Early Eocene London Clay deposits at High Ongar, Essex (Part 2)

Bob Williams (UK)

In the previous part of this article (see Early Eocene London Clay deposits at High Ongar, Essex (Part 2)), I located the beds exposed at High Ongar in Essex (TQ 556809) within the general, stratigraphic framework of the London Clay. I also argued that examining the habitats in which families of crustacea live today provides clues about the sort of habitats that may have existed when the London Clay in the pits at Aveley in Essex and Ongar (TL 562024) was deposited.

In this part, I will continue this comparison using modern lobsters, shrimps and other animals to provide clues about the habitats that may have existed at Ongar and at various other London Clay sites when their fossil relatives were alive. I will also show how one can locate a site like Ongar within the stratigraphic column.

Fig. 1. Estimated position of the clay exposures at High Ongar Essex and nearby Aveley, showing the London Clay sedimentary deposits.

At this point, it is worth bearing in mind the conditions in which the London Clay deposits are believed to have been laid down. London Clay is not one, uniform deposit. There are a number of sedimentary horizons within the deposit, each horizon reflecting the environment in which it was formed. Broadly, the London Clay is thought to have been laid down in a marine environment influenced by a tropical or subtropical climate. Water depth is thought to have averaged about 200m, but would obviously have varied locally.

Fig. 2. Locality map showing site locations and the positions of the Tertiary sediment basins that exist in the London and Hampshire regions of the UK.

It was also dependant on transgressive and regressive cycles. In fact, during the deposition of London Clay, the sea is thought to have encroached on the land and then receded five times. Each incursion of the sea is now represented by a different sedimentary horizon (that is, Divisions A to E). Also, the incursions were not of equal extent. Therefore, each horizon represents an environment of a different water depth, characterised by its own range of animals.

Comparisons with modern species and their habitats

As I started to do in Part 1, I will now consider some further modern Crustacea and some other animals, together with some physical geology, to show how these considerations might shed some light on the conditions present when the London Clay at Ongar was laid down.

Palinuridae (spiny lobsters)

This family of lobsters includes the fossil species, Trachysoma scabrum (syn. Glyphea scabra) and Linuparis scyllariformis, both of which have been found at Ongar in Division B2 of the London Clay. When examining the anatomical details of a spiny lobster, it is immediately apparent that they do not possess chelae (claws). To the uninitiated, this might seem surprising, as claws are the familiar anatomical feature of this type of animal. (This is perhaps the result of most people’s culinary experience of the Nephropid species Homarus gammarus, or the common European Lobster, as it is usually known.)

Lacking any defensive claws, this family of lobsters uses other methods of defence, the most common of which is to retreat into suitable crevices, cracks or holes. This obviously requires rocky or hard, exposed substrates that have been subject to erosion to create such features and it is on such substrates that Palinurid lobster species are found living today. In addition, none of the appendages of such lobsters are adapted for digging. This indicates that soft sediments are not frequently encountered in the environments in which they live.

These environmental features are those found in shallow coral reefs or coastal shorelines. However, spiny lobsters are also capable of living at a greater water-depth range than the other crustacea so far discussed. Some species occur at depths of up to 600m, although most are found at depths of between 10m and 100m. Therefore, there appears to be a preferred range of habitat and this supports my belief that the London Clay exposed at Ongar was likely to represent a progressively, shallow-water habitat, incorporating firm exposed surfaces as well as the soft, muddy conditions that gave rise to the London Clay itself.

Nephropidae (clawed lobsters)

The lobster family Nephropidae includes the more familiar lobsters (that is, they have claws). The fossil species Hoploparia gammaroides is a member. This is a common species in Division B2 of the London Clay and has also been found at Ongar. This family of lobsters can tolerate depth ranges of up to 475m (for example,. Homarus americanus, the American lobster) and their presence may be taken to indicate deeper-water and a more varied range of habitats than suggested merely by the presence of spiny lobsters.

Stomatopoda (mantis shrimps)

Consideration of the crustacean fauna would be incomplete without coverage of the Order Stomatopoda, the mantis shrimps. These creatures are lobster-like in general appearance, with a distinct head and thorax combined and a separate abdomen of considerable length. The cephalothorax (the fused head and thorax) is only partly covered by a carapace and at least four thoracic segments are distinctly visible behind it.

Chelae, as such, are not present as in true lobsters. However, the second pair of thoracic appendages can be highly modified into powerful raptorial claws reminiscent of the front legs of the praying mantis insect (hence, the common name, “mantis shrimp”). Modern Stomatopods are mostly marine animals, although a few species have been recorded from brackish water. They mainly inhabit shallow waters and are usually restricted to tropical and subtropical seas around islands, where they are known to frequent the sub-tidal zone.

Stomatopods are known to have existed in the Jurassic period, being recorded from the limestone deposits at Solnhofen in Germany. However, they are rare fossils and are usually found in a fragmented and imperfect condition. This results in little being known about their palaeontological history. However, they have also been found in the London Clay deposits of the UK, in particular, from the Isle of Sheppey in Kent (for example, Bathysquilla wetherelli, syn. Squilla wetherelli). The environmental conditions in which they live today strongly support the current ideas about the environment in which the London Clay is thought to have been deposited in the early Eocene period (that is, tropical and sub-tropical shallow seas).

More animals and a bit of clay

When combined with information obtained by other means, a picture of the environment that once existed may be built up. In particular, this can be done through the analysis of other faunal constituents and/or the physical geology of the sediment in question.

Other fauna found associated with the crustacea is the class of vertebrate referred to as Elasmobranchia (sharks and rays). The variety and abundance of these creatures at different sites varies, which suggests a difference in environmental conditions. For example, a low record of shark and ray genera was recorded for the sites at Ongar and nearby Aveley. I concluded that this was attributable to a lower generic population existing at the time of depositio and not because of collecting limitations. I collected at both sites for a total of eight years and it is hard to believe that fossil evidence of a larger generic population could have escaped my notice.

The sharks’ teeth found at Ongar and Aveley are believed to come from species that frequented deeper waters (for example, Otodus obliquus and Striatolamna macrota). An analysis of the generic population recorded at another London Clay site at Althorne on the River Blackwater in Essex (which is a tidal, riverbank exposure) reveals a greater generic population including many small varieties.

Why is there such an increase in the number of genera and the presence of smaller species and what might the habitat have been? In answer to the first question, it was probably because a more suitable habitat was available for all of them. In answer to the second, that habitat was probably a coral reef.

In modern marine environments, a more varied population of sharks and rays is found around a coral reef than in a deep-water environment. This is because, at the very least, there is a much more varied and ready food supply. For example, an analysis of the physical structure of the clay sediment provides a clue. The clay at Ongar is composed of very small particles that would require a low-energy environment for deposition. A low-energy environment is usually found in deeper waters where sedimentation is less likely to be affected by wave action.

When considered with the presence of crinoidal material (see below) and the lack of very shallow-water, crustacean species, this suggests to me (and, indeed, to others) a slightly deeper water habitat. This, in turn, suggests a Division B1/B2 and low Division C environment.

Ongar in the stratigraphic column

Finally, I turn to a discussion of where in the stratigraphic column Ongar is. Looking at all of the animals groups I have discussed in this part and Part 1, if one were to look at only one crustacean family, it is unlikely that one could place a particular deposit at a precise level in the stratigraphic column, even if a broad view of the general horizon might be gained. However, if more than one family is considered, the assessment will be more accurate and that accuracy is increased as more families are considered.

A basic example

The fossil Lobster species, Hoploparia gammaroides,is found throughout many horizons in the London Clay, certainly Divisions B1, B2, C and D. The crab species, Zanthopsis leachi,is found in Divisions C, D and possibly E. The crab species, Mithracia libinoides,is found in Divisions D and E, while the crab species, Dromilites bucklandi and lobster species, Linuparis scyllariformis,are both Division B2 species.

While examining an exposure of clay, if only H. gammaroides is found, then it may be concluded that the exposure is of any of Divisions B1, B2, C or D. If it occurs with the crab, Zanthopsis leachi, then Divisions B1 and B2 may be eliminated, limiting the range of horizons to Divisions C and D. If only found with Z. leachi, then it would be reasonable to assume that Division C is the horizon being examined. If found together with Mithracia libinoides, then Division D is the likely horizon. Finally, if it is found with the lobster, Linuparis scyllariformis, then Division B2 is the likely candidate.

In addition, the more of each species found, the more certain one can be of the association. This demonstrates how the consideration of more than one crustacean species and, particularly, the consideration of the relative abundance of each species are of importance in assessing the stratigraphic horizon at which they are found.

In addition, another component of the fauna has proved useful in estimating the stratigraphic position of the Ongar exposure. This is the crinoid (sea lily) Issellicrinus subbasaltiformis, which is a member of the animal phylum Echinodermata (including sea urchins, starfish and sea lilies). I was lucky enough to find a fragment of broken septarian nodule in which were preserved three separate stem sections of this crinoid. They were well preserved and are a rare find at Ongar.

This species is an index fossil or marker fossil, as it is only found in any abundance in clay sediments at the Division B1/B2 junction. It has previously been found at Ongar at a similar depth (Cooper, 1972) just above the base of the pit. As none have been found any higher in the pit, it seemed reasonable to conclude that the Division B1/B2 boundary was just below the base of the pit. I also concluded that the Ongar exposure of the London Clay was of the upper Division B2 and Division C horizons, therefore, overlapping with the stratigraphy of the Aveley site not far away (Kirby, 1974).

Finding this specimen was interesting, not just because of it rarity, but also because Chris King (1981) doubted that these animals can be found at the site. Having discussed the matter with Chris, he now proposes that this crinoid species can, in fact, be found throughout the London Clay as occasional, individual examples. However, it may still be considered as a marker fossil because of its abundance at the Division B1/B2 boundary.

From the above, it is apparent to me that the horizons of London Clay exposed at Ongar fell within Divisions upper B2 and C and that a stratigraphic overlap probably existed between the sites at Ongar and nearby Aveley (see also Kirby, 1974). The overlap occurs between Divisions B2 and C, those Divisions being exposed in the higher levels of the Aveley pit and the lower levels of the Ongar pit. When considering the close geographical proximity of these locations, this, perhaps, should be expected.

Conclusion

Two conclusions may be drawn from the ideas I have presented in this article. The first is that the study of the fossil fauna of a given sedimentary exposure (for example, an examination of the types and number of species found in the exposure and how they relate to each other in terms of abundance and whether they occur together) may provide clues that can be used in determining the stratigraphic position of the sediment being examined.

The second is that, by comparing modern examples of animals with their extinct relatives, one can get an insight into the habitats and general environment that existed at the time the sediment in which they are found as fossils was laid down.

This is not a new idea. Ammonites, for instance, have long been compared and contrasted with the pearly nautilus of the modern Pacific Ocean. However, I (and, I hope, you) find it more exciting, when collecting fossils, to be able to envisage, for example, a coral reef with myriad different creatures swimming and crawling in its protection rather than a pile of muddy clay with an alpha-numeric label somewhere between A and E.

Acknowledgements

My thanks to the former staff of the LECA company (now closed), who allowed me unrestricted access to the pit to enable collecting. Thanks are also due to Essex Rock and Mineral Society member, Derek Breden, for bringing this site to my attention and for supplying me with a fine collection of fossil material to review.

References

Cooper. J. (1972), Report of field meeting to High Ongar, Essex. Tertiary Research Group. (TRG).

Kirby. R. I. (1974), Report of project meeting and field meeting to Aveley, Essex TRG Tertiary Times. 2.(II), pp 53-67.

King. C. (1981), The stratigraphy of the London Clay and associated deposits. TRG Tertiary Research. Special paper.

Early Eocene London Clay deposits at High Ongar, Essex (Part 2)