Hymenocarina is an order of extinct marine arthropods known from the Cambrian. They possess bivalved , typically with exposed posteriors. Members of the group are morphologically diverse and had a variety of ecologies, including as filter feeders and as predators. Recent research has generally considered them to be stem or crown group members of Mandibulata, due to the presence of mandibles in well-preserved species.
Description
Hymenocarines are characterized by the combination of the following characters: bivalved, convex carapace covering
Cephalothorax (combined head and thorax) region; cephalothorax bearing multisegmented antennae (though as an exception antennae are absent in
Odaraia) and rounded mandibles and likely maxillae, post maxillae limbs with spiny, subdivided basis and
Arthropod leg (lower, leg-like braches of limbs) with well-developed terminal claws; absence of appendages between antennae and mandibles; median sclerite and lobate protrusions located between
; and posterior tagma (abdomen) with ring-like segments with the posterior of the body ending with pair of well-developed
caudal rami (often in the form of tail flukes).
The group was very diverse in shape, with some forms like Waptia somewhat resembling shrimp,
Taxonomy
Based on the interpretation of simple head region that possess only a few segments and appendages, hymenocarine taxa were thought to be part of the upper stem-group euarthropods in early and mid 2010s.
They later became widely accepted as
Mandibulata (the group that contains insects, crustaceans, centipedes and millipedes, among others) after the discovery of their
mandible-bearing mouthparts in late 2010s.
Since then, most phylogenetic analysis suggest hymenocarines represent part of the mandibulate stem-group,
with some results suggest a rather crownward position such as stem-
,
stem-
Myriapoda,
stem-
Hexapoda or somewhere in-between the former taxa.
Several subgroups within the order are recognised, including Waptiidae
List of families and genera
Cambrian bivalved arthropods are now recognised to be a polyphyletic group, with other groups of bivalved arthropods such as the Isoxyida, Bradoriida and Phosphatocopina only distantly related to Hymenocarina.
Cladogram after O'Flynn et al, 2023. showing possible placement of Hymenocarina within Mandibulata:
Cladogram of Hymenocarina, following Izquierdo-López and Caron, (2024):
