Geology 105 - Paleontology
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Lab #6: Molluscs: Cephalopods

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

1. Identify a fossil as a nautiloid, ammonoid, belemnoid, scaphopod or Tentaculites.
2. Recognize the age of an ammonoid from its sutures.
   
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.
7. Be able to describe the mathematical variables controlling molluscan growth.

Display: Cephalopods

I. Soft part morphology - For the tiny squids in the resin blocks, use fig. 13.14 in your book to identify the tentacles, the hood, the hyponome (the tube it shoots water out through), and the eyes (closed in these samples).

II. Hard part morphology - Nautilus shell and ammonites - be able to identify the septa

• A. Nautilus: (#832) find the septa, chambers, septal necks (where the siphuncle ran.  The organism uses the long soft rope of the siphuncle to regulate the amount of fluid and gas in the chambers).
• Structure of the chambers: compare the Nautilus (#832) with the three ammonite specimens, and #11 Baculites.
• A. In Nautilus, the chamber walls (septa) are geometrically very simple - just a domed curved shape.  Where the septa meets the wall of the shell, it creates a simple curved line.  If the chambers were filled with sediment or minerals, the septa would show up as that curved line - the suture.
  
• B. In these three fantastically beautifully preserved ammonite specimens, the shells were preserved with nothing filling up the chambers.  You can see how complicated the septa are - not the simple curved dome of the Nautilus, but a corrugated surface that creates a very complicated suture.  Look carefully at the septa to see the complicated structure.
• C. #11 Baculites: This is a single chamber of a straight ammonite called Baculites - actually a mold of that chamber (the chamber is an empty space, while this is the mud that filled up the chamber).  You can see the shape of the septa on each end. This specimen shows the complicated line where the septa meet the chamber wall - the sutures. 
• Siphuncle: nautiloids and ammonoids use a long strand of soft tissue the runs back through the septa to regulate the amount of fluid and gas in the shell.  The siphuncle is supported by small projections of the chamber wall, called a septal neck.
• A. Find the septal necks on the Nautilus.  In this species and in many nautiloids, the siphuncle runs near the center of the septa.
• B. If you look carefully at the large straight ammonite, you can see a tiny circle at the narrowest part of the shell. That's where the siphuncle passed through the shell.
  
• C. #11 Baculites: At the narrow end of the shell you can see a small circle of shell - that's the septal neck for the siphuncle.
  

III.  Kinds of Cephalopods

A. Primitive straight-shelled cephalopods: These organisms were once all classified as nautiloids, but are now separated in several different subclasses. ALl of these groups are Paleozoic in age, most Early to Middle Paleozoic.  Our samples are mostly too poorly preserved to recognize, but you can see these things:

• 1. Straight sutures (#1034, 982). 
  
• 2. On some specimens (#991, 1617, 1621) you can see the dome shape for the chambers.
• 3. #1100 & 1506: you can see a very small siphuncle
• 4. #994 Endoceratid with siphuncular deposits - look for the big crystals in a very wide siphuncle.  The organism used these deposits as counterweights to keep the shell horizontal.
  

B. Nautiloids: Organisms like Nautilus have existed since the Paleozoic.

• #2014 Eutrephoceras dekayi: Notice the gently curved sutures.  The outermost chamber of the nautiloid is filled with mud, and the inner coils are full of calcite.  The hole in the septa for the siphuncle was too samll to allow mud to get through, so the inner chambers were not filled with sediment when the sediment was deposited.  This often happens, but usually the shell crushed as more sediment accumulated.  In this case, the mud was micrite - calcite mud - so the water running through the sediment contained a lot of dissolved calcite that deposited calcite crystals inside the shell.
  

C. Ammonoids:

• Nautiloids v ammonoids: You already saw that ammonoids have much more complex sutures than nautiloids.  The convulted structure of the septa allowed the ammoinoids to adapt in two other ways: 1) thinner shells and 2) more streamlined shapes.  This meant that ammonoids could grow larger than nautiloids without having to produce huge amounts of shell.  They could also become more effective predators, because they could become fast, streamlined swimmers.  Compare the shapes of the ammonoids you see in this part of the display with the very round nautiloid (#2014).
  
• 1. Sutures: The first ammonoids had septa only slightly more complicated than nautiloids.  Over time, the septa became more convoluted, with more complex sutures.  Use fig. 13.17 in your book to recognize the kind of suture.
  
• a. Goniatites (#1712 Gonioloboceras, #98, #281): Goniatitic sutures are simple wavy lines.  These ammonoids are typically plump and more spherical, like nautiloids.  Goniatites are usually Late Paleozoic in age.
  
• b. Ceriatites (# 279 Phaneroceras, #49):  In the Permian and Triassic, ammonoid septa became more complex.  This produced ceriatitic sutures, where the sutures forms a rounded wiggle, and then a spiky wiggle.
• c. Ammonites (unnumbered Scaphites, unnumbered large pachydiscid, #993 Baculites, unnumbered Baculties):  Jurassic and Cretaceous ammonoids usually have much more complex septa, and so complicated sutures.   Ammonitic sutures look a bit like very fancy mittens, with many wiggles that represent the corrugations in the septa.

• 2. Ornamentation. 
• a. #416 Epenogonoceras: Very smooth, flat streamlined ammonites were probably fast swimmers.  This ammonite coiled around one point, so the outer whorls completely covered the inner whorls, leaving no indentation to create turbulence.
• b. #834 Haugia, #432, unnumbered specimen: Many fine ribs actually enhance the hydrodynamics of the shell by holding a small boundary layer of water around the shell.  This reduces drag on the water as the shell moved.  These forms were probably also fast swimmers.
• c. #398 & 21 Pervinquieria:  Very large ribs suggest that the ammonite was not very streamlined.  The large ribs may have provided some protection from predators (fish or marine reptiles) by making the shell wider and harder to bite.  However, the knobs on the shell are not thickened parts of the shell, as they are on highly ornamented gastropods.  The shell would still have been thin and vulnerable, so the ribs may not have a protective function after all.

• 3. Heteromorphs: Some families of ammonite evolved uncoiled shapes.
• a. #833 Scaphites coiled for part of its life and then grew straight.
• b. #1551 and unnumbered Baculites coiled only as an embryo in the egg, then grew straight for the rest of its life. #1551 Baculites  is a section from an organism probably about a meter long.
• c. See the pictures for some other rather spectacular and bizarre heteromorph ammonites.
• d. Because most heteromorphs are not hydrodynamic, they were probably not active predators like the other ammonites.  They may have lived high in the water column, filtering water, or lived near the bottom as scavengers.  However, Baculites may have been able to swim.
  

D. Belemnoids: Squids have a terrible fossil record except for one group of squids which grew internal skeletons: the belemnoids.

• 1. Rostrums - unnumbered specimens: The skeleton included an aragonite phragmacone and a calcite rostrum (see diagram and the picture of the entire fossilized animal).  Usually you find just the calcite rostrum, but one of these specimens still has the aragonite that originally covered the calcite.
• 2. Phragmacone - unnumbered specimens.  Some of our specimens show the conical hole where the chambers of the phragmacone were.
  
• 3. Crystalline structure of the rostrum - unnumbered broken specimens. Note the radiating calcite crystals that make up the rostrum.
  

Questions

1. #605 - This sample contains both nautiloid and ammonoid fossils. Which is which? How do you know?

2. unnumbered - What kind of sutures does this organism have? Estimate the age of the fossil.

3. #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?

4. #606, three unnumbered specimens: What kind of preservation is this?

5. #874 - What kind of cephalopod is this? What is your evidence?

6. #832 - What group of fossil cephalopods does this belong to? What is your evidence?  How is it preserved?

7. #432 - What does the ornamentation of this cephalopods tell you about how it lived?  Is it a nautiloid or an ammonite?

8.  Unnumbered - Is this an ammonite or a nautiloid?  Use the sutures to estimate the age of the fossil.

9.  Unnumbered - Was this ammonite a fast or slow swimmer?  How do you know?

10.  Unnumbered - Notice that these ammonites look a little odd.  How did these ammonites come to look like this?

11.  two unnumbered specimens - No question for these.  They are here so you can see what happens to ammonites when they weather.  The suture lines get weathered out, leaving the clay or calcite that filled the shell in a very complicated pattern.  Ranchers on the west side of the Valley sometimes think they have discovered fossil brains when they find a large weathered ammonite.

12.  unnumbered - The large ammonite labeled 12a was sexually mature when it died.  In modern Nautilus, when the animal reaches sexual maturity, it stops growing.  The last chamber is much smaller than the one before it, and the last two sutures are close together (see picture).  Look carefully at the sutures on 12a and notice how the last two sutures overlap, while the next suture is farther back. 

The ammonite fossil labeled 12b includes the body chamber of the organism - the space on the outside of the last septa where the animal's body was.  You can recognize the body chamber because it's filled with mud (the rest of the shell is filled with calcite) and it has no sutures (because there were no more chamber walls). 

Your job is to look carefully at a minimum of five of the Scaphites fossils in the boxes labeled 12c.  For each one, figure out 1) is the body chamber here? and 2) if so, was this animal sexually mature when it died?

13.  unnumbered belemnoids.  No question here - just look carefully.  Both of these belemnoid rostrums were attacked by boring organisms after the squid died.  One was attacked by clionid sponges - look for the tiny holes where the sponge dissolved a hole in the belemnoid, and then moved into the belemnoid and continued to dissolve tunnels through the belemnoid.  The other belemnoid was bored by some kind of crawling organism, leaving trackways in the surface.

14. unnumbered - This is a pretty cool specimen of a primitive straight cephalopod that was overgrown by another organism.  The cephalopod is the circular shell (on the polished ends) with the knobby shell (on the weathered side).  You can see a crescent of mud that entered the empty shell and the crystals of calcite that grew in the part of the shell that did not fill with mud.  On one of the polished ends you can see the hole of the siphuncle.  Your job is to:

• identify the organism that overgrew the cephalopod shell
• draw a little cartoon that shows what happened to this cephalopod from the time it was alive, swimming just above the ocean bottom, until it became fossilized.

15.  Just a pile of pretty ammonites to look at.  No question.

Constructional Morphology

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 aperture. In Conus, the shell overlaps itself tightly, so the aperture is reduced to only a portion of the triangle 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):

• Whorl shape
• Expansion rate (W): rate at which the cross-sectional area of the shell increases
• Distance of aperture from the axis (D): a measure of how rapidly the coil spirals outward
• Translation rate (T): rate at which the spiral moves along the axis

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 molluscan 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.

There are two sets of the same shells - CM1 and CM2.  You just need to look at one of them.

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?