Occurrence of the earliest known Sphenothallus Hall in the Lower Cambrian of Southern Shaanxi Province, China

Three-dimensionally preserved specimens of an indeterminate species of Sphenothallus Hall occur in limestones in the uppermost Guojiaba and lower Xiannudong formations (Qiongzhusian and early Canglangpuan stages, Lower Cambrian) in southern Shaanxi Province, China. The specimens consist of a small (up to 3 mm long), gradually tapered, subcircular to elliptic (transversely) tube that is broken at both ends, commonly with the fracture edge at the wide (apertural) end of each face forming a V- or U-shaped notch. Some specimens preserve the subconical upper (proximal) part of the holdfast, and all specimens are black. Combined scanning and transmission electron microscopy and energy dispersive elemental analyses show that the tube wall is finely laminated, with individual laminae measuring about 1 μm thick, and consists mainly of calcium phosphate but also contains organic material. Sphenothallus specimens from the Lower Cambrian Guojiaba and Xiannudong formations represent the earliest known record of this genus, and indicate that the origin of this taxon is coeval with and possibly related to the Early Cambrian bioradiation event.