Notostraca

The order Notostraca has just two living genera in it. Triops and Lepidura. The two genera are considered 'living fossils', as they have not changed in outward form since the Triassic.

Notostracans, or tadpole shrimps, are omnivores living on the bottom of temporary pools and shallow lakes. They live in pools of water on every continent except Antarctica. They grow very quickly, and can reach adulthood in a week. Triops only live for a short time; a record-breaking female only lived to be 100 days. When their pools dry up,adult tadpole shrimp die. Their eggs, however, stop developing for a time. When they are in water again, they come to life, and new Triops are hatched. This is because of a state known as diapause, where eggs can lie dormant for up to twenty years before hatching again.

Evolution and fossil record
The fossil record of Notostraca is extensive. They occur in a wide range of geological deposits, reaching back to the Carboniferous. The lack of major morphological change since 250 million years ago has led to Notostraca being described as living fossils.Luc Brendonck et al. 2008. Global diversity of large branchiopods (Crustacea: Branchiopoda) in freshwater. In Estelle V. Balian et al. Freshwater animal diversity assessment. Hydrobiologia: Developments in Hydrobiology 198 595 (1): 167–176. doi:10.1007/s10750-007-9119-9. .

The Notostraca abandoned filter feeding in open water, and took up a benthic lifestyle in muddy waters, taking up food from particles of sediment and preying on small animals.

Notostracans keep the ancient condition of having two separate compound eyes. These touch but do not become united, as does happens in other groups of Branchiopods.

 References 

Category:Branchiopods
Category:Living fossils