Fossil mysids have been dated as far back as the Triassic period, about 248-213 million years ago (mya). A group of fossil crustaceans known as Pygocephalomorpha, which includes a number of Paleozoic genera from the Carboniferous and Permian periods (360-248 mya), is possibly related to the mysids.
Two suborders for the order Mysidacea, the Lophogastrida and Mysida, were recognized in 1883. This arrangement persisted for nearly a century until some researchers proposed alternative taxonomic schemes to explain the relationships among mysidaceans, including raising both suborders to order level. This proposal, which has been sustained by a number of experts, is followed here.
The order Mysida includes four families: Petalophthalmi-dae, with six genera; Mysidae, with six subfamilies (one, the Mysinae, comprises seven tribes) and almost 140 genera; Lep-idomysidae, with only one genus, Spelaeomysis; and Stygiomysi-dae, also with only one genus, Stygiomysis. The order as a whole includes slightly over 1,000 described species.
Mysids are sometimes known as opossum shrimps because of the marsupium, or external pouch, formed by specially developed plates on the inner sides of the thoracic limbs of adult females.
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