Tadpole shrimps are considered living fossils because their basic body plan has not changed in the last 300 million years. In fact, some fossil species known from the Paleozoic are basically indistinguishable from modern types. The oldest fossil of a tadpole shrimp dates from the carboniferous Paleozoic, and Triops cancriformis is the oldest living animal species on the planet, with fossils of this same species dating from the Triassic. Given the innumerous changes that Earth has undergone during the last 300 million years, tadpole shrimps are considered biological marvels of survival and adaptation.
Notostracans belong to the subclass Calamanostraca, which, together with the subclasses Diplostraca and Sarsos-traca, form the three living groups of the class Branchiopoda. The fossil order Kazacharthra dates from the Jurassic and also belongs to the subclass Calamanostraca; they are distinguished from the Notostraca mainly by the shape of their carapace, which has spikes on the margin, rendering them a fierce appearance. Kazacharthran fossils were found exclusively in the now Republic of Kazakhstan, of the former USSR, and some authors consider them a very specialized group of Notostracans. Tadpole shrimps share with the other groups of the Branchiopoda the characteristic "gilled feet," which are leaf-like and divided into lobes, each containing a gill plate.
Modern Notostracans are grouped into a single family: Triopsidae. Only two genera are recognized (Triops and Lep-idurus), comprising 15 living species with 11 subspecies.
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