Inoceramids

Inoceramid bivalves (clams) range from the Permian to latest Cretaceous. On a global scale, they were widespread and locally abundant during the Jurassic and Cretaceous. They went extinct just before the end of the Cretaceous. Inoceramids lived on the ocean floor and were filter feeders.

This first image is an "average-looking" Inoceramus specimen, showing the characteristic oblique outline of the shell and the prominently raised, concentric ribs. Well-preserved shells have "mother-of-pearl" luster on the inner layer. The outer shell layer consists of well-developed prisms of calcite.

This is a cross-section of a fragment of the prismatic layer of Inoceramus. The fragment is 1 cm in thickness and 2.5 cm in length. Even small pieces of the outer layer of well-preserved Inoceramus shells can show the very characteristic, well-developed (i.e., thick) prismatic shell structure.

Plaster cast (6 cm height) of Inoceramus orientalis ambiguus Nagao and Matsumoto, 1939, Cretaceous (late Santonian), Shasta County, northern California. 

This specimen is from Upper Cretaceous (upper Campanian to lower Maastrichtian) strata in the San Diego area, southern California. The drafting pen is 12.5 cm (5 inches) in length.

Inoceramids are excellent in fossils for "age-dating" marine rocks. This is because their species evolved rapidly, commonly averaging one per 0.2 to 0.5 million years. A normal bivalve species lasts for about 2 million years. Single species of inoceramids can occur in widely scattered parts of the world. About 75% of species and subspecies have intercontinental to cosmopolitan distribution in the Cretaceous, and, in many cases, exceed the precision of geologic age correlation found in co-occurring ammonite species and subspecies or the marine planktonic microfauna! Because of their widespread distribution, however, they are not very useful in defining paleobiogeographic regions.

Some genera, many species, and many subspecies of inoceramids can be frustratingly similar looking because they have so few determinate morphologic characters. Some researchers use complex statistical analyses (e.g., multivariate analyses) to help them define and recognize species and subspecies. Before the advent of computers, these statistical analyses were overwhelming to do by hand. Now, there is software that can help paleontologists, who must first digitize the morphological data.

Some inoceramid specimens can be large and rather thick walled, like this Cretaceous specimen from Kansas.

Photo is from wikipedia.org

Some species of inoceramids grew to giant size, up to 3 m (9.84 feet = 118 inches) in length, making them the largest bivalves of all time. Their giant size was probably an adaptation for living in murky bottom waters; the gills were probably corresponding large and allowed for survival in oxygen-deficient waters. These giants are all very flat valved. The unusual color of this specimen is because of the exposure of the inner shell layer (mother-of-pearl layer).

While alive, the valves of large inoceramids could serve a shelter for schools of small symbiotic fish, which can be preserved inside as impressions. In many cases, the exterior surfaces of the shells of inoceramids are encrusted by numerous small oysters.