NACRE IN SHELLS: PART 1 (BIVALVES)

In the classification and identification of mollusk shells (clams, snails, nautiloids, ammonites, etc.), the presence of “mother-of-pearl” luster [also referred to as nacre = the material responsible for iridescent shells with rainbow colors] is very important and fundamentally useful. This beautiful aspect of some shells is related to the presence of the organic biomineral aragonite (calcium carbonate [CaCO3 + organic matrix material]). I discussed this phenomenon in one of my earlier posts dealing with biominerals, which are minerals secreted by organisms. Two of the most popularly known examples of shell nacre occur in the pearl oyster and the abalone. [Note: In one of my earlier posts I discussed and illustrated also the topic of  inorganic aragonite.]

Only certain families/genera of mollusks secrete nacre, which always occurs as an inner lining, usually protected by an organic layer covering the outside surface of a shell. These families comprise the generally least derived (least evolved) groups of bivalves and gastropods. Also, many families of freshwater clams have nacre in the interior their shells. A few slugs have internal shells, but they consist of calcite. Other mollusks having with nacre in their shells are nautiloids (straight and coiled), ammonites, sepiids, and monoplacophora.

Organic aragonite makes the strongest shells. The microstructure of organic aragonite is like stacked bricks interlayered with conchiolin. Upon death of the shell, the decomposition of the conchiolin causes the shell to deteriorate, unless the entire shell is encased in protective material like asphalt or extremely fine-grained shale. The main thing is to keep out corrosive water. Organic aragonite has been reported from rocks as old as Ordovician, but other Paleozoic locales where the aragonite has been well preserved are rare. The earliest undisputed nacre occurs in the Lower Ordovician cephalopod Isorthoceras from Cincinnatic, Ohio. Inferred nacre occurs in two Cambrian bivalves (e.g. Pojetaia), but these shells underwent replacement of the original shell material by phosphate minerals.

The fossil record shows a nearly unbroken sequence of nacre-forming mollusks from the Early Ordovician to Recent time. In my last post, I focused on the presence of nacre in Middle Pennsylvanian shells from the Buckhorn Asphalt Quarry in Oklahoma.

Because the subject matter of nacre is considerable, I decided to treat it in two separate posts. This first part shows common examples of some bivalves (marine and freshwater) that have nacre. 

NACRE IN BIVALVES

Both the exterior and interior views of each specimen are illustrated below and in that order of appearance. The caption for each pair of images precedes the images:

1) Pteria sterna [75 mm wide, 70 mm high], with a pearl blister. This species is common and found in southern California to Peru. It belongs to family Pteriidae. Collected by the author:

1)

2) Isognomon isognomum [130 mm wide, 65 mm high]. This species is common and found in the intertidal zone in the Philippines. It belongs to family Isognomonidae:

                                                     2)

3) Atrina vexillum? [90 mm wide, 100 mm high]. This species is common and found in the Indian Ocean. It belongs to family Pinnidae:

3)

4) Pinna rugosa [350 mm high] is commonly found in the Bahia Concepcion area near Loreto, Gulf of California, Baja California, Mexico. It belongs to family Pinnidae. Collected by the author.

                                                       4)

5) Proptera alata [150 mm wide, 97 mm high]. This bivalve is found in freshwaters of eastern and central North America. It belongs to family Unionidae (the freshwater bivalves):

                                                      5)