These shelled molluscs are relatives of the modern squid and, like them, possessed tentacles when they were alive. The shell is divided into a series of hollow chambers linked by a tube; in life the animal could adjust the gas pressure in these chambers and so move up and down through the water, rather in the way that submarines do today. The walls between each chamber varied from being only gently curved in the nautiloids to high folded in some of the ammonites; these sutures can often be seen on the surface of fossil ammonites, goniatites and nautiloids, which helps to distinguish them from the unchambered shells of fossil gastropods. Representatives of these three groups are common in some of the rocks of Northern Ireland, and the Ulster Museum has large collections of them.
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Ammonites, goniatites and nautiloids have been common throughout the world's seas for more than 500 million years. Ammonites and goniatites are both long extinct, while only a few species of Nautilus survive in tropical seas today. However, some of their modern relatives, the squid and cuttlefish, are still common around the shores of Britain and Ireland. |
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Ammonites were common throughout the oceans of the world from about 250 million to 65 million years ago. Although some ammonites had smooth shells, many were ornamented with ribs and spines, as in this example, and some were even uncoiled. In Northern Ireland the only rocks in which we find their fossilised shells are from part of the Lower Jurassic (from about 200-190 million years ago) and the Upper Cretaceous (from about 95-65 million years ago). The sutures, visible on the lower part of this specimen, are more complex in ammonites than in goniatites or nautiloids. |
Goniatites appeared about 380 million years ago and thrived in the sea until their extinction 250 million years ago. Most had a smooth or weakly ornamented coiled shell. In Northern Ireland they are sometimes common fossils in Carboniferous mudstones and limestones from about 360-320 million years ago. The sutures of goniatites typically form a zig-zag pattern, simpler than the sutures of ammonites but more complex than those of nautiloids. |
Nautiloids are by far the longest surviving of the three groups, having first appeared more than 500 million years ago and with a few still surviving today. Many of the early nautiloids had an uncoiled, cone-like shell but for the last 250 million most, including the present day examples, have looked pretty much like this one. They can be found as fossils in most of the marine rocks of Northern Ireland. Nautiloid sutures are the simplest of the three groups, being only gently curved. |
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