DISSOLUTION (INTERNAL MOLDS & EXTERNAL MOLDS)
Many fossils of shelly organisms (e.g., bivalves, gastropods, brachiopods, cephalopods) have had the original hard parts dissolved away. In such cases, the fossil itself is an impression of the outside of the shell (external mold) or an impression of the inside of the shell (internal mold).
Hormotoma (Mollusca, Gastropoda, Archaeogastropoda, Murchisonioidea, Murchisoniidae) (4.9 cm tall) - this fossil snail from eastern Iowa is preserved as an internal mold (= the coiled portion; fine-grained sediment has filled up the originally-hollow shell) and as an external mold (= the cavity in which the coil sits). Note the gap between the external & internal molds - that's where the original CaCO3 shell was.
Matrix: weathered fossiliferous limestone
Stratigraphy: Galena Formation, upper Middle Ordovician to lower Upper Ordovician
Sthenarocalymene celebra (Arthropoda, Trilobita, Polymerida, Calymenidae) - this fossil calymenid trilobite specimen is a nice example of dissolution. The trilobite is preserved as an internal mold in Silurian dolostone from southwestern Ohio, USA. Most trilobites found in dolostones are similarly preserved. This trilobite is Sthenarocalymene celebra (Raymond, 1916). Over the years, trilobitologists haven’t agreed on the proper generic assignment for celebra. The species has been variously assigned to the genera Calymene, Gravicalymene, Flexicalymene, and Sthenarocalymene.
Stratigraphy: Cedarville Dolomite, upper Lockport Group, upper Niagaran Series (= upper Wenlockian Series), upper Middle Silurian
Bivalve internal mold (above & below; 4.9 cm across)
Above: impression of interior of right valve. Anterior is to the right.
Below: view of hingeline and umbo/beak. Anterior is to the right.
This fossil bivalve well displays the effects of dissolution. The bivalve’s shell is gone, and all that’s left is an internal mold, representing sediment that filled up the space between the clam’s two valves. Note the absence of ornament or growth lines - a good indicator of dissolution.
