number of genetic mutations that accumulate over time, suggest that sponges must have evolved about 700 million years ago, but no convincing sponge fossils have been found in rocks that old.
This conundrum has been a subject of debate among zoologists and paleontologists for many years.
This latest discovery fills out the evolutionary tree of one of the oldest animals, explains its apparent absence in older rocks, and connects the dots on Darwin’s question of when it evolved.
Xiao’s breakthrough discovery
Xiao, a recent inductee into the National Academy of Sciences, first saw the fossil five years ago, when a collaborator texted him an image of a specimen unearthed along the Yangtze River in China. “I’d never seen anything like it,” says Xiao, a faculty member in the School of Science. “I knew right away that this was something new.”
One by one, Xiao and his collaborators from the University of Cambridge and the Nanjing Institute of Geology and Palaeontology began to rule out the possibilities that it was not a sea squirt, sea anemone, or coral. They thought it might be the elusive ancient sponge.
In a previous study published in 2019, Xiao and his team proposed Early sponges did not leave fossils because they had not yet developed the ability to produce the hard, needle-like structures called spicules that are characteristic of today’s sponges.
Xiao and his team traced the evolution of sponges through the fossil record: As they went back in time, sponge spicules became increasingly organic in composition and less mineralized.
“If we go back, the first organisms were probably mollusks with entirely organic skeletons and no minerals,” Xiao said. “If this were true, they would not have been able to survive fossilization unless there were very specific conditions where rapid fossilization outpaced degradation.”
In late 2019, Xiao and an international research group discovered a fossil sponge preserved in just such a setting: a thin layer of marine carbonate rock known for preserving a wealth of mollusks, including The earliest moving animals.
“Most of the time, these types of fossils are lost from the fossil record,” Xiao says, “so this new discovery gives us a window into what early animals were like before they developed hard parts.”
New fossil discoveries and their meaning
The surface of the new sponge fossil is studded with a complex array of regular boxes, each divided into smaller, identical boxes.
“This particular pattern suggests that the fossilized sponges are most closely related to a particular species. seed “It’s like a glass sponge,” says Xiaopeng Wang, a postdoctoral researcher at the Nanjing Institute of Geology and Palaeontology and the University of Cambridge.
Another unexpected thing about the new sponge fossil is its size. “When I was looking for early sponge fossils, I expected them to be very small,” says study collaborator Alex Liu from the University of Cambridge. “The new fossil is about 15 inches long and has a relatively complex, cone-shaped body shape, which overturns many of our expectations for what early sponges looked like.”
As well as filling in some of those lost years, the fossils also provide researchers with important guidance on how to search for these fossils, which will hopefully improve our understanding of the evolution of early animals further back in time.
“This discovery tells us that the first sponges were probably spongy but not glassy,” Xiao said. “We now know we need to broaden our search for early sponges.”
Reference: “Late Ediacaran crown group sponges”, Xiaopeng Wang, Alexander G. Liu, Zhe Chen, Chengxi Wu, Yarong Liu, Bin Wan, Ke Pang, Chuanming Zhou, Xunlai Yuan, Shuhai Xiao, 5 June 2024, Nature.
DOI: 10.1038/s41586-024-07520-y