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. 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. Arachnomorpha are considered the sister group to the crustaceans, which are increasingly being accepted as members of the mandibulate clade (including insects and myriapods).
The arachnomorph concept has been challenged by suggestions that the trilobites fall in the mandibulata stem-group.
There is no consensus as to assigning Arachnomorpha a formal Linnean rank.
A proposal contraposing many synapomorphies unites them to the Trilobita instead. Consideration of the Olenellinae as sister group to the Chelicerata has been refuted.
Classification.
Arachnomorpha [= Arachnata , = Palaeopoda ]
Phylogeny.
Using fossil data, Bergström & Hou (2003) gave an outline of arthropod relationships emphasizing trilobitomorphs (a group that includes trilobites and trilobite-like animals).