Relationships of Cambrian Arachnata and the systematic position of Trilobita

Journal of Paleontology, Mar 1999 by Edgecombe, Gregory D, Ramskold, Lars

ABSTRACT-Cladistic relationships of Trilobita, Naraodae (five ingroup taxa), Helmetda (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 helmetd relationships. A monophyletic Trilobita [="Olenellida" (Emuellida Eutrilobita)) is defined by numerous synapomorphies, including exoskeletal calcification and dorsal eyes with calcified lenses and circumocular sutures. Helmetda is a robust clade, resolved as (Helmetdae (Tegopeltidae (Saperdae Skiolddae))). Naraod monophyly is well-supported, but neither a naraod-trilobite nor a naraodRetifacies 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, naraods, and apparently helmetds is based beneath the cephalothoracic articulation. Reweighted characters favor Trilobita and Helmetda as closest relatives, with Petalopleura and then Naraodae as sister groups.

INTRODUCTION

HE SYSTEMATIC position of trilobites within the Arthropoda has long been a source of controversy. A range of mostly Cambrian taxa that have been dubbed trilobitomorphs (Stormer, 1944) or trilobitoids (Stormer, 1959) serve as the best supported candidates for closest relatives of the Trilobita. The objective of this paper is to examine cladistic relationships in trilobite-allied Cambrian Arachnata based upon parsimony analysis of 29 characters.

Among Cambrian arachnate groups that have been of uncertain phylogenetic position (because of inadequate morphological knowledge) are the Helmetdae Simonetta and Delle Cave, 1975, and Tegopeltidae Simonetta and Delle Cave, 1975. Until recently helmetds and tegopeltids were known solely from rare examples of the nominate genera, Helmetia Walcott, 1918, and Tegopelte Simonetta and Delle Cave, 1975, in the Middle Cambrian Burgess Shale. Documentation of the Early Cambrian Chengjiang fauna of Yunnan, China, revealed helmetds and tegopeltids to be widespread Cambrian groups including additional taxa. Hou (1987) identified two new helmetds in the Chengjiang fauna, Kuamaia lata and Rhombicalvaria acantha, and Hou, Ramskold and Bergstrom (1991) diagnosed a tegopeltid, Saperion glumaceum, more recently assigned to a monotypic Family Saperdae Hou and Bergstr6m, 1997. Hou and Bergstrom, 1997. Hou and Bergstrom (1997) have since added the new taxa Skoeldia aldna (Family Skoelddae Hou and Bergstrom, 1997) and Kuamaia muricata, and united the helmetds, Skioldia, Tegopelte, and Saperion as the Order Helmetda. One of the objectives of this work is to present morphological findings on the best known helmetd, Kuamia lata, and tegopeltid/saperd, Saperion glumaceum, as they bear on character analysis of the Cambrian Arachnata. We draw on recently illustrated specimens of these species (Chen et al., 1996) housed in the Early Life Research Centre of the Nanjing Institute of Geology and Palaeontology. Localities for Chengjiang specimens cited in this work are described by Chen, Zhou, and Ramskold (1995).

Our study complements the recent consideration of early arachnate relationships by Hou and Bergstrom (1997). Differences in our results are in part empirical and in part analytical. On the first score, we present differing interpretations of morphology, include new characters, and reject some characters used by those authors. Methodologically, we depart from Hou and Bergstrom in not accepting plesiomorphy as phylogenetically informative, in attempting to track all implied character transformations, in rejecting paraphyletic groups, and in avoiding erection of monotypic taxa out of adherence to Linnean categories. The basis for our study is cladistic parsimony.

TERMINAL TAXA

The history of research has seen Trilobita closely allied to a wide range of arthropod taxa. Most of these hypotheses have been convincingly refuted and are not subjected to further testing herein. All recent studies that have employed phylogenetic systematics recognize trilobites as more closely related to Chelicerata than to Crustacea (Bergstrom, 1992; Wills et al., 1994, 1998), with trilobites and related stem-group chelicerates being classified in Arachnata (Lauterbach, 1980, 1983; see Chen et al., 1997, for phylogenetic definition). An especially close (i.e., ancestor or sister group) relationship between trilobites and chelicerates (Lauterbach, 1980, 1983, 1989) has not been supported in recent work (e.g., Wills et al., 1998). Instead, trilobites are part of an imprecisely resolved grade of Early Paleozoic taxa. This analysis considers taxa that have been allied to trilobites in recent analyses and those that may occupy a nearly basal position on the lineage leading to chelicerates.