The pseudofossil Horodyskia: Flocs and flakes on microbial mats in a shallow Mesoproterozoic sea (Appekunny Formation, Belt Supergroup, western North America)

Abstract

Horodyskia moniliformis Yochelson and Fedonkin (2000) is found abundantly in its type area, the lower Appekunny Formation (Belt Supergroup; ca. 1.45 Ga) of northwestern Montana and adjacent southwestern Alberta. It has a distinct curving to meandering string-of-beads appearance on bedding planes of thin-bedded, laminated argillaceous siltstone, and has been regarded by some as one of the oldest eukaryotic organisms in the fossil record. The beads are disc-shaped to lenticular, ~1–5 mm in diameter, circular to elliptical to polygonal in outline, and composed of variably silty clay. They are interpreted as mud flocs and small intraclasts. The beads in individual specimens are fairly uniform in size and spacing, but string length varies from about one to 15 cm depending in part on bead size. The radius of curvature varies from tight to open, but strings never form loops. Isolated beads are also present and smaller ones become indistinguishable from pustules which, along with mud-cored wrinkles and blisters, are associated on many bedding surfaces. These are regarded as evidence for a benthic microbial mat with variable topography ranging from crudely linear or stellate ridges to small domes and pinnacles. Rather than representing a semi-infaunal colonial eukaryote, fungal bladders or macroalgae, Horodyskia is here interpreted to be large mud particles that were trapped on small protrusions of the microbial mat probably consisting of filament tufts. Commonly, the tufts were oriented in rows of similar-sized and generally evenly spaced elevations and these bound suspended flocs and flakes of a more or less uniform size, resulting in the features that appear organized as strings-of-beads. Horodyskia is therefore considered to be a microbially-induced sedimentary structure specific to a muddy, relatively low-energy, subtidal marine setting with just the right combination of sediment type and depositional factors.