Hearts of stone
The fossil moulds of bivalves were puzzling objects to early naturalists and common people alike. This preservational style is common in species of bivalves that had aragonitic shells. Unlike the more stable calcite shells found in Gryphaea and other oysters, aragonite is routinely dissolved by pore waters passing through the rock, leaving spaces where the shell was formerly located surrounding hardened sediment core. This core is an internal mould, often known by the German name 'steinkern'. Steinkerns are solid objects that may fall out of the rock cleanly when it is broken open.
The often bizarre and vexing appearance of bivalve steinkerns has led to misconceptions about their origins.
Steinkerns of a group of bivalves common in the Upper Jurassic of southern England when viewed in a particular orientation vaguely resemble the heads of miniature horses. Robert Plot (1677) referred to examples of these fossils from Headington near Oxford as 'Hippocephaloides' in reference to their horse head-like shape. They are now known by the scientific name Myophorella hudlestoni .
The 'eyes' of the horse are actually moulds of scars left by the muscles that originally closed the valves together, and the 'ears' are the pointed beaks of the two valves.
Steinkerns of a related species - Myophorella incurva - are conspicuous in the Portland stone of Dorset, particularly in a level called the 'Roach', which is sometimes used decoratively as a facing stone on buildings. Quarrymen on the Isle of Portland speaking in local dialect know them as 'Osses 'Eds.