A new study has revealed that a group of prehistoric sea creatures is not as ancient as we thought – their earliest fossils are actually seaweeds.
Research carried out by experts from Durham University, UK, and Yunnan University and Guizhou University in China found that the fossils, that were previously thought to be the oldest Bryozoans, are in fact green algae.
This means that Bryozoans – tentacle-bearing animals that lived in skyscraper-like underwater colonies – are millions of years younger than previously thought, only appearing in the Ordovician period (480 million years ago).
This makes them the only group of fossil animals not to appear in the Cambrian “explosion”, a rapid burst of evolution 40 million years earlier.
The delayed appearance of bryozoans shows that the Cambrian was not a unique period of innovation as conventionally thought; instead, new body plans continued to be carved out by evolution over a much longer time period.
The study findings have been published in the journal Nature.
Ancient fossil material discovered in the hills of China revealed previously unseen “soft parts” of Protomelission gateshousei, formerly believed to be the earliest Bryozoan.
This fragile tissue allowed the researchers to interpret Protomelission as a member of the green algal group Dasycladales.
Study co-author Dr Martin Smith, of the Department of Earth Sciences, Durham University, said: “We tend to think of the ‘Cambrian explosion’ as a unique period in evolutionary history, in which all the blueprints of animal life were mapped out.
“Most subsequent evolution boils down to smaller-scale tinkering on these original body plans. But if Bryozoans truly evolved after the Cambrian period, it shows that evolution kept its creative touch after this critical period of innovation – maybe the trajectory of life was not set in stone half a billion years ago.”
Study co-author Professor Zhang Xiguang, of Yunnan University, said: “Where previous fossils only preserved the skeletal framework of these early organisms, our new material revealed what was living inside these chambers.
“Instead of the tentacles we would expect to see in Bryozoans, we discovered simple leaf-like flanges – and realised we were not looking at fossil animals, but seaweeds. This means that the oldest convincing Bryozoan fossils do not evolve until the next geological period, the Ordovician.”
The researchers suggest whilst the origin of animal groups may not have been so sudden, humble seaweeds played a larger part in the early oceans than previously thought.
This study was supported by the National Natural Science Foundation of China.