[2] The two genera, Triops and Lepidurus, are considered living fossils, with similar forms having existed since the end of the Devonian, around 360 million years ago.

[1] They are found worldwide in freshwater, brackish water, or saline pools, as well as in shallow lakes, peat bogs, and moorland.

[9] The oldest known notostracan is the species Strudops goldenbergi from the Late Devonian (Famennian ~ 365 million years ago) of Belgium.

[10] The lack of major morphological change since 250 million years ago has led to Notostraca being described as living fossils.

[7] A number of other characteristics are correlated with this change, including the increased size of the animal compared to its relatives, and the loss of the ability to hinge the carapace; although a central keel marks the former separation into two valves, the adductor muscle is missing.

[7] The extant members of order Notostraca composed a single family, Triopsidae, with only two genera, Triops and Lepidurus.

[11] The problematic Middle Ordovician fossil Douglasocaris has been erected and placed in its own family Douglasocaridae by Caster & Brooks 1956, and may be ancestral to Notostraca.

[16] More recent studies, especially those employing molecular phylogenetics, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations.