The Cretaceous crab revolution was a major diversification event of brachyuran crustaceans (also known as true crabs) that took place during the Cretaceous period, from 145 to 66 million years ago.
[4] The oldest known true crabs are Eoprosopon klugi and Eocarcinus praecursor from the Early to Middle Jurassic.
[7][8] At the same time, many families of crabs evolved away from the carcinized body plan, termed decarcinization.
[1] This rapid diversification allowed true crabs to inhabit many different environments including fresh water environments, coral reefs, swimming within the water column, and many others.
Specialized shell-breaking crab claws in cretaceous seas | biology ...