Petalonema alatum is a cyanobacterium (cyanophyte, cyanoprokaryote) described and drawn first time by the Scottish author Dugald Carmichael under the taxonomic name Oscillatoria allata in the book of Scottish cryptogamic flora edited by Greville in 1826 and later published under the still valid name Petalonema alatum by Miles Joseph Berkeley in 1833.
The preservation of the fossils – and their pre-burial livelihoods – was likely facilitated by mats of the cyanobacterium Morania, which served to bind the sediment and allow anoxic conditions to quickly form.
In some species of Pseudocyphellaria the cyanobacterium is the sole photobiont, while other species also contain the green alga Dictyochloropsis and restrict the cyanobacterium to warty cephalodia on the lower surface of the lichen.