The intriguing three-pronged cavity is a natural mould of part of the shell of a brachiopod called Pentamerus oblongus, which was once entombed in the rock and then partially dissolved away.
If the Pentamerus oblongus were still there, the tip, or umbo, of the fossilized shell would protrude from the rock.
We find no pentamerus
brachiopods or cystoid crinozoans or psilopsid plants above Devonian strata, no graptolites above Mississippian strata, and no trilobites or rugose corals above Permian strata.
On the stratigraphic range of the beds with Pentamerus
oblongus and on the nature of the Late Llandoverian transgression in North Europe.
A small sliver in this block contains Silurian shallow-water sandstone and felsic tuff of the Back Bay Formation of probable late Llandoverian age on the basis of Pentamerus
A very rich association of shallow-water benthic fossils occurs in these strata, sometimes as scattered valves, sometimes as clusters of complete shells in living position, and sometimes as tempestitic accumulations of Pentamerus oblongos Sowerby (Kaljo 1970).
2B) is dominated by light-grey variously dolomitized argillaceous to bioclastic limestones with abundant shells of Pentamerus oblongus in many beds.
In western Estonia the Rumba Formation typically consists of variably argillaceous, nodular to wavy-bedded biomicritic limestones (packstone and wackestone) with accumulations of the brachiopod Pentamerus oblongus at certain levels.
Nestor (1976) when it became evident that in western Estonia the Pentamerus oblongus Beds or the Rumba Formation unconformably overlapped different subunits of the Raikkula Stage.
These correspond to the whole sequence of the Jamaja Formation in the Ohesaare core, as well as to units d-g of the Slite Beds and Pentamerus
gotlandicus Beds on Gotland (Laufeld 1974; V.