Reading:

• Textbook Chapter 15,
• Gould, S. J., 1983. Quaggas, coiled oysters and flimsy facts. In: Hen's teeth and Horse's toes, Chapter 30 (p. 376-385)

Goals of this lab: Learn to distinguish bivalve shell features used in taxonomy, and evaluate how these features are linked to the mode of life. Become familiar with common bivalve groups, in modern and ancient oceans.

There are two parts to this lab.

Part 1: Recent marine bivalves

Look at a diverse group of modern bivalves (collected on the beach of Sanibel Island, Florida), define to which order they belong (use pages 291-293 1^st edition 298-302 2^nd edition), and what their ecological role is (use figure 15.14, p. 290 1^st edition, p. 300 2^nd edition). Features to look at for each group: type of hinge structure (p. 289, 298), presence/absence of pallial sinus (p. 288, 296), number of muscle scars, and whether these scars are of equal size if more than one. (see below for table).

1. Look at some bivalve groups that are representative of Paleozoic bivalves (Nuculana arata, Carboniferous; Aviculopecten rectilaterarius, Carboniferous; Nucula varicosa, Devonian).  What do you think about the lifestyle of these shells (taking into account the nature of the sediment in which they are present)? 

Note that all three species are present in very dark-colored limestones, thus probably rich in organic matter. Note pyrite (a reduced iron-sulfur mineral, FeS₂) formation in the Aviculopecten. Both Nuculoid species (related to the nut clam, see textbook p. 291, 299) are thought to use symbiontic chemosynthetic bacteria, which use reduced sulfur as a source of energy, rather than using sunlight. These organisms thus must live in/close to a region where reduced circumstances are common. The sediments were thus probably formed in waters with a very low oxygen content. The pecten-relative Aviculopecten probably lived swimming in higher, oxygenated waters, and either dived to deep, hit anoxic waters and suffocated, or fell to the anoxic environment after its death.

2. Look at some typical, now extinct, Mesozoic bivalve groups. Note features of each, and see which features described in the text book you can see (p. 292, 301)

a. Various species of hippuritids: mode of life?

These were verious reef-forming species, with algal symbionts. One specimen has both shells preserved. One specimens is a small part of a larger colony. Also look into reading for lecture 18.

b. Various species of Gryphaea (also look at reading by Gould)

Gryphaea are related to oysters (p. 290, 300), but are much more cup-shaped. They may have lived reclining as oysters do, but in softer sediment rather than cemented on a hard substrate. Note that the valves are unequal to very unequal.

c. Various species of the Cretaceous genus Inoceramus: mode of life?

Inoceramusgrew to a huge size, and lived in the deep ocean where food is usually very hard to come by (has to make it down from the surface waters where there is light for photosynthesis). In that environment organisms can not reach a large size: simply not enough food. It is now generally accepted that Inoceramus was similar to the present-day bivalve giant clam living on chemosynthetic, symbiotic prokaryotes close to hydrothermal vents (Calyptogena magnifica; see photo below, Woods Hole). Also see web resources for lecture 12.

d. Trigonia arcuata: see textbook p. 292, 1^st edition; 301, 2^nd edition): in shifting, coarse sand. Click here for a picture of its living relative, Neotrigonia margaritacea (unusual hinge structure well visible on this web site); the genus has been thought to be extinct until its rediscovery in 1902.

3. Recent Unio: fresh water bivalve (Order Unionoidia). Check dentition (heterodont); note thick nacreous layer; note that it is unusual for fresh water dwellers (where concentrations of calcium ions are low) to secrete such thick shells.