Hymenocarina is an order of extinct marine

arthropods Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

known from the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

. They possess bivalved

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

s, 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 In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...

members of Mandibulata, due to the presence of mandibles in well-preserved species.

Taxonomy

Hymenocarines are characterized by the combination of the following characters: bivalved, convex carapace covering cephalothoracic (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 endopods (lower, leg-like parts) with well-developed terminal claws; absence of appendages between antennae and mandibles; median sclerite and lobate protrusions located between

compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

s; 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). 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 mandibulates (jawed arthropods) after the discovery of their

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

-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- pancrustaceans, stem- myriapods, stem- hexapods or somewhere in-between the former taxa. Several subgroups within the order are recognised, including Waptiidae and Protocarididae. The internal relationships of Hymenocarina are unstable, and it is unclear whether the group is

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. Cambrian bivalved arthropods are now recognised to be a

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

group, with other groups of bivalved arthropods such as the

Isoxyida Isoxyids are members of the order Isoxyida and the family Isoxyidae, a group of basal arthropods that existed during the Cambrian period. It contains two genera, ''Isoxys'', with 20 species found worldwide, and ''Surusicaris'' known from a single ...

, Bradoriida and Phosphatocopina only distantly related to Hymenocarina. '' Chuandianella'' a bivalved arthropod morphologically similar to ''Waptia'' and long thought to be closely related was reinterpreted as a non-hymenocarine euarthropod based on a restudy published in 2022, which found that it definitely lacked mandibles, characteristic of true hymenocarines. Cladogram after O’Flynn et al, 2023:

Diversity

File:20211025 Waptia fieldensis.png, Life restoration of '' Waptia'' File:Canadaspis laevigata.png, Life restoration of '' Canadaspis laevigata'' File:Tokummia.png, '' Tokummia'' is the earliest known animal bearing pincers File:Fibulacaris nereidis.jpg, Only 2 cm-long '' Fibulacaris'' is suggested to have swum upside down File:Balhuticaris.png, Large-sized '' Balhuticaris'' shows extreme multisegmentation with over 100 segments File:Pakucaris.png, Unlike other hymenocarines, '' Pakucaris'' had a separate pygidium carapace covering its posterior File:Erjiecaris.png, Unlike other taxa, eyes of possible hymenocarine '' Erjiecaris'' were probably placed over carapace File:Pseudoarctolepis.jpg, The carapace of possible hymenocarine '' Pseudoarctolepis'' had wing-like projections File:Odaraia.png, Life restoration of '' Odaraia,'' which had a trifurcate tail and probably habitually swam upside down File:Tuzoia life restoration.jpg, Life restoration of '' Tuzoia'' a large hymenocarine The group was very diverse in shape, with some forms like '' Waptia'' somewhat resembling shrimp, and others like '' Odaraia'' having a large

and trifurcate tail. The appendages showing various degrees of specialization across the group, ranging from the feathery gills of ''Waptia'' to the robust claws of '' Tokummia''. They also had a wide range of sizes with some like '' Fibulacaris'' reaching a length of up to long, while largest '' Balhuticaris'' reached long. Hymenocarines are thought to have been ecologically diverse, with various forms occupying scavenging, predatory, deposit feeding and suspension feeding niches. Cladogram of Hymenocarina, following Izquierdo-López and Caron, (2024):

List of genera

*'' Canadaspis'' *'' Clypecaris'' *?'' Dioxycaris'' *'' Ercaicunia'' *?'' Erjiecaris'' *?'' Forfexicaris'' *?'' Occacaris'' *?'' Ovalicephalus'' *'' Pectocaris'' *'' Perspicaris'' *'' Plenocaris'' *?'' Pseudoarctolepis'' *?'' Yunnanocaris'' *'' Xiazhuangocaris'' *Odaraiidae **'' Balhuticaris'' **'' Odaraia'' **'' Jugatacaris'' **'' Nereocaris'' **'' Fibulacaris'' **'' Pakucaris'' **?'' Vermontcaris'' *Tuzoiidae **'' Tuzoia'' **'' Duplapex'' *Protocarididae **'' Tokummia'' **'' Branchiocaris'' **'' Protocaris'' **'' Loricicaris'' *Waptiidae **'' Waptia'' **'' Pauloterminus'' **'' Synophalos''? **'' Chuandianella''? (lacks mandibles, may not be a hymenocarine)

References

{{Taxonbar, from=Q96381504 Cambrian first appearances Cambrian arthropods