Tabelliscolex (Cricocosmiidae: Palaeoscolecidomorpha) from the early Cambrian Chengjiang Biota and the evolution of seriation in Ecdysozoa

Abstract

Cricocosmiidae is a clade of palaeoscolecid-like worms from the Chengjiang Biota, China (Cambrian Stage 3). In contrast with palaeoscolecids sensu stricto, which exhibit tessellating microplate trunk ornamentation, cricocosmiids have larger, serially repeated sets of trunk sclerites bearing a resemblance to lobopodian trunk sclerites (e.g. Microdictyon spp.). Cricocosmiidae have therefore been proposed as stem-group Panarthropoda in some studies, but are recovered as stem-group Priapulida in most phylogenetic analyses. The affinity of cricocosmiids within Ecdysozoa is therefore of much interest, as is testing the homology of these seriated structures. We report four new specimens of the rare cricocosmiid Tabelliscolex hexagonus, yielding new details of the ventral trunk projections, sclerites and proboscis. New data confirm that T. hexagonus had paired ventral trunk projections in a consistent seriated pattern, which is also reported from new material of Cricocosmia jinningensis (Cricocosmiidae) and Mafangscolex yunnanensis (Palaeoscolecida sensu stricto). Even when the seriated sclerites and ventral projections of cricocosmiids are coded as homologous with the seriated trunk sclerites and paired appendages, respectively, of lobopodian panarthropods, our tree searches indicate they are convergent. Cricocosmiidae is nested within a monophyletic ‘Palaeoscolecida sensu lato’ clade (Palaeoscolecidomorpha nov.) in stem-group Priapulida. Our study indicates that morphological seriation has independent origins in Scalidophora and Panarthropoda.Supplementary material: Phylogenetic character list and matrix are available at https://doi.org/10.6084/m9.figshare.c.5551565Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion