| Geology 105 - Paleontology | ||||||
| Course Syllabus | Course Schedule | Journal Assignments | Course Handouts | Critter Charts | Department Home Page | Kusnick Home Page |
At the end of this lab, you should be able to:
I. Soft part morphology - For the tiny squids in the resin blocks, use fig. 15.19 to identify the tentacles, the hood, the hyponome, and the eyes (closed in these samples).
II. Hard part morphology -
Nautilus: (#832) find the septa, chambers, septal necks (where the siphuncle ran).
Primitive straight-shelled cephalopods: These organisms were once all classified as nautiloids, but are now separated in several different subclasses. Our samples are almost all too poorly preserved to recognize (#991, #1617), with these exceptions:
Coiled cephalopods: including both nautiloids and ammonoids (#1712, 432, 21, 1045, 1660)
Belemnoids: Squids have a terrible fossil record except for one group of squids which grew internal skeltons: the belemnoids (#433, 986).
1. #605 - This sample contains both nautiloid and ammonoid fossils. Which is which? How do you know?
2. #781 - What kind of sutures does this organism have? Estimate the age of the fossil.
3. no number: What kind of preservation is this?
4. #427 - There are two different classes of mollusc in this sample, both with long straight tubular shells. What are they? How can you tell them apart?
5. unnumbered: What kind of preservation is this?
6. #874 - What kind of cephalopod is this? What is your evidence?
7. #1507 - What group of fossil cephalopods does this belong to? What is your evidence?
Mollusc shells lend themselves well to mathematical modeling because of the simplicity of their construction. All mollusc shells are essentially some 2-dimensional shape coiled around an axis. What that 2-dimensional whorl shape is depends upon the organism. For example, in some snails, the whorl shape is almost a circle, as seen in the round aperature of the animal. In Conus, the whorl shape is essentially a triangle. That shape starts out very tiny and expands as the animal grows and the whorl shape is rotated about an axis. Note: this whorl shape is not necessarily the same shape as the aperature. In Conus, the shell overlaps itself tightly, so the aperature is reduced to only a portion of the triabngle that makes the shell.
The rotation of the whorl about the axis can also vary. The whorl can move out from the axis either quickly or slowly, producing whorls that overlap completely, or whorls that do not even touch. The whorls may spiral in a single dimension, producing flat shells like ammonoids, or may coil down an axis, producing spired shells, like gastropods.
We can describe mollusc growth, then, in four variables (see figs. 7.2 &7.3):
Why do we care? A couple reasons. First, if mollusc growth is controlled by only four variables, it is probably controlled by a small number of genes. It becomes easy to imagine how large changes in molluscan morphology are possible through very small genetic change, making molluscs an evolutionarily adaptable group.
Second, analyzing molluscs in this way gives us an idea of what is structurally and biologically important in constructing shells. There are many possible mooluscan forms, but only some are found in nature. This analysis gives us an idea why.
I. Examine the mollusc shells on the back tables. For each, describe the shape of the whorl and the general value (low, medium, high) or each of the other variables (W, D, T). Find the shell with the highest and lowest values of each variable. Use figs. 7.2 and 7.3 to help you.
Samples: 486, 1328, 1877, 1284, 1276, 1718, 1608, 1876, abalone.
II. Look carefully at Fig. 7.3. Notice that there no living or fossil organisms fall in either the back right quadrant of the box (high D, high W) or in the lower left quadrant (high T, high W). Why not? What would such creatures look like?