A new graptolite (Pterobranchia) from the early Cambrian Xiaoshiba Lagerstätte of China

Abstract Modern hemichordates are remarkable by their morphological and ecological disparity, encompassing single sessile, colonial, burrowing and surface-dwelling forms, However, uncertainties remain concerning the anatomy of their early representatives and the divergence of major lineages. We describe here an exceptionally preserved and nearly complete graptolite (Hemichordata, Pterobranchia) Yunotubus gemmatus gen. et sp. nov. from the 516-million-year-old Xiaoshiba Lagerstätte, China. Its tubular construction (e.g. fusellar tissues, stolon) and colonial architecture are similar to those of extant rhabdopleurids. Our phylogenetic analysis retrieves Yunotubus as a graptolithine with high confidence, although stem nodes remain poorly resolved. Our fossils confirm that key features of the pterobranch tubular construction (e.g. clusters of tubes with oblique fusellar sutures; zooids interconnected by a stolon) have remained virtually unchanged over more than 500 million years, and that pterobranchs and enteropneusts diverged and adopted radically different ecologies very early in the evolution of hemichordates.