Systematic description and phylogenetic affinity of tubular microfossils from the Ediacaran Doushantuo Formation at Weng'an, South China

This paper provides a systematic treatment of tubular microfossils collected from dolomitic phosphorites of the Ediacaran Doushantuo Formation at Weng'an, South China. These microfossils were investigated using petrographic, acid extraction and re-embedding techniques. Four morphological genera and five morphospecics are described: Ramitubus increscens gen. et sp. nov., Ramitubus decrescens gen. et sp. nov., Sinocyclocyclicus guizhouensis, Quadratitubus orbigoniatus, and Crassitubus costatus gen. et sp. nov. They are characterized by cylindrical (R. increscens, R. decrescens, S. guizhouensis and C. costatus) or tetragonal (Q. orbigoniatus) tubes with closely spaced cross-walls. Ramitubus has mostly complete cross-walls, whereas all other taxa have regularly intercalated complete and incomplete cross-walls. Some of them branch dichotomously (R. increscens and R. decrescens), others have a longitudinal ridge running along a curved tube (C. costatus). The palaeoecology, ontogeny, and phylogenetic affinity of these microfossils are still uncertain. They probably represent non-biomineralizing organisms in the Ediacaran ocean. Ramitubus (and possibly all other morphotaxa described in this paper) was probably benthic and grew unidirectionally and episodically. The incomplete cross-walls could have been formed through terminal addition or, alternatively, through intercalary insertion. It is possible that some or all morphotaxa described may represent ontogenetic or ecophenotypic variations of one or a few closely related biological species. If so, then the combination of morphological features (complete and incomplete cross-walls, tetra-radial symmetry, and longitudinal ridge) can negate a cyanobacterial or algal interpretation, and point to a ternative cnidarian affinity.