Hymenocarina is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

of extinct

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

known from the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

. 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 under ...

s, typically with exposed posteriors. Members of the group are morphologically diverse and had a variety of ecologies, including as

filter feeders Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

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 the presence of

mandibles In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

in at least some species.

Taxonomy

Hymenocarines are characterized by the combination of following characters: bivalved, convex carapace covering cephalothoracic region; cephalothorax bearing multisegmented

antennules Antennae ( antenna), sometimes referred to as "feelers", are paired appendages used for sensing in arthropods. Antennae are connected to the first one or two segments of the arthropod head. They vary widely in form but are always made of one ...

and rounded

, alongside post-maxillular limbs with spiny, subdivided basis and endopods with well-developed terminal claws; absence of appendages between antennules and mandibles; median sclerite and lobate protrusions located between

compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...

s; posterior tagma (abdomen) with ring-like segments and terminated by a pair of well-developed caudal rami. 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 anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

-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 Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. The fossil record of myriapods reaches back into the late Silurian, a ...

, stem- hexapods or somewhere in-between the former taxa. Several subgroups within the order are recognised, including Waptiidae and Protocarididae. Cambrian bivalved arthropods are now recognised to be a polyphyletic group, with other groups of bivalved arthropods such as ''

Isoxys ''Isoxys'' (meaning "equal surfaces") is a genus of extinct bivalved Cambrian arthropod; the various species of which are thought to have been freely swimming predators. It had a pair of large spherical eyes (which are the most commonly preser ...

'',

Bradoriida Bradoriids are an extinct order of small marine arthropods with a bivalved carapace, and were globally distributed, forming a significant portion of the Cambrian and Early Ordovician soft-bodied communities. Affinity Whilst the Bradoriida wer ...

and

Phosphatocopina Phosphatocopina (alternatively Phosphatocopida) is an extinct group of bivalved arthropods known from the Cambrian period. They are generally sub-milimetric to a few millimetres in size, and are typically found 3-dimensionally preserved in Orste ...

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

, characteristic of true hymenocarines.

Diversity

File:20211025 Waptia fieldensis.png, Life restoration of ''

Waptia ''Waptia fieldensis'' is an extinct species of arthropod from the Middle Cambrian Burgess Shale ''Lagerstätte'' of Canada. It grew to a length of , and had a large bivalved carapace and a segmented body terminating into a pair of tail flaps. I ...

'' File:Canadaspis laevigata.png, Life restoration of '' Canadaspis laevigata'' File:Tokummia.png, ''

Tokummia ''Tokummia katalepsis'' is a fossil hymenocarine arthropod from the Burgess Shale as found in a quarry in Marble Canyon in Canada, lived during middle Cambrian (508 million years old). The animal has maxillipeds, mandibles, ring-shaped around 5 ...

'' is the earliest known animal bearing pincers File:Fibulacaris nereidis.jpg, Only 2 cm-long ''

Fibulacaris ''Fibulacaris'' is a monotypic genus of fossil arthropod known only by one species, ''Fibulacaris nereidis'', discovered from the Cambrian Burgess Shale of Canada It was characterized by a bivalved carapace with an inverted rostrum, sandwich ...

'' is suggested to have swam upside down File:Balhuticaris.png, Large-sized ''

Balhuticaris ''Balhuticaris'' is a genus of extinct bivalved (referring to the carapace) hymenocarine arthropod that lived in the Cambrian aged Burgess Shale in what is now British Columbia around 506 million years ago. This extremely multisegmented (wit ...

'' shows extreme multisegmentation with over 100 segments File:Pakucaris.png, Unlike other hymenocarines, ''

Pakucaris ''Pakucaris'' is an extinct genus of bivalved arthropod known from a single species, ''Pakucaris apatis'', found in the Marble Canyon locality of the Burgess Shale in British Columbia, Canada. It is thought to be a member of Hymenocarina. Unlike ...

'' had a separate

pygidium The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. In groups other than insects, it contains the anus and, in females, the ovipositor. It is compo ...

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 The group was very diverse in shape, with some forms like ''

'' somewhat resembling shrimp, and others like ''

Odaraia ''Odaraia'' is a genus of bivalved arthropod from the Middle Cambrian. Its fossils, which reach in length, have been found in the Burgess Shale in British Columbia, Canada. ''Odaraia'' bore a large pair of eyes at the front of its body, and ...

'' 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 ''

''. They also had a wide range of sizes with some like ''

'' reaching a length of up to long, while largest ''

'' reached long. Hymenocarines are thought to have been ecologically diverse, with various forms occupying scavenging, predatory, deposit feeding and

suspension feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

niches.

Taxonomy

Diversity

References

See also