Rotadiscus grandis lived 518 million years ago during the Cambrian period.

Rotadiscus grandis lived 518 million years ago during the Cambrian period. Image © Holly Sullivan

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New fossils could provide missing early evolution puzzle piece

Analysis of well-preserved Rotadiscus fossils sheds light on the origin and evolution of major groups alive today.

Scientists have described new specimens of the enigmatic animal Rotadiscus from an exceptionally preserved fossil assemblage in China. Their analysis revealed new anatomical details that could be key to understanding how chordates – the major group that includes humans – evolved. 

The fossils date to the Cambrian, a key geological period beginning approximately half a billion years ago. This was a pivotal point in the evolution of life which saw the emergence of nearly all the major groups of animals seen today. 

Following their research, scientists, including Dr Imran Rahman, Principal Researcher at the Natural History Museum and Dr Frankie Dunn, Research Fellow at Oxford University Museum of Natural History, have classified Rotadiscus and its closest fossil relatives as early members of an animal group called the Ambulacraria, which also includes modern invertebrates like starfish, sea urchins and acorn worms. 

Ambulacraria sit within the superphylum Deuterostomia, as do Chordata (the group humans and other vertebrates belong to).

Deuterostomes are one of the most widely divergent groups of animals – including starfish, pandas, crows, snakes and humans – making it difficult to predict ancestral characters. As a result, the origin and earliest evolution of the group has long remained ambiguous. 

The analysis revealed that Rotadiscus exhibited a combination of ambulacrarian and chordate characteristics, including a previously unknown double spiral structure. Up to this point, there had been competing theories on how to classify Rotadiscus. 

Dr Iman Rahman, Principal Researcher at the Natural History Museum, and lead author of the study, said: ‘This is the first time we have been able to confidently place a group of fossils as early ambulacrarians, filling an important gap in the tree of life.  

‘This research demonstrates once again the vital role ancient fossils play in mapping the evolutionary journey of animals still alive today – including the major group that humans belong to.’ 

It also allowed the scientists to conclude that the post-anal tails of chordates and hemichordates were acquired independently. 

Dr Frankie Dunn, Research Fellow at Oxford University Museum of Natural History, said: ‘Work on these fossils helped us to resolve a palaeontological mystery. Rotadiscus sheds light on the origin of major animal groups today, including starfish, and tells us what their ancestor may have looked like more than half a billion years ago.’

The paper Cambrian stem-group ambulacrarians and the nature of the 2 ancestral deuterostome is published in Current Biology. 


Notes to editors

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