- A new discovery from the Isle of Wight reveals fossilised embryos preserved within the gills of ancient freshwater molluscs with two shells, known as bivalves
- The embryos show that these animals were already protecting and incubating their young within their shells during the age of dinosaurs
- Dating from the Cretaceous Period, this discovery provides the oldest known fossil evidence of maternal care in shellfish such as clams, mussels and oysters
A complete specimen of one of the Cretaceous fossils
An international team of scientists has uncovered the oldest known evidence of maternal care in shellfish, revealing that some freshwater species were protecting and incubating their young more than 125 million years ago.
Published today in Scientific Reports , the new research found fossilised soft tissues preserved inside ancient shells. This is a remarkable discovery because such tissues normally decay soon after an animal dies.
Among these tissues were microscopic embryos and larvae preserved within the gills.
Dr Martin C Munt is a visiting academic at the University of Portsmouth and a specialist in fossil molluscs. He said: "This is the earliest known fossil evidence that these shellfish cared for and protected their developing young. Until now, this reproductive strategy was known only from living species.
"Not only does this discovery provide a rare glimpse into how ancient freshwater shellfish reproduced, but it also helps explain how these animals successfully adapted to life in rivers and lakes millions of years ago.
"For decades, scientists have searched for direct evidence of reproduction in ancient freshwater mussels, making this find a significant breakthrough in understanding their evolutionary history."
This is the earliest known fossil evidence that these shellfish cared for and protected their developing young. Until now, this reproductive strategy was known only from living species.
Dr Martin C Munt , Specialist in fossil molluscs
The study documents an exceptionally preserved reproductive system in Margaritifera valdensis, a freshwater bivalve distantly related to modern freshwater pearl mussels. It was found in fossil deposits on the Isle of Wight in the United Kingdom, a site famous for its rich Cretaceous-age fossils, including dinosaurs such as Iguanodon.
The fossil preserves not only the shell, but also delicate internal structures associated with reproduction, including brood chambers, gill tissues, mineralised supports, and different stages of embryonic and larval development.
The international team of researchers was led by Dr Graciela Delvene of the Geological and Mining Institute of Spain (CSIC).
Modern freshwater mussels exhibit one of the most remarkable reproductive cycles among invertebrates. Their larvae initially develop within the maternal gills before being released to temporarily parasitise fish, a necessary stage in completing their life cycle.
Thin section of one of the "pregnant" fossil bivalves from the Isle of Wight
Dr Aleksandra Skawina, a Polish researcher on the team who is a specialist in fossil bivalves, said: "These new fossils demonstrate that this complex reproductive strategy had already evolved by the Early Cretaceous."
The researchers found evidence of several stages of development, from the earliest embryo-like cells to more advanced larvae. They also discovered clues showing how these ancient shellfish provided calcium to their developing young. Tiny mineral deposits preserved within the gills appear to have acted as calcium stores, helping the larvae build their shells, much like modern freshwater mussels do today.
The study also sheds new light on a mysterious dark material known as "molluskite", first described in the nineteenth century by the famous British palaeontologist Gideon Mantell.
"We found that this material is actually made up of fossilised soft tissues and reproductive structures that have been exceptionally preserved by minerals," said Rafael P. Lozano, a geochemist at the Geological and Mining Institute of Spain (CSIC).
Today, freshwater mussels are among the most threatened groups of animals on Earth. These fossils therefore offer a unique window into the origins and long history of a group that plays a vital role in freshwater ecosystems but now faces an uncertain future.
The research was carried out by an international team of scientists from the University of Portsmouth (UK), the University of Warsaw (Poland) and the Geological and Mining Institute of Spain (CSIC). The study is published today in Scientific Reports: Exceptionally preserved embryos reveal maternal care in freshwater bivalves since the Cretaceous | Scientific Reports
