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Arachnomorpha; Fossil range: Cambrian - Present Day
	"Arachnida" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Kingdom:	Animalia;
Phylum:	Arthropoda;
(unranked):	Arachnomorpha;
	Trilobita Walch 1771; Megacheira; Strabopida Hou & Bergström, 1997; Aglaspida Walcott, 1911; Cheloniellida Broili, 1932; Chelicerata Heymons 1901 Class Xiphosura Latreille, 1802; Class Chasmataspida Caster & Brooks, 1956; Class Eurypterida Burmeister, 1843; Class Arachnida Lamarck, 1801; ;

Arachnomorpha ("Spider Form") Lameere 1890 is a subdivision or clade of Arthropoda, containing 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] ^[3] . 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 ^[4] Arachnomorpha are considered the sister group to the crustaceans, which are increasingly being accepted as members of the mandibulate clade (including insects and myriapods).^[3]

There is no consensus as to assigning Arachnomorpha a formal Linnean category. It could be a Subphylum or a Superclass and then its subgroups Cheliceromorphora and Trilobita would be ranked as various infrataxa and the Chelicerata sensu stricto would be a Class.

A proposal^[5] to consider the Olenellinae as sister group to the Chelicerata has been refuted^[6], which contraposed many synapomorphies uniting them to the Trilobita instead.

Synonyms include Arachnata Paulus, 1979 and Palaeopoda Packard 1903.

[edit] Classification of the Arachnomorpha

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

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

Trilobita Walch 1771
Strabopida Hou & Bergström, 1997
Aglaspida Walcott, 1911
Cheloniellida Broili, 1932
Chelicerata Heymons 1901 [= Euchelicerata Weygoldt & Paulus, 1979]
- Class Xiphosura Latreille, 1802
- Class Chasmataspida Caster & Brooks, 1956
- Class Eurypterida Burmeister, 1843
- Class Arachnida Lamarck, 1801

[edit] References

^ Cotton, T.J., & Braddy, S.J. 2004. Transactions of the Royal Society of Edinburgh: Earth Sciences, 94: 169–193
^ Cotton, Trevor J. (2003). "The phylogeny of arachnomorph arthropods and the origin of the Chelicerata". Transactions of the Royal Society of Edinburgh Earth Sciences 94. doi:10.1017/S0263593300000596.
^ ^a ^b Hendricks, J.R.; Lieberman, B.S. (2008), "New Phylogenetic Insights into the Cambrian Radiation of Arachnomorph Arthropods", Journal of Paleontology 82 (3): 585–594, doi:10.1666/07-017.1, http://www.bioone.org/perlserv/?request=get-abstract
^ Dunlop, J.A. & Arango, C.P. 2005. Journal of Zoological Systematics and Evolutionary Research, 43: 8-21.
^ Lauterbach, K.E. 1980. Abhandlungen des Naturwissenschaftlichen Vereins Hamburg (NF), 23:163-327.
^ Fortey, R.A., & Whittington, H.B. 1989. Historical Biology, 2:125–138.

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[0] Cotton, T.J., & Braddy, S.J. 2004. Transactions of the Royal Society of Edinburgh: Earth Sciences, 94: 169–193

[1] Cotton, Trevor J. (2003). "The phylogeny of arachnomorph arthropods and the origin of the Chelicerata". Transactions of the Royal Society of Edinburgh Earth Sciences 94. doi:10.1017/S0263593300000596.

[Hendricks2008-2] Hendricks, J.R.; Lieberman, B.S. (2008), "New Phylogenetic Insights into the Cambrian Radiation of Arachnomorph Arthropods", Journal of Paleontology 82 (3): 585–594, doi:10.1666/07-017.1, http://www.bioone.org/perlserv/?request=get-abstract

[3] Dunlop, J.A. & Arango, C.P. 2005. Journal of Zoological Systematics and Evolutionary Research, 43: 8-21.

[4] Lauterbach, K.E. 1980. Abhandlungen des Naturwissenschaftlichen Vereins Hamburg (NF), 23:163-327.

[5] Fortey, R.A., & Whittington, H.B. 1989. Historical Biology, 2:125–138.

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