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

Problematic Mesoproterozoic fossil Horodyskia from Glacier National Park, Montana, USA
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.
0301-9268
125-142
Retallack, Gregory J.
50b8bdfd-6699-4d36-9469-50c6188c9066
Dunn, Kimberley L.
01ae1dae-a460-44f1-bd87-30b4f4896599
Saxby, Jennifer
6adc15f3-2079-491c-aa1a-58ffc2c9f15f
Retallack, Gregory J.
50b8bdfd-6699-4d36-9469-50c6188c9066
Dunn, Kimberley L.
01ae1dae-a460-44f1-bd87-30b4f4896599
Saxby, Jennifer
6adc15f3-2079-491c-aa1a-58ffc2c9f15f

Retallack, Gregory J., Dunn, Kimberley L. and Saxby, Jennifer (2013) Problematic Mesoproterozoic fossil Horodyskia from Glacier National Park, Montana, USA. Precambrian Research, 226, 125-142. (doi:10.1016/j.precamres.2012.12.005).

Record type: Article

Abstract

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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Published date: March 2013
Organisations: Ocean and Earth Science

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Local EPrints ID: 358824
URI: http://eprints.soton.ac.uk/id/eprint/358824
ISSN: 0301-9268
PURE UUID: 40e5e59f-29ba-40a2-a748-43b0a4f7d618

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Date deposited: 11 Oct 2013 13:22
Last modified: 09 Jan 2022 03:27

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Author: Gregory J. Retallack
Author: Kimberley L. Dunn
Author: Jennifer Saxby

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