AN EDIACARAN-LIKE ARTHROPOD FROM THE KAILI BIOTA (MIDDLE CAMBRIAN, GUIZHOU PROVINCE, CHINA) - A MISSING LINK FOR ASSESSING THE EARLY EVOLUTION OF ARTHROPODS

Parvancorina minchami was one of the earlier fossils described from Neoproterozoic rocks in the Ediacara Hills of South Australia, but its affinity with Phanerozoic metazoans is poorly understood. The discovery of a Parvancorina-like arthropod in the Middle Cambrian strengthens the hypothesis that some arthropods originated in the Precambrian and survived through the Neoproterozoic-Cambrian transition.

Nonmineralizing Parvancorina-like fossils are found in silty, micaceous mudshales of the middle Kaili Formation near Balang, Guizhou Province, China, and are associated with age-diagnostic early Middle Cambrian trilobites (e.g., Orytocephalus indicus, Pagetia prolata and Xingrenaspis). Some Kaili specimens are broken along the exoskeletal surface and show external relief, while others are split though the fossil and display internal soft parts preserved as color stains. Various degrees of taphonomic preservation in the Kaili materials allow a useful comparison with Neoproterozoic and other Phanerozoic taxa. Kaili specimens have an anchor-shaped anteromedial ridge on an undifferentiated dorsal shield, presumed homologous with the anchor-shaped structure of Parvancorina. Other inferred closely related taxa occur elsewhere in the Cambrian strata (e.g., Skania from the Burgess Shale Biota and Primicaris from the Chengjiang Biota). Fossils of these primitive arthropods collectively have a single-piece exoskeleton with a marginal rim, a labrum, and paired, transverse linear stains reflecting appendage positions.

Interpretation of Parvancorina-like fossils from Kaili as part of a larger Neoproterozoic – Cambrian arthropod clade supports the following notions: 1) at least one group of arthropods had originated prior to the Cambrian “Explosion”, 2) post-depositional modifications complicate the paleobiological and taxonomic interpretation of Parvancorina and other Ediacaran taxa, and 3) the Kaili materials, representing a taphonomic bridge between Ediacaran fossils and fossils from other Cambrian konservat-lagerstätten, permit external and internal examination of this primitive arthropod group, and allow separate assessments of body segments and dorsal shield morphology.