The Jurassic/Cretaceous boundary interval is represented in Lincolnshire by the Spilsby Sandstone Formation, a shallow water marine deposit that spans the Volgian stage of the Jurassic to the Berriasian stage of the Cretaceous (Hopson et al. 2008). The ammonite faunas of this formation are of particular interest, exhibiting affinities with correlative forms in both Russia on the Siberian plain, as well as Greenland and Canada (for example, Casey, 1973; Mikhail Rogov, personal communication 2015).
Exposures of the Spilsby Sandstone Formation are currently almost absent. The last substantial exposure that allowed collecting was at Castle Top Quarry in Nettleton (TF1198), which, following its closure in 1998, was subsequently backfilled. The most complete sequence of the Spilsby Beds is present in south Lincolnshire (Casey, 1973) To the north of the county, at Nettleton, the beds have been eroded down to the level of the ammonite zone of Subcraspedites lamplughi of Lower Cretaceous (Berriasian) age. However, it is beyond the scope of this account to deal with the complex biostratigraphy of the Spilsby Formation as a whole, and the present account will focus on my observations at Castle Top Quarry.
The Spilsby Formation here is about 8.5m thick, consisting of the Basal Spilsby Nodule Bed at the base, succeeded by a largely undifferentiated, coarse, friable sandstone, with no obvious bedding or marker horizons. This sandstone is characterised by common belemnites such as Borioteuthis sp. (Simon F Mitchell, personal communication 2014) as well as rarer ammonites that suggest faunal links with the regions discussed above.
An ammonite that I found loose appears very close to Subcraspedites sowerbyi (Spath), although the inner whorls are absent. If this identification is correct, it is the first definitive record of this species from Nettleton and, more importantly, it is indicative of the preplicomphalus zone at Nettleton. Casey (1973) assigned over four metres of the Spilsby beds at Nettleton to this zone, although, interestingly, found no age-diagnostic fossils. Even more interesting is the fact that I managed to recover a partial ichthyosaur, most probably from the preplicomphalus zone, which has also been recently described (Green and Lomax, 2014), this being the most complete ichthyosaur known from this formation. Forrest and Oliver (2003) previously described ichthyosaur fragments from this formation, but these are very rare.
The lower horizon of the Spilsby Beds at Nettleton is especially friable and, aside from the calcitic guards of belemnites, contains relatively poorly preserved fossils. The topmost two metres of the Spilsby Beds consist of concretionary horizons assigned to the lamplughi zone, based on the ammonite faunas (Casey, 1973). This represents the most fossiliferous section of these beds at Nettleton, and my observations reveal ammonites showing evolutionary variation from the evolute (where the outer whorl of the shell largely does not cover the preceding whorls), relatively inflated Subcraspedites through to compressed, involute forms (where the outer whorl of the shell largely does cover the preceding whorls) representing Volgidiscus spp. and related forms. Casey (1973) described many of these genera, in addition to what he described as “undescribed forms”. The ammonites found by me exhibit a number of changes. Subcraspedites spp. is an ammonite genus characterised (in general terms) by being relatively evolute, with prominent umbilical nodes and fine ribbing on the ventro lateral shoulders. Volgidiscus sp., in addition to being compressed and involute, exhibits weak or no ornament on the venter and demonstrates coarser umbilical nodes (Casey, 1973). Some of the ammonites are smooth, compressed forms (?Volgidiscus spp), with little apparent ornament and very weak umbilical nodes, which may represent some of the “undescribed forms” noted by Casey (1973), although variations in preservation must be taken into account regarding true identification. It is also unfortunate that I was unable to carry out bed-by-bed collecting (invaluable for biostratigraphical purposes) and the majority of fossils were obtained from loose blocks. Further work is needed on this topic and comparison with ammonites in comparative sections elsewhere could well repay further study. Other notable fossils from the lamplughi zone include large belemnites (Borioteuthis sp.), as well as the pectinid bivalve, Entolium sp., and poorly preserved moulds of the burrowing bivalves, Pleuromya sp. and Pinna sp.
This overview of the fauna of the Spilsby Sandstone Formation at Nettleton in Lincolnshire is of particular significance, especially in light of the fact that exposures of this formation are at present largely unavailable. The fossils I documented from this formation, in particular, the exceptional ichthyosaur remains as well as the rare ammonite faunas, form an important record of this unique rock that will be available for future generations of geologists to study. In this way, the role of the amateur geologist is often invaluable in documenting such exposures and recording the fossils they contain before they, like the Castle Top Quarry, disappear forever.
Thanks are due to Dr Mikhail Rogov of the University of Moscow, Russia, for his interest in my collection of Spilsby Sandstone ammonites, as well as for informal discussion and helpful comments. Prof Simon Mitchell of the University of the West Indies is also thanked for his help with belemnite identification.
Casey, R. 1973. The ammonite succession at the Jurassic–Cretaceous boundary in eastern England. In Casey, R. and Rawson, PF (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.
Green, JP and Lomax, DR, 2014. An ichthyosaur (Reptilia: Icthyosauria) specimen from the Lower Cretaceous (Berriasian) Spilsby Sandstone Formation of Nettleton, Lincolnshire, UK. Proceedings of the Geologist’s Association 125, 432 – 436.
Hopson, P.M., Wilkinson, I.P, Wood, M.A., 2008. A Stratigraphical framework for the Lower Cretaceous of England. British Geological Survey Research Reports, Keyworth, Nottingham, pp. 77.