Article: Upper Proterozoic microfossils from the Summer Isles, N.W. Scotland

A structurally preserved and distinctive microbiota composed of sphaeromorphic acritarchs and filamentous microfossils has recently been recovered from shales of the Aultbea Formation, Torridon Group, on Tanera Beg, Summer Isles, N.W. Scotland. The dominant sphaeromorphs comprise single vesicle; plurivesicular aggregates, and envelopes containing up to several tens of vesicles. They closely resemble those occurring in shales of the Roper Group, Northern Territory, Australia. Detailed study of the variety of sphaeromorphs has revealed that they probably represent stages in the life cycle of a single species of coccoidal, endospore-forming pleurocapsalean blue-green alga Torridoniphycus lepidus gen. et sp. nov. The filaments are hollow tubes ranging from 1.5 up to 53.0 um wide. They are assigned to the taxa Eomycetopsis crassiusculum (Horodyski) comb, nov., Siphonophycus beltensis Horodyski, Siphonophycus sp., and unnamed larger filaments. Most of them are interpreted as representing discarded sheaths of filamentous oscillatoriacean cyanobacteria and are similar to those preserved in shales of the lower Belt Supergroup in the Little Belt Mountains, Montana and the Dismal Lakes Group in Arctic Canada. This microbiota is thought to be of cyanophyte affinity. The low taxonomic diversity and high dominance of a few species may indicate an unusual and restricted aquatic ecosystem.