True crabs (known as Brachyurans) stand in contrast to “false crabs” (called Anomurans) that aren’t technically crabs but are still sometimes called by the name (think hermit crabs or king crabs).
Previous fossil records, which mainly consist of bits and pieces of claws, suggested that nonmarine crabs came onto land and into freshwater about 75 to 50 million years ago. This new discovery pushes that back to at least 100 million years ago, answering Luque’s initial question of what this crab was doing in the jungle and bringing the fossil record in line with long-held theories on the genetic history of crabs.
“If we were to reconstruct the crab tree of life — putting together a genealogical family tree — and do some molecular DNA analysis, the prediction is that nonmarine crabs split from their marine ancestors more than 125 million years ago,” Luque said. “But there’s a problem because the actual fossil record — the one that we can touch — is way young at 75 to 50 million years old … So this new fossil and its mid-Cretaceous age allows us to bridge the gap between the predicted molecular divergence and the actual fossil record of crabs.”
The researchers now believe that what is known as the Cretaceous Crab Revolution —when crabs (true or not) diversified worldwide and started evolving their characteristic, crabby-looking body forms — was not a one-time event, as previously thought. This new research brings the tally of when different crab species independently evolved to live outside their marine habitat to at least 12 separate times.