Classifying the extinct ones is tough because the classification of modern ones is based on appendages, which are usually missing in the fossils because they are not heavily calcified. One of the most interesting modifications of the arthropod body plan is by the crabs. Their abdomen is reduced in size and folded underneath their thorax. This protects the gills from clogging by sediment and allows crabs to live in muddier environments and to burrow quickly. The degree of calification of the exoskeleton is correlated with how quickly the animal moves.
Faster ones are more lightly calcified.
The decapods appear in the Triassic. These are heavily calcified and are opened to allow the animal to extend its appendages for feeding, locomotion and breeding. The carapace is usually kidney or bean shaped and about 3 mm long. The appendages of the cephalic part assist in locomotion and those of the thorax assist in feeding. They lack abdominal appendages altogether.