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Natural History: ​500-Million-Year-Old Fossil Reveals Ancient Origins of Starfish and Sea Urchins

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10 July 2025
By James Hamilton

​A remarkable fossil unearthed in the mountains of Morocco is rewriting what we know about some of the ocean’s most unusual creatures.

Scientists from the Natural History Museum in London have discovered Atlascystis acantha, a 500-million-year-old fossil that could be the missing link in understanding how starfish, sea urchins, and their relatives evolved into the five-armed wonders we know today.
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Modern echinoderms, like starfish and sea cucumbers, have a unique fivefold symmetry that sets them apart from most other animals, including humans, who have bilateral symmetry (left and right sides that mirror each other). But how these marine invertebrates made the evolutionary leap from bilateral to radial symmetry has long been a mystery.

Enter Atlascystis acantha, the oldest known echinoderm fossil to display a bilateral body structure. This ancient creature bridges a crucial gap between the bilateral ancestors of echinoderms and the distinctive star-shaped species alive today.

“This fossil helps us understand, step-by-step, how the body plans of starfish and their relatives evolved,” explained Dr. Imran Rahman, Principal Researcher at the Natural History Museum and co-lead author of the study.

Discovered in Morocco’s Anti-Atlas mountains, Atlascystis had a flattened, spiny body and two specialized rows of skeletal plates - features seen in modern echinoderms that help them move and feed. These structures, called ambulacra, had never been clearly identified in such early bilaterally symmetrical forms, until now.

A New Chapter in Evolutionary Biology
Using advanced 3D imaging, growth studies, and evolutionary modeling, the research team uncovered a major twist in the story of echinoderm evolution.

Rather than being evolutionary side-branches, early bilateral echinoderms like Atlascystis were foundational, sitting right at the base of the echinoderm family tree. According to the study, today’s five-rayed starfish likely evolved later, as a result of duplicating one of these early ambulacra and losing a defined "head-to-tail" axis.

“The fossil record is our most powerful tool for tracing the history of life,” said Dr. Frances Dunn, senior researcher at Oxford University Museum of Natural History. “This discovery sheds light on the early steps that led to one of nature’s most iconic body plans.”

Dr. Jeff Thompson, a lecturer at the University of Southampton and co-author of the study, added, “By studying how Atlascystis grew, we were able to place it confidently within the evolutionary tree, offering vital clues to how these animals developed over time.”

Why This Matters
While genetics tells us how organisms function today, fossils like Atlascystis acantha give us the rare opportunity to look back in time and see how those organisms came to be. This discovery shows just how much the fossil record still has to offer in unlocking evolutionary mysteries.

The study, titled "A new Cambrian stem-group echinoderm reveals the evolution of the anteroposterior axis", is published in Current Biology. It’s part of the wider Evolution of Life research initiative, aimed at exploring the deep-time processes that have shaped life on Earth.

Credit: Samuel Zamora

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