Relationships of Cambrian Arachnata and the systematic position of Trilobita

Cladistic relationships of Trilobita, Naraoiidae (five ingroup taxa), Helmetiida (five ingroup taxa), Xandarellida, and the Cambrian arachnatesRetifacies, Sinoburius, Emeraldella, andSidneyiaare investigated based on 29 characters. Documentation of appendage morphology and other ventral structures inSaperionfrom 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-Retifaciesclade are parsimonious, the latter grouping (“Nectopleura”) being explicitly paraphyletic. A sister group relationship between Xandarellida andSinoburiusis 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.