Problematic Mesoproterozoic fossil Horodyskia from Glacier National Park, Montana, USA

String-of-beads fossils (Horodyskia moniliformis and Horodyskia williamsii) from the 1.48 Ga lower Appekunny Argillite of Glacier National Park have been re-examined, and collected from both scree, which yielded most prior specimens, as well as outcrops. The fossils come from laminated silty shales and carbonaceous-swirl shales, with local sandstone paleochannels, interpreted as a very shallow lake margin. Very weakly developed paleosols also are present, but do not contain Horodyskia, which lived in very shallow water, seldom exposed and rilled. Chemical index of alteration at horizons with Horodyskia are evidence of a warm temperate to subtropical humid paleoclimate, unlike arid and cool paleoclimates at other stratigraphic levels in the Belt Supergroup. Thin section examination reveals that the beads are associated with a system of tubes, including connecting strings, and other tubes radiating outward from each bead. Partial burial and branching of these tubes may be evidence of a benthic sessile life style. A variety of explanations for Horodyskia are falsified by our new observations: including pseudofossil, dubiofossil, prokaryotic colony, foraminifera, slime mold, puffball fungus, brown alga, sponge, hydrozoan or bryozoan colony, or metazoan fecal string. Our remaining working hypothesis is that Horodyskia beads were endolichen bladders, comparable with living Geosiphon pyriformis (Archaeosporales, Glomeromycota, Fungi), which has heterocystic cyanobacterial photosymbionts (Nostoc punctiforme). This hypothesis is not without problems, because bladders of Geosiphon are mostly erect and clavate, but beadlike only in early growth stages, form clusters or close strings rather than elongate strings, and are terrestrial rather than aquatic. Nevertheless this new hypothesis for Horodyskia is compatible with what little is known about fungal evolution, and testable by additional studies of its paleoenvironments and associated fossils.

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► String-of-beads fossils Horodyskia come from shales of a shallow lake.
► Chemical index of alteration is evidence of warm temperate to subtropical humid paleoclimate.
► Beads are associated with connecting strings, as well as radiating tubes.
► We suggest that Horodyskia beads were endolichen bladders like living Geosiphon.