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Description of ammonites and nautiles
Both marine cephalopods molluscs, cousins of extant octopuses and squids with partitioned univalve external shell and tentacles arranged around the head.
They are found worldwide in large quantities in the fossil record, with an important diversity of species. Due to their rapid species evolution and their large number, ammonites are widely used as markers to identify specific geological strata, specifically for the identification of Jurassic epochs.
The shell chambers are filled with gas, except the last (and largest) one, which is the living chamber, where the body of the animal is. When the animal grows, it adds a new and larger chamber to the open end of the Coil.
The animal remains connected to the first chamber via the siphon.
In palaeontology, it is common to base our interpretation of the possible way of life of fossil organisms on the knowledge of morpholocically close present-day organisms. This can lead to major biases, as in the case of ammonites, for which we have long imagined an ecology close to that of the nautile. This view has now been widely challenged, with ammonites being more closely related to octopuses and squids than to nautiles (Engeser, 1996; Jacobs and Landman, 1993; Saunders and Ward, 1994; Ward et al., 2023).
Anatomy of ammonites and nautiles
Figure 1. Schematic representation of the anatomy in a transverse cut of both a nautile (on top) and an ammonite (on bottom).
Figure 2. Schematic representation of a front (on top) and side (on bottom) view of an ammonite (fossil).
Figure 3. Schematic representation of the suture lines. From A, the simplest form to C, the more complicated one.
These lines correspond to the contact betweeen the walls of the chambers (called septa) and the internal part of the shell. Nautiles do only have simple suture lines (Figure 1A) while ammonites go from very simple to very elaborated suture lines (Figure 1A to C).
These lines are only visible when the sediments fossilises inside the shell. The external part of the shell disappears and we only find the internla part, showing the suture lines.
While the nautiluses are all very involute (Figure 4), the variety of species in the ammonites kingdom is large and we find all the different coiling types.
Figure 4. Schematic representation of the different types of shell coiling found in the fossil record.
Differences between ammonites and nautiles
Ammonites (Ammonoidea)
First appearance in the fossil record during the Devonian, 410 million years ago.
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Extinct (over 10,000 species described), disappeared during the cretaceous – Paleogene extinction crisis 65 million years ago, at the end of Cretaceous (at the same time as dinosaures). The closer we get to the Cretaceous, the more complex their structure is.
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Tend to have complex suture lines.
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Septa with bulges and indentations, (to varying degree) convex when seen from the front (Procoele, oriented forward).
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Shells with various ornaments (grooves, carinae, tubercles, nodules, ribs, etc.), possibly related to their ecoogy (Cowen et al., 1973; Ritterbush et al., 2014).
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Maximum known diametre of 1,80 m (Ifrim et al., 2021) (and smallest one is 10 mm).
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Siphon runs along the ventral periphery of the septa and camerae.
Nautiles (Nautilidea)
First appearance in the fossil record during the late Triassic, 230 million years ago.
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Still existing (only 9 species).
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Simple suture lines.
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Simple septa, dish-shaped and concave when seen from the front (Opisthocoele, oriented backward).
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Smooth and colourful shell.
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Maximum known diametre of 25 cm for living species, and maximum known diametre of 2.5 m in the fossil record.
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Siphon runs through the center of the septa and camerae.
Bibliography / Bibliographie
Cowen, R., Gertman, R., and Wiggett, G.: Camouflage patterns in Nautilus , and their implications for cephalopod paleobiology, LET, 6, 201–213, https://doi.org/10.1111/j.1502-3931.1973.tb01193.x, 1973.
Engeser, T.: The Position of the Ammonoidea within the Cephalopoda, in: Ammonoid Paleobiology, vol. 13, edited by: Landman, N. H., Tanabe, K., and Davis, R. A., Springer US, Boston, MA, 3–19, https://doi.org/10.1007/978-1-4757-9153-2_1, 1996.
Ifrim, C., Stinnesbeck, W., González González, A. H., Schorndorf, N., and Gale, A. S.: Ontogeny, evolution and palaeogeographic distribution of the world’s largest ammonite Parapuzosia (P.) seppenradensis (Landois, 1895), PLoS ONE, 16, e0258510, https://doi.org/10.1371/journal.pone.0258510, 2021.
Jacobs, D. K. and Landman, N. H.: Nautilus - a poor model for the function and behavior of ammonoids?, LET, 26, 101–111, https://doi.org/10.1111/j.1502-3931.1993.tb01799.x, 1993.
Ritterbush, K. A., Hoffmann, R., Lukeneder, A., and De Baets, K.: Pelagic palaeoecology: the importance of recent constraints on ammonoid palaeobiology and life history, Journal of Zoology, 292, 229–241, https://doi.org/10.1111/jzo.12118, 2014.
Saunders, W. B. and Ward, P. D.: Nautilus is not a model for the function and behavior of ammonoids, LET, 27, 47–48, https://doi.org/10.1111/j.1502-3931.1994.tb01554.x, 1994.
Ward, P., Barord, G. J., Schauer, A., and Veloso, J.: Comparative Trophic Levels of Phragmocone-Bearing Cephalopods (Nautiloids, Ammonoids, and Sepiids), Integrative And Comparative Biology, 63, 1285–1297, https://doi.org/10.1093/icb/icad125, 2023.
June 2025
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