Dinocaridida
Dinocaridida is a proposed fossil taxon of basal arthropods, which flourished during the Cambrian period and survived up to Early Devonian. Characterized by a pair of frontal appendages and series of body flaps, the name of Dinocaridids refers to the suggested role of some of these members as the largest marine predators of their time. Dinocaridids are occasionally referred to as the 'AOPK group' by some literatures, as the group composed of Radiodonta, Opabiniidae, and the "gilled lobopodians" Pambdelurion and Kerygmachelidae. It is most likely paraphyletic, with Kerygmachelidae and Pambdelurion more basal than the clade composed of Opabiniidae, Radiodonta and other arthropods.
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 apparently unsegmented and had a pair of 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 Opabiniidae. Based on their preocular position and putative protocerebral origin, the frontal appendages are generally thought to be homologous to the labrum of euarthropods and primary antennae of onychophorans, while subsequent evidence also suggests a deutocerebral origin. 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 lobopodian genera, 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. The dinocaridid brain is relatively simple than those of a euarthropod, it is thought to be comprised either 1 or 2 cerebral ganglions.Classification
Although some authors may rather suggest 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 arthropod crown 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 Opabiniidae and Radiodonta are more derived and closely related to the arthropod crown group, with the latter even having significant arthropod affinities such as arthropodization and head sclerites.In the original description, Dinocaridida was composed of only Opabiniidae and Radiodonta. With the exclusion of questionable taxa, the former were known only by Opabinia, while all radiodont species were grouped under a single family: Anomalocarididae. In later studies, the gilled lobopodians Pambdelurion and Kerygmachela were also regarded to be dinocaridids, two new opabiniid genera, Utaurora and Mieridduryn were described, other strange dinocaridids like Parvibellus, many radiodonts were reassigned to other new families, and a new family, Kerygmachelidae, was named.