Panarthropoda is aclade comprising the greatest diversity ofanimal groups. It contains the extantphylaArthropoda (Euarthropoda),Tardigrada (water bears) andOnychophora (velvet worms),[3][4][5] although the relationships among these remained uncertain according to studies published in 2023 and 2024.[4][5]Panarthropods also includeextinctmarine legged worms known as lobopodians ("Lobopodia"), aparaphyletic group where thelast common ancestor and basal members (stem-group) of each extant panarthropod phylum are thought to have risen.[6][7][8][9][10][11] However the term "Lobopodia" is sometimes expanded to include tardigrades and onychophorans as well.[7]
There are three competing hyphotheses for the interrelationship between the extant panarthropod phyla, each known asTactopoda (Arthropoda+Tardigrada),Antennopoda (Arthropoda+Onychophora), and thesister relationship between Onychophora and Tardigrada (Lobopodiasensu Smith & Goldstein 2017).[24][5]
Tactopoda had been supported bymitochondrial gene arrangements,[25] palaeontological[26][6] and neuroanatomical evidences, specifically the presence of segmentedganglia shared by arthropods and tardigrades.[27] Antennopodia united by the presence of specialized head appendages and deutocerebrum (additional second section of the brain), but subsequent anatomical studies suggest these features wereconvergently evolved between onychophoran and arthropod lineages.[17][28] Onychophorans and tardigrades shared somelobopodian traits (e.g. soft cuticle, lobopodous appendages and peripheral nerve roots), but these were generally considered to beplesiomorphies traced back to thelast common ancestor of Panarthropoda orEcdysozoa.[6][7][29][8] 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,[12] while the others mostly recovering either Antennopoda or Onychophora+Tardigrada.[12][15][16] A study published in December 2024 supported the sister relationship of Onychophora and Arthropoda.[5]
Nervous system of aremipedearthropod, showing the presence of both deutocerebrum (dc) and ventral nerve cord (vnc) organized by segmented ganglia
The "Lobopodia" areparaphyletic; they include the last common ancestor of arthropods, onychophorans and tardigrades.[30]
Sialomorpha, a genus of microinvertebrate discovered inDominican amber in 2019, is also considered to be a panarthropod. However, due to the unusual combination of tardigrade andmite-like characteristics, its exact placement is uncertain.[31]
^abcdeOrtega-Hernández, Javier (2016). "Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848: Upper and lower stem-Euarthropoda".Biological Reviews.91 (1):255–273.doi:10.1111/brv.12168.ISSN1464-7931.PMID25528950.S2CID7751936.
^Dunn, C. W.; Hejnol, A.; Matus, D. Q.; Pang, K.; Browne, W. E.; Smith, S. A.; Seaver, E.; Rouse, G. W.; Obst, M.; Edgecombe, G. D.; Sørensen, M. V.;Haddock, S. H. D.; Schmidt-Rhaesa, A.; Okusu, A.; Kristensen, R. M. B.; Wheeler, W. C.; Martindale, M. Q.; Giribet, G. (10 April 2008). "Broad phylogenomic sampling improves resolution of the animal tree of life".Nature.452 (7188):745–749.Bibcode:2008Natur.452..745D.doi:10.1038/nature06614.PMID18322464.S2CID4397099.
^Persson, Dennis K. (November 2012). "Neuroanatomy ofHalobiotus crispae (Eutardigrada: Hypsibiidae): Tardigrade brain structure supports the clade panarthropoda".Journal of Morphology.273 (11):1227–1245.doi:10.1002/jmor.20054.PMID22806919.S2CID5260983.
^Poinar, George; Nelson, Diane R. (2019). "A new microinvertebrate with features of mites and tardigrades in Dominican amber".Invertebrate Biology.138 (4) e12265.doi:10.1111/ivb.12265.S2CID204157733.
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