Kingdom Animalia Phylum Arthropoda Subclass Branchiura | Subphylum Crustacea Rank Order | |
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Similar Branchiura, Euthycarcinoidea, Thylacocephala, Tantulocarida, Mystacocarida | ||
Cyclida (formerly Cycloidea, and so sometimes known as cycloids) is an order of fossil arthropods that lived from the Carboniferous to the Cretaceous. Their classification is uncertain, but they are generally treated as a group of maxillopod crustaceans.
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Description
Cycloids have a "striking" resemblance to crabs, and are thought to have inhabited a similar ecological niche, and to have been driven to extinction when crabs became widespread and diverse. The largest members are over 6 centimetres (2.4 in) across the carapace. Their gills are often preserved in three dimensions, and do not resemble those of other crustaceans. Cycloid taxa differ in the number of walking legs, in the form of the mouthparts and in other significant ways.
Affinities
There is considerable debate about the placement of cycloids within the Arthropoda. While they are generally considered to be crustaceans of some kind, doubts have been expressed about the homology of cycloids' respiratory structures with those of other crustaceans, and parallels drawn instead with chelicerates.
The first description of a cycloid was in the 1836 treatise Illustrations of the Geology of Yorkshire by John Phillips, where Phillips described "Agnostus ? radialis" among the trilobites, with the text "ribs radiating, with acute puncta; abdomen mucronate". In 1838, Hermann von Meyer described a species of trilobite, albeit in the genus Limulus, and later transferred it to a new genus, Halicyne, recognising that it was something different. In 1841, Laurent-Guillaume de Koninck transferred Phillips' species to a new genus, Cyclus, away from the trilobites, although he later described a second species of Cyclus which was later recognised as the hypostome of a trilobite. Cycloids were later considered to be members of the Xiphosura, true crabs, and branchiurans.
In an unpublished dissertation, Neil D. L. Clark proposed in 1989 that cycloids were copepods. In 1997, Frederick Schram and his co-authors classified them as the sister group to copepods, within the Maxillopoda, and in 2008, Jerzy Dzik placed them as an order within the maxillopod suborder Branchiura, which previously contained only the modern fish lice.
Taxa and stratigraphy
Cycloids are known from deposits ranging from Carboniferous (Cyclus spp.) to Maastrichtian (Maastrichtocaris rostrata). They are one of only three groups of "generally Palaeozoic" arthropods to survive the Permian–Triassic extinction event, the others being the freshwater-living Euthycarcinoida and the marine Thylacocephala.
The order Cyclida contains 15 genera. In approximate age order, they are:
Stagmacaris quenstedti, a species from the late Kimmeridgian of southern Germany, has been reinterpreted as part of the abdomen of a hermit crab, rather than a cycloid.