Geology 105 - Paleontology
Course Syllabus	Course Schedule	Journal Assignments	Course Handouts	Critter Charts	Department Home Page	Kusnick Home Page

Lab #6: Molluscs

At the end of this lab, you should be able to:

1. Identify a fossil as a gastropod or bivalve, and be able to identify both groups to the order using the charts provided.
2. Be able to identify the probable life habit of a gastropod or bivalve.
3. Know the skeletal structure and material of each of these animals.
4. Know the ecological characteristics of each of these animals.
5. Know the geologic range of each of these groups.
6. Know a few important genera (mentioned by name in this handout) for each group.

Display: Gastropods and Bivalves

Gastropods

I. Soft part morphology: In the resin block, identify the head and the rest of the foot.

II. Hard part morphology: Find these features: whorl, aperature, callus, siphonal notch, siphonal canal

III. Classification: A new cladistic classification of molluscs was published in 2005. It is complex and requires knowledge of the soft tissue morphology of the animals. For that reason, we will dip a tiny bit into the traditional orders of the Prosobranchs: the archeogastropods, mesogastropods and neogastropods. There are few features of the shell that are universal within each of these groups. Fossil gastropods are placed into orders by identifying the family, then looking at which order that family is placed in. In cladistic analysis, these orders just fall apart. YOU DO NOT NEED TO MEMORIZE THE CLASSIFICATION. I want you to be familiar enough with the morphology of snails to place them into an order given the descriptions of the orders.

Archeogastropods (#579, 1305, 578): Paleozoic to Recent, herbivores

• gastropods with a rapid expansion rate and little or no coiling: limpets and abalone
• several groups with low translation rates and simple aperatures
• top and turban shells

Mesogastropods (#1604, 474): Late Paleozoic to Recent, herbivores and carnivores

• rounded to high spired forms
• simple to complex aperature
• sometimes siphonal notch

Neogastropods (#459, 1303, 473): Jurassic to Recent

• usually elongate aperatures
• siphonal notches and canals

Pulmonates (no #): land snails

• thin smooth shells
• simple aperatures

IV. Life habits and morphology

• uncoiled or flat-bottomed shells (#1189, abalone) - often live attached to rock in intertidal or subtidal
• smooth rounded shapes (#458)- deep burrowing in sand and mud
• high unornamented spires (brown unnumbered)- shallow burrowing in sand
• thick ornamentation (#20, 1555)- defense against crushing predators (like crabs), so living out in the open either on rocky or sandy substrates

Bivalves

I. Soft part morphology. In resin block look at the specialized morphology of the mussel. Compare to the more general bivalve morphology in the diagram in our book.

II. Hard part morphology. Find these features: dentition, hinge, pallial line and sinus, muscle scars.

III. Classification: The traditional classification of bivalves is in even more of an uproar than the gastropods. Recognize a few groups by their common names (below).

• Mussels (987): live in the rocky intertidal, attached to rocks with threads of a strength unrivalled by any other creature's constructions, including humans. No siphons, reduced foot.
• Pectins (no #): free swimmers. A single large muscle closes the shell (the cylindrical "scallop" you eat. Look for the muscle scar.
• Oysters (#1704, no #): some are attached, others rest in soft substrate. Wide environmental tolerances. Assymetrical shells, often irregular.
• Ark shells (#21): long straight hinge, taxodont dentition, heavy ribs. Live attached to rocks
• Veneroids: venus clams (eating clams) (#1608, 1196), razor clams (#496), giant clams (no #), and other groups. Heterodont dentition. "Clam"- shaped burrowers.
• Rudists (Hippuritoida) (no #): extinct clams shaped like corals, with one valve as "lid" on cone-shaped lower valve. Reef builders.

IV. Life Habits: shells match the requirements of the environment

• burrowing bivalves (#1236, 25): symmetric valves, two equal adductor muscles, distinct pallial line
  • deep burrowers - deep pallial sinus, thin, smooth shell
  • shallow burrowers - shallow pallial line, thick or heavily ornamented shells
  • fast burrower (shifting sediment, many predators) - small shell, flattened shell
  • slow burrower - large shells, bulbous shell
• swimming bivalves (pectin box, #596): thin shell, symmetric valves with "wings" (auricles), single large adductor muscle
  • flat swimmers: one valve flatter than the other
  • vertical swimmers: valves equally rounded
  • shallow swimmers: ribs
  • deep swimmers: smooth
• epifaunal bivalves:
  • byssal (attched with threads) (no #): adductor muscles of unequal size, gape for byssus, either very pointy beak or long straight hinge with "wings"
  • cemented (#382): highly assymetrical valves, spines, may have single muscle
  • soft substrate (#1738): highly assymetrical valves, lid-like smaller valve on deeply concave lower valve.

Display: Other Molluscs

Scaphopods (#843) are called tusk shells. The organism lives head-down anchored in the sediment. They have changed very little over geologic time, and have never been very diverse.

Tentaculites: (#155) this organism is one of several poorly known and enigmatic early creatures which are interpreted as molluscs. The tiny conical shell is calcite. Tentaculites lived during the early Paleozoic.

Questions

1. #1739 - This mesogastropod is in the family Natacidae, the moon snails. These snails are predators, catching bivalves and drilling holes through the shell, then digesting the contents. Look carefully at the natacid and its prey, and conclude whether this snail is epifaunal or infaunal.

2. # 123 -The beautiful mother of pearl nacreous layer inside the shell marks where the soft tissue of the snail was attached to the shell. What was this snail's life habit? What is your evidence?

3. #1329 - How is this gastropod preserved?

4. This pulmonate has a typically thin smooth shell. Why would pulmonate snails evolve shells like this while marine snails have been steadily evolving thicker, more heavily sculptured shells?

5. Place each of these gastropods in the order Archeogastropoda, Mesogastropoda, or Neogastropoda: #1242, 1307, 1667, 1017, 1877.

6. #1633 - Notice the asymmetric shape of this pectin. How do you explain this?

7. #26 - Notice the marks on this shell of some boring animal. What are the defenses this bivalve has evolved against such attacks? What is your evidence?

8. No # - Name some possible functions of the spines on this bivalve.

9. Identify the probable life habit of each bivalve and the morphological evidence supporting your hypothesis: #1074, 22, 25, 592.