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.

When the rostrum is broken, the internal structure can be observed (Fig. 2). It is composed of a series of fibrous calcite crystals arranged at right angles to the surface pointing towards the apical line running along the centre. Concentric patterns, also observed in horizontal cross-sections, are interpreted as growth rings. 𝛿¹⁸O measurements taken between these bands have been interpreted as seasonal variations in temperature. It is estimated from counting growth lines that the lifespan of a belemnite was about a year, comparable to those of living squids.

Grooves are found along the rostrum and are used to distinguish the different belemnite suborders (see below). The reason for these grooves is not yet fully understood, although they are usually interrupted as attachment scars for soft tissues such as blood vessels (Iba et al., 2014).

Comprising the middle section of the belemnite skeleton is the thin-walled aragonitic phragmocone, which fits inside the alveolus on the front end of the rostrum, housing the belemnite animal. Because it was initially made of aragonite (a less stable form of calcium carbonate than calcite) means the phragmocone has a much lower preservation potential than the rostrum. The phragmocone is conical, containing large chambers used for buoyancy control that are separated by septa (that is, thin walls or partitions between the internal chambers of the shell). A thin siphuncle passed through the septa connecting camerae (which are spaces or chambers enclosed between two adjacent septa) allowing gas and liquids to leave and enter the chamber to adjust buoyancy.

The most anterior part of the belemnite skeleton is the pro-ostracum. Studies of the microstructure of the pro-ostracum suggest that it was initially composed of organic matter rather than calcium carbonate (Doguzhaeva, 2012). The pro-ostracum was a flat, tongue-shaped projection of the phragmocone which, presumably, covered the top of the belemnite’s “head”. These three components of the belemnite skeleton, unlike the shells of ammonites, were internal, being surrounded by soft tissue.

Soft tissue preservation is extremely rare in the fossil record, as soft tissues breakdown and disintegrate rapidly. Sites of exceptional preservation are called Konservat-Lagerstätten. One widely studied Lagerstätten that has yielded exceptionally well-preserved belemnites is the Early Jurassic Posidonienschiefer in Baden-Württemberg, just south of Stuttgart in Germany. Here, details that would typically not be preserved, such as arms and ink sacs (in some case with still usable ink) are found (Fig. 3).

Ecology

Belemnites are often assumed to have been fast, epipelagic (that is, inhabiting the upper layer of the water column of the open ocean) swimmers based on their superficial resemblance to living squids. Variation in rostra shape was likely the result of different ecological niches and possibly water depths. Genera with short, thick rostra may have been nektobenthic (that is, swimming near the bottom of the sea) and slender, elongate forms being epipelagic swimmers (Rexfort and Mutterlose, 2009). For locomotion, belemnites used their arms and were also capable of jet propulsion by using a syphon to release jets of water as many cephalopods do today. It is also speculated that belemnites had manoeuvrable fins to help with stability just as squids do today. However, despite often being included in life reconstruction, no such structures have been found so far in the fossil record.

Belemnites had ten arms each, covered in approximately 40 hooks called onychites. It is thought that these hooks were used in capturing prey as well as being used to establish sexual dimorphism (that is, when the two sexes of the same species exhibit different characteristics other than differences in their sexual organs). A possible example of sexual dimorphism in belemnites comes from the Posidonienschiefer.

Of the four belemnites with soft tissues known from the Posidonienschiefer, one had a greatly enlarged onychite at the base of the arm crown, which was interpreted by Engeser and Clarke (1988) as an example of sexual dimorphism. In modern squids, mega-onychites (that is, large hooks) develop during sexual maturity and are only found in males, suggesting that they play a role in reproduction rather than predation, so we can assume that they played a similar role in belemnites. Some squids have protective membranes covering the smaller onychites to avoid the arms becoming snared by the razor-sharp hooks. Possibly due to the delicate nature of these structures, none have yet been found in the fossil record, but it is unknown whether such a membrane existed on belemnites.

Being rather common meant that belemnites were an important food source for many predators, including sharks and marine reptiles. The fossilised stomach contents of these large vertebrates commonly contain the indigestible onychites but rarely rostra. This is because the rostra were regurgitated after being eaten so that the edible mantel and arms (that is, soft tissues) were retained to be digested. An amazing example of this was a juvenile Ichthyosaurus communis from the Lower Jurassic of Britain described by Dean Lomax and colleagues (2017) that had thousands of onychites preserved in its stomach.

An interesting example of predation on belemnites comes from the Posidonienschiefer. Here, around 250 rostra were found in the stomach cavity of a Hybodus shark (Fig. 4). It is thought that Hybodus swallowed the belemnites whole and regurgitated the indigestible hard parts later like modern sperm whales. Belemnite larvae were microscopic and planktonic, forming the foundation the marine food chains fulling an ecological need now occupied by holoplanktonic gastropods such as sea butterflies (suborder Thecocomata – a group of small pelagic sea snails).

Belemnites, like modern squids, were nektonic organisms (that is dwelling in the water column rather than living on the seafloor) and active swimmers. Today, squid inhabit a diverse range of marine habitats from the warm shallow waters around the equator to cold Antarctic waters and everything in between, so it may seem reasonable to think that belemnites also inhabited a comparable range of environments. However, the evidence suggests otherwise. For example, in 1973, Gerd Westermann studied the strength of belemnite shells and he found that belemnites lived in shallow waters with a maximum depth range of between 50m to 200m. An exception to this was the genus Cylindroteuthis, which may have descended to a maximum depth of up to 400m. The accumulation of hundreds or thousands of belemnite rostra (Belemnitenschlachtfeld, which is a literal translation from German meaning belemnite battlefield) in shallow marine deposits helps support the theory that belemnites were restricted to shallow shelf waters.

Applications

Belemnites can also provide information about ancient climates and ocean circulation. Calcite fossils have been playing an essential role in palaeoenvironmental studies since Harold C Urey first proposed in 1947 that past ocean water temperatures could be reconstructed using a carbonate-based oxygen isotope thermometer. Urey and colleagues later conducted the first palaeotemperature study using belemnites in 1951. The abundances and ratios of stable isotopes (calcite, oxygen and carbon) and trace elements (magnesium, strontium and manganese) is dependent on seawater chemistry and environmental conditions, such as temperature (in the case of oxygen), the carbon cycle (in the case of carbon) and global erosion rates (in the case of strontium).

Belemnites are particularly useful in such studies as they are quite large, at least compared to foraminifera (single-celled protists commonly used in climate research), their rostra have well-organised growth bands, making it possible to track climate change during an individual’s life, and their low-Mg calcite rostra are relatively resistant to diagenetic alteration.

The stable isotope most often used for calculating palaeotemperatures is δ¹⁸O. The ratio of ¹⁸O and ¹⁶O in biomineralized elements (that is bones, shells and teeth) is temperature dependent. During cool periods, the lighter ¹⁶O isotope will evaporate at a much faster rate than the heavier ¹⁸O, which is left behind in the water and is thus incorporated into shells and tests. Concentrations of isotopic species is represented by the relative abundancy ratios of the heavy to the light isotopes in a given sample to the ratio of a given standard. The values of the 𝛿 can be either positive, showing an enrichment of the heavier isotopes relative to the standard, or negative, showing depletion of the heavier isotopes relative to the standard.

The ratio of ¹⁶O to ¹⁸O decreases as temperatures and evaporation rates increases. In general, a 𝛿¹⁸O increase of 1.0‰ is equivalent to approximately 4°C of cooling.

The following equation is used to calculate the 𝛿-value of oxygen: 

If you know the 𝛿¹⁸O, it is possible to calculate the palaeo-temperature of Mesozoic oceans using the following equation: T (°C) = 16.5 – 4.3𝛿 + 0.14𝛿².

For example, thousands of Cylindroteuthis rostra from Greenland described by Peter Alsen and Jörg Mutterlose (2009) suggest that the Boreal region was a migration route for belemnites. Migration of Tethyan species in the Arctic may hint at a proto-Gulf-stream bringing warm water to the Arctic as early as the Lower Cretaceous. This has significant implications for the interpretations of Lower Cretaceous climates and ocean currents.

Before being used for palaeoclimate studies, the belemnite must undergo diagenetic screening to ensure that the specimen has not been chemically altered. Diagenesis is the term that encompasses all the chemical changes that occur to a fossil after burial. Diagenetic processes include dissolution, recrystallisation and replacement by other minerals – all of these processes can affect the isotopic ratios in a given substance. Samples are screened by measuring the amount of iron (Fe), and manganese (Mn), as the ratios of these two elements are higher in poorly persevered material. An excellent review of the screening techniques, including trace element concentrations and isotopic ratios used to identify evidence of diagenetic change in low-Mg calcite, is provided by Ullmann and Korte (2015).

Belemnites can also be used as zone fossils for use in biostratigraphy, that is dating rocks based on the temporal ranges of fossil species. For example, the genera Belemnella, Belemnitella and Gonioteuthis are used to date the Campanian and Maastrichtian stages of the Upper Cretaceous. Despite being common in Mesozoic secessions and many species having short time ranges (both requirements for a reliable zone fossil), they are less morphologically diverse than ammonites and so it is harder to distinguish individual belemnite taxa past the genus level. The use of belemnites for biostratigraphy is summarised in a 1995 paper published in the journal Palaeontology by Peter Doyle and Matthew Bennett.

Origins and extinction

Belemnites are cephalopods classified into the suborder Coleoidea, along with squids and octopuses. The first belemnites are traditionally thought to have appeared in Northern Europe during the lowermost Jurassic Hettangian Stage (201 to 199 Ma). However, a study published in the journal PLoS ONE (Iba et al., 2014) suggests that belemnites originated in Asia during the Carnian – 33 million years earlier than previously thought. Belemnites remained restricted to the southern margin of Laurasia until the Pliensbachian (191 to 183 Ma) before spreading out.

The order Belemnitina is monophyletic (consisting of a common ancestor and all its descendants) with no living representatives. Belemnites are divided into two suborders based on the presence of alveolar grooves: Belemnopseina, which have alveolar grooves, and Belemnitina, which lack alveolar grooves instead having a groove towards the tip of the rostrum. Belemnitina was the more dominate group during the Jurassic and Early Cretaceous.

It has been suggested that belemnites gave rise to modern Coleoidea – the belemnoid root-stock theory that was first proposed by Voltz (1830). After conducting a detailed morphological and phylogenetic analysis of belemnites and phragmocone-bearing coleoids, Fuchs et al. (2013) concluded that Late Cretaceous spirulids evolved from diplobelid belemnoids, with Longibelus being a transitional species between the two. This conclusion challenges the idea that Spirulida evolved during the Carboniferous and is contradicted by molecular evidence that suggests a Palaeozoic origin squids and octopuses (Strugnell et al., 2006).

Belemnites, like 75% or more of all species, fell victim to the end-Cretaceous mass extinction ending 160 million years of belemnoid diversification. Belemnite diversity, as with ammonites, collapsed after the Chicxulub impact. Following their extinction, there was an adaptive radiation of modern types of cephalopods (squids, cuttlefish and octopus) in the Cenozoic (Iba et al., 2011). Neil Landman (1988) noticed this drop in cephalopod numbers and concluded that their small embryonic shells could not survive the acidic condition that resulted from the K-Pg impact.

Acknowledgements

Thanks go to Prof Stephen Hesselbo and Dr Clemens Ullmann (Camborne School of Mines, University of Exeter) for being my project supervisors; and to Prof Christoph Korte (University of Copenhagen) for supplying belemnite samples from southern Germany for my Master’s research.

About the author

Jack Wilkin is a graduate researcher at the Camborne School of Mines (University of Exeter) in the United Kingdom. His research focuses on the use of isotopes from macrofossils, particularly belemnites, to reconstruct ancient climates.

References

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