Eocarcinus praecursor

Eocarcinus praecursor is a Jurassic species of decapod crustacean, sufficiently distinct from its relatives to be placed in its own family (Eocarcinidae). Once considered the oldest true crab, it is now considered to be an early member of the Anomura.

It lived during the Pliensbachian age (Lower Jurassic), and has been found in rocks at two sites in the United Kingdom – Mickelton Tunnel (near Aston Magna), Gloucestershire and Runswick Bay, Yorkshire.

In many of its characters, it represents a transitional stage between the Glypheoidea and the Middle Jurassic crabs in the Prosopidae. Since its ancestors were long-tailed decapods, and its successors were short-tailed crabs, Eocarcinus has been described as "the lobster who decided to become a crab". Previously considered to be the oldest known true crab, a 2010 revision concluded that Eocarcinus could not be accommodated among the Brachyura, and was instead transferred to the Anomura.