Wooster’s Fossil of the Week: a very large clam (Upper Cretaceous of South Dakota, USA)

Our version above of the bivalve Inoceramus is actually rather small compared to how big it can get. The record holder is a specimen 187 centimeters in diameter (over six feet) in the Geological Museum of Copenhagen. This Wooster Inoceramus is from the Pierre Shale of South Dakota, a unit my colleague Paul Taylor and student John Sime once explored in some detail.

Inoceramus means “strong pot”, which I assume must refer to its unusually thick shell with calcite prisms oriented perpendicular to the surface. They also had concentric “wrinkles” that make them easily identifiable even in small fragments. In fact, we can even recognize the isolated prisms of inoceramids in thin-sections of sedimentary rocks. This genus was widespread during the Late Cretaceous, being found from British Columbia to Germany. The had very large gill systems that enabled them to live in poorly-oxygenated waters. It makes sense that they are so common in the dark, carbon-rich sediments of the Pierre Shale.
Inoceramus was named by the dapper James Sowerby (above) in 1814, so it is a genus we have known for a very long time. Sowerby (1757-1822) was an Englishman skilled in natural history as well as scientific illustration. He named the first species of the genus as Inoceramus cuvieri to honor the French scientist Georges Cuvier. His illustration of I. cuvieri is below.
Inoceramus was one of the first invertebrate fossils to be the subject of an evolutionary study in a modern way. Woods (1912) studied various species of Inoceramus in the Cretaceous, noting that it apparently underwent rapid intervals of change. My former student Colin Ozanne and his advisor (and my friend) Peter Harries studied Inoceramus and its relatives in the Western Interior Seaway. Their study, published in 2002, showed that inoceramids were greatly stressed by parasites and predators before their final extinction in the Maastrichtian.

References:

Ozanne, C.R and Harries, P.J. 2002. Role of predation and parasitism in the extinction of the inoceramid bivalves: an evaluation. Lethaia 35: 1–19.

Sowerby, J. 1822. On a fossil shell of a fibrous structure, the fragments of which occur abundantly in the chalk strata and in the flints accompanying it. Transactions of the Linnean Society of London XIII: 453-458. Plate XXV.

Woods, H. 1912. The evolution of Inoceramus in the Cretaceous Period. Quarterly Journal of the Geological Society 68: 1-20.