Introduction to belemnites

Jack Wilkin (UK) Belemnites are an extinct group of cephalopods that first appeared during the Triassic and became extinct at the end of the Cretaceous period. Their closest living relatives are squid and cuttlefish. Belemnites, unlike modern squids, have a hard bullet-shaped calcified internal skeleton consisting of three parts: a rostrum (plural ‘rostra’) or guard at the back of the animal (the term rostrum will be used throughout this article); the phragmocone in the middle; and the pro-ostracum at the front (Fig. 1). Fortunately, much is known about belemnites as their rostra are robust and readily preserved. Anatomy The rostrum was a bullet-shaped cylinder of solid calcite, which tapered at the end and was indented at the front by the alveolus that housed the phragmocone. The rostrum was the largest and most posterior part of the belemnite shell, and the most commonly preserved part of the animal. The rostrum is comprised of low-Mg calcite, a material relatively resistant to diagenetic alteration (that is, the changes in the chemistry or structure of a fossil once it has been buried) making it ideal for extracting isotopes to provide information about ancient climates and ocean circulation (see below). The rostrum likely acted as a counterbalance to the head and tentacles while swimming. Fig. 1. Reconstruction of the belemnite animal showing both the soft tissues and the three-part belemnite skeleton. Notice how, unlike ammonites, the skeleton is wholly internal surrounded by soft tissues. When the rostrum is broken, the internal structure can be observed … Read More

To access this post, you must purchase Annual subscription, 12 Month Subscription or Monthly subscription.
%d bloggers like this: