Emeraldella brocki, from Walcott (1912).

Belongs within: Euarthropoda.
Contains: Pycnogonida, Euchelicerata.

The Cheliceriformes are here recognised as a group uniting the Chelicerata with related Palaeozoic fossil forms. Members of this group have a postabdomen lacking appendages, and commonly have dorsal, sessile (unstalked) eyes. Living members of this clade belong to the Chelicerata, which ancestrally have the first pair of head appendages formed as pincered chelicerae with the terminal segment opposed to a spinose projection of the penultimate segment (Cotton & Braddy 2004). The sister group of the Chelicerata may be the Vicissicaudata, a group of early Palaeozoic arthropods with a differentiated postabdomen, commonly with a long telsonic spine or appendicular derivatives on the pretelson.

Cheliceriformes [Protochelicerata]
    |--Chelicerata KBR09
    |   |--Pycnogonida RS10
    |   `--Euchelicerata RS10
    `--Vicissicaudata [Emeraldellidea] LSE13
         |--Sanctacaris Briggs & Collins 1988 CB04 [Sanctacarida, Sanctacaridea B95]
         |    |--‘Utahcaris’ orion Conway Morris & Robison 1988 B95
         |    `--Sanctacaris uncata Briggs & Collins 1988 WD02
         `--Emeraldellacea B95
              |--+--Sidneyia Walcott 1911 LSE13, CB04 [Limulavida, Sidneyidea, Sidneyiida]
              |  |    `--S. inexpectans Walcott 1911 CB04
              |  `--+--+--Molariida B95
              |     |  |    |--Molaria Walcott 1912 LSE13, CB04
              |     |  |    `--Emeraldoides Simonetta 1964 B95
              |     |  `--Emeraldella Walcott 1912 LSE13, CB04 [Emeraldellida]
              |     |       |--E. brocki Walcott 1912 CB04
              |     |       `--E. brutoni LSE13
              |     `--Cheloniellida [Cheloniellonida] LSE13
              |          |--Neostrabops Caster & Macke 1952 LSE13, H75
              |          `--+--Pseudoathron LSE13
              |             `--+--Cheloniellon Broili 1932 LSE13, B95
              |                |    `--C. calmani Broili 1932 CB04
              |                `--+--Triopus [Triopida] LSE13
              |                   `--+--Paraduslia LSE13
              |                      `--Duslia Jahn 1893 LSE13, S60
              |                           `--*D. insignis Jahn 1893 S60
              `--Aglaspidida [Aglaspida] LSE13
                   |  i. s.: Paleomerus hamiltoni Størmer 1956 CB04
                   |         Borchgrevinkium B95
                   |         Strabops [Strabopidae] B95
                   |         Khankaspis bazhanovi Repina & Okuneva 1969 CB04
                   |         Lemoneites Flower 1968 CB04
                   |--Kwanyinaspis LSE13
                   `--+--Quasimodaspis LSE13
                      `--+--+--Beckwithia LSE13
                         |  `--Kodymirus vagans Chlupác & Havlícek 1965 LSE13, CB04
                         `--+--Uarthrus LSE13
                            `--+--+--Flobertia kochi Hesselbo 1992 LSE13, CB04
                               |  `--+--Tremaglaspis LSE13
                               |     `--Chlupacaris LSE13
                               `--+--+--Australaglaspis LSE13
                                  |  `--Chraspedops LSE13
                                  `--+--Aglaspella LSE13
                                     `--+--Aglaspis Hall 1862 LSE13, B95
                                        |    `--A. spinifer Raasch 1939 CB04
                                        `--Glypharthrus LSE13

Inorganic: Sidneyia groenlandica Cleaves in Cleaves & Fox 1935 H75

*Type species of generic name indicated


[B95] Bousfield, E. L. 1995. A contribution to the natural classification of Lower and Middle Cambrian arthropods: food-gathering and feeding mechanisms. Amphipacifica 2: 3–34.

[CB04] Cotton, T. J., & S. J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 169–193.

[H75] Häntzschel, W. 1975. Treatise on Invertebrate Paleontology pt W. Miscellanea Suppl. 1. Trace Fossils and Problematica 2nd ed. The Geological Society of America: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[KBR09] Kühl, G., D. E. G. Briggs & J. Rust. 2009. A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsrück Slate, Germany. Science 323: 771–773.

[LSE13] Legg, D. A., M. D. Sutton & G. D. Edgecombe. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4: 2485.

[RS10] Regier, J. C., J. W. Shultz, A. Zwick, A. Hussey, B. Ball, R. Wetzer, J. W. Martin & C. W. Cunningham. 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature 463: 1079–1083.

[S60] Smith, A. G. 1960. Amphineura. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I41–I76. Geological Society of America, and University of Kansas Press.

[WD02] Waloszek, D., & J. A. Dunlop. 2002. A larval sea spider (Arthropoda: Pycnogonida) from the Upper Cambrian ‘Orsten’ of Sweden, and the phylogenetic position of pycnogonids. Palaeontology 45 (3): 421–446.

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