The modern definition of anomurans could be "pseudo-crabs", as the abdomen
is not greatly calcified and is almost always reduced and they
usually have enlarged chelae on the first pereiopods. Other features
used to distinguish anomurans are the fact that the third pereiopods
are always achelate and the pleon (while retaining a tailfan)
is held beneath the cephalothorax. They include (in order of the
images below) the porcelain crabs, squat lobsters and hermit crabs.
Anomurans are known from the Early Jurassic to the present day.
images are taken from www.nhm.org/guana/bvi-invt/bvi-surv/deca-inf.htm