A recent discovery could upend our understanding of the evolution of early animal life forms. Researchers have unearthed a sea sponge fossil dating back 550 million years, revealing crucial clues about sponge evolution and filling a 160-million-year gap in the fossil record.
Holotype of Helicolocellus, photographed under reflected light.
Credit: Yuan Xunlai
The team, led by Shuhai Xiao from Virginia Tech, unveiled this fossil, suggesting that early sponges lacked mineral skeletons, thereby influencing paleontologists' research methods.
The apparent simplicity of marine sponges hides a puzzle. Although molecular clocks indicate an emergence around 700 million years ago, sponge fossils older than 540 million years had been elusive, leaving an unexplained gap.
In an article published on June 5 in
Nature, Shuhai Xiao and colleagues described their discovery of a 550-million-year-old sea sponge. This find indicates that early sponges had not yet evolved mineral skeletons, redefining the criteria for searching for missing fossils.
Estimates based on molecular clocks posed a paradox: how could sponges exist without leaving fossil traces? This fossil fills that gap.
Reconstructed life position of Helicolocellus on the Ediacaran seafloor.
Credit: Yuan Xunlai Shuhai Xiao was intrigued by a photo of a specimen found in China. Together with colleagues from the University of Cambridge and the Nanjing Institute of Geology and Palaeontology, they ruled out other possibilities, culminating in the identification of an ancient marine sponge.
In a 2019 study, Shuhai Xiao and his team suggested that early sponges, lacking mineral spicules, left no fossils except in rapid fossilization conditions that outpaced degradation.
This unique fossil, measuring 15 inches (38 centimeters) long, features complex structural patterns, seeming to be related to present-day glass sponges, expanding our understanding and guiding future research.