McGill University
EPSC 334 Invertebrate Paleontology

	LABORATORY EXERCISE # 9: class CEPHALOPODA (phylum MOLLUSCA)

Typical living cephalopods include the squid, octopus, cuttlefish, and pearly nautilus. These animals have a head equipped with tentacles that bring food to the mouth. The mantle encloses a relatively large cavity that housed the gills and is connected to the water by a narrow opening called the funnel. Animals like the squid are capable of rapid movement backward by jetting water from the funnel and slower swimming motions using a fin along the side of the body.

The single shells secreted by cephalopods consist of a series of chambers added as the animal grows. The modern Nautilus lives in the last chamber of its large shell and its soft parts are much like those of the squid.  The septa (singular =  septum) that divide the shell into chambers represent successive positions of the back of the animal as it grows to a mature size at which time it stops secreting the shell. The animal is in communication with the abandoned chambers by means of a thin strand of tissue (siphon) in a tube (the  siphuncle) leading through the septa. The tube consists of short backward extensions of the septa (septal necks) and connecting rings.

Nautilus shells: THESE ARE FRAGILE -- DO NOT DROP THEM!
Examine the shell fragments that show the interior and the polished specimen of a Recent or a fossil coiled nautiloid. Orient the shell in its living position and draw a cross  section through the plane of the siphuncle of the shell and label the following: septum, septal neck, septal foramina, position of the siphon (you may dot it in since the connecting rings are not preserved), living chamber, initial chamber (1). 

Note the striped protective colouring on the exterior. These  small specimens are immature. How does the colour pattern change in the mature animal (2a)? What is the adaptive value of the pattern (2b)? Why should it change as the animal matures (2c)? 

The septa of nautiloids are simple concave plates. This is best seen in fossil specimens in which the chambers have been filled with sediment and then fell apart. Examine  the specimen that shows these separated chambers.

The  "pearly" aragonite inner layer of Nautilus is only visible where the dull outer layer has worn off. The outer layer is commonly removed from specimens sold in shops to enhance their appearance. Examine the specimen from which the outer layer has been stripped away.  Was a similar layer present on the inside of some bivalves? Whas was its name and mineralogy (3)?

	FOSSIL NAUTILOIDS

  Many of the nautiloids of the Paleozoic were straight cones, not coiled like Nautilus but some were coiled even early in nautiloid history. To balance the shell, straight cephalopods built up deposits in the shell away from the body chamber. Most of these  deposits are in the siphuncle whose connecting rings are generally enlarged into the chambers to accommodate them. The siphuncle is examined by cutting the specimen along its axis.

Examine the silicified and polished specimens of Armenoceras and other orthoconid (i.e. straight-shelled) nautiloids.  Draw a longitudinal section showing the outer wall, septa, septal  necks, connecting rings, deposits in the greatly enlarged siphuncle (4).  Examine also specimens showing siphuncular deposits (Actinoceras). Other specimens show "inflated"  siphuncles but apparently no deposits in them.

Endoceras was a large straight nautiloid with a wide central opening in a massive siphuncle. What type of deposits were secreted to weight the shell (5a)? 

What would have been the living position of nautiloids without the presence of siphuncular deposits (5b)? Paleozoic nautiloids have a variety of forms in addition to coiled and straight. The position of all nautiloids depended on the vertical alignnent of their centres of gravity (in the body chamber) and of buoyancy (in the gas-filled shell).

Find in one of the textbooks one example each of Paleozoic nautiloids that are tightly coiled, slightly curved,  or shortened. Examine the plaster cast of Winnipegoceras. Draw a sketch to show how the distribution of chambers and body chamber would orient its shell in the water (5c), assuming that no siphuncular deposits were present.

AMMONOIDS

These cephalopods increased the strength of the septa by folding the edges into patterns of increasing complexity. The junction of the septa with the outer wall of the shell outlines the suture; if the septal edges are folded the suture is wavy. The suture is only revealed where the outer wall is worn away but commonly the wall of ammonoids is so thin that it is not preserved in all parts of their fossils and the sutures are obvious. Ammonoids of the 3 major suture type succeed each other in time, from the late Paleozoic through the Mesozoic. The great majority of ammonoids have shells closely coiled in one plane.

How would you describe the suture of Goniatites (Late  Paleozoic) (6a)? Make a sketch of the side view of one of the genera of goniatites from the collection showing the sutures clearly (6b).

How has the pattern changed in Ceratites (Triassic) (6c)? Part of the suture lines are intricately folded and the other is a simple fold. How are these different parts of the suture called (6d)?

How is the pattern further modified in Ammonites (Jurassic-Cretaceous) (6e)?

Examine also the chambers of ammonites that have been broken apart so that the septal face can be seen.  What preservation process gave rise to these fossils (7a)?  Is it possible to determine the shell mineralogy from them (7b)? Note how complex a suture is  produced by the progressively more intricate folding of the septum as it reaches the outer wall.

Examine the 3 specimens of ammonites that have been cut and polished. Keeping in mind the dynamics of the shell and the relative length of the living chamber, draw  the cross section of one of them in the living position (8a). Note the thinness of the walls and the shape of the septa. What adaptive value has been proposed for the increase in suture complexity (8b)? Why is it not always possible to see the siphuncle (9a)? Why is the internal chamber generally filled with a different material than the earlier chambers (9b)?

Heteromorphs:  Most  ammonoids are coiled in a flat  spiral (planispiral) like a watch spring and Nautilus.  In some groups less regular patterns evolved, adapted to different positions in the water.  Examine the following genera which show some of these patterns:

Hamites - no coiling, 2 sharp bends,
Turrilites, Plesioturrilites - coiled like a gastropod,
Scaphites - uncoiled from original tight coil then bent back at maturity,
Spiroceras - open coil.

What link have some scientists made between this spectacular diversity and the timing of extinction of these lineages (10a)? How is this interpreted nowadays (10b)?

Although nautiloids and goniatites commonly have smooth shells, ceratites and ammonites have shells ornamented with knobs, ribs, and  keels.  What explanation(s) have been given for the adaptive values of these features 10c)?

Draw one of the following ammonites showing the surface ornament (11a). Can you also see the sutures on any specimen? Why (or why not) (11b)?

Dactylioceras -simple transverse  ridges, all whorls visible fron the side.
Amaltheus, Arnioceras  - transverse ridges, keel on outer edge.
Scaphites - sharp nodes from which 3 ridges branch outward
from the side. Compare with Nautilus.

Placenticeras - smooth, thin, discus-shaped shell.
Acanthoceras - coarse ribs and nodes.

BELEMNITES

These are cigar-shaped fossils of calcite with concentric and  radial structure which are commnon in Mesozoic rocks. The animal was much like a squid but had a much heavier shell. The main part of the fossil is called the guard and appears to have acted as a counterweight like the siphuncle deposits of the nautiloids.  The chambered part of the shell, which is not commonly preserved, was small and fitted into a conical depression at the end of the guard. A third part of the shell, equivalent to the pen which is all that remains of the shell in the modern squid, extended forward supporting the body from the guard. The whole shell was internal, that is, it was covered by the soft parts of the animal and is smooth, locally with vascular markings.

Examine the fragments of large belemnites that show the chambered shell and its position in the guard. Examine and sketch the specimen from the Solenhofen limestone that shows the guard, chambers, and  pen which supported the front of the animal but is very rarely preserved (12).

"Argonauts" are a group of extant cephalopods.  Examine their paper-thin shell. Is it homologous with the shell of Nautilus(13a) What is its purpose (13b)?