‘Strings of beads’

‘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, US. Calling them ‘problematic bedding-plane markings’, Horodyski described the features fully, and used their characteristics 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 records being those in Montana (~1480 million years old) and Tasmania (1450 to 1330 million years old).

‘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. Although poorly constrained, both of these units are considered Mesoproterozoic in age. The ‘strings of beads’ are relatively plentiful in both of these units, and have been recorded from at least 49 separate localities; GSWA holds an extensive collection of ‘strings of beads’ from numerous sites in its Paleontology Collection.

There have been numerous suggestions of inorganic processes that could form the ‘strings of beads’, including current (‘sole’ or ‘tool’) marks, rain drop prints, crystal growths and mineral grain trails. Most of the problems with these interpretations relate to the regularly spaced, rounded, statistically similar sized impressions seen in the ‘strings of beads’, a level of regularity that is virtually never observed in inorganic structures. Crystal growths tend to be singular, not linearly aligned, and both these growths and those in mineral trails are usually irregularly shaped and randomly spaced. Further, current marks would always be expected to be aligned with the water flow directions in the rocks, which is not the case for the ‘strings of beads’.

An additional problem of relief also exists for inorganic surface impressions. Although rain drop prints might form shapes superficially similar to the ‘strings of beads’, being rounded, with a depressed rim and raised central pimple, they only form negative relief on the top surface of a bed; on a bottom surface of a bed, where many of the ‘strings of beads’ are preserved, they would always form features of positive relief. Conversely, the bead examples preserved on the top bedding surfaces do not preserve the rims or dimples seen in a rain print. This question of preserved relief also precludes the ‘strings of beads’ as being interpreted as a number of types of trace fossils, such as walking trails, and sediment-surface feeding trails.

The consistency and regularity seen in the ‘strings of beads’ have therefore been used as one of the main arguments that the features were formed by an organism. Suggestions have included trace fossils such as faecal pellets or burrows, and body fossils of algae, fungi, primitive animals, or other unknown multicellular organisms. A lack of sediment disturbance surrounding the Western Australian beads (caused by animals burrowing or moving through the sediment) would suggest that the ‘strings of beads’ are not trace fossils, supported by the idea that such burrows and faecal trails would be unlikely to be as regularly arranged as the ‘beads’. This is further supported by the discovery of a complex internal structure within the Horodyskia specimens from the US, although this structure has not been confirmed for the Western Australian examples.

As a result, Horodyskia is generally considered to represent a body fossil, although there is still no consensus on what type of organism the fossils represent. Suggestions that possibly connected variously shaped dimple features seen alongside Horodyskia williamsii are holdfasts of some kind, tethering the ‘strings of beads’ to the sediment surface, would indicate a link to sessile animals or seaweeds. However, there is no unambiguous evidence that the strings are in fact attached to the dimple features, and the dimples have not been noted in association with Horodyskia from Montana.

Therefore, the ‘string of beads’ fossils remain a fascinating but mysterious form of past life. As research continues on these fossils, it is hoped that the identity of the forms will be elucidated in time.