Knobs – spherical structures that extend from ribs in places.Ribbing – ribs running across the whorls.In cases where the shell does survive, mineralisation of this can lead to some beautiful ammonite specimens, for example, those of the Gault Clay in England and the Bearpaw Formation of Canada and the USA.Īmmonite shells often have ornamentation, consisting of some or all of the following: These are basically a copy of what the outside of the shell looked like, only a tiny bit smaller. The actual aragonite shell of the ammonite creature is often lost during fossilisation, to leave us with internal mould fossils. Heteromorph ammonites had a variety of shapes, including the basic ammonite spiral, only uncoiled (each new whorl not touching the last), a spiral that became significantly uncoiled in the lead up to the animal becoming mature, and upwards coiling spirals, like the type we see in snail shells, to mention but a few. If it does not, the specimen is said to be ‘evolute’ (for example, Dactylioceras). If the outer whorl of an ammonite shell largely covers the preceding whorls, the specimen is said to be ‘involute’ (for example, Anahoplites). Some are fairly square (for example, Acanthoceras), while others are more rounded (for example, Dactylioceras) and some very slim (for example, Placenticeras). In transverse cross section, the whorls of different ammonite species can show various profiles. In other words, they are generally a flattish spiral, which gets thicker with each whorl. The shell – generalĪmmonite shells (apart from in the heteromorphs) all follow a similar basic design, which is that of an expanding cone, spiralling around a centre point, in a single plane. Ammonites (Ammonitida) were shelled cephalopod molluscs that lived throughout the world’s oceans during the Mesozoic (Triassic, Jurassic and Cretaceous periods) and they were an extremely successful order, as evidenced by the frequency with which their remains are found.
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