GEOLOGY | FOSSILS | key fossil groups
belemnites
Belemnites belong to the phylum Mollusca and the class Cephalopoda. Belemnites were abundant in the the Jurassic and Cretaceous. The Jurassic Period began about 201 Mya and the Cretaceous Period ended about 66 Mya, together, these represent a time interval of about 135 Ma.
The belemnites became extinct at the end of the Mesozoic. We know a lot about them because they are commonly found as fossils found in sedimentary rocks.
Belemnites are a commonly found fossil in Oxfordshire.
Morphology
Belemnites were marine animals belonging to the phylum Mollusca and the class Cephalopoda. Their closest living relatives are squid and cuttlefish.
They had a squid-like body but, unlike modern squid, they had a hard internal skeleton. In the animal’s tail, the skeleton formed a bullet-shaped feature referred to as a rostrum. These are the parts that are normally found as fossils.
1. rostrum
The internal, bullet-shaped rostrum (or guard) probably acted as a counterbalance to the head and tentacles during swimming.
The rostrum was composed of fibrous calcite crystals arranged at right-angles to the surface. There may also be a concentric pattern, like the growth-rings of a tree trunk. These ring-like patterns represent the growth of the animal, probably over a period of months.
2. Arm hooks
As well as the rostrum, strongly curved, sharp hooks made of chitin are sometimes preserved as fossils. These hooks were attached to the belemnite’s tentacles and were probably used for grasping and holding prey such as other molluscs, small fish and crustaceans. Larger hooks may have been used in mating.
Belemnites had 10 arms/tentacles, each equipped with 30–50 pairs of hooks.
3. phragomocone
At the anterior (wider) end of the rostrum was a fragile, multi-chambered structure constructed of aragonite called the phragmocone. In fossilised belemnites, the phragmocone has often dissolved out leaving a cone-shaped void. The phragmocone records the growth stages of the animal, the juvenile belemnite would have started life in the protoconch and the final chamber housed the animal until death. As the animal grew it would move into successively larger chambers, sealing off the discarded chamber behind it. The empty chambers would then be used for buoyancy. A soft tube, the siphuncle, linked all the chambers and was used to regulate the proportions of gas and liquid in each.
The protoconch was the first chamber lived in by the juvenile animal.
The rostrum is the most commonly found body part in the fossil record. In life it would have been an internal calcite mass.
The proostracum was an extension to the rostrum.
Siphuncle: the soft, delicate tube that ran from the protoconch through the phragomocone to the body chamber.
The phragomocone is the empty chambers, once inhabited, but abandoned by the animal as it grew.