File:Foerstia fossil algae on mudshale (Ohio Shale, Upper Devonian; Glen Echo Ravine, Columbus, Ohio, USA) (36542600921).jpg

Foerstia sp. - fossil algae on shale in the Devonian of Ohio, USA.

This flattened black disc is a fossil macroalga called Foerstia (also known by the subjective senior synonym "Protosalvinia" - I'm skeptical of the latter name being a senior synonym). An interval of relatively common Foerstia algae occurs in the Late Devonian-aged Ohio Shale. The formation has been subdivided into three parts (in ascending order): Huron Shale Member, Chagrin Shale Member, and Cleveland Shale Member. The outcrop appearances of the three different members are best developed in northeastern Ohio, in the Cleveland area. The Huron and Cleveland are black shales and the Chagrin is gray shale. In central Ohio, the Chagrin Shale lithofacies occurs in a relatively thin, poorly developed interval - it is referred to as the Three Lick Bed. The Three Lick consists of interbedded gray and black shales. The Foerstia zone occurs in the Three Lick Bed.

These rocks were deposited in a moderately deep, anoxic seafloor environment. Black shale was widespread lithofacies during the Late Devonian. The Ohio Shale is equivalent to the Chattanooga Shale, the Antrim Shale, and the New Albany Shale in surrounding states.

Foerstia itself is an enigmatic organism - it is often referred to taxonomically as "Problematica" or "Incertae Sedis" (= "Uncertain Placement"). Paleobotanists have reported tetrads of spores occurring in depressions or conceptacles at the ends of appendages in some Foerstia/Protosalvinia specimens - this is a feature of brown algae (Phaeophyta).

Stratigraphy: Ohio Shale, Famennian Stage, upper Upper Devonian