analysis of Mesozoic ammonites
by Robert Sturm, Austria
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Sampling, preparation and determination of ammonites is one of the most fascinating working fields within the palaeontological discipline. From a systematical point of view, ammonites belong to the cephalopods, representing the most highly developed molluscs. Ammonites colonized marine habitats during the late Palaeozoic (Devon-Perm) and Mesozoic period and became one of the many victims of the meteorite impact 65 million years ago (as we know this event was also responsible for the extinction of the dinosaurs). Their main characteristic is the shell, forming lots of shapes with highly varying complexity. The shells of most species are convoluted in a plane, for which also the term “nautilicon” is used. Concerning their size ammonites were marked by great variability, with shell diameters ranging from few centimetres (e.g. Cheiloceras) to 2.5 metres (Douvilleiceras, Pachydiscus). Except for Nautilus, recent cephalopods have only preserved shell rudiments, which in the case of Sepia and Loligo are carried under the integument and in the case of Octopus are reduced to two small calcium carbonate sticks.
In the contribution presented here, the shells of some Mesozoic ammonite species are presented, using the stereographic technique. This method enables the interested viewer to get a more spatial and plastic impression of the shell architecture and of respective individualities. For a successful perception of the 3D effect stereo viewing glasses (left: red, right: blue) are needed, which can be ordered via the World Wide Web or self-produced, using red- and blue-coloured transparencies.
a …Amaltheus sp. (Jurassic, Plienbach, diameter: 3.5 centimetres)
b …Crossouvria sp. (Jurassic, Callov, diameter: 1.4 centimetres)
c …Staufenia sp. (Jurassic, Aalen, diameter: 2.5 centimetres)
d …Hildoceras sp. (Jurassic, Toarc, diameter: 3 centimetres)
The biology of the ammonites has to be regarded as a thrilling feature of palaeontology, because most of the current knowledge was obtained from the investigation of numerous fossil samples and the implementation of recent cephalopod biology. First of all, ammonites seemed to be characterized by a kind of sex dimorphism, with females often being much bigger than males. The radula, representing the chewing apparatus of molluscs, was found out to be quite small with a low number of teeth, thereby not serving as an appropriate tool for the breaking up of the nutriment. A very specific feature of ammonites were the so-called aptychs (aptichi) and anaptychs (anaptichi), which represent flap-like structures made of calcite or chitin and which are recently interpreted as parts of the jaw apparatus of the animals. However, besides shell morphology these structures became essential for ammonite determination.
a …Catacoeloceras sp. (Jurassic, Toarc, diameter: 1.5 centimetres)
b …Joannites sp. (Triassic, Anis-Karn, diameter: 1.5 centimetres)
c …Oppelia sp. (?) (Jurassic, Bajoc, diameter: 1 centimetre)
d …Harpoceras sp. (Jurassic, Toarc, diameter: 3 centimetres)
The nutriment of ammonites included among others smaller ammonites (cannibalism can be found quite often in the animal world), protozoa (foraminifers), crustaceans (ostracods), and small echinoderms. However, ammonites did not primarily act as predators, but more likely occurred as grazers, taking up all eatables crossing their way. Besides benthic organisms living on the sea ground they also ingested various kinds of zooplankton. The most prominent enemies of adult ammonites were among others aquatic dinosaurs (e.g. Mosasaurus and Plesiosaurus) as well as fishes, turtles, large crustaceans, and bigger cephalopods. Juvenile ammonites and eggs were mainly taken up by fishes and cephalopods. As an important difference to recent cephalopods, ammonites probably did not have any ink glands, enabling their escape from potential enemies by releasing high amounts of ink into the water.
a …Dactyloceras sp. (Jurassic, Toarc, diameter: 2 centimetres)
b …Pseudothurmannia sp. (Cretacious, Hauterive – Barenne, diameter: 2.3 centimetres)
c …Strenoceras sp. (Jurassic, Bajoc – Bathon, diameter: 1.5 centimetres)
d …Grammoceras planula (Jurassic, Toarc, diameter: 3 centimetres)
Regarding their biotope, most ammonites colonized the outer part of the shelf sea and lived near the sea ground, where a sufficient supply with nutriments was guaranteed. Neither in vertical nor in horizontal direction their movement was fast enough to act as predators. According to the recent knowledge it is rather assumed that some ammonite species were creeping ground inhabitants.
Another important feature for the successful determination of single ammonite species is the so-called suture line, representing the intersection between the partition wall of single chambers and the outer wall of the shell itself (that sounds as complicated as it really is). This suture line was subject to a development in Mesozoic ammonites, thereby continuously increasing its complexity. Today, most determination keys of Mesozoic ammonites are based on the suture line concept, whose understanding requires a long and extensive attention to the subject.
Comments to the author, Robert Sturm, are highly welcomed. Also any exchange of information on ammonite determination and biology is gratefully acknowledged by the author.
Published in the September 2005 edition of Micscape.
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