Pliosaurs and predation at Smallmouth Sands: a collaborative collecting approach

Heather Middleton, Mike Harvey and Steve Snowball (UK)

This article describes a series of pliosaur remains recovered from the Lower Kimmeridge Clay Formation at Smallmouth Sands, Weymouth (Dorset). The material, collected as isolated elements from a single locality within the Rasenia cymodoce Biozone, includes cervical and caudal vertebrae, ribs and associated teeth.

Several specimens preserve clear evidence of predation and scavenging, including bite marks attributable to both large marine reptiles and hybodontid sharks. Comparison with previously described material suggests similarities with Pliosaurus kevani, although firm assignment remains uncertain (Benson et al., 2013).

These finds demonstrate the continuing scientific value of systematic field collecting at classic coastal localities and provide new insights into predator behaviour in the Late Jurassic seas of southern Britain.

Introduction

The low cliffs and foreshore at Smallmouth Sands, on the southern side of Weymouth, have long been recognised as a productive source of Jurassic fossils. Within the mudstones of the Lower Kimmeridge Clay, a diverse assemblage of marine reptiles, fish and invertebrates is continually revealed by coastal erosion. While the site has been studied for over two centuries, it remains an active collecting ground where new material is regularly exposed, often as isolated elements weathered from the clay and concentrated on the beach.

In recent years, renewed interest in Dorset’s pliosaurs, sparked in part by spectacular discoveries elsewhere along the coast (e.g. Benson et al., 2013; Martill et al., 2023), has drawn attention back to Smallmouth Sands. Here, careful and systematic collecting has produced a series of pliosaur remains from a single locality within the Rasenia cymodoce Biozone (a subdivision of geological time based on characteristic ammonite fossils). Several of these specimens preserve clear evidence of predation and scavenging, offering insights into predator behaviour, as well as the processes by which vertebrate remains are accumulated and preserved. In this article, we describe this material and place it within its geological and biological context.

Geological context

The Lower Kimmeridge Clay Formation is widely exposed along the south Dorset coast, including at Black Head, Ringstead Bay and Smallmouth Sands. These deposits consist mainly of fossiliferous mudstones deposited in a shallow marine shelf environment (Wignall, 1995), and are internationally recognised for their rich vertebrate assemblages, including marine reptiles, fish and pterosaurs.

At Smallmouth Sands, the succession is particularly accessible, with low cliffs and foreshore exposures revealing the Rasenia cymodoce and underlying Pictonia baylei Biozones. The site has yielded a diverse range of vertebrate material and continues to produce important specimens through natural erosion, making it an increasingly significant locality within the Kimmeridge Clay outcrop.

The sediments accumulated at water depths of approximately 50-150m under relatively low-energy conditions. Periodic erosion releases fossils from the clay, concentrating them on the foreshore, particularly following storms or episodes of cliff collapse.

In practice, the exposures at Smallmouth Sands are highly variable, with slumping of the low cliffs and redistribution of material across the foreshore complicating the relationship between in situ strata and derived fossils. Careful observation of marker beds, combined with repeated visits under different conditions, is therefore essential in building a reliable understanding of the succession and the origin of collected material.

The present study focuses on a restricted area within Bay 4 at Smallmouth Sands (Fig. 1). The locality and its stratigraphy have been described by Snowball and Middleton (2023), and form the basis for the collecting framework used here. Repeated and systematic collecting over many years has produced a consistent assemblage of pliosaur material from the Rasenia cymodoce Biozone.

Fig. 1. Bay 4 at Smallmouth Sands, Weymouth, showing the low, slumped cliffs of the Lower Kimmeridge Clay and the foreshore ‘ledge’ from which many fossils are derived.

Correlation with other Dorset sections is possible using marker beds and biozones (Figs. 2 and 3).

Fig. 2. Sketch map of the Kimmeridge Clay outcrop in the Weymouth area, with simplified stratigraphy of the *Rasenia cymodoce* and *Pictonia baylei* Biozones at Smallmouth Sands.

Fig. 3. Simplified cliff section at Smallmouth Sands, showing the position of the *Rasenia cymodoce* and *Pictonia baylei* Biozones.

For recording purposes, the embayments along the site have been numbered (Fig. 4), allowing finds to be tied to specific locations along the exposure.

Fig. 4. Numbered embayments along Smallmouth Sands, used for recording fossil localities, with GPS positions.

Material and identification

The material described here consists of isolated pliosaur remains collected from a restricted area within Bay 4. These include cervical (neck) and caudal (tail) vertebrae, ribs and several loose teeth (Figs. 5 and 7). All specimens were recovered ex situ (that is, weathered out of the rocks) from the Rasenia cymodoce Biozone.

This material has been recovered over an extended period of collecting, with particular attention paid to recording the position of finds within the numbered embayments and their relationship to exposed horizons. Although the specimens are ex situ, consistent recovery from a restricted area supports their derivation from a limited stratigraphical interval within the Rasenia cymodoce Biozone.

Fig. 5. Pliosaur vertebrae from Bay 4: (1-3) cervical vertebrae with associated ribs; (4) sacral vertebra; (5-6) caudal vertebrae. Anterior views.

The teeth are typically sub-trihedral in cross-section (Fig. 6), a morphology associated with large pliosaurs including Pliosaurus kevani (Benson et al., 2013).

Fig. 6. Sub-trihedral pliosaur teeth from Bay 4, typical of large Late Jurassic pliosaurs.

However, all teeth were found loose and lack roots, indicating that they were shed and may not be directly associated with the vertebral material. It is therefore possible that the teeth and skeletal elements represent more than one individual.

One cervical vertebra preserves an ammonite imprint (Fig. 7), allowing precise placement within the Rasenia cymodoce Biozone.

Fig. 7 Ammonite (*Rasenia involuta*) imprint preserved on a pliosaur cervical vertebra, confirming its position within the *Rasenia cymodoce* Biozone.

Additional material includes well-preserved cervical vertebrae with associated ribs (Fig. 8).

Fig. 8. Cervical vertebra with associated rib from Bay 4, showing close articulation between rib and centrum.

The vertebrae are large and robust. Notably, the neural arches are not fused to the centra. While this has traditionally been interpreted as indicating a juvenile individual, recent work suggests that unfused elements may instead reflect paedomorphosis (Vincent et al., 2024), that is, the retention of juvenile features in the adult animal. Given the size of the specimens, a juvenile interpretation seems unlikely.

Predation marks and interpretation

Several of the vertebrae show clear evidence of damage caused by feeding activity (Fig. 9-12). These include grooves, scratches, gouges and areas where bone has been broken away entirely.

Fig. 9. Cervical vertebra showing localised damage consistent with predation or scavenging. (Drawing by Mike Harvey.)

Two caudal vertebrae show particularly well-preserved evidence of feeding traces (Figs. 10 and 11).

Fig. 10. Caudal vertebra (specimen 6) after preparation: (A) lateral; (B) dorsal; (C) ventral; (D) posterior views. (Drawing by Mike Harvey.)

Fig. 11. Caudal vertebra (specimen 5) after preparation: (A) anterior; (B) ventral; (C) posterior; (D) lateral views. (Drawing by Mike Harvey.)

Fig. 12. Two caudal vertebrae, posterior views, showing contrasting preservation and predation damage.

In several cases, the anterior faces of the vertebrae are well preserved, while the posterior faces display extensive damage. This suggests partial burial in sediment, protecting one surface, while leaving the other exposed to scavengers.

Many of the marks consist of shallow, parallel grooves consistent with the action of closely spaced teeth. These are most likely attributable to hybodontid sharks, which are well represented in the Kimmeridge Clay and are known to have fed by repeatedly biting and tearing at carcasses.

Other damage includes larger gouges and areas where bone has been removed entirely. These are consistent with feeding by large marine reptiles. Pliosaurs, as apex predators with powerful jaws and robust teeth, are the most likely candidates (Clarke and Etches, 1992; Martill et al., 2023).

Examination of comparative material in museum collections further supports this interpretation. Jaws of hybodontid sharks show closely spaced dentition capable of producing the repeated, parallel scoring observed on several specimens. In contrast, the more widely spaced, robust teeth of large marine reptiles are consistent with the deeper gouges and areas of bone removal seen on the vertebrae. While precise attribution of individual marks is not always possible, the combined evidence points to the involvement of multiple predators.

One cervical vertebra shows particularly extensive damage to a rib attachment point, with a large section of bone removed to reveal the internal structure. Immediately adjacent to this area are two narrow, parallel grooves separated by a ridge of intact bone. These features may represent the passage of a tooth across the bone surface during a biting or tearing action.

Additional evidence is provided by a turtle carapace fragment from the same locality (Fig. 13), which shows a combination of deep puncture marks and closely spaced scoring.

Fig. 13. Turtle carapace fragment showing bite marks and scoring, interpreted as feeding traces from both large reptiles and sharks.

Taken together, the evidence indicates that the carcass was subject to extensive scavenging, involving multiple predators.

The role of amateur collectors

In recent years, the role of non-institutional fossil collectors has been the subject of increasing debate. Concerns are often raised that privately held material may be inaccessible to researchers. However, the situation in practice is more nuanced.

At sites such as Smallmouth Sands, much of the available material is recovered through sustained, careful field collecting over many years. Amateur collectors are often well placed to document local stratigraphy, recognise recurring fossil horizons and recover material that might otherwise be lost to erosion.

The work presented here reflects such long-term, site-based collecting, where repeated visits and detailed recording have allowed the accumulation of a coherent assemblage from a single locality.

The question of long-term curation remains important. Regional collections, such as the Etches Collection of Jurassic Marine Life at Kimmeridge, provide an effective model, combining local relevance with high standards of conservation and research access.

Ultimately, the greatest scientific value arises when well-documented specimens are made accessible for study. Collaboration between collectors, researchers and museums remains key.

Conclusion

The material described here highlights the continuing importance of detailed field collecting at classic coastal sites. Careful documentation, combined with collaboration between collectors and researchers, allows even fragmentary material to contribute meaningfully to our understanding of Jurassic marine ecosystems. As coastal erosion continues to expose new material, sites such as Smallmouth Sands remain valuable windows into the past.

Acknowledgements

The authors extend their thanks to the following for their invaluable contributions: Noel Morris (NHMUK) for identifying the ammonites. Andrew Wass for donating some pieces of the pliosaur cervical ribs. Dr Steve Etches (Etches Collection of Jurassic Marine Life) for invaluable information on pliosaurs. Richard Forrest for his useful comments on pliosaurs. Maria Snowball for help with the scale bars.

References

Benson, R.B.J., Evans, M., Smith, A.S., Sassoon, J., Moore-Faye, S., Ketchum, H.F. and Forrest, R. (2013). A giant pliosaurid skull from the Late Jurassic of England. PLoS ONE, 8, e65989.

Clarke, J. and Etches, S. (1992). Predation amongst Jurassic marine reptiles. Proceedings of the Dorset Natural History and Archaeological Society, 113, 202–205.

Martill, D.M., Jacobs, M.L. and Smith, R.E. (2023). A gigantic pliosaur from the Kimmeridge Clay Formation of England. Proceedings of the Geologists’ Association, 134, 361–373.

Snowball, S. and Middleton, H. (2023). A Field Guide to the Fossils of the Fleet Lagoon, Dorset. Siri Scientific Press, Manchester.

Vincent, P., Poncet, D., Rard, A., Robin, J.-P. and Allemand, R. (2024). New remains of Liopleurodon and paedomorphosis in pliosaurids. Palaeontological Society Papers.

Wignall, P.B. (1995). Benthic palaeoecology of the Late Jurassic Kimmeridge Clay of England. Special Papers in Palaeontology, 43.