Tactopoda

Tactopoda is a proposed clade of protostome animals that includes the phyla Tardigrada and Euarthropoda, supported by various morphological observations.[1][2]

Tactopoda
The tardigrade Hypsibius dujardini
The blue crab Callinectes sapidus, an arthropod
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
Superphylum:	Ecdysozoa
(unranked):	Panarthropoda
(unranked):	Tactopoda Budd, 2001 [1]

The competing hypothesis is that Arthropoda sensu lato[3] (= Euarthropoda + Onychophora, the arthropods and the velvet worms) is monophyletic.[4]

Anatomic arguments for the tactopoda monophyly include similarities in the anatomies of head, legs, and muscles between the arthropods and the tartigrades. Anatomic arguments against it include that tartigrades lack the kind of circulatory system (including a dorsal heart) which the arthropods and the velvet worms share. Graham Budd argued that the lack of this system in recent tartigrades is due to their miniature size, which makes a complex circulatory system superfluous; thus, the loss of this feature would be a secondary property, acquired as tartigrade stem group turned smaller, and both the Euarthropoda+Onychophora circulatory system and a relatively large size should be a feature of the last common ancestor of all three groups.[1] However, Gregory Edgecombe also invoked phylogenomic evidence in favour of the alternative Euarthropoda+Onychophora grouping.[4]

Proposed classification

• Phylum Tardigrada
  • Class Eutardigrada
  • Class Heterotardigrada
  • Class Mesotardigrada
• Phylum Arthropoda
  • Class Pycnogonida (?)
  • Clade Arachnomorpha
    • Class †Trilobita
    • Order †Aglaspida
    • Order †Strabopida
    • Class Pycnogonida (?)
    • Order †Cheloniellida
    • Subphylum Chelicerata
      • Class Xiphosura
      • Class †Eurypterida
      • Class †Chasmataspidida
      • Class Pycnogonida (?)
      • Class Arachnida
  • Clade Mandibulata
    • Order †Euthycarcinoidea
    • Subphylum Myriapoda
      • Class Chilopoda
      • Class Symphyla
      • Class Diplopoda
      • Class Pauropoda
    • Clade Pancrustacea
      • Clade Oligostraca
        • Class Ostracoda
        • Some of polyphyletic class Maxillopoda
      • Clade Altocrustacea
        • Clade Multicrustacea
          • The rest of polyphyletic class Maxillopoda
          • Class Malacostraca
        • Clade Allotriocarida
          • Class Branchiopoda
          • Clade Miracrustacea
            • Superclass Xenocarida
              • Class Cephalocarida
              • Class Remipedia
            • Subphylum Hexapoda
              • Paraphyletic class Entognatha
              • Clade Ectognatha
                • Class Insecta

Phylogeny

Panarthropoda

Velvet worms (Onychophora)   Tactopoda Tardigrada   Eutardigrada     Heterotardigrada     Mesotardigrada     Arthropoda   †Trilobitomorpha/Trilobita     ?Pycnogonida   Chelicerata   Xiphosura         †Eurypterida     †Chasmataspidida         Arachnida     ?Pycnogonida         Mandibulata Myriapoda   Symphyla     Chilopoda       Diplopoda     Pauropoda       Pancrustacea Oligostraca   Ostracoda     Some of Maxillopoda     Altocrustacea Multicrustacea   The rest of Maxillopoda     Malacostraca     Allotriocarida Xenocarida   Remipedia     Cephalocarida       Branchiopoda   Hexapoda Ellipura   Collembola     Protura       ?Diplura       ?Diplura   Ectognatha   Insecta

References

1. Graham E. Budd (2001). "Tardigrades as 'stem-group arthropods': the evidence from the Cambrian fauna" (PDF). Zoologischer Anzeiger. 240 (3–4): 265–279. doi:10.1078/0044-5231-00034. Archived from the original (PDF) on 2016-03-03.
2. Smith, M. R.; Ortega-Hernández, J. (2014). "Hallucigenia's onychophoran-like claws and the case for Tactopoda". Nature. 514 (7522): 363–366. doi:10.1038/nature13576. PMID 25132546.
3. J. Ortega-Hernández, "Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848," Biol. Rev., vol. 91, no. 1, pp. 255–273, 2016.
4. Gregory D. Edgecombe (2010). "Arthropod phylogeny: An overview from the perspectives of morphology, molecular data and the fossil record". Arthropod Structure & Development. 39: 74–87.