Aenigmastacus

Aenigmastacus crandalli is a species of fossil freshwater crayfish. It was found in Eocene lake deposits in British Columbia, and was described in 2011. It is the first member of the Gondwana-distributed family Parastacidae to be found in the Northern Hemisphere, and is the only species in the genus Aenigmastacus. Twelve specimens are known, with a total body length of 3–5 cm (1.2–2.0 in). On some specimens, details of the internal anatomy can be seen due to the exceptional preservation.

Aenigmastacus was discovered in the McAbee Fossil Beds of British Columbia, Canada, at 50°47.181′N 121°8.568′W / 50.786350°N 121.142800°W / 50.786350; -121.142800. The McAbee beds comprise a 30-metre (100 ft) thickness of siliceous sediments within a sequence dominated by volcaniclastic rocks. They are believed to have formed from lake deposits, based on the presence of five families of freshwater fish and other terrestrial taxa, and the fossil-bearing sediments have been dated to the Ypresian (Lower Eocene). The deposition of a silica-rich sediment deriving from diatoms indicates a calm environment with little influx of terrestrial silt. There is an alternation of dark and light layers, which may indicate a seasonal change, such as freezing over in winter. The bottom of the lake was probably anoxic, which may have aided the preservation of fossils.