Belemnotheutis

Belemnotheutis is an extinct coleoid cephalopod genus from the middle and upper Jurassic, related to but morphologically distinct from belemnites. Belemnotheutis fossils are some of the best preserved among coleoids. Remains of soft tissue are well-documented in some specimens, even down to microscopic muscle tissue. In 2008, a group of paleontologists even recovered viable ink from ink sacs found in several specimens.

This genus was the subject of a dispute between several eminent 19th century British paleontologists, notably between Richard Owen and Gideon Mantell. Some authors incorrectly spell the genus Belemnoteuthis following the usual spelling teuthis (τευθίς) for 'squid'.

The genus Belemnotheutis is characterized by an internal shell consisting of a conical phragmocone covered apically by a thin rostrum, or guard, homologous to the bullet-shaped rostrum of true belemnites, a short forward projecting proostracum, and ten hook bearing arms of equal length.

Belemnotheutis fossils are sometimes found in remarkable states of preservation, some specimens retaining permineralized soft tissue. The mantle, fins, head, arms, and hooks are well-documented from remains preserved in Lagerstätten. One specimen recovered from Christian Malford, Wiltshire and currently displayed in the Paleontology Department of the Natural History Museum in London is fossilized clasping a fish.

Belemnotheutis is not a 'true' belemnite (suborder Belemnitina) but a closely related coleoid. Both belemnotheutids and belemnites resembled modern squids except that they had chambered internal skeletons called phragmocones.

The apical portion of the Belemnotheutis internal skeleton is called the rostrum (plural: rostra) or the guard. The rostrum of Belemnotheutis differs significantly from that of true belemnites. Unlike the bullet-shaped dense guards of belemnites, the rostrum of Belemnotheutis is only present as a very thin sheath. It was also composed of aragonite rather than the heavy calcite of belemnites. In large specimens the rostrum can reach a maximum of only 1 mm (0.039 in) in thickness near the tip. The outer surface was covered by a thin organic layer in the live animal. In true belemnites, the large dense rostra acted as a counterbalance, keeping the animal horizontally oriented when swimming. It was long assumed that Belemnotheutis were confined to shallow waters, unable to venture into deeper waters due to the absence of the heavy rostra. The discovery of cameral deposits in the phragmocones of Belemnotheutis in 1952 made it clear that they were capable of controlling buoyancy.

The phragmocone of Belemnotheutis is short and blunt, measuring around 35 mm (1.4 in) to 86 mm (3.4 in) in length. The outer wall of the phragmocone is called the conotheca, distinct from the rostrum. It begins approximately 50 mm (2.0 in) from the tip of the phragmocone and consists of a nacreous outer layer and an inner lamellar layer. The outer layer gradually thins from 0.6 mm (0.024 in) in thickness to only about 0.1 mm (0.0039 in) thick at about 65 mm (2.6 in) further down the shell until it eventually disappears around the opening of the phragmocone (the peristome). Sometimes there is a hollow gap between the rostrum and the lamellar layer of the conotheca, indicating either organic content that have since disappeared or disintegration of the lamellar layer itself. The phragmocone of Belemnotheutis had about 50 chambers that were originally aragonitic, though they are usually replaced by calcium phosphate during the process of fossilization.

At the very tip of the phragmocone beneath the rostrum is an embryonic shell known as the protoconch. In Belemnotheutis, like in other belemnotheutids, the protoconch is roughly cup-shaped and sealed. This was thought to be another method of distinguishing it from other belemnites which usually have ball-shaped protoconchs. However, it is probably a taphonomic artefact, with the protoconch being spherical like other belemnites.