Bathynellcea have no fossil record. Their closest relatives are the Anaspidacea in the Southern Hemisphere. Both groups are confined to fresh water, but have had marine ancestors. This is evidenced by their fossil relatives, the Palaeo-caridacea, which, during the Carboniferous and Permian, inhabited the littoral of the then tropical seas in the Northern Hemisphere. Transition into fresh water was achieved independently, first by the Bathynellacea and later also by the Anaspidacea. Unlike the Anaspidacea, the Bathynellacea have disappeared almost completely from surface waters today.
When compared with adults of related Crustacea taxa, Ba-thynellacea appear very different, but they share a remarkable similarity with the larvae of these other Crustacea. In fact, Bathynellacea have the appearance of larvae, indicating that they have had larval-like ancestors. There is good evidence that the postembryonic development of their surface-living ancestors passed through a series of several larval stages. In the course of adaptation to life in the groundwater, abbreviation of this development caused by precocious sexual maturity at an early stage of larval development marked the beginning of the evolution of subterranean Bathynellacea. Today they reach adulthood at a stage that corresponds to the last larval stage of their ancestor. A morphological consequence of this adaptational process was that their body became continually smaller and its structure increasingly simpler. As a result, they finally fitted into the small spaces between sand particles of the groundwater-bearing strata. Ecologically they progressed from a benthic life through a colonization of coarse substrates with large interstitial spaces to an existence in ever smaller sediments with ever narrower spaces between the particles. The ancestral form of the Ba-thynellacea had conquered a completely new habitat and marked the beginning of an impressive radiation leading to a worldwide colonization of the groundwater.
The order Bathynellacea comprises two families, 60 genera, and about 200 species. The two families are Bathynellidae and Parabathynellidae.
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