Geology 105 - Paleontology
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Lab #5: 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 bivalves to the order level using the chart 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:

• A. In the resin block, identify the head and the rest of the foot.  Don't forget to turn it over to look at the bottom of the snail.
• B. #1880 Littorina & #1326: The operculum is a little trap door that some snails make.  It can be either protein or calcified.  The operculum is attached to the foot of the snail, and when it pulls itself into its shell, the operculum seals the aperture.
  

II. Hard part morphology: # 486 & unnumbered Fasciolaria. Find these features: whorl, aperature, callus, siphonal notch, siphonal canal.  Note that the insides of the modern snails in the display are smooth and shiny.  Gastropods typically lay down a smooth layer of nacre (a mixture of aragonite and protein) on the part of the shell that their body regularly contacts.

• A. Unnumbered Fasciolaria
  
• B. #486 - Fasciolaria
  

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, you will not be required to learn any taxonomy for gastropods.

IV. Life habits and morphology: Gastropods live in a broad range of environments.  As a group, they have some of the widest environmental tolerances of any invertebrate.  Much of the wide range of morphology in gastropods are adaptations to different environments:

• A. rocky surfaces in high energy water: 
  
• 1. wide shells with dome-shaped spires, or broad triangular cross-sections, and flat bottoms that clamp down effectively on rock: turbans (#1305 & #1189 Tegula) and tops (unnumbered pink shell)
  
• 2. uncoiled or flat-bottomed shells (#1879 Collisella, abalone) - often live attached to rock in intertidal or subtidal
• B. deep burrowing in sand and mud:
  
• smooth rounded shapes (#458 Ampullonatica, #474 Natica), streamlined forms (unnumbered olive shells).  Many of these burrowing snails are predators on clams, chasing the clams through the sediment and drilling into them.  Olives are the fastest burrowers among gastropods.
  
• C. shallow burrowing in sand - these gastropods move with their foot just under the surface and the shell partially buried. 
  
• high smooth spires, or high spires with corrugations that help hold the shell in place as the gastropod's foot pushes forward through the sand (brown unnumbered, four white unnumbered specimens)
  
• D. thick  or complicated ornamentation
  
• 1. thickening the shell with knobs and bumps can be a defense against crushing predators (like crabs) for snails living out in the open either on rocky or sandy substrates (#1555 Busycon, #20 Salterella, #1645 Ecphora, four unnumbered specimens)
• 2. for snails living in shifting sand or mud, snowshoe projections can stabilize them (three unnumbered specimens)
  

• A. Paleozoic forms: (#578 Straparolus, #579 Worthenia, #1328, #1668 Trepospira, #1771-75 Bellerophon, #981 Busanopsis).  Some early gastropods did not form spires, but coiled almost in a single plane (#578, #981, #1771-75).
• B. Cowries:  The nacreous outside surface of the shell tells you that the soft tissue of the snail wrapped around the outside of the shell in life, camouflaging the shell.  Cowries are grazers, many of them hiding in the crevices of reefs by day and only emerging at night.  It's hard to see the coil of the snail in an adult cowrie because the outer edge of the shell thickens and curves inward when the animal reaches sexual maturity.
• C. Cones:  Cone shells are predators.  The radula - the hard rasp-shaped mouth part that many other snails use to scrape algae off rocks - in Conus is specialized into a harpoon that injects toxins into prey.  Many Conus are dangerous to humans.
• D. Crepidula: These gastropods attach themselves to shells of living or dead creatures and filter feed.  They can form large mating stacks, where each snail becomes the opposite sex of the snail it is stacked on top of.
• E. Vermetids: These gastropods are very weird.  They are neither grazers nor predators.  Instead they build long tubes instead of coiled shells, and filter feed out one end, like many marine worms.  Their shells look like large serpulid worm tubes, but the interiors have shiny nacre, like other gastropods.
• Some gastropods do not build mineralized skeletons, including slugs on land and nudibranchs and sea hares in the ocean.  They have no fossil record, but we will probably see some on our field trip.
  

Bivalves

I. Soft part morphology. In the 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 (E), hinge (F), muscle scars (C).  The shiny line that runs from one muscle scar to the other is the pallial line (B), the place where the mantle (the tissue that secretes the shell) attached to the shell. 
  
• One of these clams was a burrower, and the clam needed a place to pull in its siphons.  That place is the pallial sinus (A) where there was a large recess for the clam to retract the siphons.  In clams that don't burrow, like our other sample here, there is no pallial sinus, and the pallial line runs around the edge of the shell from muscle scar to muscle scar.
  

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).

• A. Mussels (#987 Myalina, unnumbered Mytilus): live in the rocky intertidal, attached to rocks with threads of a strength unrivaled by any other creature's constructions, including humans. No siphons, reduced foot.
• B. Pectens (# 1676 Pecten, unnumbered): free swimmers. A single large muscle closes the shell (the cylindrical "scallop" you eat. Look for the muscle scar.
• C. Oysters (#1470, #1704, #1444 Exogyra, unnumbered Gryphea): some are attached, others rest in soft substrate. Wide environmental tolerances. Asymetrical shells, often irregular.
• D. Ark shells (#21): long straight hinge, taxodont dentition, heavy ribs. Live attached to rocks
• E. Veneroids: venus clams (eating clams) (#1608, 1196), razor clams (#496), giant clams (unnumbered), and other groups. Heterodont dentition. "Clam"- shaped burrowers.
• F. Rudists (Hippuritoida) (unnumbered small specimen, unnumbered oversize specimen): 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

• A. burrowing bivalves (#1236, 25): symmetric valves, two equal adductor muscles, distinct pallial line
  • 1. deep burrowers (unnumbered)  - deep pallial sinus, thin smooth shell (inaccessible to crushing predators, or shells with ridges parallel to shell edge for digging
  • 2. shallow burrowers (#25, #1236 Glycymeris) - pallial sinus is small or absent, thick or heavily ornamented shells
  • 3. fast burrower in shifting sediment with many predators (unnumbered Nasuta) - small shell, flattened shell
  • 4. slow burrower (unnumbered Clinocardium) - bulbous shell, thick ornamentation(these are often shallow burrowers)
    
• B. swimming bivalves (pectin bag, #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
• C. epifaunal bivalves:
  • 1. byssal (attached with threads) (#1074 Mytilus): adductor muscles of unequal size, gape for byssus, either very pointy beak or long straight hinge with "wings"
  • 2. cemented (#590 Ostrea, unnumbered rock pectin): highly assymetrical valves, spines, may have single muscle.  Notice on the rock pecten the very normal looking pecten buried in the craziness of the newer shell.  This species swims as a young pecten, then settles down, cements it self to the rock, and grows an oyster-like shell for the rest of its life (you can also see where some boring organism cut into the shell, but the pecten kept htickening the shell on the inside to escape predation).
  • 3. soft substrate (#589 Gryphea, unnumbered Exogyra): highly assymetrical valves, lid-like smaller valve on deeply concave lower valve.

V.  Evidence of predation or parasites on shells

• A. Bivalves that do not burrow suffer many attacks by predators.  Because of that, these bivalves usually build shells that are thicker, with heavy ornamentation or corrugation to resist both crushing and drilling predators.  They may have spines to discourage predators like starfish or snails.
• B. Some bivalves fall prey to snails that drill holes through the shell, inject digestive juices and then slurp out the semi-digested clam.
• C. Other bivalves are attacked by parasites that use the shell of the clam.  These include clionid sponges, that dissolve holes in the shell and then move into the tunnels opened up, and boring clams that drill holes into the shell that they live in.
  

Display: Other Molluscs

Chitons (unnumbered): Despite the name and the many segments, these organisms are mollusks, not arthropods.  Their skeleton is calcium carbonate, just like other mollusks, not chitin like arthropods.  The body of the organism is much like an abalone.  They typically like in rocky environments and clamp onto the rocks.  This one is covered in encrusting spnge (red) and calcareous algae (pink)

Scaphopods (unnumbered) 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.  Notice the thickness of the shell, especially in the fossil sample.  Late you may want to confuse these with some kinds of cephalopods, but the shell on scaphapods is typically much thicker.

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 (land snail) 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. What can you say about the mode of life of each of these gastropods?
#1667:
#1017:
#2010:
#1307:
#1311:
#1879:
#1242:
#1877:
unnumbered brown and white:

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 (deep burrow, shallow burrow, fast burrow, slow burrow, swimmer, epifaunal soft substrate, epifaunal cemented) of each bivalve and the morphological evidence supporting your hypothesis:
#1472
#1504
#14
#20
#592
unnumbered
tiny white