One of the other major differences lies at the core of success for all living organisms: reproductive strategy. The Pearly Nautilus of today contrasts very sharply with other living cephalopods in breeding technique. The females produce no more than a dozen, large (25-35mm) eggs per season enclosed in a milky white capsule that is then anchored to a stable surface. In about a year, a 25mm Nautilus hatches, looking like a miniature copy of its parents. Even fossil nautiloids show a hatching size of 2 to 19mm. (This can be inferred by changes in shell shape and composition at the time of hatching.) Ammonoids, in contrast, are only 1 to 3mm at emergence. Modern cephalopods with similar hatchling size produce thousands to tens of thousands of eggs  per year.
     Whatever the function of these differences, Ammonites soon began competing heavily with their ancestral nautiloid kin. At the end of the Paleozoic Era, both groups dropped to only a few species in the fossil record. The Mesozoic era proved to be a prolific time for cephalopods. Both taxa rebounded during the Triassic period, producing a wealth of shell shapes and sutures. Both groups became nearly extinct again at the beginning of the Jurassic. After this point, nautiloids are seen only with flat, spiral shells. Ammonoids, however, continued to evolve many different shapes, ornaments and sutural convolutions. At the end of the Mesozoic era, the infamous boundary between the Cretaceous and Tertiary (that some say was the occasion of a catastrophic event such as a meteor impact), the ammonoids disappeared, perhaps from similar causes that precipitated the demise of the last Dinosaurs. Nautiloids experienced a renaissance after the disappearance of their cousins, but slowly dwindled until today they only inhabit the deep waters in a small region of the world. Today, all of the ammonoids are gone, but their descendants may be the common cephalopods seen today. It is believed that modern octopus, squid and cuttlefish (coleoids) are more closely related to the extinct ammonoids than the living Nautilus species. The similarity between the breeding mechanisms of ammonoids and coleoid cephalopods supports the ancestral hypothesis, but I do wonder how many inferences one can make of the history of creatures that are so soft bodied that they have left no fossil impressions that have yet to be discovered. If the Mesozoic waters had hosted large numbers of octopus, we would never be the wiser.
 

References

Saunders, W. B., and N. H. Landman, eds. 1987. Nautilus: the biology and paleobiology of a living fossil. Plenum, New York.

Ward, P. D., 1992. On Methuselah’s Trail. W. H. Freeman and Company, New York

Ward, P. D., 1987. The Natural History of Nautilus. Allen and Unwin, London
 

Photographs and Illustrations

Title Picture - Christopher Broski 1998 - 165 million year old (Jurassic period) Dactiloceras fossil from Whitby, Yorkshire, England.

Figure 1. Waikiki Aquarium - Nautilus Belauensis

Figure 2. “Paleozoic Lecture” by Dr. Jon Anderson - A cut away nautilus shell showing the camera, body chamber and remnants of the siphuncle.

Figure 3. Christopher Broski 1998 - Orthoceras, a straight shelled Nautiloid from Morocco. (These are pretty common and cheap if you want to buy one. Two Guys Fossils has them. I have seen them dated from the Silurian and Devonian periods.)

Figure 4. Christopher Broski 1997

Figure 5. Paleo Place - One can observe the complex sutural patterns on the surface of this Placenticeras fossil.

Figure 6. “The Cephalopod Page” - A juvenile Hamites subrotundus. The adult form was paper clip shaped.
 

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