Abstract

Fossil crayfish burrows exhibiting simple to complex architectural morphology from Paleocene (about 65 to 55 million years ago) alluvial deposits in the Washakie, Great Divide, and Hanna Basins, Wyoming, USA, contribute to reconstructions of the depositional environment and regional water table levels. The architectural and surficial burrow morphologies are similar to those of modem crayfish burrows. Simple burrows are generally vertical with diameters of 20 mm to 94 mm and lengths of 915 mm to 3048 mm. The upper portions of these burrows have multiple openings that converge downward to a central shaft. In contrast, complex burrows exhibit central chambers with multiple openings to the surface and one or more shafts that extend downward. Diameters of complex burrows are comparable to burrows with relatively simple architecture and exhibit chamber diameters from 100 mm to 500 mm. Surficial burrow morphologies of both burrow types display scrape and scratch marks, knobby textures, and mud-liners. Relatively shallow, complex burrows (Type 1) are assigned to the ichnogenus Cambarygma symplokonomos Hasiotis & Mitchell 1993. Extremely long, vertical burrows (Type IIIB) are assigned to the ichnogenus C. eumekenomos Hasiotis & Mitchell 1993. Burrows intermediate in architecture, quasi-vertical in orientation (Type IIIA), and much rarer in occurrence are assigned to the ichnogenus C. araioklados Hasiotis & Mitchell 1993. The burrow length and architecture were formed in response to the seasonal depth and fluctuation of the regional water table in the Washakie Basin. The presence of crayfish burrows correlates with the distribution of non-coal-bearing deposits, whereas the absence of burrows correlates with the distribution of coal-bearing deposits in the Washakie Basin. Non-coal-bearing sandstones, siltstones, and mudstones contain intensely burrow horizons that indicate regionally lower water tables (300 mm to 3000+ mm in depth). The presence of coal and carbonaceous shales without crayfish burrows indicate regionally high water tables (nearfy at or above the surface) and swampy conditions. The paleoecological and paleohydrological interpretation of crayfish burrows derived from Paleocene deposits in the Washakie and Hanna Basins can be applied to other Cenozoic (age of rocks from 65 million years ago to present) deposits, as well as Mesozoic (age of rocks from 245 to 65 million years ago) deposits with crayfish burrows. Crayfish burrows and other freshwater-aquatic and terrestrial trace fossils enhance interpretations of paleohydrologic settings in ancient ecosystems.

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