A tiny crab is trapped in this lump of amber from Myanmar. It remained almost unchanged for 100 million years – and is now turning out to be an important testimony to the evolution of crabs
They crawl sideways over the silt of the Wadden Sea or can be seen in the shallow water of the beach: Today we mostly encounter crabs on the ocean beach, most of the species of this cancer group are sea creatures. But there are exceptions: some of these short-tailed crabs (brachyura) are so well adapted to rural life that they spend almost their entire adult life on dry land. This is possible because their gills are encased in a sack-like cover that turns them into a kind of lung.
But when did the first crabs begin to leave their original ocean habitat and also to colonize fresh water or even dry land? So far there has been conflicting data on this. Molecular dating based on genetic comparisons suggest that the first crabs left the sea 130 million years ago and adapted to a way of life in freshwater and on land. But the oldest fossil finds of such crabs to date are only around 50 million years old.
The tiny cancer trapped in this lump of amber is now closing this gap. The crab, just a few millimeters in size, is already 100 million years old and therefore much older than previous fossil records of such crustaceans. Nevertheless, the fossil named Cretapsara athanata is strikingly similar to modern crabs and belongs to the group of more highly developed real crabs. “This is the oldest find of a real crab in amber,” report Javier Luque from Yale University and his colleagues.
The fact that this little crab was once enclosed in tree sap also reveals that this crab did not live in the sea, but already on land or at least in shallow water near trees. Unlike modern land crabs, Cretapsara athanata still had real gills and no lungs. “It is obviously an animal that is not marine, but not yet completely terrestrial,” says Luque. This amber fossil is therefore an important link in the crab evolution, as it proves that this group of animals left the sea 100 million years ago.