Hymenocarina is an order of extinct arthropods 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 ag ...

. They possess bivalved carapaces, 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 Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda (millipedes and others), Crustacea and Hexapoda (insects and others). Mandibulata is currently believed to be the sister group of the clad ...

, due the presence of mandibles 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 and rounded mandibles, 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-

pancrustacea Pancrustacea is the clade that comprises all crustaceans and hexapods. This grouping is contrary to the Atelocerata hypothesis, in which Myriapoda and Hexapoda are sister taxa, and Crustacea are only more distantly related. As of 2010, the P ...

ns, stem- myriapods, 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 A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

group, with other groups of bivalved arthropods such as '' Isoxys'',

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 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.

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 swam 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 The group was very diverse in shape, with some forms like '' Waptia'' somewhat resembling shrimp, and others like '' Odaraia'' having a large carapace 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 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