Belemnites
Belemnites (BEL-em-NIGHTS) would have been a very common site during the Jurassic and Cretaceous periods. Their fossilised remains are abundant in Mesozoic marine rocks and are often found in similar areas to that of their not to distance cousins, the Ammonites.
Belemnites would have lived during the Devonian period and their well-formed belemnite guards have been found in rocks dating from the Early Carboniferous period, right through to the Cretaceous period. Belemnites were a very successful species and it wasn’t until the end of the Cretaceous period, Belemnites became extinct along with many other sea creature including the Ammonites.
About Belemnites
Belemnites which can also be referred to as belemnoids are an extinct group of marine cephalopod. Belemnites are very similar in many ways to the modern squid and closely related to the modern cuttlefish.
Like modern day squids and cuttlefish, belemnites had an ink sac which they would have used for defence, allowing them to escape predators quickly. Belemnites had a couple of major differences when compared to modern day squids. They had ten tentacle like arms of roughly equal length, and these arms had hooks instead of suckers on them. These hooked arms allowed Belemnites to grab their prey in a similar fashion to modern day squids, quickly firing out their arms to grab their prey.
Dmitry Bogdanov, CC BY-SA 3.0, via Wikimedia Commons
Effective Hunters
Belemnites were efficient carnivores that caught small fish and other marine animals with their arms. Once caught they would have used their beak-like jaws, that weren’t too dissimilar to that found on modern day squids.
Belemnites were also food for a lot of other marine reptiles such as Ichthyosaurs, with remains of Belemnites, such as their arms being found frequently in many fossilised stomach of marine reptiles.
How Big Were The Belemnites?
As only fossilised remains of the guard or rostrum have been found, it is still unclear as how big these animals could get. Scientist estimate that Belemnites could range from around 19 inches or 46cm right up to 10ft or 3 metres from the tips of the arms to the top of guard or rostrum.
Belemnites Fossils
When fossilised remains of Belemnites are discovered it usually portrays only the back part of the shell which is called the guard or rostrum. The guards or rostrum is shaped like a bullet, elongated and are pointed at one end.
The hollow region at the front of the guard or rostrum is termed the ‘alveolus’ and this houses a chambered conical-shaped part of the shell called the phragmocone. The phragmocone is usually only found on well preserved Belemnites specimens.
Ra’ike (see also: de:Benutzer:Ra’ike), CC BY-SA 3.0, via Wikimedia Commons
Projecting forwards from one side of the phragmocone is the thin pro-ostracum. The guard, phragmocone and pro-ostracum were all internal to the Belemnite, forming a skeleton which was enclosed entirely by soft muscular tissue.
The Belemnites would have been larger than the fossilized shell remains that have been discovered, with a long streamlined body and prominent eyes. The guard / rostrum would have been in place towards the rear of the Belemnite, with the phragmocone located behind the head and the pointed end of the guard / rostrum facing backwards.
FURTHER READING & STUDY
- Rene Hoffmann, Kevin Stevens: (2019) The palaeobiology of belemnites – foundation for the interpretation of rostrum geochemistry.
- British Geological Survey: Belemnites
- Yasuhiro Iba: (2014) The Early Evolutionary History of Belemnites: New Data from Japan
- Christian Klug: (2021) Fossilized leftover falls as sources of palaeoecologically data.