Crustaceans thumb|Eryon, a polychelidan decapod crustacean from the Late Jurassic of Germany. The Jurassic was a significant time for the evolution of decapods. The first true crabs (Brachyura) are known from the Early Jurassic, with the earliest being Eocarcinus praecursor from the early Pliensbachian of England, which lacked the crab-like morphology (carcinisation) of modern crabs, and Eoprosopon klugi from the late Pliensbachian of Germany, which may belong to the living family Homolodromiidae. Most Jurassic crabs are known only from carapace pieces, which makes it difficult to determine their relationships. While rare in the Early and Middle Jurassic, crabs became abundant during the Late Jurassic as they expanded from their ancestral silty sea floor habitat into hard substrate habitats like reefs, with crevices in reefs providing refuge from predators. Hermit crabs also first appeared during the Jurassic, with the earliest known being Schobertella hoelderi from the late Hettangian of Germany. Early hermit crabs are associated with ammonite shells rather than those of gastropods. Glypheids, which today are only known from two species, reached their peak diversity during the Jurassic, with around 150 species out of a total fossil record of 250 known from the period. Jurassic barnacles were of low diversity compared to present, but several important evolutionary innovations are known, including the first appearances of calcite shelled forms and species with an epiplanktonic mode of life.
Crustaceans thumb|Eryon, a polychelidan decapod crustacean from the Late Jurassic of Germany. The Jurassic was a significant time for the evolution of decapods The first true crabs (Brachyura) are known from the Early Jurassic, with the earliest being Eocarcinus praecursor from the early Pliensbachian of England, which lacked the crab-like morphology (carcinisation) of modern crabs, and Eoprosopon klugi from the late Pliensbachian of Germany, which may belong to the living family Homolodromiidae. Most Jurassic crabs are known only from carapace pieces, which makes it difficult to determine their relationships. While rare in the Early and Middle Jurassic, crabs became abundant during the Late Jurassic as they expanded from their ancestral silty sea floor habitat into hard substrate habitats like reefs, with crevices in reefs providing refuge from predators. Hermit crabs also first appeared during the Jurassic, with the earliest known being Schobertella hoelderi from the late Hettangian of Germany. Early hermit crabs are associated with ammonite shells rather than those of gastropods. Glypheids, which today are only known from two species, reached their peak diversity during the Jurassic, with around 150 species out of a total fossil record of 250 known from the period. Jurassic barnacles were of low diversity compared to present, but several important evolutionary innovations are known, including the first appearances of calcite shelled forms and species with an epiplanktonic mode of life.