Order Cyclida (Crustacea: Maxillopoda: Branchiura)

About of Cyclida

Cyclids are enigmatic extinct marine crustaceans that existed from the Early Carboniferous to the end of the Cretaceous (DZIK 2008). Due to their rarity, they are a poorly studied group of invertebrates with weakly developed taxonomy.

The first description of a member of the Cyclida was made by to J. PHILLIPS (1836), who depicted and described the carapace as the trilobite Agnostus? radialis. Later, DE KONINCK (1841) came to the conclusion that this animal is not a trilobite, although he had no clear idea about its taxonomic placement. One year later, DE KONINCK (1842) described another species, Cyclus brongniartianus, but subsequently reported by WOODWARD (1870) as a trilobite hypostome. Cyclids were later considered to be members of Xiphosura or true crabs (PACKARD 1872; OOSTERINK 1978). SCHRAM et al. (1997) classified the cyclids as a sister group of crustaceans within the Maxillopoda, and DZIK (2008) placed them in a subclass Branchiura, which contained only one modern family of fish lice, Argulidae.

Taxonomy of Maxillopoda
Modern concept of systematics class Maxillopoda
The place of cyclids according to Dzik
The place of cyclids according to Dzik (2008)
Members of the order Cyclida have some resemblance to brachyurans and most likely, inhabited a similar ecological niche (SCHWEIGERT 2007) and were also outcompeted by them at the end of the Mesozoic. The largest size of a cyclid carapace was reported for Opolanka decorosa DZIK, 2008, which exceeds 6 cm (DZIK 2008: 1501). There is a dispute about the relationship of the order Cyclida with other arthropods. Despite the fact that they are considered crustaceans, doubts were expressed about homology and their relationship to chelicerates (BOXSHALL & JAUME 2009). Nevertheless, considering cyclids as an order within the subclass Branchiura, we herein use the classification proposed by DZIK (2008).