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These molluscs first appeared in the Cambrian and have been important ever since. Most fossil forms had internal or external shells that were divided into gas-filled chambers which kept the creatures buoyant in the water. These chambers are a useful way to distinguish cephalopod fossils from those of gastropods, which lack them. Most modern cephalopods, such as squids and octopuses, have lost theis shells.
Unlike most molluscs, the cephalopods have well developed nervous systems and sense organs, and swim actively by squirting water out of a muscular tube called the siphon - basically a primitive type of jet propulsion. They have well-developed tentacles and many are sharpeyed predators which compete with modern fish. Modern forms range from mere millimetres to the giant squid, the size of a small whale. All cephalopods are marine.
The earliest known types are the Nautiloids, which were widespread in the Ordovician and Silurian Periods. They had straight or gently curved shells up to 5 metres long. They became rarer after the Devonian, but a few species with coiled shells (e.g. the Nautilus) persist to the present day.
While the nautiloids were in decline during the later Palaeozoic, another cephalopod group, the Ammonoids, which first appeared in the Upper Silurian, were becoming more abundant. Ammonoids usually had tightly coiled spiral shells. The internal chambers of the shells met the surface as elaborate wiggly patterns called suture lines. These are useful for identification. Palaeozoic ammonoids include the goniatites, which are often abundant in Devonian and Carboniferous rocks. The most famous ammonoids, however, are the ammonites, which were prolific throughout the Mesozoic, but died out in the late Cretaceous. They are useful to geologists for dating rocks because they evolved rapidly and show great variety in their shells. No Triassic ammonites are known in Britain though they were common in other parts of the world. Vast numbers of Jurassic and Cretaceous types are known though, ranging in size from a couple of millimetres to well over a metre across. Most ammonites had spiral shells like those of goniatites, ornamented in seemingly endless ways, but some of the later, Cretaceous species adopted bizarre shell shapes, including hooks and snail-like spires.
Another important Mesozoic group of cephalopods, also now extinct, are the squid-like belemnites. These had pointed, bullet-shaped internal shells which are very abundant in some Jurassic beds. Occasionally, well preserved fossils show the soft tentacles and ink sacks.
Modern cephalopods, such as squids and octopuses, are poorly known as fossils because their soft shell-less bodies were unsuitable for preservation. We know that octopuses existed in the Cretaceous, however, because a slab of rock from Lebanon shows the faint imprint of the earliest known specimen.
These are a very important group of molluscs. They are characterised by a single coiled shell, although some types, e.g. the limpets, have a simple dome-shaped shell, and in others such as the slugs, the shell is virtually absent. Most gastropods are marine, though there are numerous freshwater types as well as land-living forms (e.g. the common and generally unpopular garden slugs and snails) which have developed 'lungs' to enable them to breathe out of water. Most gastropods feed by scraping up plant material, but many are flesh eaters which use their mouths to drill through the shells of other molluscs to reach their soft bodies.
Many fossil species are known, varying greatly in the nature of the coiling of the shell and the surface ornamentation. The first fossils are found in the Cambrian Period. In the rest of the Palaeozoic they are relatively uncommon, but are more abundant in the Mesozoic. It is in the Caenozoic that they really proliferate - some Eocene beds in Southern England are packed with the remains of numerous types, many of which are still alive in modern tropical seas.
Most gastropod fossils are fairly small (the smallest being about one millimetre long), but occasional 'giants' such as Campanile from the Tertiary were over 5Ocm in length.
Bivalves are molluscs with hinged shells and include the familiar, and edible, cockles, oysters and mussels. They first appeared in the Cambrian, though for much of the Palaeozoic they are quite rare as fossils. The largest specimens can be well over a metre across (e.g. the modern Giant Clam). Most bivalves are marine, though some are freshwater. None live on land. They feed by straining small edible particles out of the water using sieve-like organs called ctenidia.
Bivalves tend to be very sluggish creatures which either burrow in the mud or attach themselves to solid objects such as rocks, and they have poorly developed sense organs. An exception to this are the scallops, which are able to swim rapidly away from danger by flapping the valves of their shells. They also have primitive eyes located around the inside edges of the valves.
Bivalves are exceedingly abundant in Mesozoic and Tertiary rocks. Sometimes whole beds are made up of their accumulated shells.
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