A rare Cretaceous ichthyosaur from Lincolnshire

As many amateur and professional palaeontologists are aware, ichthyosaurs are well-known aquatic reptiles from the Mesozoic era, which are especially common in Jurassic marine deposits in the UK. They are particularly conspicuous in the Charmouth and Whitby Mudstone Formations of the Lias (Lower Jurassic), as well as the Oxford and Kimmeridge Clay Formations (Upper Jurassic). These horizons have yielded numerous complete and fragmentary remains that grace many private and museum collections across the UK.

By contrast, the record of ichthyosaurs in Britain from the Jurassic/Cretaceous boundary interval is somewhat scanty, and only rare and fragmentary remains having been discovered. Any remains discovered from this time interval are therefore of great potential significance.

Back in August 1995, during one especially hot summer’s day, I was fortunate to discover fragmentary ichthyosaur remains at a small quarry at Nettleton, Lincolnshire. This quarry exposed the Lower Spilsby Sandstone Formation, which is a shallow water marine deposit that embraces the Jurassic/Cretaceous boundary interval in Lincolnshire (Gaunt et al., 1992).

The specimen was discovered at a single horizon, 3.5m above the base of the formation, which (based upon stratigraphical grounds) falls within the ammonite zone, Subcraspedites preplicomphalus (Casey, 1973). This was formerly considered to fall within the Volgian stage of the latest Jurassic (Gaunt et al., 1992), although more recent work by Hopson et al. (2008) has placed it within the Berriasian stage of the Early Cretaceous.

Fig. 1. Isolated tooth crown. Reproduced from Green and Lomax, 2014.

A number of disassociated complete and fragmentary vertebrae were recovered, along with numerous fragmented ribs and apparent gastralia. Of particular interest was an isolated tooth crown found preserved under the largest preserved caudal vertebra. This could be indicative of extreme post mortem disarticulation or possible predation by another ichthyosaur. Other remains included a large isolated phalange (the bone from the forefin). On informing the quarry manager of my discovery, I received the disconcerting news that bone material had been observed previously at the same level and working face, indicating further remains that were subsequently removed by quarrying.

These fossils remained in my private collection until 2013, when it occurred to me that this specimen was a potentially rare find that warranted further study. I therefore contacted palaeontology and ichthyosaur expert, Dean Lomax, of Doncaster Museum and the University of Manchester, who readily agreed that this could indeed be the case. In February 2014, I brought the specimen over to Doncaster Museum, where Dean and I set about measuring and examining the specimen, with a view to recording our conclusions within a scientific publication.

We both became aware that this was a significant find, representing the most substantial ichthyosaur remains known from the Spilsby Sandstone Formation, which was quite exciting indeed. The previous records are limited to an isolated humerus from the base of the formation, in addition to a single basioccipital (the bone at the base of the skull) recovered from the overlying Subcraspedites primitivus zone. Both of these finds also originated from the Nettleton area (Forrest and Oliver, 2003). These authors, when discussing marine reptile remains from this formation, noted apparent evidence of reduced physical size. This they attributed to environmental and ecological stress, as a result of a restricted marine environment caused by the palaeogeography of the time (Ziegler, 1990). However, in connection with my specimen, Dean estimated its size (based on the preserved elements) to have been around, perhaps 4.5m (or more) in length, suggesting a fairly large ichthyosaur. Obviously, this contradicts the observations of Forrest and Oliver (2003), although preservation biases could also be a contributing factor to this discrepancy.

Fig. 2. Isolated phalange. Reproduced from Green and Lomax, 2014.

Assigning the specimen to a genus or species was impracticable, given the lack of diagnostic characteristics. Therefore, it was assigned by Dean to Ichthyosauria indet, meaning that the specimen could be identified as an ichthyosaur, but more complete remains were required for specific identification. Given that the specimen was discovered in an incoherent sandstone matrix, its preservation is all the more remarkable, given that the sandy facies of a marginal marine environment are largely unsuited to vertebrate preservation.

As we would be recording the specimen within a scientific journal, it would require housing within an appropriate establishment. Therefore, we decided to contact Yorkshire Museum, who agreed that this fossil represented a rare find from a geological interval during which ichthyosaurs are scarce and, moreover, originated from a locality (and formation) that is no longer accessible. As a result, the ichthyosaur now resides in, and is cared for, by the Yorkshire museum as specimen no. YORYM: 2014.212.

Acknowledgements

Thanks are due to Dean Lomax for his assistance and for co authoring the scientific publication about the specimen; and to Dr Sarah King of the Yorkshire museum.

Fig. 4. Locality map of the Castle Top quarry. Reproduced from Green and Lomax 2014; image originally reproduced. by permission of the council of the Yorkshire Geological Society.

References and further reading

Casey, R, 1973. The ammonite succession at the Jurassic cretaceous boundary in eastern England. In: Casey, R, and Rawson, P.F. (Eds), The Boreal Lower Cretaceous. The Boreal Lower Cretaceous Geological Journal Special, No. 5 193 – 266.

Forrest, R, and Oliver, N., 2003. Ichthyosaurs and plesiosaurs from the Lower Spilsby Sandstone Member (Upper Jurassic) of Nettleton, north Lincolnshire. Proceedings of the Yorkshire Geological Society, 54, 269 – 275.

Gaunt, G.D, Fletcher, T.P, and Wood, C.J., 1992. Geology of the country around Kingston – upon – Hull and Brigg: Memoir for 1: 50,000 geological sheets 80 and 89 (England and Wales). Natural Environment Research Council, pp. 172.

Green, J.P, and Lomax, D.R, 2014. An ichthyosaur (Reptilia: Ichthyosauria) specimen from the Lower Cretaceous (Berriasian) Spilsby Sandstone Formation of Nettleton, Lincolnshire, U.K. Proceedings of the Geologist’s Association, 125, 432 – 436.

Hopson, P.M., Wilkinson, I.P., and Wood, M.A., 2008. A stratigraphical framework for the Lower Cretaceous of England. British Geological Survey Research Reports, Keyworth, Nottingham, pp. 77.

Ziegler, P.A., 1990. Geological Atlas of Western and Central Europe, second edition. Shell Internationale Petroleum Maatschappij, Netherlands, pp, 239 – 249.


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