Subclass Ammonoidea

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Juvenile - hamitid ammonite

By Dr. Neale Monks, coauthor of the book Ammonites

Ammonoids are a group of externally shelled cephalopods probably more closely related to the coleoids than to the nautilus. Evidence for this comes from the structure of the radula as well as features of shell growth (especially of juveniles) [Engeser 1996].

Cretaceous

Ammonites are a group within the Ammonoidea characterized by very complex septa, the walls between the chambers of the shell. In nautiloids this wall is basically a shallow hemisphere. Throughout the ammonoidea, and especially in the ammonites, this wall becomes more and more intricately folded. This reason for this is not clear. It has generally been thought to be an adaptation to living in deep water [Batt 1989]. It is likely that it also allowed ammonites to have thinner shells than nautiloids without losing mechanical strength.

Heteromorph ammonites (figured in the two sketches) have uncoiled shells. These shells are often helical in the juvenile phase and planar when mature. Hamites maximus (the left illustration is of a young H. maximus) is helical when young. At this stage it was probably planktonic with the head orientated downwards. When mature, the helix was a small part of the shell of H. maximus. The photograph on the right shows the juvenile coil of a similar species, Hamites subrotundus. The rest of the shell was basically straight but folded twice, and looked rather like a paper-clip. Some heteromorphs are helical at all stages and may have been planktonic (or vertically migrating) even as adults. [Klinger 1980]

Other species, like Hamites incurvatus (the right illustration) are planar throughout growth. Some palaeontologists think that this was an adaptation to make the shell very stable in the water, and that these species could not swim at all. Instead, they may have drifted along catching zooplankton, perhaps like modern cranchid squids [Westermann 1996]. Others believe that these forms lived on the sea floor, and were able to tilt the shell by moving water within the chambers [Kakabadzé and Sharikadzé 1993] or simply by the position of the body in the shell [Monks & Young, in prep]. Helical

The life habits of ammonites are much in debate, right down to whether they could swim or not! Some believe that that after an initially planktonic stage the juveniles became benthonic and crawled about on their arms like octopuses [Ebel 1990, 1992]. We do know that they ate small benthic crustaceans, molluscs and echinoderms. They may also have scavenged [Lebrun 1996].

The final image is my of my paperweight. Ahem, um, Neale tells me it is a...

The paperweight is probably a "goniatite", an early offshoot from the ammonoid lineage. It has relatively simple sutures compared with an ammonite. It looks very like the specimens widely traded that come from the Devonian of Morocco.

Neale, you hit the nail on the head, my paperweight says "Made in Morocco". I'm not entirely sure how the Moroccans were able to make a fossil though... :-)

Also see the Fossil Cephalopods FAQs and Neal's Ammonite Gallery. There is a nice collection of online images of ammonites on the Ammonites at the Black Hills Institute web page.

References and Credits

Credits

Thank you to Neale Monks for helping with this page.

References

Batt R. J. 1989. Ammonite shell morphotype distributions in the Western Interior Greenhorn Sea and some paleoecological implications. Palaios, 4: 32-42.
Ebel K. 1990. Swimming abilities of ammonites and limitations. Palãontologische Zeitschrift, 62: 25-37.
Ebel K. 1992. Mode of life and soft body shape of heteromorph ammonites. Lethaia, 25: 179-193.
Engeser T. 1996. The Position of the Ammonoidea within the Cephalopoda, p.3-19. In Landman N. H., Tanabe K., Davis R. A. (eds.), Ammonoid Paleobiology, Topics in Geobiology Volume 13. Plenum Press, New York, USA.
Kakabadzé M. V, and Sharikadzé M. Z. 1993 On the mode of life of heteromorph ammonites heterocone, ancyclocone, ptychocone), p. 209-215. InS. Elmi, C. Mangold and Y. Almeras (eds.), 3-eme Symposium internationalsur le Cephalopodes actuels et fossiles. Geobios Memoire Special 15.
Klinger, H. C. 1980. Speculations on Buoyancy Control and Ecology in some Heteromorph Ammonites, p. 337-355. In House, M. R. and Senior, J. R.(eds.), The Ammonoidea. Systematics Association Special Volume 18. Academic Press, London, UK.
Lebrun P. 1996. Ammonites. Minéraux et Fossiles, Hors-Série 4: 1-96.
Westermann G. E. G. 1996. Ammonoid life and habit, 607-707. In Landman N.H., Tanabe K., Davis R. A. (eds.), Ammonoid Paleobiology, Topics in Geobiology Volume 13. Plenum Press, New York, USA.

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