Panarthropoda

Panarthropoda is a clade comprising the greatest diversity of animal groups. It contains the extant phyla Arthropoda (Euarthropoda), Tardigrada (water bears) and Onychophora (velvet worms),<ref name=Telford2008/><ref name=WuPisaDono23/><ref name=GiacVeccGuidRebe24/> although the relationships among these remained uncertain according to studies published in 2023 and 2024.<ref name=WuPisaDono23>Template:Cite journal</ref><ref name=GiacVeccGuidRebe24>Template:Cite journal</ref> Panarthropods also include extinct marine legged worms known as lobopodians ("Lobopodia"), a paraphyletic group where the last common ancestor and basal members (stem-group) of each extant panarthropod phylum are thought to have risen.<ref name=":0"/><ref name=OrtegaHernandez2016/><ref name=":3">Template:Cite journal</ref><ref name=":4">Template:Cite journal</ref><ref name=":5">Template:Cite journal</ref><ref name=":6">Template:Cite journal</ref> However the term "Lobopodia" is sometimes expanded to include tardigrades and onychophorans as well.<ref name=OrtegaHernandez2016/>

Common characteristics of the Panarthropoda include a segmented body, paired ladder-like ventral nervous system, and the presence of paired appendages correlated with body segments.<ref name=":1"/><ref name="OrtegaHernandez2016"/><ref name=":3"/><ref name=":5"/>

Taxonomy

Not all studies support the monophyly of Panarthropoda,<ref name="nature phylo">Template:Cite journal</ref> but most do, including neuroanatomical,<ref>Template:Cite journal</ref> phylogenomic<ref name=":1">Template:Cite journal</ref><ref name=":17" /><ref name=":18" /> and palaeontological<ref name=":2">Template:Cite journal</ref><ref name=":0">Template:Cite journal</ref><ref name="OrtegaHernandez2016"/><ref name=":3"/> studies. At least a close relationship between onychophorans and arthropods is widely agreed upon, but the position of tardigrades is more controversial.<ref name="Giribet2017"/> Some phylogenomic studies have found tardigrades to be more closely related to nematodes.<ref name="Laumer2019"/><ref name="Smythe2019"/> Traditionally, panarthropods were considered to be closely related to the annelids, grouped together as the Articulata (animals with body segments), but subsequent phylogenomic studies consistently place them closer to cycloneuralians (nematodes, nematomorphs, loriciferans, kinorhynchas and priapulids), grouped together as Ecdysozoa.<ref>Template:Cite journal</ref><ref name=":19" /><ref name=":17"/><ref name=":18"/> While annelids are placed among the Spiralia (making them more closely related to mollusks, flatworms and such),<ref name=":19">Template:Cite journal</ref> having evolved their segmented bodies convergently.<ref>Template:Cite journal</ref>

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 relationship between Onychophora and Tardigrada (Lobopodia sensu Smith & Goldstein 2017).<ref>Template:Cite journal</ref><ref name=GiacVeccGuidRebe24/>

Tactopoda had been supported by mitochondrial gene arrangements,<ref>Template:Cite journal</ref> palaeontological<ref>Template:Cite journal</ref><ref name=":0"/> and neuroanatomical evidences, specifically the presence of segmented ganglia shared by arthropods and tardigrades.<ref>Template:Cite journal</ref> 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.<ref name=":2" /><ref>Template:Cite journal</ref> 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 last common ancestor of Panarthropoda or Ecdysozoa.<ref name=":0" /><ref name="OrtegaHernandez2016"/><ref name=":7">Template:Cite journal</ref><ref name=":3"/> 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,<ref name=":1"/> while the others mostly recovering either Antennopoda or Onychophora+Tardigrada.<ref name=":1"/><ref name=":17">Template:Cite journal</ref><ref name=":18">Template:Cite journal</ref> A study published in December 2024 supported the sister relationship of Onychophora and Arthropoda.<ref name=GiacVeccGuidRebe24/>

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  Anatomy of onychophoran anterior region, showing specialized appendages (ant, md) and deutocerebrum (D)
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  Nervous system of a tardigrade, showing ventral nerve cord with segmented ganglia (ga1-4)
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  Nervous system of a remipede arthropod, showing the presence of both deutocerebrum (dc) and ventral nerve cord (vnc) organized by segmented ganglia

The "Lobopodia" are paraphyletic; they include the last common ancestor of arthropods, onychophorans and tardigrades.<ref>Template:Cite journal</ref>

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-like characteristics, its exact placement is uncertain.<ref>Template:Cite journal</ref>

See also

• List of bilaterial animal orders