Problematic Fossil Horodyskia of Mesoproterozoic Age from Glacier National Park, Montana, USA

Australia: The Land Where Time Began

Problematic Fossil Horodyskia of Mesoproterozoic Age from Glacier National Park, Montana, USA

Re-examined string-of-beads fossils, Horodyskia moniliformis and Horodyskia williamsi, that were collected from the 1.48 Ga lower Appekunny Argillite of Glacier National Park, and collected from both outcrops and scree, most prior fossils being found in the scree. The fossils were found in silty shales and carbonaceous swirl-shales, with local sandstone palaeochannels, the site being interpreted as a lake margin that was very shallow. There are also palaeosols present that are very weakly developed, but they do not contain Horodyskia, which lived in very shallow water, and was not often exposed and rilled. At horizons with Horodyskia chemical index alteration provides evidence of a palaeoclimate that was humid, warm temperate to subtropical, which differs from the conditions that are indicated from other stratigraphic levels in the Belt Supergroup, where the palaeoclimates appear to have been cool and arid. The beads are revealed by thin section examination to be associated with a system of tubes, which includes connecting strings, as well as other tubes that radiate outwards from each bead. A benthic sessile lifestyle is suggested by the partial burying and branching of these tubes. The new observations by Retallack et al. are said by them to falsify a variety of explanations for Horodyskia: These falsified explanations for Horodyskia are pseudofossil, dubiofossil, prokaryotic colony, foraminifer, slime mould, puffball fungus, brown alga, sponge, colony of hydrozoa or bryozoa, or faecal string of a metazoan. The remaining working hypothesis is that Horodyskia beads were endolichen bladders, which are comparable to those of extant Geosiphon pyriformis (Archaeosporales, Glomeromycota, Fungi), which has heterocystic cyanobacterial photosymbionts (Nostoc punctiforme). There are some problems associated with this hypothesis, as Geosiphon bladders are mostly erect and clavate, though only in early growth stages that are beadlike, forming clusters or close strings instead of elongate strings, and they are terrestrial, not aquatic. This new hypothesis for Horodyskia is compatible with the little that is known of fungal evolution and subject to testing by additional studies of its palaeoenvironments and associated fossils.