The groundbreaking discovery of a 550-million-year-old marine sponge fossil offers new insights into the sponge’s evolution and leads to future fossil searches. Reconstructed life position of Helicolocellus on the Ediacaran seafloor. Credit: Yuan Xunlai
The research offers 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 that sheds light on a 160-million-year gap in the fossil record. This fossil, which suggests that early fungi lacked mineral skeletons, provides new insight into the evolution of one of the oldest animals and influences how paleontologists search for ancient fungi.
At first glance, a simple sea sponge is not a mysterious creature. Without brain. No gut. There is no problem dating it back 700 million years. Still, conclusive fossils of the fungus only go back about 540 million years, leaving a gap of 160 million years in the fossil record.
In an article published 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 suggest that the oldest sea sponges have not yet developed mineral skeletons, offering new parameters for searching for missing fossils.
The mystery of the missing sea sponges centered on a paradox. Molecular clock estimates, which involve measuring the number of genetic mutations that accumulate over time, suggest that fungi must have evolved about 700 million years ago. And yet, no conclusive fossils of the fungus have been found in such old rocks.
For years, this puzzle has been the subject of debate between zoologists and paleontologists.
This latest discovery fills in the evolutionary family tree of one of the oldest animals, explains its apparent absence from older rocks, and connects the dots back to Darwin’s questions about when it evolved.
Xiao’s breakthrough discovery
Xiao, who was recently inducted into the National Academy of Sciences, first saw the fossil five years ago when a co-worker texted him a picture of a specimen dug up 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 it was something new.
Xiao and collaborators from the University of Cambridge and the Nanjing Institute of Geology and Paleontology began ruling out the possibilities one by one: not a sea squirt, not a sea anemone, not a coral. Did it occur to them that it might be an elusive ancient sea sponge?
Virginia Tech geobiologist Shuhai Xiao and his colleagues have announced a 550-million-year-old marine sponge fossil that fills a gap in the evolutionary family tree of one of the oldest 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 evolve the ability to form the hard, needle-like structures known as spicules that characterize marine sponges today.
Xiao’s team members followed the evolution of the fungus through the fossil record. As they progressed through time, the fungal spicules became increasingly organic in composition and less mineralized.
“If you extrapolate back, then maybe the first of them were soft-bodied creatures with a completely organic skeleton and no minerals at all,” Xiao said. “If that were true, they would not have survived fossilization except in very special circumstances where rapid fossilization overtook degradation.”
Later in 2019, Xiao’s international research team found a sponge fossil preserved in just such circumstances: thin beds of marine carbonate rocks known to preserve abundant soft-bodied animals, including some of the earliest mobile animals.
“Most often, this type of fossil would be lost in the fossil record,” Xiao said. “The new finding offers a window into early animals before they developed hard parts.”
The discovery of new fossils and its consequences
The surface of the new mushroom fossil is dotted with a complex array of regular boxes, each of which is divided into smaller, identical boxes.
“This specific pattern suggests that our fossilized sea sponge is most closely related to a certain one.” species glass sponges,” 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 looking for fossils of early fungi, I expected them to be very small,” said Alex Liu, a collaborator at the University of Cambridge. “The new fossil is about 15 inches long with a relatively complex, cone-shaped body plan, challenging many of our expectations about the appearance of early fungi.”
While the fossil fills in some of the missing years, it also provides researchers with important guidance on how to look for these fossils—hopefully expanding understanding of early animal evolution further back in time.
“The discovery suggests that perhaps the first fungi were spongy but not glassy,” Xiao said. “Now we know we have to broaden our view when looking for early fungi.”
Link: “Sponge from the Late Ediacaran Group” by Xiaopeng Wang, Alexander G. Liu, Zhe Chen, Chengxi Wu, Yarong Liu, Bin Wan, Ke Pang, Chuanming Zhou, Xunlai Yuan and Shuhai Xiao, 5 Jun 2024, Nature.
DOI: 10.1038/s41586-024-07520-y