Virginia Tech scientists have proposed the absence of a mineral skeleton as an explanation for the 160-million-year gap in the marine sponge fossil record.
At first glance, the simple sea sponge is not a mysterious creature. It has no brain or intestines. There is no problem dating it to 700 million years ago. However, convincing sponge fossils only date back about 540 million years.
In a paper published June 5 in the journal Nature, Virginia Tech geobiologist Shuhai Xiao and his collaborators report on a 550-million-year-old marine sponge and conclude that the former had not yet developed mineral skeletons, offering new parameters. for the search for lost fossils.
The mystery of the lost sea sponges centered on a paradox.
Molecular clock estimates, which involve measuring the number of genetic mutations that accumulate over time, indicate that sponges must have evolved about 700 million years ago. And yet, no convincing sponge fossils had been found in such ancient rocks.
For years, this enigma was a topic of debate among zoologists and paleontologists.
This latest discovery completes the evolutionary family tree of one of the first animals, explaining its apparent absence in older rocks and connecting the dots with Darwin’s questions about when it evolved.
Xiao first saw the fossil five years ago, when a collaborator texted him a photo of a specimen excavated along the Yangtze River in China.
SOMETHING NEW
“I have never seen anything like it before,” Xiao, a faculty member in the College of Science, said in a statement. “Almost immediately, I realized it was something new.”
Xiao and collaborators at the University of Cambridge and the Nanjing Institute of Geology and Paleontology began ruling out possibilities one by one: It wasn’t a sea squirt, it wasn’t a sea anemone, it wasn’t a coral. They wondered if it could be an ancient and elusive sea sponge.
In a previous study published in 2019, Xiao and his team suggested that the first sponges did not leave behind fossils because they had not evolved the ability to generate the hard, needle-like structures, known as spicules, that characterize today’s marine sponges.
Members of Xiao’s team traced the evolution of sponges through the fossil record. As they went back in time, the sponge spicules became more organic in composition and less mineralized.
“If you extrapolate back, then maybe the first ones were soft-bodied creatures with completely organic skeletons and no minerals at all,” Xiao said. “If this were true, they would not survive fossilization except in very special circumstances where rapid fossilization outweighs degradation.”
Later in 2019, Xiao’s international research group found a sponge fossil preserved in just such a circumstance: a thin bed of marine carbonate rocks known to preserve abundant soft-bodied animals, including some of the first mobile animals.
“Most of the time, this type of fossil would be lost in the fossil record,” Xiao said. “The new find offers a window into early animals before they developed hard parts.”
The surface of the new sponge fossil is dotted with an intricate series 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 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 fossils of primitive sponges, I expected them to be very small,” said Alex Liu, a collaborator at the University of Cambridge. “The new fossil measures about 38 centimeters long and has a relatively complex conical body plan, which challenged many of our expectations about the appearance of primitive sponges.”
While the fossil fills in some of the missing years, it also provides researchers with important guidance on how to search for these fossils, which will hopefully expand understanding of early animal evolution further back in time.
“The discovery indicates that perhaps the first sponges were spongy, but not glassy,” Xiao said. “We now know that we need to broaden our vision when looking for primitive sponges.”
