An Early Ordovician clonal organism from China with a zig-zagged suture on branches

Abstract

An enigmatic clonal organism from the earliest Floian (Arenig) Fenxiang Formation at Tianjialing in Hubei Province, China and here named Crinisdendrum sinicum gen. et sp. nov., shows a puzzling combination of anatomical characters. Lateral colony branches that originate from tubular stolons are built of C-shaped serial oblique units resembling fuselli and merging along a zig-zagged suture, a set of features generally believed to be unique to pterobranch hemichordates. At least most branches taper near their tips and are closed in a manner resembling the termination of thecae previously reported in the extant pterobranch Rhabdopleura normani. The main drawback of the pterobranch model is that previous observations of terminating thecae in R. normani have neither been confirmed by more recent zoological studies nor explained in functional and developmental terms. Pyritized Crinisdendrum specimens preserved in shale were scanned using X-ray microtomography, enabling restoration of early stages of colony development (astogeny). Pyritic internal moulds of probable thecae, showing their interiors in negative relief, and phosphatized walls of thecae overgrown by black corals were recovered chemically from samples of calcareous intercalations in the Fenxiang Formation. Both sets of specimens exhibit a thread-like cylindrical structure resembling a stolon inside the theca and located below the zigzagged suture. A similar organization of branches also characterizes the feathery colonies of Webbyites from the Early Ordovician Klabava Formation of Bohemia, although these colonies are known in less detail. A new family, Crinisdendridae is proposed to encompass these two genera. Further testing of the hypothesis of a pterobranch affinity for these 470-million-year-old organisms will require a better understanding of the anatomy and growth of extant clonal hemichordates.