Tactopoda

Tactopoda is a proposed clade of protostome animals that includes the phyla Tardigrada and Euarthropoda, supported by various morphological observations.[1][2] The cladogram below shows the relationships implied by this hypothesis.

Tactopoda
Waterbear.jpg
The tardigrade Hypsibius dujardini
The Childrens Museum of Indianapolis - Atlantic blue crab.jpg
The blue crab Callinectes sapidus, an arthropod
Scientific classification e
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Superphylum: Ecdysozoa
(unranked): Panarthropoda
(unranked): Tactopoda
Budd, 2001 [1]
Panarthropoda

Onychophora

Tactopoda

Tardigrada

Euarthropoda

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

Panarthropoda

Tardigrada

Arthropoda s.l.

Onychophora

Euarthropoda

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 classificationEdit

PhylogenyEdit

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

ReferencesEdit

  1. ^ a b c 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. ^ a b 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.