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Belemnites: A Quick Look

by Phil Eyden, 2003

By Phil Eyden

Note: Phil welcomes discussion on this article in the Cephalopod Fossils forum.



Introduction

Belemnites are probably the most well known extinct cephalopod after the ammonites. They are quite common fossils and have a worldwide distribution. They are a very characteristic and easily recognisable fossil usually resembling a bullet in shape, although this only represents the extreme 'tail' of the animal.

The name 'Belemnite' is derived from the Greek word belemnon which means javelin or dart due to the obvious resemblance in the shape of the fossil. It was a common folklore tale that belemnites were formed from the point of strike of lightning bolts into the ground; hence they are frequently referred to as 'thunderbolts'.

Belemnites are grouped amongst the Order Coleoidea along with the squid, octopus, cuttlefish and argonaut. Belemnites were very squid-like in shape, sharing the same streamlined torpedo shape, but this came about through convergent evolution rather than squids being descended from belemnites. In fact, the closest living relatives to the belemnites are probably the cuttlefish and the strange little squid Spirula, both of which have a chambered internal shell structurally similar to that of the belemnite though in both cases highly modified in their own ways. Although the method of employing a chambered shell for buoyancy control amongst the belemnites is representative of many cephalopod groups, (e.g. Nautilus, Spirula, ammonoids), the use of a counterweight at the rear of the body was an unusual feature.

Coleoids and ammonoids are more closely related to each other than either are to the nautiloids, despite the superficial similarities of ammonoids with nautiloids. However, the coleoids and ammonoids diverged long ago, probably during the Silurian Period, with the ammonoids being the dominant group until their extinction at the end of the Cretaceous. The Belemnoidea were a large group and currently contain about 2000 described species although it is possible that only half of these are actually valid.

Physiology

Perhaps somewhat surprisingly, much more information is known about the physiology of the belemnite than the ammonite. This is because unlike the ammonite, examples of belemnites displaying soft-bodied anatomy have been discovered. Notable examples in the 155 million year old Jurassic Solnhofen limestones in Germany and the Late Jurassic Oxford clays in the UK display creatures with ten arms, each equipped with 30-50 hooks that are slightly recurved to ensnare prey and prevent it from struggling free. As with almost all modern hooked squids these hooks are normally arranged in pairs on each arm and are arranged in a V-shape; it is believed that in at least some species sexual dimorphism is demonstrated with these hooks. With those particular species although both male and female belemnite have mainly identical hooks, the males have in addition a few large smooth hooks; and it has been speculated that these may have been used to grasp hold of the female during mating. Unlike most squid belemnites are not found with tentacles and all ten arms tend to be of same length. Belemnites and their close relatives do seem to have two fins on either side of the mantle and some German examples have also displayed traces of ink sacs.

The internal anatomy of the belemnite differed from the squid to a high degree. Whereas the only trace of an internal shell in most modern squid is the gladius or pen, the belemnite had a complex and complete internal shell that was divided into three sections. These were the rostrum (or guard), the phragmocone and the pro-ostracum. The rostrum (pl.rostra) was the posterior bullet shaped section of the shell and is the fossil that is most commonly found. It is normally straight sided and tapers to a point, although in some species the rostrum can be conical or even slightly curved and bladelike in profile. If sliced open the rostrum often displays concentric rings much like a tree trunk; this almost certainly represents growth rings and it has been estimated from these that the belemnite lived about four years. The rostrum is composed of calcite, a weighty material that probably acted as a counterbalance for the head and arms whilst swimming. Some genera of belemnite have long ventral grooves cut into the rostrum (e.g. Actinocamax). The slight differences in profile of the rostrum, the shape of the cross section and shape of the tip are one method palaeontologists use to differentiate species of belemnite. The rostrum is believed to have accounted for roughly a third of the length of the animal.

In some species, such as Cylindroteuthis and Belemnitella, cut into the anterior end of the rostrum is a conical-shaped socket known as the alveolus. This held the thin-walled conical-shaped phragmocone, which the animal used to control its buoyancy in the water. In other species the alveolus was not present and the phragmocone was butted onto the tapered end of the rostrum. This phragmocone contained chambers, which the animal used to bleed in gases to keep the animal buoyant via the use of a siphuncle or a thin tube in a manner similar to the modern Nautilus. The phragmocone is a delicate structure and is rarely preserved; it was often longer than the rostrum such as in the species Neohibilites.

The third part of the shell was the pro-ostracum which was a long tongue shaped extension of the phragmocone that projected forwards towards the anterior of the animal. Some writers have pointed out similarities between this structure and the gladius of the modern squid Loligo. It is important to remember that this three part shell was wholly internal and the whole three part structure would have been surrounded in a fleshy mantle, giving the animal a very squid like appearance.




HISTORY

The earliest true belemnites (Belemnitida) date from the early Jurassic period and they became extremely widespread before their extinction at the end of the Cretaceous period, along with the ammonites. The early evolution of the belemnites is poorly understood, it appears that group may have derived from an earlier group known as the Aulacocerida which thrived from the Devonian to the early Jurassic. These in turn probably originated from the straight-shelled orthoconic nautiloids that dominated the seas as top predators in the Ordovician. The Aulacocerida display characteristics that seem transitional between the orthoconic nautiloids and the later belemnites. It is thought that the nautiloid shell slowly thickened and the phragmocone slowly expanded to reach the highly developed state in the belemnitida. This is particularly clear in the Triassic species Ausseites which has a phragmocone which very closely resembles the earlier nautiloids.

The belemnites, along with the related families Phragmoteuthida and Belemniteuthina, probably diverged from the Aulacocerida as far back as the late Devonian or Early Carboniferous, this forming one main branch of the coleoid family tree. The other branch, which is speculated to have arisen at roughly the same time, led to all the coleoids we are familiar with today. It seems likely that the modern Ramshorn squid Spirula represents a coleoid in a primitive state with the cuttlefish and teuthids fairly recent (post-Cretaceous) descendants from Spirula's lineage. The fact that Spirula and the modern cuttlefish retain vestiges of their internal shell indicates that these are descendants of the most primitive coleoids and closer to the belemnites than the squid and octopus. It is interesting to note that the modern hooked squid Moroteuthis (of the family Onychoteuthidae) also has a cylindrical rostrum composed of chitin at the end of its' shell which looks strikingly similar to that of the calcite rostrum of the belemnite.

It is worth mentioning here that the octopus pursued a very different path along the second branch of the coleoid family tree. It is speculated that the octopus evolved from the vampyromorphs in the late Jurassic, today this group is represented by Vampyroteuthis, the only known living example. The vampyromorphs themselves probably diverged from an animal very much closely related to the Aulococerida during the split of the ancestral lineage in the late Devonian or early Carboniferous.

The age of the belemnites proper was the Jurassic to the Cretaceous period (208-65mya) making them contemporary with the heyday of the true ammonites. They rapidly diversified in the Jurassic and although failed to achieve anything approaching the degree of variation in form as with the ammonites, they were clearly a very successful animal and have been found widespread across the world in sedimentary deposits. As with the ammonites, towards the end of the Cretaceous the belemnites suffered a gradual reduction in diversity and were a very much-depleted group before their final extinction. They became increasingly confined to higher latitudes in the north and south hemispheres with the very last family, the belemnitellidea, known from northern Europe only at the Maastrichtian at the very end of the Cretaceous. The belemnite Belemnitella mucronatus found in the Upper chalk in Britain is one such example.



The reason why belemnites became extinct is not entirely clear. It is seems highly possible that the belemnites had planktonic larvae; these would have been dependant on the phytoplankton for survival as part of the food chain. Planktonic levels would have suffered a heavy depletion following the impact of the asteroid 65 million years ago that would have vaporised millions of tons of rock and probably caused a blackout of the sun for months, if not years. This would have limited the ability of the phytoplankton to photosynthesise with a knock-on effect up the food chain. As it appears that the belemnite was purely restricted to European waters at that time, then the populations would have been especially vulnerable. Perhaps the squid and octopi, which also have planktonic larvae, had a greater geographic distribution and were less vulnerable to an ecological catastrophe.

There is a controversial fossil interpreted as a belemnite known as Bayanoteuthis that has been dated to the very early Tertiary, but it seems that this has been misinterpreted and may actually be a piece of sea-pen. Besides Bayanoteuthis, belemnite fossils from Jurassic and Cretaceous rocks can also be found in Tertiary and younger sediments. Because they are so durable, belemnite guards persist after the original sediments that contained them are destroyed by water or ice; for example the Jurassic belemnite Cylindroteuthis puzosiana are among the commonest fossils found in Pleistocene Boulder Clays of Southern England. However, the absence of unambiguously original rather than reworked belemnite fossils from Tertiary sediments does seem to indicate that belemnites probably went extinct at the end of the Cretaceous.

Facts and Figures/Lifestyle

Belemnites would have formed a major component of the Mesozoic oceanic eco-system. They were a major source of food for many marine creatures at the time; examples of belemnite hooks have been discovered in the stomach of plesiosaurs and fossils of the Jurassic shark Hybodus have been discovered with stomachs packed with belemnite rostra. An example of the Jurassic pliosaur Liopleurodon was described in 1987 from Peterborough, UK, that displays belemnite hooklets amongst its stomach contents. The ichthyosaurs probably represented the main predator of belemnites with species such as the Jurassic Ophthalmosaurus possessing jaws packed with a rows of very small teeth set into grooves, perfectly adapted for predation upon cephalopods.

It has been theorised that ichthyosaurs vomited belemnites following digestion as the hard calcite rostra could have caused damage to the stomach and intestines, or perhaps they were simply too hard to digest. A parallel could be drawn with the modern sperm whale that vomits the indigestible parts of squid, notably the beak. An interesting example of alleged ichthyosaur vomit was announced in February 2002 that had been discovered in a quarry in Peterborough in the UK and was dated to 160mya. This consisted of mass of juvenile belemnite rostra that had suffered acid damage; it is believed that the stomach acids of the ichthyosaur caused the pitting and scarring.

However, there are other causes that could cause depositions of these 'belemnite battlefields'. One theory is that after spawning the belemnites died en masse in a manner similar to most modern squid. Alternatively these mass mortalities could sometimes be caused by mudslides on the Mesozoic continental shelves burying communities together. Perhaps all these theories are valid; in the Peterborough specimen the fact that the belemnites were mostly juvenile would rule out the spawning theory. It would also seem evident from these mass mortalities that belemnites probably gathered in shoals. It is true that with many of these mass mortalities, the specimens tend to be aligned more or less in a similar direction, this indicates the direction of the prevailing current (known as the palaeocurrent), a useful tool for geologists.



It is estimated that most belemnite species grew to around 300-500mm in length but there were much smaller examples. The Early Cretaceous belemnite Neohibilites minimus has a rostrum about 30mm long implying the adult animal was probably around 100mm or so. On the other hand, there were also giants. The Jurassic Megateuthis is estimated to have reached a length of two to three metres and another example from Indonesia has a rostrum that was 46cm long. As this was only the posterior part of the animal, the whole belemnite may well have approached four or five metres in length depending on the length of the arms. It is important to remember that both the dwarf and giant forms of belemnite were uncommon and atypical, the majority of species were of a mid-range size, much as with the majority of modern squid.

The belemnite was probably an active hunter; the presence of hooks on the arms implies that the arms were designed to prevent prey from struggling free. It therefore seems likely that the smaller belemnites would have fed on a diet of ostracods, with the larger belemnites able to tackle fish and crustaceans. Although the animal was streamlined, as with modern fast-swimming squids, because the body had to contain the buoyant phragmocone as well as the mantle cavity, a belemnite probably couldn't suck in and squirt out as much water with each contraction as could a similar sized squid. As a result they probably weren't able to swim quite so fast. On the other hand because they were neutrally buoyant they didn't need to expend energy to prevent themselves from sinking, as most squids do, which would have allowed them to be efficient. Besides being a counterweight, the guard also supported the fins, which could have been used both for active swimming and underwater gliding, allowing the belemnite to coast on ambient currents. So although they were probably less speedy than modern oceanic squids, they were nevertheless well adapted to efficient swimming and coasting in open water.




And finally...

As mentioned at the beginning of this article it was a common folklore tradition belemnites were formed from the point of lightning strikes into the ground hence were given the nickname 'thunderbolts', 'Devil's Fingers' or 'St Peter's Fingers'. This, however, is not the only tradition surrounding these fossils.

In Scandinavia belemnites were commonly known as vatteljus which means gnomes candles as it was once believed that elves and pixies used them to make light. In some parts of western Scotland they were known as Bat Stones and were believed to cure horses of distemper by giving them water to drink into which the specimen had been soaked for sometime. Belief in the healing powers of these fossils was also common in southern England, with a similar method believed to cure rheumatism. The crushing of these fossils to form a powder and then blown into a sore eye was another treatment. One can only imagine the counterproductive effect this would have had!



References

Bassett, M, 1982 'Formed Stones', Folklore and Fossils. National Museum of Wales.
Clarkson, ENK; 1998. Invertebrate Palaeontology and Evolution (4th ed.).Blackwell.
Cox, B; Doyle P. 1996 Fossil Focus: Belemnites British Geological Survey.
MacLeod ,N. 2003 PalaeoBase Macrofossils pt.2: Mollusca. Blackwell.
Monks, N; Palmer P. 2002. Ammonites. The Natural History Museum.
Walker,C; Ward D;1992 Fossils (Eyewitness Handbook) Dorling Kindersley.

Dr Theo Engeser's Fossil coleoidea Page
Dr Neale Monks (pers.com) and comments on this site!
BBC News

And a big 'Thankyou' to Dr Neale Monks for his contributions, suggestions and guidance!

Phil Eyden 9/14/03
Published:
Dec 29, 2013
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