Cyclida (formerly Cycloidea, and so sometimes known as cycloids) is an extinct order of crab-like fossilarthropods 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 superclassMulticrustacea.
Cyclidans have a "striking" resemblance to crabs,[1] with circular to ovoid carapaces, which vary from flat to domed.[2] The carapace covers the entire body.[3] The largest members are over 6 centimetres (2.4 in) across the carapace.[4] Their gill plates are arranged in a horseshoe shape beneath the carapace in at least some species.[3] The head and thorax were fused into a single unit, the cephalothorax.[3] 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).[2][3] The abdomen is short with either one or two segments, which ends with a pair of posterior structures, dubbed "caudal rami".[3]
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.[5] 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.[2]
The first description of a cycloid was in the 1836 treatise Illustrations of the Geology of YorkshirebyJohn Phillips,[6][7] where Phillips described "Agnostus ? radialis" among the trilobites, with the text "ribs radiating, with acute puncta; abdomen mucronate".[8] 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.[6] 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.[6] Cycloids were later considered to be members of the Xiphosura, true crabs, and branchiurans.[9]
In an unpublished dissertation, Neil D. L. Clark proposed in 1989 that cycloids were copepods.[9] In 1997, Frederick Schram and his co-authors[6] classified them as the sister group to copepods, within the Maxillopoda.[5] In 2008, Jerzy Dzik placed them as an order within the maxillopod suborder Branchiura, which previously contained only the modern fish lice,[4] however, later studies have rejected this placement, due to the similarities between the two groups only being superficial.[3] 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.[10]
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.[2] 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.[3]Aparasitic mode of life has sometimes been suggested for cyclidans,[4] but this has been considered unlikely due to their large size and well calcified skeletons.[3]
Schramine Dzik, 2008[4] comprises two species from the Carboniferous of North America and three species from the Early Triassic of Madagascar, some of which were formerly included in Halicyne.
Ambocyclus Schweitzer, Mychko & Feldmann, 2020 contains three species from the Carboniferous of Ireland, North America and Russia
Prolatcyclus Mychko, Feldmann, Schweitzer & Alekseev, 2019 Contains two species from the lower Carboniferous of England and Russia
Cyclus de Koninck, 1841 formerly contained 17 species found in Carboniferous deposits of Europe, Asia and North America, now defined to only include the species Cyclus radialis from the lower Carboniferous of Europe
Carabicyclus Schweitzer, Mychko & Feldmann, 2020 known from two species from the Lower Carboniferous of the British Isles
Litocyclus Schweitzer, Mychko & Feldmann, 2020 definitely two and possibly four species, known from the Lower Carboniferous of the British Isles and the Upper Carboniferous of North America
Chernyshevine Schweitzer, Mychko & Feldmann, 2020 known from a single species from the Lower Carboniferous of Tajikistan
Tazawacyclus Schweitzer, Mychko & Feldmann, 2020 known from a single species from the Lower Carboniferous of Japan
Uralocyclus Mychko & Alekseev, 2018 contains two species from the Lower Carboniferous of the British Isles and the Earliest Permian of Chelyabinsk, Russia
Family Halicynidae Gall & Grauvogel, 1967
Halicyne von Meyer, 1844 contains six species from the Middle Triassic of Europe.[4]
Carcinaspides Glaessner, 1969 contains a single species found in limestone rocks in the Isar valley of Bavaria (Germany) and is probably of MidtoLate Triassic age.[4]
Americlus Dzik, 2008 contains A. americanus (formerly Cyclus americanus),[4] a well known species from Mazon Creek,[6] as well as two possible other species, the holotypes of which are now lost, which all known from the Late Carboniferous of North America.
Yunnanocyclus Feldmann, Schweitzer & Hu, 2017 single species known from the Middle Triassic of China
Brittaniclus Schweitzer, Mychko & Feldmann, 2020 Contains three species from the Carboniferous of Britain
Dziklus Schweitzer, Mychko & Feldmann, 2020 Contains a single species known from the Upper Carboniferous of North America
Malayacyclus Tang et al. 2021[11] Contains one species from the Lower Carboniferous of Malaysia
Family Alsasuacaridae van Bakel, Jagt, Fraaije & Artal, 2011 (Note: The placement of these taxa within Cyclida has been questioned, and some authors argue that these taxa may be true crabs instead).[3]
Maastrichtocaris Fraaije, Schram & Vonk, 2003 comprises a single species found in Maastrichtian (late Cretaceous) rocks from that period's type locality in the southern Netherlands.[9]
Family Hemitrochiscidae Trauth, 1918
Cyclocarcinides Glaessner, 1969 comprises 5 species known from the Upper Triassic of Europe
^Schweitzer, Carrie E.; Schram, Frederick R.; Feldmann, Rodney M.; Clark, Neil D L. (2020). "Redescription of Americlus rankini (Woodward, 1868) (Pancrustacea: Cyclida: Americlidae) and interpretation of its systematic placement, morphology, and paleoecology". Journal of Crustacean Biology. 40 (2): 181–193. doi:10.1093/jcbiol/ruaa001.
