Relationships of Cambrian Arachnata and the systematic position of Trilobita

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.

Gravitational Constraints on the Burial of Chengjiang Fossils

“…The taxo− nomic sampling used by Cotton and Braddy (2004) included several megacheiran genera, representative chelicerates, a number of taxa representing groups that have been proposed as possible chelicerate relatives (e.g., Sidneyia, Aglaspis, Cheloniellon) in other analyses (Dunlop and Selden 1997;Hou and Bergström 1997), and members of the major lineages of the Artiopoda sensu Hou and Bergström (1997), i.e., trilo− bite−allied lamellipedians. Because artiopodan inter−relation− ships have remained quite stable through a series of phylogen− etic analyses (Edgecombe and Ramsköld 1999;Cotton and Braddy 2004;Hendricks and Lieberman 2008;Paterson et al 2010) and artiopodans are now often regarded as less closely related to Megacheira than is Chelicerata Cotton and Braddy 2004;Dunlop 2006), we have reduced the sampling of Artiopoda in the present analysis. To augment the diversity of "short great appendage" arthropods, Haikoucaris is added based on the description of H. ercaiensis by Chen et al (2004).…”

Trilobite body patterning and the evolution of arthropod tagmosis

“…(9,10) This clade, together with mandibulate arthropods, form the Euarthropoda. The position of the uniramous insects and myriapods within the Mandibulata has been contentious, but recent data suggest myriapods as the most basal mandibulate clade and insects as the most derived.…”

The Cambrian explosion: Its timing and stratigraphic setting

“…The base of the provisional Series 2 and Stage 3 is considered to approximately coincide with the appearance of the first trilobite sensu Fortey and Whittington (1997) and Edgecombe and Ramsköld (1999), related to the calcification of its exoskeleton. This definition excludes different Terreneuvian ichnofossils attributed to trilobites.…”

“…La base de la Série 2 et de l'Étage 3 provisoires est estimée comme coïncidant approximativement avec l'apparition du premier trilobite sensu Fortey et Whittington (1997) et Edgecombe et Ramsköld (1999), en lien avec la calcification de son exosquelette. Cette définition exclue différents ichnofossiles terreneuviens attribués aux trilobites.…”