Arachnomorpha

Arachnomorpha is a subdivision or clade of Arthropoda, comprising the monophyletic group formed by the trilobites, other great appendage arthropods and trilobite-like families (Helmetiidae, Xandarellidae, Naraoiidae, Liwiidae, and Tegopeltidae), and a diverse sister clade including the chelicerates.[1][2] Great debate is held on the position of the Pycnogonida, which are currently thought not to be placed in the immediate vicinity of the Chelicerata.[3] Arachnomorpha are considered the sister group to the crustaceans, which are increasingly being accepted as members of the mandibulate clade (including insects and myriapods).[2]

Arachnomorpha Temporal range: Cambrian - Recent PreꞒ Ꞓ O S D C P T J K Pg N
"Arachnida" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Arachnomorpha Lameere, 1890
Synonyms
Arachnata Paulus, 1979 Palaeopoda Packard, 1903

The arachnomorph concept has been challenged by suggestions that the trilobites fall in the mandibulata stem-group.[4]

There is no consensus as to assigning Arachnomorpha a formal Linnean rank.

A proposal contraposing many synapomorphies unites them to the Trilobita instead.[5] Consideration of the Olenellinae as sister group to the Chelicerata has been refuted.[6]

Classification

• ?Pycnogonida Latreille, 1806 (incl. Pantopoda Gerstaecker, 1863)

Arachnomorpha Lameere, 1890 [= Arachnata Paulus, 1979, = Palaeopoda Packard, 1903]

• †Trilobita Walch, 1771
• †Megacheira Hou & Bergstrom, 1997
• †Strabopida Hou & Bergström, 1997
• †Aglaspida Walcott, 1911
• †Cheloniellida Broili, 1932
• Chelicerata Heymons 1901
  • Family †Sanctacarididae Legg & Pates, 2017
  • Class Pycnogonida Latreille, 1810
  • Clade Euchelicerata Weygoldt & Paulus, 1979
    • Genus †Offacolidae Sutton et al., 2002
    • Clade Prosomapoda Lamsdell, 2013

Phylogeny

Using fossil data, Bergström & Hou (2003) gave an outline of arthropod relationships emphasizing trilobitomorphs (a group that includes trilobites and trilobite-like animals).[7]

Arthropods Arachnomorpha   Limulavida Sidneyia       Aglaspidida     Chelicerates     Cheloniellon       Trilobitomorpha (s.s.) Marrellomorpha Acercostraca Vachonisia   Marellida   Mimetaster     Marrella       Emeraldellida Emeraldella   Retifaciida Retifacies   Core trilobitomorphs Concilioterga  Order Helmetiida   Helmetia     Kuamaia       Sapenion     Tegopelte       Nectopleura (e.g. Nektaspida) Naraoia   Petalopleura  Order Xandarellida   Cindarella       Xandarella     Phytophilaspis         Trilobites, subclass Trilobita                 Hexapoda     Crustaceans       Yohoia       Canadaspis     Fuxianhuia

References

1. Trevor J. Cotton & Simon J. Braddy (2003). "The phylogeny of arachnomorph arthropods and the origin of the Chelicerata". Transactions of the Royal Society of Edinburgh: Earth Sciences. 94 (3): 169–193. doi:10.1017/S0263593300000596.
2. Jonathan R. Hendricks & Bruce S. Lieberman (2008). "New phylogenetic insights into the Cambrian radiation of arachnomorph arthropods". Journal of Paleontology. 82 (3): 585–594. doi:10.1666/07-017.1.
3. J. A. Dunlop & C. P. Arango (2005). "Pycnogonid affinities: a review" (PDF). Journal of Zoological Systematics and Evolutionary Research. 43 (1): 8–21. doi:10.1111/j.1439-0469.2004.00284.x.
4. Gerhard Scholtz & Gregory Edgecombe (2005). "Heads, Hox and the phylogenetic position of trilobites". Crustacea and Arthropod Relationships. CRC Press. pp. 139–165. CiteSeerX 10.1.1.130.2585.
5. Karl-Ernst Lauterbach (1980). "Schlüsselereignisse in der Evolution des Grundplans der Arachnata (Arthropoda)" [Key events in the evolution of the ground plan of the Arachnata (Arthropoda)]. Abhandlungen des Naturwissenschaftlichen Vereins in Hamburg. NF (in German). 23: 163–327.
6. R. A. Fortey & H. B. Whittington (1989). "The Trilobita as a natural group" (PDF). Historical Biology. 2 (2): 125–138. doi:10.1080/08912968909386496.
7. Bergström, J. & Hou, X. G. (2003). "Arthropod origins" (PDF). Bulletin of Geosciences. 78 (4): 323–334. Archived from the original (PDF) on 2016-03-04. Retrieved 2014-03-03.