Cyclida

Cyclida Temporal range: Carboniferous–Maastrichtian PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Maxillopoda
Subclass:	Branchiura
Order:	† Cyclida Glaessner, 1928
Genera
Alsasuacaris Van Bakel et al., 2011 Americlus Dzik, 2008 Apionicon Schram et al., 1997 Carcinaspides Glaessner, 1969 Cyclocarcinoides Cyclus de Koninck, 1841 Halicyne Meyer, 1847 Hemitrochiscus Schauroth, 1854 Juracyclus Schweigert, 2007 Maastrichtocaris Fraaije, Schram & Vonk, 2003 Mesoprosopon Stolley, 1914 Oonocarcinus Gemmellaro, 1890 Opolanka Dzik, 2008 Paraprosopon Gemmellaro, 1890 Schramine Dzik, 2008

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.

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.

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.

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