Temporal range: Carboniferous–Maastrichtian
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.
In an unpublished dissertation, Neil D. L. Clark proposed in 1989 that cycloids were copepods.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:
- 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 Posidonienschiefer 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.
- Alpheus Spring Packard (1885). "The Syncarida, a group of Carboniferous Crustacea". The American Naturalist. 19: 700–703. doi:10.1086/274005. JSTOR 2450109.
- Günter Schweigert (2007). "Juracyclus posidoniae n. gen. and sp., the first cycloid arthropod from the Jurassic" (PDF). Journal of Paleontology. 81 (1): 213–215. doi:10.1666/0022-3360(2007)81[213:JPNGAS]2.0.CO;2.
- Jerzy Dzik (2008). "Gill structure and relationships of the Triassic cycloid crustaceans" (PDF). Journal of Morphology. 269 (12): 1501–1519. doi:10.1002/jmor.10663. PMID 18690662.
- Geoff A. Boxshall; Damià Jaume (2009). "Exopodites, epipodites and gills in crustaceans" (PDF). Arthropod Systematics & Phylogeny. 67 (2): 229–254.
- Frederick R. Schram; Ronald Vonk; Cees H. J. Hof (1997). "Mazon Creek Cycloidea". Journal of Paleontology. 71 (2): 261–284. JSTOR 1306460.
- Frederick R. Schram; Arjan C. Boere; Natalie Thomas (2006). "Cycloids of the Mississippian Bear Gulch limestone of central Montana" (PDF). Contributions in Science. 504: 1–8.
- John Phillips (1836). Part 2. The Mountain Limestone District. Illustrations of the Geology of Yorkshire. London: John Murray.
- René H. B. Fraaije; Frederick R. Schram; Ronald Vonk (2003). "Maastrichtiocaris rostratus new genus and species, the first Cretaceous cycloid". Journal of Paleontology. 77 (2): 386–388. doi:10.1666/0022-3360(2003)077<0386:MRNGAS>2.0.CO;2. JSTOR 4094744.
- Barry W. M. van Bakel, John W. M. Jagt, René H. B. Fraaije & Pedro Artal (2011). "A new family, genus and species of cyclid (Crustacea, Branchiura, Cyclida) from mid-Cretaceous reefal deposits in northern Spain" (PDF). Bulletin of the Mizunami Fossil Museum. 37: 47–49.