THE MIDDLE PROTEROZOIC FOSSIL HORODYSKIA FROM NORTH AMERICA AND AUSTRALIA

In Glacier National Park, Montana, where this fossil was first found, the fossiliferous beds are dated as 1.5 billion years. Dating is less certain in Western Australia, but the fossils are in excess of 1 billion years. Specimens from both areas are informally termed "string of beads," and differences are currently judged to be at the species level. In the Montana species seemingly alternate beads episodically disappear, for the ratio of bead size to spacing is nearly constant, allowing for some individual variation among strings. In the Western Australia species, seemingly there is no loss of beads, but the beads are not so large as some from Montana. Both species occur in sandstone, but the Montana sandstone is finely laminated and no such lamination occurs in the Australian outcrops. Both may have lived in relatively well-oxygenated water, with the Montana occurrence in quieter water. The organisms cannot be certainly assigned to either "plants" or "animals," though the latter interpretation is preferred by us. In either assignment, they suggest that coloniality appeared early in the geologic record. As indicated by widespread distribution in Western Australia, this genus may have utility as a guide fossil in ancient clastic strata which are essentially barren of fossils.