Jurassic Fossil Cephalopods Preserving Aragonite Shells

Aragonite is a common mineral in the skeletons of marine organisms (Cusack & Freer 2008), but is not typically well preserved in the fossil record due to its tendency to recrystallise as its more stable polymorph calcite. Certain circumstances however can enhance the preservation of the original aragonite, including reducing conditions (Jordan et al. 2015), low temperatures (associated with limited burial depths), and burial in impermeable sediments (Hall 1967, Vendrasco et al. 2018). 

Four Middle to Lower Jurassic examples of preservation of aragonite shells, consisting of two belemnoids and two ammonites, are illustrated here below. Consideration of these examples also leads to a discussion on preservation of soft parts in fossil cephalopods.

Middle Jurassic Oxford Clay of Christian Malford

The Middle Jurassic of Christian Malford (Wiltshire) is celebrated for the exceptionally preserved fauna it contains (Pearce 1842, Wilby et al. 2004, 2008). J. Chaning Pearce read his paper on the discovery at the Geological Society on 5th January 1842, stating that "his attention was first directed to this part of the railway by the impression of a crushed Ammonite procured at Cheltenham in April 1841, but that he was prevented from examining the locality for three or four months." 

Most spectacular are the soft-bodied coleoid cephalopods such as Belemnotheutis antiquus (Pearce, 1847) and Mastigophora brevipinnis Owen, 1856, but the ammonites with preserved aragonitic shells also drew early attention, particularly those with highly modified apertures. The Christian Malford biota occurs in the Peterborough Member of the Oxford Clay Formation, belonging to the Callovian Stage of the Middle Jurassic. According to Pearce (Pearce 1842), the best preserved fossils came from a 6 feet (1.8 m) thick part of the succession consisting of ‘four or five bands of laminated clay alternating with sandy clay, almost entirely composed of broken shells’. They occur at a level within the phaeinum Subzone of the athleta Zone. Discovered early in the 1840s during the construction of the Great Western Railway, this interval was still being collected as late as 1854.

A famous feud resulted from Richard Owen's assertion (Owen 1844) that the Belemnotheutis specimens discovered by Pearce and others (though Owen did not allude to Pearce's earlier description) represented the front parts of belemnites detached from their calcitic guards (Donovan & Crane 1992). Joseph Pearce, James Scott Bowerbank and Gideon Mantell all corrected Owen and Mantell in particular suffered Owen's sustained and bitter vindictiveness as a consequence. 

Figure 1. A Belemnotheutis antiquus (spelled Belemnoteuthis on this early label) specimen from Christian Malford on display in the Yorkshire Museum, York, showing soft body preservation in calcium phosphate and phragmocone (at left) preservation in aragonite

Preservation of both muscle tissue and aragonitic shells is excellent at this site. Kear et al. (1995) documented fibrous muscle structure preserved in calcium phosphate in Belemnotheutis and Mastigophora  specimens and Doguzhaeva et al. (2006) used scanning electron microscopy to investigate the fine structure of the aragonitic shell (rostrum, conotheca and pro-ostracum) in Belemnotheutis antiquus.

Belemnotheutis antiquus is relatively common at Christian Malford, but belemnites are rarer and those complete with phragmocones are rarer still (Wilby et al. 2008). The specimen of a belemnite from Christian Malford shown below does not display soft body preservation, but is remarkable for the presence of both the complete (though flattened) chambered phragmocone with its original aragonitic composition, and the associated calcitic guard behind.

Such preservation has enabled improved reconstruction of the marine palaeo-temperatures experienced by these animals, using stable isotope data measured from both the calcite (guard) and aragonite (phragmocone) phases. Price et al. (2015) reconstructed palaeo-temperatures in this way, but found a discrepancy between data from associated phragmocones and guards, with data from the guards suggesting temperatures cooler than those from the phragmocones; they were unsure whether the temperatures derived from the aragonite were too warm or from the calcite too cool. Vickers et al. (2021) subsequently showed, using clumped isotope palaeothermometry, that the inferred calcite temperatures were too cold and that the belemnites precipitated both aragonite and calcite in warm, open ocean surface waters. 

Figure 2. Cyclindroteuthis sp, belemnite with complete calcitic guard and flattened aragonitic phragmocone, 16.5 cm long, Christian Malford, Wiltshire (author's collection)

The lack of soft part phosphatization in the Christian Malford belemnites (in contrast with the Belemnotheutis and Mastigophora material) prompts the question 'why?' A compelling line of argument presented by Clements et al. (2017) is that the majority of modern squid families (decabrachians, i.e. ten-armed coleoids) exchange sodium for ammonium to create a low-density fluid that imparts lift for neutral buoyancy, whereas the eight-armed vampyropods (octopuses and vampire squid) do not. Presence of ammonia decreases the likelihood that a sufficiently high pH would be reached during decomposition of the carcasses to favour phosphatization. If belemnites and ammonites used ammonium for buoyancy control, this could explain the very limited record of soft part preservation in these two groups.

The well-known zonal ammonite Kosmoceras phaeinum from Christian Malford, with its characteristic large lappets developed in the microconchs (inferred to be males), like the belemnite lacks soft body preservation, but again (as in the belemnite phragmocone) the original nacreous aragonite shell is preserved, crushed flat by compaction, without alteration to calcite. A complete example is shown below.

 

Figure 3. Kosmoceras phaeinum, 6 cm ammonite microconch from Christian Malford with highly elongate lappet showing preservation of nacreous aragonitic shell  (author's collection)

Lower Jurassic of Watchet, Somerset and Holzmaden, Germany

Preservation of originally aragonitic shells is also known from Lower Jurassic mudrocks of England and, to a lesser extent (Lautenschlager 2024), Germany.

Watchet

The ammonite Psiloceras planorbis is one of the earliest ammonites found in the British Isles, with only a very small number of earlier specimens (Neophyllites lavernockensis) known from deeper horizons (Hodges 2021). The specimen of Psiloceras planorbis shown here, from Watchet in Somerset, illustrates the colourful iridescence often present when the aragonite in ammonite shells has been preserved. The effect is produced by the interference of light with the numerous thin, flat platelets of aragonite. This style of preservation contrasts strongly with three-dimensionally preserved shells with all aragonite converted to calcite, as seen on the Yorkshire coast.

Figure 4. Psiloceras planorbis, 4 cm, preserving the highly iridescent aragonitic nacre shell, from the Lower Jurassic Blue Lias, Planorbis Zone, Hettangian. Watchet, Somerset (author's collection).

Figure 5. By way of contrast, this 33 mm specimen of Psiloceras plicatulum in a calcareous concretion from the Lower Lias of Redcar, Yorkshire, is fully three-dimensional and converted to calcite, with no aragonite remaining (author's collection). The detail below shows the suture pattern and calcite-filled chambers.

Holzmaden

Another Jurassic deposit famed for soft body preservation of coleoids is the Lower Jurassic Posidonia Shale Formation of Holzmaden, Germany (e.g. Fuchs et al. 2013). As in the Christian Malford example, the crushed phragmocone of Clarkeiteuthis conocauda from there is clearly seen in the specimen shown below, although only small patches of iridescent aragonite remain. The large ink sac is complete in this piece, but is only partially exposed. While the head of the animal has been preserved, there is no record of the soft parts of the arms, but their positions are faithfully recorded by the in situ preservation of black hooklets ('onychites'). This is a wonderful example of anaerobic microbial decay, in the absence scavengers, but with no associated early diagenetic mineralization to preserve a record of the soft parts.

Figure 6. Clarkeiteuthis conocauda, 20 cm with phragmocone below showing (in its frontal portion) patchy preservation of remnant iridescent aragonite and complete set of arm hooklets (author's collection)

Lautenschlager (2024) recently described an unusual occurrence of preserved aragonitic shells of ammonites in the Posidonia Shale Formation at the Hesselberg Quarry in Southern Germany. The preservation of aragonite at this locality is in strong contrast to the Posidonia Shale Formation elsewhere, where ammonite shells are dissolved and only remains of the organic periostracum (the thin organic outer layer) are preserved. 

In summary

Together, the fossils illustrated above display preservation of aragonitic shells in Jurassic belemnoids and ammonites. In each of the depositional settings mentioned, conditions of oxygen depletion and impermeable clay-rich enclosing sediment were satisfied, thus preventing the conversion from aragonite to its more stable polymorph calcite. From an aesthetic perspective, the colourful iridescence in such fossils can be a pleasure to observe. From a scientific perspective the exceptional preservation has permitted more accurate estimates of the Jurassic marine palaeo-temperatures experienced by the animals and documentation of microscopic shell structure.

References

Clements, T., Colleary, C., De Baets, K., & Vinther, J. 2017. Buoyancy mechanisms limit preservation of coleoid cephalopod soft tissues in Mesozoic Lagerstätten. Palaeontology, 60, 1–14.

Cusack, M., & Freer, A. (2008). Biomineralization: Elemental and Organic Influence in Aragonite Formation. Elements, 4(6), 439–444.

Doguzhaeva, L.A., Donovan, D.T & Mutvei, H. 2006. Ultrastructure of Belemnotheutis from the Oxford Clay (Callovian), England, as a key for elucidating the origin of the pro-ostracum. Acta Universitatis Carolinae - Geologica, 49, 

Donovan, D.T. & Crane, M.D. 1992. The Type material of the Jurassic cephalopod Belemnotheutis. Palaeontology, 35, 273-296.

Fuchs, D., Donovan, D. T., Keupp, H. 2013. Taxonomic revision of ?Onychoteuthis? conocauda Quenstedt, 1849 (Cephalopoda: Coleoidea)". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 270, 245–255. doi:10.1127/0077-7749/2013/0368.

Hall, A., Kennedy, W. J. & Taylor, J. H. 1967. Aragonite in fossils. Proceedings Royal Society B, 168.

Hodges, P. 2021. A new ammonite from the Penarth Group, South Wales and the base of the Jurassic System in SW Britain. Geological Magazine, 158, 1109-1114.

Jordan, N., Allison, P.A., Hill, J., Sutton, M.D. 2015: Not all aragonitic molluscs are missing: taphonomy and significance of a unique shelly lagerstatte from the Jurassic of SW Britain. Lethaia, Vol. 48, pp. 540 548.

Kear, A.J., Briggs, D.E.G. & Donovan, D.T. 1995. Decay and fossilisation of non-mineralised tissue in coleoid cephalopods. Palaeontology, 38, 105-131.

Kimmig, J. & Schiffbauer, J.D. 2024. A modern definition of Fossil-Lagerstätten. Trends in Ecology & Evolution 39(7):621-624.

Lautenschlager, S. 2024. Deposits. https://depositsmag.com/2024/03/31/shining-white-ammonites-remarkably-preserved-ammonites-from-the-posidonia-shales-of-southern-germany/

Owen, R. 1844. A description of certain belemnites, preserved, with a great proportion of their soft parts, in the Oxford Clay at Christian Malford, Wilts. Philosophical Transactions of the Royal Society, 125, 65-85.

Pearce, J.C. 1842. On the mouths of ammonites, and on fossils contained in laminated beds of the Oxford Clay, discovered in cutting the Great Western Railway, near Christian Malford in Wiltshire. The Annals And Magazine Of Natural History, 9, 578-579.

Price, G.D., Hart, M.B., Wilby, P.R. and Page, K.N. 2015 Isotopic analysis of Jurassic (Callovian) Mollusks from the Christian Malford Lagerstätte (UK): Implications for Ocean Water Temperature Estimates Based on Belemnoids. PALAIOS, v. 30, 645–654.DOI: http://dx.doi.org/10.2110/palo.2014.106

Seilacher, A. 1970. Begriff und Bedeutung der Fossil-Lagerstätten. Neues Jahrb. Geol. Paläontol. Monatsh, 34–39.

Vendrasco, M., Checa, A., Squires, R. & Pina, C.M. 2018. Unaltered nacre from the Pennsylvanian Buckhorn Asphalt, and implications for the arms race between Mollusks and their predators. Palaios, 33, 451-463.

Vickers, M.L., Bernasconi, S.M., Ullmann, C.V. et al. Marine temperatures underestimated for past greenhouse climate. Sci Rep 11, 19109 (2021). https://doi.org/10.1038/s41598-021-98528-1

Wilby, P. R., Hudson, J. D., Clements, R. G., and Hollingworth, N. T. J. 2004. Taphonomy and origin of an accumulate of soft-bodied cephalopods in the Oxford Clay Formation (Jurassic, England), Palaeontology, 47, 1159–1180.

Wilby, P. R., Duff, K., Page, K., and Martin, S. 2008. Preserving the unpreservable: a lost world rediscovered at Christian Malford, UK, Geology Today, 24, 95–98.

A note on the use of the terms 'exceptionally preserved biota' and 'Lagerstätte':  The German word  'Lagerstätte', from Lager 'storage' and Stätte 'place' (directly translatable as 'deposit' in a sedimentological context) is often used in English to refer to instances of exceptional fossil accumulations, even though the word and its etymology offer no useful connection to exceptional preservation of fossils. Seilacher (1970) used the composite term 'konservat-Lagerstätte' for deposits displaying exceptional preservation (“Rock bodies, which in quality and quantity preserve an unusual amount of paleontological information”), but the English words already function perfectly well. I prefer to use 'exceptionally preserved biota' (EPB) instead of the highly inelegant 'fossil konservat-Lagerstätte'.  Kimmig & Schiffbauer (2024) suggested refinements to Seilacher's concept, but a better option in my opinion is to simply stop using this unwieldy and unhelpful term.