Morphology, Ontogeny and Phylogeny of the Phosphatocopina (Crustacea) from the Upper Cambrian "Orsten" of Sweden

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About 2500 specimens of different Phosphatocopina species from the Upper Cambrian "Orsten" of Sweden were investigated using scanning electron microscopy (SEM) with reference to their morphology, ontogeny, systematics and phylogeny. Major morphological aspects of the shields and partly of the ventral morphology are described and documented. Ontogeny was described and documented in detail for the best preserved species, Hesslandona unisulcata Müller, 1982. Characters of all species were compared and a character matrix was drawn up, forming the basis for a computer-aided phylogenetic analysis of the Phosphatocopina.

According to this analysis, the Phosphatocopina can be reconstructed as a monophyletic taxon, characterised by a special all-embracing bivalved shield and small antennulae (both autapomorphies). A recently discovered species of Phosphatocopina from the Lower Cambrian of England is, according to the analysis, the sister taxon to all investigated phosphatocopines, for which the name Euphosphatocopina is proposed. The characteristic interdorsum of several species, a cuticular plate separating the two shield halves, turned out to be an ingroup character, not present in the ground pattern of the Phosphatocopina, and still in the ground pattern of Euphosphatocopina only a simply furrow is developed. The next evolutionary stage has triangular plates anteriorly and posteriorly, which eventually become elongated into the dorsal interdorsum. This character state characterises the ground pattern of the Hesslandonina, the sister group of Falites fala Müller, 1964.

This intensive investigation of the Phosphatocopina and the phylogenetic analysis allows a better determination of the ground patterns of the taxa Labrophora sensu Siveter et al. ( = Eucrustacea + Phosphatocopina) and Eucrustacea sensu Walossek (1999). The stem species of Labrophora is characterised by a large set of autapomorphies compared with any of the stem lineage taxa of the Crustacea (serving as outgroup, taxa), such as the labrum, the sternum with paragnaths, and the development of coxal portions on the antennae and the mandibles (and only on these limbs). Besides the monophyletic Phosphatocopina, its sister taxon Eucrustacea can also be characterised by autapomorphies of its stem species, namely the development of a specific oligomeric hatching stage, the orthonauplius, and the modification of the first post-mandibular limb into a "mouthpart", the so-called maxillula, both characters plesiomorphically missing in the Phosphatocopina. Phosphatocopina can, therefore, be excluded from being an ingroup, taxon of the Eucrustacea, nor are they Ostracoda, into which they were originally placed due to their superficially similar bivalved shield. They are instead a monophyletic taxon, restricted to the early Palaeozoic, and the sister group of the Eucrustacea.

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