Coming from the Green River shale, this stromatolite formed comparatively recently (for stromatolite). The Green River shale is a laggerstat fossil site noted for its exquisitely preserved fish and other fauna. It was formed by an extensive fresh-water lake system.
The stromatolite is widely believed to have been formed by green algae, that is, Chlorophycophyta, Chlorellopsis sp. Science is still debating whether algae is a plant or deserves its own group - the gene sequencing and nucleotide homology comparisons go on. With nucleated cells, algae are Eukaryotes like animals, fungi and plants, unlike photosynthetic cyanobacteria that lack a cell nucleus. This Eocene-age stromatolite was formed long, long after stromatolite-building organisms had to compete with other life forms, and hence had become uncommon on earth, similar to modern times.
This specimen has been cut along one vertical plane and highly polished one side to show the considerable intricate detail.