Notostraca

Notostracans are long, with a broad carapace at the front end, and a long, slender abdomen. This gives them a similar overall shape to a tadpole, from which the common name tadpole shrimp derives. The somites on thorax II are fused into "rings", which varies in number between species and gender and appear to be body segments, but do not always reflect the underlying segmentation. The form of the telson varies between the two genera: in Lepidurus, a rounded projection extends between the caudal rami, while in Triops there is no such projection. ==Life cycle==

Within the Notostraca, and even within species, there is variation in the mode of reproduction, with some populations reproducing sexually, some showing self-fertilisation of females, and some showing a mix of the two. The frequency of males in populations is therefore highly variable. In sexual populations, the sperm leave the male's body through simple pores, there being no penis. The eggs are released by the female and then held in the cup-like brood pouch. The eggs are retained by the female only for a short time before being laid, and the larvae develop directly, without passing through a metamorphosis. ==Ecology and distribution==

Notostracans are omnivorous, eating small animals such as fishes and fairy shrimp. ==Evolution and fossil record==

The fossil record of Notostraca is extensive, occurring in a wide range of geological deposits. The oldest known notostracan is the species Strudops goldenbergi from the Late Devonian (Famennian ~ 365 million years ago) of Belgium. The lack of major morphological change since has led to Notostraca being described as living fossils. are closely related to notostracans, and may belong within the order Notostraca, or alternatively are placed as their sister group within the clade Calmanostraca. The "central autapomorphy" of the Notostraca is the abandonment of filter feeding in open water, and the development of a benthic lifestyle in muddy waters, taking up food from particles of sediment and preying on small animals. 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. --> ==Taxonomy==

The extant members of order Notostraca composed a single family, Triopsidae, with only two genera, Triops and Lepidurus. Many putative species have been described based on morphological variation, such that by the 1950s, as many as 70 species were recognised. and Longhurst in 1955 – synonymised many taxa, and resulted in the recognition of only 11 species in the two genera. This taxonomy was accepted for decades, More recent studies, especially those employing molecular phylogenetics, have shown that the eleven currently recognised species conceal a greater number of reproductively isolated populations. • Lepidurus, worldwide except Antarctica Extinct • Apudites, (Formerly "Notostraca" minor, often referred to as Triops cancriformis minor, or "Triops" minor in historic literature) Lower Triassic, Grès à Voltzia, Vosges Mountains, France; Hassberge Formation, Germany, Late Triassic (Carnian) • Brachygastriops Dabeigou Formation, China, Late Jurassic or Early Cretaceous • Chenops Yixian Formation, China, Early Cretaceous (Aptian) • Dikelocephala, Lower Triassic of North China • Discocephala, Lower Triassic of North China • Heidiops, Lower Permian of the Lodève Basin, France • Jeholops Yixian Formation, China, Early Cretaceous (Aptian) • Lynceites Germany, Canada, Carboniferous • Prolepidurus, Late Jurassic?-Lower Cretaceous, Transbaikal, Russia • Strudops Strud locality, Belgium, late Devonian (Fammenian) • Thuringiops, Upper Oberhof Formation, Thuringian Forest Basin, Carboniferous Germany • "Calmanostraca" hassbergella Hassberge Formation, Germany, Late Triassic (Carnian) ==See also==