Arachnomorpha

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;

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]

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

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.
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.
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.
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.
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.
R. A. Fortey & H. B. Whittington (1989). "The Trilobita as a natural group" (PDF). Historical Biology. 2 (2): 125–138. doi:10.1080/08912968909386496.
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.

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[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.

[Hendricks2008-2] 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.

[B&H2003-7] 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.