Biology:Tactopoda

Short description: Group of ecdysozoan animals

Tactopoda Temporal range: Fortunian–Present PreЄ Є O S D C P T J K Pg N
The tardigrade Hypsibius dujardini
The blue crab Callinectes sapidus, an arthropod
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
Clade:	Protostomia
Superphylum:	Ecdysozoa
Clade:	Panarthropoda
(unranked):	Tactopoda Budd, 2001

Tactopoda or Arthropodoidea is a proposed clade of protostome animals that includes the phyla Tardigrada and Euarthropoda, supported by various morphological observations. The cladogram below shows the relationships implied by this hypothesis.

Panarthropoda	Onychophora     Tactopoda   Tardigrada     Euarthropoda

The competing hypothesis is that Antennopoda (= Euarthropoda + Onychophora, the arthropods and the velvet worms) is monophyletic, and tardigrades lie outside this grouping.

Panarthropoda	Tardigrada     Antennopoda   Onychophora     Euarthropoda

Anatomic arguments for the tactopoda monophyly include similarities in the anatomies of head, legs, and muscles between the arthropods and the tardigrades. Anatomic arguments against it include that tardigrades 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 tardigrades 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 the tardigrade 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. However, Gregory Edgecombe also invoked phylogenomic evidence in favour of the alternative Euarthropoda+Onychophora grouping.

Etymology

Budd formed the suggested clade name 'tactopoda' from Greek taktos, ordered, and poda, feet, "with reference to the alleged well-formed stepping motion that characterises the group".

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
        • Class Ichthyostraca
      • Clade Altocrustacea
        • Clade Multicrustacea
          • Class Hexanauplia
          • Class Malacostraca
          • Class Thecostraca
        • 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     Ichthyostraca     Altocrustacea Multicrustacea   Hexanauplia     Malacostraca     Thecostraca     Allotriocarida Xenocarida   Remipedia     Cephalocarida       Branchiopoda   Hexapoda Ellipura   Collembola     Protura       ?Diplura       ?Diplura   Ectognatha   Insecta

References

Wikidata ☰ Q15974773 entry