Some shells are coiled while others are straight. The shape and nature of the coiling can be a defining characteristic to help distinguish between different types of organisms:
- Planispiral: Each coil within the shell is in the same plane, looking like a flattened spiral.
- Conispiral: Each successive coil is not in the same plane as the prior coil, creating a cone shape.
Shells are vulnerable to mechanical breakage (e.g. by being tossed around on a beach) and to predators who may try to bite through a shell. The more crenulated (like corrugated cardboard) the shell, the stronger it is. Flat shells of the same thickness are weaker. Think of the increased “dipping strength” of a ripple potato chip over a regular flat potato chip.
The inside of a coiled shell may be separated into individual chambers by the same material as the outside shell. Chamber separations act as internal reinforcement for the shell. The more crenulated the chamber separations, the stronger the shell will be. Chambers may also be used to hold gas used for buoyancy compensation where a shell is heavier than the organisms living within it. Holding gas inside the shell prevents a swimming animal from having to exert energy to continually swim upwards to not sink.
What To Do
Inspect the fossil specimens in Sample Set 5 (images available below), paying special attention to the features you are asked to examine. Answer questions (a) to (g) as you work through Sample Set 5.
Note: If the image slide-decks fail to load, click the sample title to open the slide deck in a new window.
Sample Set 5 & Questions
A. Sample 121-38: Cephalopod (Ammonite)- Whole
Sample 121-38 can be picked up but please handle with care.
B. Sample 121-51: Cephalopod (Ammonite)- Cut in half
Sample 121-51 can be picked up but please handle with care.
a. What form of coiling does this organism display?
b. What kind of symmetry does Sample 121-38 exhibit?
c. Examine Sample 121-51. Notice the individual chambers. The ammonite animal lived inside the last chamber (it has broken off in this specimen). As the animal grew, it added a larger chambers. What were the old chambers used for?
d. In Sample 121-51, what method of preservation is helping to preserve the inner parts of the coil?
C. Sample 1: Gastropod (Snail)
Sample 1 can be picked up but please handle with care.
e. What kind of coiling is this?
f. You cannot see it, but the interior is a continuous and unchambered spiral from end to end. If the same animal was living in a straight shell, which would be stronger and why?
D. Sample 121-3: Straight Cephalopod (Nautiloid)
Please do not handle Sample 121-3