Oldest Fossils of Mysterious Prehistoric Creature Aren’t What Scientists Thought

Mineralised Fossils of Protomelission gatehousei

Mineralized fossils of Protomelission gatehousei from Wirrealpa, Australia. Credit: Reproduced from Zhang et al. (Nature, 2021)

A recent study has revealed that the origins of a prehistoric group of sea creatures are not as ancient as previously thought. The earliest fossils of these creatures have been determined to be seaweeds.

Experts from Durham University in the UK and Yunnan University and Guizhou University in China conducted research and discovered that the fossils, previously believed to be the oldest Bryozoans, are actually green algae.

This implies that Bryozoans, which were tentacle-bearing animals that lived in underwater colonies resembling skyscrapers, are millions of years younger than previously estimated and only emerged during 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.

Bryozoan Field Site

Collecting fossils from the Xiaoshiba biota field locality. Credit: Yang Jie

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.

New Fossils of Protomelission From the Xiaoshiba biota

New fossils of Protomelission from the Xiaoshiba biota, showing attachment of the alga to a brachiopod shell. Credit: Zhang Xiguang

“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 realized 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.

Reference: “Protomelission is an early dasyclad alga and not a Cambrian bryozoan” by Jie Yang, Tian Lan, Xi-guang Zhang and Martin R. Smith, 8 March 2023, Nature.
DOI: 10.1038/s41586-023-05775-5

The study was funded by the National Natural Science Foundation of China.

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