Cephalopoda: Belemnitida

BELEMNITES

Cephalopoda: Belemnitida

Belemnites are extinct cephalopod molluscs. The internal shells from belemnites (called a guard or rostrum) are commonly found in Jurassic and Cretaceous deposits, preserved in dense, fibrous calcite.

The name is derived from the Greek word for dart and refers to the pointed end of many belemnite guards.

They were a highly successful group of exclusively marine animals, and were most abundant during the Jurassic and Cretaceous periods. They shared the same fate as the dinosaurs, marine and flying reptiles, and ammonites at this time. There is no fossil evidence of these animals beyond the end of the Cretaceous period, 65 million years ago.

Living members of the Cephalopoda include squid, octopus, cuttlefish, nautilus and spirula. The nautilus is the only living cephalopod with an external shell.

Belemnite guards can be conical or bullet-shaped, the main body of which acted as a counterweight to the chambered phragmocone. In more complete specimens, the hollow alveolus or chambered phragmocone may be partly preserved at the proximal end of the guard, but it is rarely complete. This conical structure allowed the animal to be horizontally balanced in the water.

Neohibolites minimus, Lower Cretaceous (199-142 mya) Folkstone, Kent

The thin, transverse partitions or septa, divide the interior of the cone into a succession of narrow chambers, traversed near the lower margin by the siphuncular tube.

Most specimens are found broken, showing the radial prismatic fibres of the mineral calcite, which make up the shell. These are crossed by concentric growth layers.

The guard was secreted as a series of sheathing layers around and behind the phragmocone. It is the equivalent to the tiny thorn-like spike at the distal end of the present day ‘cuttle bone’. Well-preserved, uncrushed belemnite guards, may still have the apical part of the phragmocone still in position at the end of the guard. Sometimes only the conical cavity (alveolus) remains, where the phragmocone once was. This is often found infilled with a plug of matrix that has taken its place.

Belemnites attracted attention long before their true nature was understood. Their guards, especially when weathered out and found loose on the surface of the ground, have sometimes been popularly known as ‘thunderbolts’. These fossils are often mistaken for fossil teeth because of the often sharply pointed guard and hollow cavity at the opposite end. However, the similarities are purely superficial as most teeth have diagnostic ornament, keels or cusps. Below the pro-ostracum and in front of the last septum were the main soft parts still further forward were the head and arms.