Belemnites were around from the late Carboniferous to the end of the Cretaceous, a period spanning over 200,000,000 years. They were cephalopods, related to the better-known ammonite. Their outward appearance was essentially that of a squid, or elongated cuttlefish. They were around in vast numbers - the fossilised remains of these creatures can often be found en masse. These mass mortality sites are occasionally referred to as 'belemnite battlefields' due to the fossil remains' resemblance to bullets, or lance-tips. For a reconstruction of what the living animal looked like, the British Geological Survey Website has a good representation.
What Did They Look Like?
Belemnites were cephalopods - marine molluscs which include octopoids, squid, cuttlefish and the extinct ammonite (to name but a few). Outwardly, belemnites could easily be mistaken for squid. They are more or less the same shape, having tentacles extending away from the mouth. Belemnites, unlike squid, had hooked tentacles, with which they could maintain a firm grip on their prey.
The chief fossil evidence left behind by these creatures are the 'guards'. The guards resemble an elongate bullet with an exaggerated point. This was the most solid part of the creature, and therefore the part most prone to the fossilisation process. At one end of the guard - the blunt end - is the phragmocone. This is a 'cup-within-cup' arrangement which housed the internal organs of the creature. Leading out from the phragmocone is the pro-ostracum, an extension of the guard/phragmocone assembly. The phragmocone is occasionally preserved, but the pro-ostracum is only rarely found.
Although the internal organs were stored in this arrangement, it was completely hidden from view in the living animal, much in the same way that a cuttlefish body covers its internal support (the white calcareous objects frequently inserted between the bars of a budgie cage). One remarkable fossil indicates the presence of an ink-sac - another thing in common with some modern cephalopods.
How Did They Live?
It is largely supposed that they swam in groups or shoals, as their modern (squid) relatives do. Another reason to suppose this is that the fossilised guards turn up in large numbers. These 'battlefields' have been taken as evidence that the belemnite had a similar mating procedure to that of the modern squid. The female squid die off in their thousands after their eggs have hatched. This could indicate that the majority of belemnites found in large groups are female, although even with such a clear analogue as this, it would still be conjecture.
Another explanation for the mass mortalities is that the belemnites have been consumed by a larger animal, which then vomited out the indigestible parts. Indeed, a fossilised shark has been found which contained over 250 belemnite guards in its stomach. These were probably the cause of death. The shark may well have swallowed the belemnites in one go, as modern predators frequently attack the vast numbers of squid that gather to mate. Other marine predators would have included ichthyosaurs. Not only have fossilised hooks from belemnite tentacles have been found adhering to ichthyosaur rib-cages, but some of the smaller belemnite 'battlefields' have been identified as regurgitation sites. It seems that the ichthyosaur ate a lot of belemnites and then regurgitated the most indigestible part - the guard. It is quite possible that the shark with the belly full of belemnite guards fed the same way, but died before it could regurgitate them.
However, given all the other similarities, it is reasonable to assume belemnites hunted in the same manner as squid. There were fins each side of the mantle (the long, rear body part), which, when undulated, could propel the creature through the water. In addition to this, water could have been forced through a funnel at the head end, propelling the animal away from danger at high speeds. Their diet is assumed to have included plankton, crabs and small fish.
Why Did They Die?
Nobody knows. They were highly successful, and were around for roughly 200,000,000 years. They died at the end of the Cretaceous, along with the dinosaurs. Why they went extinct while squid and other cephalopods survived is yet another mystery the K-T extinction event left behind. It could be that their reproductive cycle was sufficiently different from that of squid to be affected by the bolide impact/extreme volcanism of that period.
Where Can They Be Found?
Belemnite fossils are easy to find (in the UK). Although they can be found in many places, the easiest to get to for most people is Lyme Regis. They are so numerous there that a whole bed is named after them (the Belemnite Marls). These can be found at Black Venn, a promontory between Charmouth and Lyme Regis. The Belemnite Marls can be reached by climbing two-thirds of the way up Black Venn, until you are above the Black Venn Marls, which are dark black and friable. This is not recommended however, as there are frequent slides along this stretch of coast. It is safer to look along the beach under Black Venn for obvious bullet/spear-point like objects. They are black in colour, and can be anything from a couple of inches to around six inches in length. When you get them home and they dry off, they can look rather plain. To get them gleaming, get a tin of Duraglit or Brasso. Make sure it's the one with the polish soaked up into little packets of wire wool, and give your belemnite a careful rub. The wire-wool polishes the scratches out of the surface of the calcite, and the polish, being slightly oily, ensures a gloss finish.