Dinocaridida[Greek for deinos "terrible" and Latin for caris "crab" – sometimes informally spelt Dinocarida, but the second 'id' is linguistically correct – see ] is a proposed fossil taxon of basal ,
Anatomy
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 apparently unsegmented
and had a pair of specialized frontal appendages just in front of the mouth and eyes. The frontal appendages are either lobopodous (soft as in gilled lobopodians) or arthropodized (hardened and segmented as in
Radiodonta) 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
and primary antennae of
onychophoran,
while subsequent evidence also suggest a deutocerebral origin (homologous to the jaws of onychophora and
/antennae/
chelicerae of euarthropods).
The trunk possessed multiple segments, each with its own gill branch and swimming flaps (lobes).
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 (lobopod) 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 (3-segmented), it is thought to be comprised either 1 (only protocerebrum
) or 2 cerebral
(protocerebrum and deutocerebrum).
Classification
Although some authors may rather suggest different taxonomic affinities (e.g. as
cycloneuralian relatives
), most of the
phylogenetic studies suggest that dinocaridids are
stem group .
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.
In general, the gilled lobopodian genera
Pambdelurion and
Kerygmachela which have lobopodian traits (e.g. lobopodous appendage, annulation) 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.
Distribution
The group was geographically widespread, and has been reported from
Cambrian strata in
Canada,
United States,
Greenland,
China,
Australia and
Russia,
as well as the
Early Ordovician to Middle Ordovician of
Morocco and
Wales and the
Early Devonian of
Germany.
Notes
