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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 . On some specimens, details of the internal anatomy can be seen due to the exceptional preservation.
==Origin and description==
''Aenigmastacus'' was discovered in the McAbee Fossil Beds of British Columbia, Canada,〔Feldmann ''et al.'' (2011), p. 320〕 at .〔 The McAbee beds comprise a 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.〔
Twelve "nearly complete" specimens of ''Aenigmastacus'' have been recovered,〔 with a total body length of .〔 The carapace is thin, but the form of the rostrum is not known.〔 The antennae are as long as the carapace, but the eyes have not been preserved on any of the known specimens.〔 The first pair of pereiopods are larger than the others, and are armed with a nearly symmetrical pair of chelae (claws).〔 The pleon (abdomen) is generally smooth, and ends in a broad, slightly tapering telson with no diaeresis.〔 The specimens of ''Aenigmastacus'' show exceptional preservation. In some specimens, dark areas formed of fluorapatite can be seen, which appear to represent soft tissue in a pattern congruent with the endophragmal skeleton of extant crayfishes.〔Feldmann ''et al.'' (2011), p. 329〕 The trace of the alimentary tract is also visible in some specimens.〔
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