Hesslandona angustata (Phosphatocopida, Crustacea) from the Upper Cambrian of western Hunan, South China, with comments on phosphatocopid phylogeny

摘要

Although fossils of 'Orsten'-type preservation from the Cambrian of China represented by Skaracarida and Phosphatocopida were first reported from localities in western Hunan in 2005 (DONG et al. 2005), the species of Phosphatocopida have not been appropriately described and elucidated in terms of evolution so far. Here, we re-describe the phosphatocopid Hesslandona angustata, originally described from Swedish 'Orsten' limestone, based on new specimens with preserved soft-body details. Two specimens have four pairs of limbs and thus represent head larvae. Based on the previously published material and the present one, comparative morphology investigation of limbs mainly within Phosphatocopida is accomplished. As a main result of the analysis a two-divided mandibular limb stem in younger stages, a plesiomorphic state retained from the stem species of Labrophora, should probably be present in many phosphatocopid taxa, and should be regarded as a synapomorphy of Phosphatocopida and Eucrustacea. The two parts of the limb stem, i.e. a coxa and a basipod, become fused with each other to form an undivided limb stem in later developmental stages of all known euphosphatocopids. Phosphatocopids in an 'Orsten' type of preservation have been studied for over 30 years. However the only computer-based phylogenetic analysis of this taxon (MAAS et al. 2003) is now six years old and, therefore, does not include new data brought up in China in the last four years. Accordingly, a new phylogenetic analysis is accomplished herein. Based on the analysis we re-establish the phylogeny of Phosphatocopida and discuss the evolution within this group of crustaceans.