See text 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.

[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.

[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.

Cyclidans are suggested to have had a similar ecology to modern crabs, including as detritivores, scavengers, predators and consumers of plant matter.

[3] A parasitic 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.