Middle Proterozoic (1.5 Ga) <i>Horodyskia moniliformis</i> Yochelson and Fedonkin, the Oldest Known Tissue-Grade Colonial Eucaryote

Abstract

“Problematic bedding-plane markings” discovered by the late R.J. Horodyski from the Appekunny Formation in Glacier National Park, Montana, and dated at approximately 1.5 giga-annum (Ga), were never formally named. We are convinced the specimens are biogenic and have placed them within Linnaean nomenclature as <i>Horodyskia moniliformis</i> Yochelson and Fedonkin. An apt description of the locally abundant fossils is “string of beads.” On each string, beads are of nearly uniform size and spacing; proportionally, bead size and spacing remain almost constant, regardless of string length or size of individual beads. They may not be related to any other known fossil, and their position within highest levels of the taxonomic hieararchy is enigmatic. We judge they were multicellular, tissue-grade, colonial eucaryotes. Similar strings have been reported from Western Australia, but nowhere else. The general geologic setting in Montana, details of sedimentation, and taphonomy suggest the organisms were benthonic, growing upward about 1 cm through episodically deposited eolian dust. During life, specimens were stiff and relatively strong, but show no evidence of a mineralized skeleton. They lived in poorly oxygenated water with the body progressively subjected to anaerobic conditions. Their energy source is obscure; their mode of growth and several features of interpreted environment lead us to speculate that <i>Horodyskia</i> likely lived primarily by ingesting chemosynthetic bacteria rather than by photosynthesis. This notion should be tested by searching red, fine-grained, subaqueous arenites of approximately the same age throughout the world for additional occurrences.