New Miaolingian tubicolous fossils from North China and the persistence of the Ediacaran “worm-world”

Abstract

The oldest undisputed metazoan body fossils are often represented by tubular remains, which led to the characterization of early ecosystems as the Ediacaran ‘worm-world’ fauna. Tubular body fossils continued to dominate during the Cambrian Bioradiation Event, primarily as three-dimensionally preserved small skeletal fossils, although their preservation varied widely. Most tubular fossils, however, lack soft tissue preservation, making it challenging to determine their biological affinities based solely on tube morphology. Here we describe well-preserved tubular fossils, Longgangia bilamellata gen. et sp. nov. and Selkirkia cf. columbia, from the Cambrian Mantou Formation (Miaolingian Series, Wuliuan Stage) in Burgess Shale-type preservation. Longgangia bilamellata gen. et sp. nov. features a double-layered tube wall, transversal annulations, and Y-shaped branching of half-ring annulations. The new species shows importance because of its resemblance in construction and preservation with Ediacaran tubular fossils, such as Sinospongia, Sinotubulites, and Sabellidites, therefore providing evidence for the persistence of the tube-dwelling lifestyle from the Ediacaran to the Cambrian (Miaolingian). This also highlights the evolutionary and ecological significance of tube-forming organisms during the Cambrian Bioradiation Event, an interval characterized by explosive metazoan diversification, biomineralization, genetic innovation, and niche specialization. The potential affinity with cnidarians, annelids, and hemichordates is discussed and an assignment of Longgangia gen. nov. to Annelida is considered most plausible. The new material also reveals that annelids are capable to create tubes by half-ring construction with Y-branchings of growth lines, thus emphasizing this tube construction is not exclusive to hemichordate pterobranchs.