Modem crabs are common inhabitants of shallow subtidal, intertidal, and supratidal environments, and many crabs are capable of producing traces that can be preserved in the rock record. The first crabs, Early Jurassic in age, probably were not fossorial. By Cretaceous time, however, diverse endobenthic lineages were established. Many representatives of these lineages undoubtedly produced domiciles that are preserved in shallow marine to quasimarine sediments and that should be useful in characterizing the depositional environment of the sediments. Nonetheless, most such dwelling structures have been studied little and remain essentially unnamed.
The ichnogenus Psilonichnus Fiirsich is amenable to the taxonomic concept of several forms of crab burrows; presently recognized ichnospecies include P. tubiformis Fiirsich and P. upsilon (n. ichnosp.). Future work may reveal the need for further ichnospecific differentiation. The occurrence of Psilonichnus upsilon and related burrow forms should prove to be a useful criterion for the identification of marine-margin facies in the rock record.
Certain crabs also produce domiciles referable to Thalassinoides, Gyrolithes, and Skolithos, and possibly Macanopsis and Spongeliomorpha. Except for Skolithos, such structures traditionally have been attributed to shrimp, lobsters, or stomatopods. Ethologic and taxonomic re-evaluation of these burrow forms is needed
