Contributions to Zoology, 67 (4) 237-256 (1998)G. A. Bishop; R. M. Feldmann; F. Vega: The Dakoticancridae (Decapoda, Brachyura) from the Late Cretaceous of North America and Mexico

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The morphology of crabs results from complex patterns of development and adaptations to complex physical and biological conditions. Schäfer (1954; fide translation of 1976) categorized four groups of brachyuran decapods as: 1) "Laufers suf dem Boden," crabs which crawl and run on the substrate surface, 2) "des Grabens im Boden," crabs which burrow into the sediment, 3) "des Kletterns auf zerkluftetem Untergrund," crabs which climb on vertical, fissured, hard substrates, and 4) "des Schwimmens im freien Wasser," crabs which swim in open waters. Each has a characteristic body-plan enabling the crab to move, to feed, to reproduce, and to defend itself. The form of the body parts constrain the mode of life of the organism. Carapace shape and proportions, angle between sagittal plane and sternites, arrangement of sternites, lateral crossection, and other body aspects vary in a general way with mode of life.

The carapace of "Laufers," epifaunal crawlers, is equant, reflexed, with serrated or toothed front. The angle between the sagittal plane and the sternites is 76°. Sternites are somewhat radial relative to the sagittal plane leading to sideways movements of the legs. Lateral cross sections show these crabs have a convex bottom with the body embedded within the legs. Chela are always formed so as to fit against the cephalothorax. Walking legs become longer in deep sea forms and setose with the weight borne by folding the dactyli over forming "snow-shoes."

The pincers, or chelae, of crabs serve multiple functions and hence are highly variable in their size and morphology. Those with specific functions may have longitudinal shearing teeth, flat molariform crushing teeth, comb-like raking teeth, or club-like exterior projections for breaking into shells. Shäfer (1954) discussed the morphology of chelae of crabs and distinguished seven morphotypes of teeth. Sometimes obvious conclusions can be made about specific morphologies, but for most crabs the claws are generalized tools for many roles.