A palaeontological solution to the arthropod head problem | Semantic Scholar

New evidence from the Cambrian stem-group euarthropod Parapeytoia is presented to suggest that an originally protocerebral appendage persisted well up into the upper stem- group of the euARthropods, which prompts a re-evaluation of widely-accepted segmental homologies and the interpretation of fossil central nervous systems.

The presence of crustaceomorph traits in the Cambrian larvae of various clades basal to Mandibulata is reinterpreted as evidence for the existence of distinct ontogenetic niches among stem arthropods and Hymenocarines now illustrate that the subdivision of the basipod and the presence of proximal endites are likely to have been ancestral conditions critical for the evolution of coxal and pre-coxal features in mandibulates.

The head structures of several important taxa, Fuxianhuia, Canadaspis, Odaraia, Chengjiangocaris and Branchiocaris are redescribed, revealing the essential similarity between them and an anterior sclerite, usually bearing eyes, appears to be a widespread feature of basal arthropods.

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Morphologically some characters – the presence of gonopores on the trunk and absence of a labrum, nephridia and intersegmental tendons – support Cormogonida (Euarthropoda excluding pycnogonids), yet even the putative autapomorphy of chelicerae needs to be treated with caution.

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The present analysis supports the basal position of the Stomatopoda, and suggests a sister-group relationship is suggested between Euphausiacea and Peracarida (including Thermosbaenacea), with the Syncarida as the sister group to both taxa.

The phylum Tardigrada is an engaging but enigmatic group composed of minute metazoans with four pairs of stubby lobopodous appendages that share important derived characters with arthropods but, as a consequence, the phylogenetic position of the taxon remains uncertain.