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Non-marine crabs began to live on land at least 100 million years ago, according to recently published research by evolutionary biologists, including Javier Luque, a researcher at the Harvard Museum of Comparative Zoology.

The researchers studied an amber fossil of a complete crab dating back to the Cretaceous Period, which began almost 150 million years ago. Amber fossils form when organisms are preserved in tree resin, indicating that crabs had migrated onto land by the time of this fossil’s formation.

Researchers had previously predicted that crabs began colonizing land at least 125 million years ago, but the earliest previously known fossil record of non-marine crabs only dated to roughly 75 million years ago — leaving a 50 million year gap.

Luque and his colleagues — including researchers from China University of Geosciences, Yale University, Lynn University, Yunnan University, The University of Regina and the Longyin Amber Museum — published their findings in Science Advances last month.

Luque said the fossil is the most complete fossil of a crab ever discovered.

“It is virtually complete with antennas, eyes, mouth parts, and even super tiny hairs all over the body,” Luque said. “Moreover, it has preservation of soft to lightly mineralized tissues that are seldom preserved, such as the lungs, and the gills inside the body.”

The Longyin Amber Museum acquired the fossil in 2015 after discovering it on the jewelry market. Luque said he was contacted about the discovery in 2018.

“So these researchers from China discovered it in the market, they recognized the importance, they rescue it, and they had it in their museum,” Luque said. “One of them contacted me because they knew I was studying the crab evolution and asked me what did I think and if I wanted to study it and that's why we started the project in 2018.”

The fossil is being studied as part of a larger project working to piece together the evolutionary history of crabs through the study of living crabs and fossils.

Luque said the Cretaceous period was a “crucial moment for the evolution of crabs,” as the species became a “bigger player in the evolution of life in the oceans and inland” by moving into new non-marine habitats.

“One of the things that this animal challenges is our understanding of how many times and in which groups the invasion into novel habitats has occurred,” Luque said.

Luque said this discovery is part of the “cretaceous crab revolution” — the mass expansion and species diversification of crabs that occurred a hundred million years ago.

“One of the things we want to see is why certain groups of crabs are so diverse and have hundreds of species, whereas a few other groups have only one species,” Luque said. “And the question is, ‘What can we predict about the evolution of crab forms through time based on their genes ecology?’”