On the basis of evidence from the fossil order Lipostraca, and the Upper Cambrian species Rehbachiella kinnekullensis, the anostracan line apparently split off at a very early stage from the rest of the Branchiopoda, about 500 million years ago. Fairy shrimps are widely considered the most primitive living crustaceans. Currently, scientists count eight families in two suborders within the Anostraca. This number is lower than the 11 families listed in older reference works. The change in taxonomy is the result of molecular investigations, which led researchers to group the families Artemiidae and Parartemiidae together to form the suborder Artemiina; and combine the families Chirocephalidae, Branchinectidae, Thamnocephalidae, Branchipodidae, Tanymastigidae, and Streptocephalidae to form the suborder Anostracina. DNA analysis also showed that the (sub)arctic genera Polyartemia and Polyartemiella, which have 17 and 19 pairs of limbs respectively as opposed to the usual 11, do not necessarily represent the most primitive anostracans. Rather, these genera may have resulted from mutations in homeobox genes, which are DNA sequences whose function is to divide an embryo into bands of tissue that will develop into specific organs. In females of the genus Parartemia, the eleventh pair of limbs may be reduced in size or completely missing.
The ancestral form Rehbachiella is known from horizontally banded limestone nodules found in the Orsten formation on the Baltic shore of Sweden. It has a rather large (more than 11) number of limbs, but no or few free abdominal segments. Unlike modern Anostraca, Rehbachiella had paired but sessile (located within the head) eyes. Even in extant fairy shrimps, however, rare cyclopic (one-eyed) mutants occasionally occur. The primitive ancestral anostracan may thus have had two sessile eyes that merged into a single median eye, which separated again at a later stage of evolution into two eyes on stalks.
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