| Tactopoda | |
|---|---|
 | |
| The tardigradeHypsibius dujardini | |
 | |
| The blue crabCallinectes sapidus, an arthropod | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Clade: | Nephrozoa |
| Clade: | Protostomia |
| Superphylum: | Ecdysozoa |
| (unranked): | Panarthropoda |
| (unranked): | Tactopoda Budd, 2001[1] |
Tactopoda orArthropodoidea is a proposedclade ofecdysozoan animals that includes the phylaTardigrada andEuarthropoda, supported by various morphological observations.[1][2][3] Thecladogram below shows the relationships implied by this hypothesis.
The competing hypothesis is thatAntennopoda[4][5] (=Euarthropoda +Onychophora, the arthropods and the velvet worms) is monophyletic,[6] and tardigrades lie outside this grouping.
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 adorsalheart) 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 tardigradestem 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 invokedphylogenomic evidence in favour of the alternative Euarthropoda+Onychophora grouping.[6]
Budd formed the suggested clade name 'tactopoda' from Greektaktos, ordered, andpoda, feet, "with reference to the alleged well-formed stepping motion that characterises the group".[1]
| Panarthropoda |
| ||||||
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