Crustacea have the head and thorax joined into a cephalothorax and have legs with two branches. Arthropods are known from the earliest Cambrian, and crustacea soon afterwards.

Ostracods are microfossils typically tenths of millimetres long and are a class of the phylum Crustacea. They are shrimp-like arthropods living in a carapace comprising two oval valves hinged at the upper, dorsal, margin. This picture shows the side view of the right-hand valve. The ostracod, which has not yet been identified, comes from the Cretaceous Atherfield Clay exposed during building works in Maidstone, Kent, England.