The ‘string of beads’ fossil (Horodyskia) in the mid-Proterozoic of Tasmania

Horodyskia has been found at a single Tasmanian locality, in the Cassiterite Creek Quartzite (ca. 1300–800 Ma), part of a thick Proterozoic, mildly deformed, low greenschist facies, marine shelfal siliciclastic succession known as the Rocky Cape Group. The rock hosting the fossils is thinly interbedded and interlaminated dark grey slaty shale and quartzose siltstone. The sharp-based, graded siltstone layers are interpreted as distal storm surge deposits on an outer marine shelf. The ‘strings of beads’ are mostly preserved at the base of the siltstone layers, in concave hyporelief (external moulds) on the soles of the event beds, and as convex epirelief (casts) on the tops of the underlying shale beds. The casts comprise shale identical to the underlying bed. The beads average 1.7 mm in diameter, and the gap between the borders of adjacent beads tends to be approximately equal to the bead diameter. Occasionally, the fossils are preserved within shale, as wholly flattened beads delineated by a subtle darkened halo. The Tasmanian ‘strings of beads’ have most of the morphological attributes of previously described Horodyskia, including regularity of size and spacing of beads in any one string, lack of branching, and in some instances, ‘haloes’ and casts with apical depressions. The strings on at least one bedding plane have a strong N–S preferred orientation of unknown origin. Tectonic deformation has resulted in 30% shortening in a SW–NE direction. The morphologic similarity, but differing mode of preservation of the Tasmanian Horodyskia to the two previously described Mesoproterozoic species is strong evidence for a biologic origin for the string of beads phenomenon. After morphological and morphometric comparisons with other species of Horodyskia, the Tasmanian specimens are assigned to Horodyskia williamsii.