Dinocaridida

Dinocaridida Temporal range: Cambrian–Middle Devonian PreꞒ Ꞓ O S D C P T J K Pg N
Assembles of Dinocaridids (Anomalocaris, Opabinia, Pambdelurion and Kerygmachela)
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	†Dinocaridida Collins, 1996
Subgroups
Kerygmachela Pambdelurion Opabiniidae Radiodonta

Part of a series on
The Cambrian explosion
Fossil localities Burgess Shale Chengjiang Sirius Passet Doushantuo
Key organisms Ediacaran biota Dickinsonia Kimberella Vernanimalcula Burgess-type Marrella Anomalocaridids Halwaxiids Opabinia Odontogriphus Small shelly fauna Helcionellids
Evolutionary concepts Trends Cambrian substrate revolution Themes Cladistics Convergent evolution Stem and crown groups
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Dinocaridida^{[derivation 1]} is a proposed fossil taxon^[1] of basal arthropods that flourished in the Cambrian period with occasional Ordovician^[2] and Devonian records.^[3] Characterized by a pair of frontal appendages and series of body flaps, the name of Dinocaridids comes from Greek, "deinos" and "caris" ("terrible crab"), referring to the suggested role of some of these members as the largest marine predators of their time.^[1] Dinocaridids are occasionally referred to as the 'AOPK group' by some literatures,^[4][5][6] as the group compose of Radiodonta (Anomalocaris and relatives), Opabinia, Pambdelurion and Kerygmachela.^[7] It is most likely paraphyletic, with Kerygmachela and Pambdelurion more basal than the clade compose of Opabinia, Radiodonta and other arthropods.^[4][8][9]

Anatomy[]

Eyes (deep blue), brain (light blue) and digestive system (yellow) of Radiodonta (left) and Kerygmachela (right)

Dinocaridids were bilaterally symmetrical, with a mostly non-mineralized cuticle and a body divided into two major groupings of tagmata (body-sections): head and trunk. The head comprised only one segment (ocular somite)^[8][9] and had two specialized frontal appendages just in front of the mouth and eyes. The frontal appendages are either lobopodous (in gilled lobopodians) or arthropodized (in Radiodonta) and usually paired, but highly fused into a nozzle-like structure in Opabinia.^[7] 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.^[10][9] The trunk possessed multiple segments, each with its own gill branch and swimming flaps (lobes).^[11] It is thought that these flaps moved in an up-and-down motion, in order to propel the animal forward^[12] in a fashion similar to the cuttlefish. In gilled lobopodian genera, the trunk may have borne a lobopodous limb (lobopod) underneath each of the flaps.^[11] The midgut of dinocaridids had paired digestive glands similar to those of siberiid lobopodians and cambrian euarthropods.^[13] 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 lateral region respectively.^[10][14]

Classification[]

Although some authors may rather suggest a different taxonomic affinities (e.g. as cycloneuralian relatives^[15][7]), most of the phylogenetic studies suggest that dinocaridids are stem group arthropods.^{[16][17][18][19][11][8][9][20][21][22]} Under this scenario, Dinocaridida is a paraphyletic grade in correspond to the arthropod crown group (Euarthropoda or Deuteropoda) and also suggest a lobopodian origin of the arthropod lineage.^[23][8] In general, the gilled lobopodian genera Pambdelurion and Kerygmachela which have lobopodian traits (e.g. lobopodous appendage, annulation) 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.^[22][24][8]

In the original description, Dinocaridida compose of only Opabiniidae and Radiodonta.^[1] With the exclusion of questionable taxa (e.g. the putative opabiniid Myoscolex^[25]), the former were known only by Opabinia, while all radiodont species were grouped under a single family: Anomalocarididae (hence the previous common name 'Anomalocaridids'^[16]). In later studies, the gilled lobopodians Pambdelurion and Kerygmachela were also regarded to be dinocaridids,^[7] and many radiodonts were reassigned to other new families (Amplectobeluidae, Tamisiocarididae and Hurdiidae).^[19]

Distribution[]

The group was geographically widespread, and has been reported from Cambrian strata in Canada, United States, Greenland, China, Australia and Russia,^[26] as well as the Ordovician of Morocco^[2] and Devonian of Germany.^[3]

Notes[]

References[]

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