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Relationships of Cambrian Arachnata and the systematic position of Trilobita

Published online by Cambridge University Press:  20 May 2016

Gregory D. Edgecombe  and
Lars Ramsköld
Gregory D. Edgecombe
Affiliation:
Centre for Evolutionary Research, Australian Museum, 6 College Street, Sydney South, NSW 2000, Australia
Lars Ramsköld
Affiliation:
Museum of Palaeontology, University of Uppsala, Norbyvägen 22, S-752 36 Uppsala, Sweden
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Abstract

Cladistic relationships of Trilobita, Naraoiidae (five ingroup taxa), Helmetiida (five ingroup taxa), Xandarellida, and the Cambrian arachnates Retifacies, Sinoburius, Emeraldella, and Sidneyia are investigated based on 29 characters. Documentation of appendage morphology and other ventral structures in Saperion from the Chengjiang fauna permits an appraisal of helmetiid relationships. A monophyletic Trilobita [=“Olenellida” (Emuellida + Eutrilobita)] is defined by numerous synapomorphies, including exoskeletal calcification and dorsal eyes with calcified lenses and circumocular sutures. Helmetiida is a robust clade, resolved as (Helmetiidae (Tegopeltidae (Saperiidae + Skioldiidae))). Naraoiid monophyly is well-supported, but neither a naraoiid-trilobite nor a naraoiid-Retifacies clade are parsimonious, the latter grouping (“Nectopleura”) being explicitly paraphyletic. A sister group relationship between Xandarellida and Sinoburius is endorsed, although character support is novel compared to previous groupings of these taxa. The fourth postantennal limb pair in trilobites, naraoiids, and apparently helmetiids is based beneath the cephalothoracic articulation. Reweighted characters favor Trilobita and Helmetiida as closest relatives, with Petalopleura and then Naraoiidae as sister groups.

Type
Research Article
Information
Journal of Paleontology , Volume 73 , Issue 2: Papers from the Second International Trilobite Conference, August 1997 , March 1999 , pp. 263 - 287
Copyright
Copyright © The Paleontological Society 

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