Phosphatocopina (alternatively Phosphatocopida) is an extinct group of bivalved arthropods known from the Cambrian period. They are generally sub-milimetric to a few millimetres in size. They are typically only known from isolated carapaces, but some found in Orsten-type phosphatized preservation have their bodies preserved in high fidelity in three dimensions.
Description
Reconstruction of Dabashanella
The phosphatized bivalved carapace covered the entire body.[1] Members typically grew to a maximum of 1–3 millimetres (3⁄64–1⁄8 in) in length, though Cyclotron grew up to 6 mm (1⁄4 inch).[2] In some species, spines were present on the carapace.[3] The head either bore a pair of stalked eyes or a pair of dome-shaped medial eyes.[4] The first appendage pair, dubbed the "antennulae", were uniramous, with the remaining appendage pairs being biramous. The basipods and endopods of the biramous limbs had prominently developed endites, while the exopods were typically annulated, and bore setae.[3] The earliest larval stages of phosphatocopines are known as "head larva", due to them only having the four pairs of cephalic appendages, a feature that is a shared groundplan with most other arthropod groups.[1][3] Fossilised eggs likely to belong to phosphatocopids have been found in Furongian deposits in Poland.[5]
Ecology
Phosphatocopines are generally thought to have been nektobenthic (swimming close to the sediment), and have been suggested to have fed on small particulate organic matter, using the endites and spines on their limbs to trap particles.[6] They are thought to have been tolerant of hypoxic environments, which was probably their preferred habitat.[2]
Taxonomy
When phosphatocopines were first described, they were suggested to be ostracods, but this was rejected after their soft tissue was described.[7] They have often been suggested to be close relatives of crustaceans, with the proposed clade containing the two groups dubbed Labrophora.[4] However, their mandibles and maxillae are not strongly morphologically differentiated from the other trunk limbs, with differentiated mandibles and maxillae characterising most crown-groupmandibulates, including crustaceans, and as such have been alternatively suggested to be stem-group mandibulates.[8] The fact that specimens with preserved soft tissue all appear to be larval instars makes their exact placement uncertain.[9] Several subgroups have been proposed, such as Hesslandonidae and Vestrogothiidae.[1]
