DIFFERENTIAL PRESERVATION OF CRABS (BRACHYURA, RANINOIDA) IN ASH-RICH CONCRETIONS FROM THE EOCENE OF WEST COASTAL USA

Two species of frog crab (Brachyura: Raninoida: Raninidae, Lyreididae) from the west coast of North America were examined. One is Raninoides vaderensis Rathbun, 1926, from the Eocene Hoko River Formation, Washington, USA. The second crab is Lyreidus alseanus Rathbun, 1932, from the Eocene Alsea Formation in Oregon. Both crabs are preserved in volcanic ash-rich concretions. However, the Hoko River crab is encased in a finely compacted gray siltstone with ash, whereas the other is surrounded by iron-stained and heavily banded ash. We hypothesize, based upon previous work, that ash greatly delays the decay process because of how fine it is, completely encasing the organisms and obstructing pore spaces where decay material may escape. Thin sections of the specimens will be analyzed to observe ash and cuticle structure. A scanning electron microscope (SEM) and Energy Dispersive Spectroscopy (EDS) will be used to determine chemical differences between the two samples. In addition, the degree and type of preservation and what body parts of the crabs remain can help determine how useful volcanic ash is in fossilizing an organism. These specimens will be compared against other crabs fossilized in both ash and non-ash bearing concretions to note any similarities and differences in preservation styles. Preliminary analysis of whole concretions and multiple thin sections has determined that the concretion surrounding the Alsea crab is extremely light and its cuticular remains are very small. The Hoko River crab concretion was found to have fecal pellets throughout, an interesting find that will lead to a possible connection to how well ash fossilizes organisms and their traces.