Discovered by fossil hunter Jonathan Bow on Lavernock Beach, the find turned out to be part of a rare fossil jaw containing developing teeth.
The specimen is now helping researchers uncover new details about fish that lived more than 200 million years ago.
Cindy Howells, curator of geology at Amgueddfa Cymru, said: “Many teeth from Saurichthys were once mistaken for those of an amphibian, and that misunderstanding continued well into the 20th century.
“This lovely little jaw caused great confusion when it was first found as it bore no resemblance to any known species.”
For generations, fossil teeth found in rocks around the Bristol Channel have been difficult to identify. Some early scientists even believed they belonged to giant amphibians rather than fish, due to their unusual shape.
Researchers from Amgueddfa Cymru, the University of Bristol, and the State Museum of Natural History Stuttgart worked together to study the fossil.
Their research revealed that fossils previously grouped under one name actually belonged to two different species of fish from the Late Triassic period.
Jacob Quinn, lead researcher, said: “These rocks were formed during storms more than 200 million years ago, so it is remarkable that such a delicate fossil survived.
“Although British Triassic fossils have been studied for over 200 years, this is the first bone from Saurichthys ever found in these rocks.”
The discovery has led to the publication of a scientific paper sharing the team’s findings.
Professor Mike Benton, a co-author, said: “This delicate fossilised jaw has now been compared with specimens in museum collections across Britain and Europe to provide clear evidence that fossils originally thought to belong to the same fish actually came from two separate species.”
The fossil was sent to the University of Bristol for further study.
Dr David Whiteside, another co-author, said: “These predators lived in a shallow tropical sea filled with small limestone islands.
“Saurichthys was probably a fast and agile hunter, while Birgeria was larger and more powerful, likely feeding on a wider variety of prey.”
Dr Sam Cooper, also a co-author, said: “These fish have a remarkably complicated history.
“It’s amazing that a single fossil, found by Jonathan Bow during a spring tide, has helped clear up nearly 200 years of confusion.”
The fossil will go on display at National Museum Cardiff later this year.
Ms Howells said: “Being able to share this incredible discovery with the public will capture people’s imagination and hopefully inspire the next generation of fossil hunters to explore the amazing finds that can be discovered along Wales’ coastline.”
