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The early history and phylogeny of the chelicerates



Jason A. Dunlop & Paul A. Selden
Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK


Summary:
1. Chelicerata Heymons, 1901 belongs to a broad clade of arthropods, the Arachnomorpha Heider, 1913.
2. Chelicerata is monophyletic, diagnosed as above, and is restricted to Lemoneites, Xiphosura, Chasmataspida, Eurypterida and Arachnida.
3. Eleven opisthosomal segments appears to be primitive for Chelicerata, as it occurs in Lemoneites, Xiphosura and outgroups such as Paleomerus. Chasmataspids are not xiphosurans and are placed as the sister group of Eurypterida + Arachnida, with which they share the synapomorphy of 12 opisthosomal segments.
4. Xiphosura, Eurypterida, Chasmataspida and Scorpiones all possess autapomorphies and thus none is ancestral to any other chelicerate group. Arachnida emerges as a monophyletic taxon while Merostomata is paraphyletic.
5. The Ordovician Lemoneites appears very close to the origins of the chelicerates, but is too poorly preserved to resolve whether it is a primitive synziphosurine, sister group to chasmataspids, eurypterids and arachnids, or a sister group to all other chelicerates.
6. Presence of chelicerae is difficult to determine in many Lower Palaeozoic fossils and the presence of median eyes (or their tubercle) and some degree of differentiation in the opisthosoma into a preabdomen and postabdomen are better characters for recognising a fossil chelicerate.
7. Neostrabops, Duslia, Triopus, Pseudarthron and Cheloniellon form a monophyletic group, Cheloniellida, placed as sister group to Chelicerata in this analysis, and is united on the character of a procurved posterior carapace margin.
8. (Paleomerus, Strabops + Aglaspidida) represents the sister group to Chelicerata and Cheloniellida. Aglaspidids represent a poorly defined group which warrants revision and it is difficult to resolve the position of aglaspidids relative to chelicerates and the Paleomerus/Strabops lineage.
9. Cheloniellida, Aglaspida and Chelicerata represent three distinct arachnomorph taxa, all of which cpuld be derived from a Paleomerus/Strabops-like lineage, tough their relationships to the other Arachnomorpha remain uncertain.

Arthropod Relationships, pp. 237-245, R. A. Fortey & R. H. Thomas (eds.), Systematics Association Special Volume Series 55 (1997)

Authors comments:

Some of this work, especially that relating to arachnid monophyly and the position of the chasmataspids has now been superceded somewhat by my more recent papers. However, the general aim of this work relating to Chelicerata as a whole remains sound.

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