Dinocaridida is a proposed taxon of extinctfossil arthropod-like marine animals found, with one exception, in the Cambrian and Ordovician. The name of Dinocaridids comes from Greek, "deinos" and "caris", meaning "terror shrimp" or "terror crab", due to their crustacean-like appearance and the hypotheses suggesting that members of this class were amongst the dominating and most diverse apex predators of their time. Dinocaridids occasionally referred as 'AOPK group' by some literatures, as the group compose of Radiodonta, Opabinia, Pambdelurion and Kerygmachela. It is probably paraphyletic, with Kerygmachela and Pambdelurion more basal than the Opabinia and Radiodonta clade.
Anatomy
Dinocaridids were bilaterally symmetrical, with a mostly non-mineralized cuticle and a body divided into two major groupings of tagmata : head and trunk. The head comprised only one segment and had two specialized frontal appendages just in front of the mouth and eyes. The frontal appendages are either lobopodous or arthropodized and usually paired, but highly fused into a nozzle-like structure in Opabinia. Based on their preocular position and protocerebral origin, the frontal appendages are most likely homologous to the labrum of euarthropods and primary antennae of onychophora. The trunk possessed multiple segments, each with its own gill branch and swimming flaps. It is thought that these flaps moved in an up-and-down motion, in order to propel the animal forward in a fashion similar to the cuttlefish. In gilled lobopodiangenera, the trunk may have borne a lobopodous limb underneath each of the flaps. The midgut of dinocaridids had paired digestive glands similar to those of siberiid lobopodians and cambrian euarthropods. Based on the neuroanatomical evidence from Kerygmachela and Lyrarapax, the dinocaridid brain comprised only a protocerebrum, with frontal appendage nerves and optic nerves connected to its anterior and lateralregion respectively.
Classification
Although some authors may rather suggest a different taxonomic affinities, most of the phylogenetic studies suggest that dinocaridids are stem group arthropods. Under this scenario, Dinocaridida is a paraphyletic grade in correspond to the arthropodcrown group and also suggest a lobopodian origin of the arthropod lineage. In general, the gilled lobopodian genera Pambdelurion and Kerygmachela which have lobopodian traits occupied the basal position; while Opabinia and Radiodonta are more derived and closely related to the arthropod crown group, with the latter even have significant arthropod affinities such as arthropodization and head sclerites. Only one order, Radiodonta, had been founded for Dinocaridida. Traditionally, Radiodonta included all dinocaridid taxa and all radiodonts were grouped under a single family: Anomalocarididae. In later studies, Opabinia, Pambdelurion and Kerygmachela were excluded from Radiodonta, and many radiodonts were reassigned to other new families.
Distribution
The group was geographically widespread, and has been reported from Cambrian strata in Canada, China and Russia, as well as the Ordovician of Morocco and Devonian of Germany.
