Page 358 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 345

color siphuncular
siphuncle bands cord septum
mural part chambers or camerae
of septum stomach
body tissue and
septal cavities shaded
line hood
intestine
eye
growth lines
mouth
oesophagus
radula
body chamber
dorsal tentacles
nacreous layer gills mantle cavity hyponome
(a) (b)

orthoconic cyrtoconic orthoconic cyrtoconic lituiticone gyrocone torticone
longicones brevicones
(c)
Figure 13.14 (a) Features of the shell and (b) internal morphology of a living Nautilus. (c) Shell shapes
of the nautiloids.

Box 13.6 Living Nautilus

Living Nautilus has allowed biologists and paleontologists to model the functions and life modes of
the ancient ammonites by using a modern analog. But despite the similarity of their respective shells,
coleoids are, in fact, more closely related to ammonites than modern nautiloids, and thus better
behavioral analogs may be found within the coleoids (Jacobs & Landman 1993). It is probable that
coleoid-type swimming mechanisms probably evolved prior to the loss of the body chamber in the
coleoids. Ammonoids thus probably had a coleoid-like mantle and thus may have operated quite
differently from living Nautilus. But how far could an empty ammonite shell travel? Ryoji Wani and
his colleagues (2005) have demonstrated that the phragmocone of living Nautilus pompilius becomes
waterlogged only after the mantle tissue decomposes. Water is then sucked into the shell because of
its lower internal gas pressures. This is actually more common for smaller shells, generally with

diameters less than 200 mm, and these fill up with water more quickly. Only larger shells had the
ability to drift long distances. Since the ratio of volumes of the body chamber to the phragmocone
in nautiloids is similar to that of the ammonoids, they probably behaved similarly. The small shells
sank and the large shells drifted.

ontogeny of the animal. The phragmocone is welded to the shell, a suture is developed,
chambered, with each chamber marking suc- commonly with a complex pattern of frilled
cessive occupation by the animal, and sealed lobes and saddles.
off from previous chambers by a septum, Five main sutural types are recognized
complex in structure at its margins, like a among cephalopods (Fig. 13.17). The ortho-
sheet of corrugated iron. Where the septum is ceratitic pattern, with broad undulations or