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Brachyuran Taphonomy in a Modern Tidal-Flat Environment: Preservation Potential and Anatomical Bias

¹ Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061-0420; jstempie{at}vt.edu

The scarcity of fossil crabs compared to heavier calcified taxa implies either: (1) crab remains are rarely preserved (taphonomic hypothesis); (2) crab remains are frequently overlooked/misidentified (taxonomic hypothesis); or (3) crabs were less abundant in ancient ecosystems (ecological hypothesis). To evaluate the taphonomic hypothesis, the preservational potential of the yellow shore crab, Hemigrapsus oregonensis, was evaluated in a modern tidal pool of False Bay, San Juan Islands, Washington, USA. Crab remains were compared to molluscan taxa, which served as a taphonomic reference standard. The surface crab remains (4.2 parts per m²) displayed an anatomical bias: carapace and cheliped remains were more numerous than predicted, while leg remains were underrepresented. Crab remains often were disarticulated, but other alterations (bioerosion, dissolution, or encrustation) were virtually absent. This contrasts with non-crab material (bivalves, gastropods, and barnacles), which was abundant in the tidal pool (41 parts per m²), and dominated (94.7%) by highly taphonomic altered remains, suggesting long exposure at, or near, the surface. Ratios of non-crab to crab remains increased from 10: 1 at the surface to 154:1 in the subsurface. Low taphonomic alteration of crab remains, low density of surface crab material, an increase in non-crab to crab ratio in the subsurface, and a severe anatomical bias imply nearly complete loss of crab remains prior to burial. This suggests a low probability of preservation, especially when compared to co-occurring mollusks, providing quantitative support for the taphonomic hypothesis.