Strings of beads

The ‘strings of beads’, named Horodyskia williamsii by Grey and others in 2010, are enigmatic and iconic Western Australian fossils found in the southern Pilbara and northern Midwest regions. Found in rocks between 1455 and 1298 million years old, they potentially represent the oldest evidence for multicellular life in Western Australia, although their origin is debated.

History of discovery

‘Strings of beads’ (also known as ‘chain of beads’) were first identified in 1982 by geologist Robert Horodyski (University of Tulane) from outcrops in Glacier National Park, Montana, USA. Calling them ‘problematic bedding-plane markings’, Horodyski described the features fully, and used this information to carefully weigh the possibilities of whether they were formed by organic or inorganic processes.

After this discovery, similar forms began to be described from other parts of the world, including India, China, and Australia (Western Australia and Tasmania). Most of these records are Mesoproterozoic in age, with the oldest being those in Montana (~1480 million years old) and Tasmania (1450 to 1330 million years old).

The 'strings of beads' in Western Australia

‘Strings of beads’ were first collected in Western Australia in 1985 by Geological Survey of Western Australia (GSWA) geologist Ian Williams. Although at first found exclusively within sandstones of the Stag Arrow Formation, Collier Basin, the ‘beads’ were later also observed in the age-equivalent Backdoor Formation in the same basin.

The age of both geological units is poorly constrained but is considered Mesoproterozoic. The ‘strings of beads’ are relatively plentiful in both units and have been recorded from at least 49 separate sites. We hold an extensive collection of ‘strings of beads’ from many of these sites in our Palaeontology Collection.

Map showing sites where ‘strings of beads’ have been collected within the Collier Basin of the southern Pilbara and northern Midwest regions.

Interpreting the ‘strings of beads’

Pictures of two sandstone slabs, each preserving multiple examples of ‘strings of beads’ fossils on their surface. A 5cm scale bar next to each image is provided for reference.

The age and simple form of these impressions have led to extensive debate over whether the traces are biogenic (formed by living organisms), or whether they are pseudofossils created by the action of environmental factors. When first described in 1982, Horodyski could not decide if they were formed by organisms and so designated them ‘dubiofossils’ of probable inorganic origin.

Additional evidence from new material uncovered worldwide has now shifted the consensus towards considering these features body fossils, although what kinds of organism is still unknown.

For now, the ‘strings of beads’ fossils remain a fascinating mystery. We continue to study them, and with further research, we may one day understand what they were and how they formed. 

A reconstruction of a ‘string of beads’ fossil showing the interpreted shape of the original organism (seen centrally), and the various ways the fossil is preserved in the sediment (shown as orange blocks at the top and bottom of the figure).

Do your part to protect Western Australia’s fossils by staying up to date with the restrictions and responsibilities related to fossils and State geoheritage, following all laws and regulations, and spreading the word to others (including tourists).