Belemnitellidae: Reaching the End of an Era

Fossil cephalopods have featured on this site numerous times in the past. I've talked about nautiloids, I've talked about ammonoids. But one group of cephalopods that I haven't given that much time to to date is the group including the majority of living species: the coleoids. In coleoids, the ancestral cephalopod shell has become reduced and internalised (one group, the octopods, has lost the shell entirely) so it should not come as much of a surprise that their fossil record is more limited than that of other cephalopod groups. Nevertheless, the coleoid lineage does include at least one group known from an abundant fossil record: the Mesozoic belemnites.

Fossil guard of Belemnitella americana, from here, in ventral view with the ventral opening of the alveolus visible as a longitudinal fissure.

Belemnites were a significant part of the marine fauna during the Jurassic and Cretaceous. Externally, they were similar in overall appearance to modern squid, as demonstrated by rare finds of specimens with preserved soft body parts. However, whereas squid have the internal shell reduced to the thin, non-calcified pen, belemnites possessed a well-developed internal shell. The posterior end of the shell was a solid, bullet-shaped rostrum or guard, in front of which was a chambered section known as the phragmocone. Being completely calcified, the rostrum of a belemnite was readily preserved and isolated rostra make up the greater part of the belemnite fossil record (the more delicate phragmocone was less likely to survive the fossilisation process). Different belemnite taxa may be recognised by variations in rostral shape and structure and several families are recognised from various parts of the Mesozoic. The latest surviving belemnite family was the Belemnitellidae.

Reconstruction of a typical belemnite showing the life position of the shell (not actually visible externally), copyright Charlotte Miller.

Belemnitellids are characterised by rostra with an alveolus or pseudoalveolus (an anterior conical depression into which the phragmocone would have originally fit) that opens through a ventral fissure, and longitudinal dorsolateral impressions (Christensen 1997, 2002). The earliest belemnitellids appeared during the early part of the Cenomanian epoch of the Cretaceous period, about 98 million years ago (Christensen 1997). They reached their peak of diversity during the lower Santonian, about 86 million years ago, but they persisted in one form or another right up to the end of the Cretaceous, eventually disappearing in the giant colossal environmental clusterbump that brought that period to a close. Throughout their history, belemnitellids were restricted to the Northern Hemisphere, being known from what is now Europe and North America. By the late Cretaceous, of course, the modern continents were definitely approaching their modern forms and positions but were not quite there yet. For a large chunk of this period, sea levels were higher than they are now so much of modern Europe and the central part of North America were covered by shallow seas. The North Atlantic was still a developing prospect; it looks like there still would have been something of a continental shelf connection between what is now its two sides during the Santonian. This continental shelf and shallow seas was the habitat of the belemnitellids; it appears that they never made the shift to deeper waters. Hence their geographical restriction as the deeper Tethys Ocean still separated Eurasia from Africa and India. When the belemnitellids first appeared, these deeper Tethys waters were home to another belemnite family, the Belemnopseidae (the belemnitellids would make some inroads to the northern coast of the Tethys after the belemnopseids became extinct during the Cenomanian but never anything extensive). A third family, the Dimitobelidae, occupied the position of the belemnitellids in the Southern Hemisphere.

The earliest belemnitellids are known from northern Europe where they presumably evolved from belemnopseid ancestors (Christensen 1997). There do appear to be some questions about whether the belemnitellids as currently recognised represent a monophyletic group or whether the belemnopseid invasion happened more than once. However it be, northern Europe would remain the centre of diversity for the group. They reached North America during the Turonian, about ninety million years ago, but for whatever reason never quite diversified there as much as they did in their homeland. During the Campanian, from about 83 million years ago, there is a period of close to ten million years where belemnitellids disappeared from the North American fossil record entirely. Presumably this represents a local extinction followed by a later recolonisation from Europe.

North American belemnitellids also failed to quite make it to the end of the Cretaceous, dropping out about one or two million years earlier. In Europe, however, three species are known from the period's closing hours. Though not at their earlier levels of success, belemnitellids were diversifying right to the end: the distinctive Fusiteuthis polonica appears well within the last couple of million years. Nevertheless, there was precious little from that part of the world at that time in history that did not have the word DOOM stamped firmly on its forehead and belemnitellids were no exception. Their passing marked the final end of the belemnite hegemony and the stage was now completely clear for the more modern coleoids to rise.

REFERENCES

Christensen, W. K. 1997. The Late Cretaceous belemnite family Belemnitellidae: taxonomy and evolutionary history. Bulletin of the Geological Society of Denmark 44: 59–88.

Christensen, W. K. 2002. Fusiteuthis polonica, a rare and unusual belemnite from the Maastrichtian. Acta Palaeontologica Polonica 47 (4): 679-683.