EVALUATION OF THE PRESERVATION POTENTIAL OF THE YELLOW SHORE CRAB, HEMIGRAPSUS OREGENENSIS, IN TIDAL FLAT ENVIRONMENTS

Brachyurans have a fossil record dating back to the Jurassic and occur today within a wide array of ecological niches. However, the brachyuran fossil record is limited to occasional claw remains and rare concretions. The scarcity of fossil crabs implies that either 1) crab material is highly susceptible taphonomic filtration, 2) that fossil crab remains have not been properly identified, or 3) that crabs were less abundant in ancient ecosystems. This study investigates preservation potential of the yellow shore crab, Hemigrapsus oregenensis, in a modern tidal flat environment. This was done by comparing crab and molluscan surface material using surface and subsurface samples across a tidal pool in False Bay, San Juan Islands. Crab material was collected along a 2-meter wide transect, 60 meters long. The transect was resampled one week later to estimate the rate of input of bioclastic material. Four subsurface samples were collected to a depth of 30 cm below the sediment-water interface at regular intervals. These were wet sieved and all macroscopically recognized molluscan, brachyuran, and other biological material were recovered.

Different durability properties of crab parts create a bias already in the surficial assemblage, as durable parts such as sections of the cheliped were the most abundant on the surface (4.3 parts/m²). The surface crab material was often disarticulated, but no surface alteration was seen. The amount of material brought into the tidal pool within a week is estimated at 3 parts/ m². Low taphonomic alteration and density of surface material despite a high input, indicate a high turnover rate of crab remains. This contrasts with the molluscan material that is abundant (41 parts/ m²) and often displays high surface alteration. Within the subsurface samples a total of three cheliped parts were found, while an individual sample on average consisted of 106 bivalve remains. The extreme scarcity of crab remains seen within the subsurface collection suggests that crab material is removed quickly at the surface before it can be buried. Preliminary results suggest the probability of crab material being incorporated below the sediment surface in tidal flat settings is extremely low relative to mollusks and points to an overall low probability of brachyuran remains entering the fossil record.