Panarthropoda is a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

comprising the greatest diversity of

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

groups. It contains the extant

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

Arthropod 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 ...

a (Euarthropoda),

Tardigrada Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged Segmentation (biology), segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who calle ...

(water bears) and

Onychophora Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...

(velvet worms), although the precise relationships among these remained uncertain according to studies published in 2023 and 2024. Panarthropods also include

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

marine legged worms known as lobopodians ("

Lobopodia Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as ...

"), a

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 ...

group where the

last common ancestor A most recent common ancestor (MRCA), also known as a last common ancestor (LCA), is the most recent individual from which all organisms of a set are inferred to have descended. The most recent common ancestor of a higher taxon is generally assu ...

and basal members ( stem-group) of each extant panarthropod phylum are thought to have risen. However the term "Lobopodia" is sometimes expanded to include tardigrades and onychophorans as well. Common characteristics of the Panarthropoda include a segmented body, paired ladder-like ventral nervous system, and the presence of paired

appendages An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

correlated with body segments.

Taxonomy

Not all studies support the

monophyly 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 comm ...

of Panarthropoda, but most do, including neuroanatomical, phylogenomic and palaeontological studies. At least a close relationship between onychophorans and arthropods is widely agreed upon, but the position of tardigrades is more controversial. Some phylogenomic studies have found tardigrades to be more closely related to

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

s. Traditionally, panarthropods were considered to be closely related to the

annelid The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

s, grouped together as the Articulata (animals with body segments), but subsequent phylogenomic studies consistently place them closer to cycloneuralians (nematodes, nematomorphs,

loricifera Loricifera (from Latin, ''wikt:lorica, lorica'', corselet (armour) + ''ferre'', to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals with 43 described species and approximately 100 more that hav ...

ns, kinorhynchas and priapulids), grouped together as

Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerates (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phylum (biology), phyla. The grouping of these animal phyla into a single ...

. While annelids are placed among the

Spiralia The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other Taxon, taxa. The term ''Spiralia'' is applied to those phyla that exhibit canonical spiral clea ...

(making them more closely related to

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

s and such), having evolved their segmented bodies convergently.

Interrelationships

There are three competing hyphotheses for the interrelationship between the extant panarthropod phyla, each known as Tactopoda (Arthropoda+Tardigrada), Antennopoda (Arthropoda+Onychophora), and the

sister A sister is a woman or a girl who shares parents or a parent with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to ref ...

relationship between Onychophora and Tardigrada (

''sensu'' Smith & Goldstein 2017). Tactopoda had been supported by mitochondrial gene arrangements, palaeontological and neuroanatomical evidences, specifically the presence of segmented

ganglia A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there a ...

shared by arthropods and tardigrades. Antennopodia united by the presence of specialized head appendages and deutocerebrum (additional second section of the brain), but subsequent anatomical studies suggest these features were convergently evolved between onychophoran and arthropod lineages. Onychophorans and tardigrades shared some lobopodian traits (e.g. soft cuticle, lobopodous appendages and peripheral nerve roots), but these were generally considered to be plesiomorphies traced back to the

of Panarthropoda or

. While most phylogenomic analyses support the monophyly of Panarthropoda, the results of interrelationship between the three phyla are less correlated—some of them inconsistently placing Tardigrada within Arthropoda, while the others mostly recovering either Antennopoda or Onychophora+Tardigrada. A study published in December 2024 supported the sister relationship of Onychophora and Arthropoda (the second cladogram above). File:Britannica 1911 Arthropoda - Peripatus head.png, Anatomy of onychophoran anterior region, showing specialized appendages (ant, md) and deutocerebrum (D) File:Tardigrade zenuw.jpg,

Nervous system In biology, the nervous system is the complex system, highly complex part of an animal that coordinates its behavior, actions and sense, sensory information by transmitting action potential, signals to and from different parts of its body. Th ...

of a

tardigrade Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them . In 1776, th ...

, showing ventral nerve cord with segmented ganglia (ga1-4) File:20200213 Remipedia Nectiopoda central nervous system.png,

of a remipede

arthropod 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 ...

, showing the presence of both deutocerebrum (dc) and ventral nerve cord (vnc) organized by segmented ganglia The "

" are

; they include the last common ancestor of arthropods, onychophorans and tardigrades. '' Sialomorpha'', a genus of microinvertebrate discovered in Dominican amber in 2019, is also considered to be a panarthropod. However, due to the unusual combination of tardigrade and

mite Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as eac ...

-like characteristics, its exact placement is uncertain.

References

{{Taxonbar, from=Q134870 Extant Cambrian first appearances Ecdysozoa unranked clades

Taxonomy

Interrelationships

See also

References