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.

What's in a name?

What’s in a name? That question is extremely important in the world of paleontology.

The principle of scientific naming started with the binominal system of nomenclature established by Carl Linnaeus in 1758. For a species to be officially recognized, it must be given a latinized generic name and specific name (e.g., Turritella andersoni Dickerson, 1916). Note that these names must be italicized. The genus name is always capitalized, whereas the species name is not (this rule is internationally recognized although magazine and newspaper articles commonly do not follow it). The genus name can be abbreviated to a single letter (e.g., T. andersoni Dickerson, 1916), whenever the full name has already been mentioned, so that the reader is aware of the genus name.

Carl Linnaeus, also known as Carl von Linné (1707–1778), a Swedish naturalist.

Image modified from Wikipedia (2018).

The whole concept of naming involves taxonomy and systematics, which are basically synonymous. Taxonomy (or systematics) consists of two distinct components: Nomenclature, which deals with the purely legalistic aspect of names, and classification, which deals with the ranking or grouping of various categories of names (e.g., order, superfamily, family, genus, species). In modern times, DNA studies have revolutionized classification schemes.

The rules of nomenclature of genus and species names are given in the “International Code of Zoological Nomenclature” (ICZN). It provides widely detailed information about accepted procedures dealing with how names should be correctly established, which name must be used in case of name conflicts, and how scientific literature must cite names. The first edition of the Code was published in 1961. The present edition (4^th) was published in 2000. A pdf of the Code is available online.

Title page of the ICZN.

If you are interested in the details of nomenclature, Wikipedia.org, has a very informative overview of the principles (e.g., priority, homonymy, type specimens upon which names are based [e.g., the holotype is the "name bearer" of a species], gender agreement of names, etc.). Also, you can Google the phrase “international code of nomenclature.”

Wikipedia also has a good overview of what a synonymy (list of equivalent names) is. Just "Google" the word “synonymy.”