HORODYSKIA MONILIFORMIS OF THE BELT SUPERGROUP: A CASE STUDY OF THE RELATIONSHIPS BETWEEN PRECAMBRIAN MICROBIALLY INDUCED AND PHYSICAL SEDIMENTARY STRUCTURES IN A FLOCCULATING MUDDY ENVIRONMENT

Horodyskia moniliformis is found extensively in the lower part of the Appekunny Formation (lower Belt Supergroup; ca. 1.4 Ga) of northwestern Montana and adjacent southwestern Alberta. Known for its distinct string-of-beads appearance, it is regarded by some as one of the oldest Precambrian multicellular organisms from the fossil record, but there is considerable disagreement as to its affinity, for example if it represents eukaryotic colonies, agglutinated foraminifera, or fungi, or if it is an inorganic pseudofossil. The host rock is laminated, very fine-grained sandy mudstone; Horodyskia does not occur higher in the Appekunny Formation where it is dominated by mudstone locally with coarse sandstone interbeds. The beads consist of non-porous lenses composed of very fine sand, silt and clay, which stand out from the surrounding matrix due to differential weathering. The diameters and spacing vary in size but tend to be uniform in individual strings. These varying string sizes have been identified in the past as belonging to different species, such as H. minor and H. williamsii. A complex biological origin begins to degrade when the relationships between the beads, surrounding sediment, and locally associated microbially induced sediment structures (as matgrounds) become apparent. New analysis utilizes high-resolution micro-CT scans and X-ray projections in addition to traditional methods of SEM and optical microscopy. This shows the distribution of muddy lenses comprising the strings-of-beads and co-occurring isolated beads. Isolated beads prove to be pervasive in the host beds, whereas the string-of-beads form, while striking, is much less common. Their relationship to textures known to be of microbial mat structures such as wrinkle marks, pustules, domes and petee structures suggests their formation is linked. We suggest the beads accreted in a clay-flocculating environment as part of, or aided by microbial activity. Mucous coatings and filamentous microbial growth resulted in the string-of-beads form, although most beads were deposited as isolated flocs. Horodyskia moniliformis in the Appekunny Formation is therefore not a body fossil or an ichnofossil.