Cambrian Naraoiids (Arthropoda): Morphology, Ontogeny, Systematics, and Evolutionary Relationships

Abstract

Naraoiids, defined as lightly sclerotized arthropods with a dimidiate tergum of two sclerites separated by a single transverse articulation, have been found in the Cambrian and Silurian. During the Cambrian they had a wide distribution coinciding with trilobite realms. This pattern may be related to the breakup of a Neoproterozoic supercontinent, probably Pannotia, which implies that naraoiids originated before the Cambrian “explosion.” Based on new observations on the original material from the Burgess Shale (Middle Cambrian, British Columbia),Naraoia haliais reconsidered as a valid species. The validity is further confirmed by a new record of the occurrence of this species in the Chengjiang Lagerstätte (Lower Cambrian, China). In addition, some structures ofN. compactaof the Burgess Shale have been reinterpreted. Two more naraoiid species are redescribed in detail from the Chengjiang Lagerstätte on the basis of more than 1,000 well-preserved specimens.Naraoia spinosashows dimorphism andMisszhouia longicaudataexhibits geographical variation in the overall shape of the dorsal exoskeleton. Naraoiids may have a protaspis-like larva, but the previously assigned protaspis has proven to be a separate taxon,Primicaris. In dorsal view, naraoiids resemble a giant “degree 0” meraspis (i.e., without thorax), and could have originated from different heterochronic processes, neoteny or hypermorphosis. Naraoiids are generally accepted as vagrant benthos. A predatory/scavenging life mode is supported by functional morphology and recent analogues. A healed injury inM. longicaudatasuggests that they could be the prey of larger predators, most likely anomalocaridids. We suggest that differences in exopod composition might represent evolutionary changes through the Early–Middle Cambrian. The monophyly of the Naraoiidae is not firmly established. Similarity to liwiids, supposed to be the close relatives of naraoiids, is limited to overall shape. We exclude naraoiids from the Trilobita, though there do exist a number of similarities between them.