Early Cretaceous belemnite from California

Belemnites are fossil cephalopods (include nautiloids, ammonoids, squids, octopus) that were abundant during the Mesozoic. Belemnites had an internal shell enclosed by the soft body. The shell consisted of a straight, tapering chambered part (phragmocone) that was embedded into the most massive part of the skeleton, the cigar-shaped rostrum or guard.

Some well preserved specimens have been found that show that belemnites had 10 arms (tentacles). Belemnites were fast-moving nektonic (swimmers) carnivores (meat eaters) that resembled squids. The largest belemnites had a total length of approximately 18 cm. Although some hypothetical reconstructions show flattened "hands" at the end of two of their tentacles (as shown above), some well preserved specimens that actually have impressions of the arms (click HERE) show no "hands" (as shown in the commercially available model below).

The geologic time range of belemnites is shown below:

Most belemnite specimens are like the one shown above; only the posterior part (rostrum) of the shell is present.  This is an incomplete rostrum (length 9.3 cm, diameter 2.7 cm) of the belemnite Acroteuthis kernensis Anderson, 1938 from the Hex Formation in the Devils Den District of central California. This formation is of late Early Cretaceous age (i.e., approximately 128 million years old and corresponding to what geologists refer to as the late Barremian Stage of the Early Cretaceous). The rostrum consists of solid, dense calcite (calcium carbonate). It acted as a counterbalance for the heavy head and tentacle areas of the belemnite shell; otherwise, the animal would have swam head down (not a desirable trait for an otherwise streamlined creature).

This is the ventral view of the specimen shown above. The ventral groove (function unknown) is well developed. Not all belemnites shells have a ventral groove.

This sketch shows the alveole, which is where the phragmocone (the chambered part of the shell) fits into the rostrum.  

Alveoles are rare at the Devils Den District locale, but I was able to find this interesting specimen (3.9 cm in total length) that shows the interior of the rostrum with an impression (a negative feature that can hold water) of the alveoli.

This is a cross-section view of the specimen shown above in the first two pictures of Acroteuthis kernensis. You can readily see how the calcite that comprises the rostrum was secreted in successive concentric layers (like tree rings) by the animal. The genus Acroteuthis is restricted to the Early Cretaceous between 140 and 125 million years ago. It has been found mostly in Canada (British Columbia, Yukon, and Northwest Territories), as well as in Greenland, Germany, and Russia.