Cyclida

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 Cretaceous. Their classification is uncertain, but they are generally interpreted as crustaceans, likely belonging to the superclass Multicrustacea.

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 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In an unpublished dissertation, Neil D. L. Clark proposed in 1989 that cycloids were copepods.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. A 2020 study based on well preserved remains of ''Americlus'' from the Carboniferous of North America found that Cyclida had the greatest affinities with Copepoda and Malacostraca, and likely represented an independent branch of the Multicrustacea.Redescription of ''Americlus rankini'' (Woodward, 1868) (Pancrustacea: Cyclida: Americlidae) and interpretation of its systematic placement, morphology, and paleoecology Neil D L Clark, Rodney M Feldmann, Frederick R Schram, Carrie E Schweitzer ''Journal of Crustacean Biology'', Volume 40, Issue 2, March 2020, Pages 181–193, https://doi.org/10.1093/jcbiol/ruaa001

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:
*''Cyclus'' contains a number of species found in Carboniferous deposits.
*''Schramine'' comprises three species formerly included in ''Halicyne'' of mid to late Carboniferous age.
*''Americlus'' contains several species from the Carboniferous, including ''A. americanus'' (formerly ''Cyclus americanus''), a well known species from Mazon Creek.
*''Apionicon apioides'' is found in Pennsylvanian shales in Illinois, including Mazon Creek.
*''Hemitrochiscus paradoxus'' is found in Permian dolomite near Wünschendorf/Elster (Germany).
*''Oonocarcinus insignis'' and ''Paraprosopon reussi'' are found in the Permian limestones of the Sosio valley on Sicily (Italy).
*''Halicyne'' contains several species from the Triassic.
*''Carcinaspides pustulosus'' is found in limestone rocks in the Isar valley of Bavaria (Germany) and is probably of Mid to Late Triassic age.
*''Opolanka decorosa'' is found in Late Triassic rocks at Krasiejów in Poland.
*''Cyclocarcinoides serratus'' and ''Mesoprosopon triasinum'' are found in the grey Norian (late Triassic) limestones of Bad Ischl in the Austrian Salzkammergut.
*''Juracyclus posidoniae'' is found in the Jurassic ' near Tübingen, southern Germany.
*''Alsasuacaris nostradamus'' was found in late Albian to early Cenomanian (Cretaceous) rocks near Alsasua (Navarre, Spain).
*''Maastrichtocaris rostrata'' is found in Maastrichtian (late Cretaceous) rocks from that period's type locality in the southern Netherlands.

''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.

Further reading

*

References

External links

«Database of Cyclida»

{{Taxonbar, from=Q5198245

Maxillopoda
Crustacean orders
Prehistoric crustaceans
Prehistoric arthropod orders
Maastrichtian extinctions