Extinct order of crustaceans
Cyclida (formerly Cycloidea, and so sometimes known as cycloids) is an extinct order of crab-like fossil arthropods that lived from the Carboniferous to the Jurassic and possibly Cretaceous. Their classification is uncertain, but they are generally interpreted as crustaceans, likely belonging to the superclass Multicrustacea.
Description.
Cyclidans have a "striking" resemblance to crabs, with circular to ovoid carapaces, which vary from flat to domed. The carapace covers the entire body. The largest members are over across the carapace. Their gill plates are arranged in a horseshoe shape beneath the carapace in at least some species. The head and thorax were fused into a single unit, the cephalothorax. The eyes, when present, were stalked, with the head also bearing two pairs of antennae, the first of which (dubbed the antennulae), are significantly longer than the second. There are at least eight pairs of appendages excluding the antennae and antennulae. The anterior appendages (including the pairs of maxillae and maxillipeds) are modified into chelae-like structures, with the posterior appendages being walking legs, which have sharp terminal segments (dactyli). The abdomen is short with either one or two segments, which ends with a pair of posterior structures, dubbed "caudal rami".
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. However, other workers have interpreted the gills as similar to those of crustaceans, and the possession of two pairs of antennae is considered characteristic for many groups of crustaceans.
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. In 2008, Jerzy Dzik placed them as an order within the maxillopod suborder Branchiura, which previously contained only the modern fish lice, however, later studies have rejected this placement, due to the similarities between the two groups only being superficial. A 2020 study based on well preserved remains of Americlidae from the Carboniferous of Britain found that Cyclida had the greatest affinities with Copepoda and Malacostraca, and likely represented an independent branch of the Multicrustacea.
Ecology.
While most cyclidans are known from marine environments, a handful are known to have inhabited freshwater environments. Many marine species occupied reef habitats, while other are known to have lived in deep marine settings. The anterior chelae like appendages are suggested to have been used for grasping and manipulating food items. Cyclidans are suggested to have had a similar ecology to modern crabs, including as detritivores, scavengers, predators and consumers of plant matter. A parasitic mode of life has sometimes been suggested for cyclidans, but this has been considered unlikely due to their large size and well calcified skeletons. 
Taxa and stratigraphy.
Cycloids are known from deposits ranging from the Early Carboniferous to the very end of the Cretaceous (Maastrichtian). 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.
Families and genera.
After Schweitzer, Mychko &amp; Feldmann, 2020.
References.
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