Three head-larvae of Hesslandona angustata (Phosphatocopida, Crustacea) from the Upper Cambrian of western Hunan, South China and the phylogeny of Crustacea

We describe three head-larvae of the phosphatocopid Hesslandona angustata of ‘Orsten’-type preservation from the Upper Cambrian Bitiao Formation in Wangcun section, western Hunan, South China. The head-larvae are small (around 300 μm long) and bear four pairs of functional limbs, presumably representing the first ontogenetic stage. A comparison with the head-larvae of Vestrogothia spinata from the same locality and horizon indicates that these head-larvae are identical in terms of appendage design. These head-larvae allow further comparison with phosphatocopids from other ontogenetic stages and other localities. We take these two species as representatives of Euphosphatocopida and include their ontogenetic information, combined with other ‘Orsten’-type stem-lineage crustaceans and crown-group crustaceans, into a computer-based cladistic analysis to explore the relationships of early crustaceans. The phylogeny of the Crustacea sensu lato is reconstructed and the autapomorphies of each node on the phylogenetic tree are given. The occurrence and development of the proximal endite (and its derivatives) can be used to trace the evolution of crustaceans. The evolutionary path from the origin of crustaceans to crown-group crustaceans consists of five steps. These steps are (1): the proximal endite occurred but only on the third pair of limbs and only in later larval stages, (2): the proximal endite occurred on the third pair of limbs since the first ontogenetic stage and later also on the second pair of limbs, (3): the proximal endite occurred on all the post-antennular limbs simultaneously since the first ontogenetic stage, (4): the proximal endites on the second and third pairs of limbs were enlarged to form a coxa and (5): the proximal endites on post-mandibular limbs were enlarged to form a coxa in the malacostracan eucrustaceans.