One-hundred and sixty million years ago, most of Europe - including the British Isles – was underwater, flooded by a warm, shallow sea populated by astonishing marine reptiles, gigantic fish and a diverse invertebrate fauna. The Oxford Clay Formation, one of the UK's most famous fossil-bearing rock units, provides an extensively researched window into this Jurassic marine ecosystem.
|Extent of the Oxford Clay across the UK, with major localities. Whittlesey, the source of the marine reptile skeleton behind this and the preceding blog post, is highlighted in red.|
The Oxford Clay Formation is an extensive succession of dark mudrocks with intermittent limestones which crop out almost continuously from Dorset to Yorkshire. Further exposures are found on the seabed of the English Channel and in Normandy, France. The entire Oxford Clay sequence is of late Middle Jurassic to lower Upper Jurassic age (164-159 Ma) and fossils occur throughout, although most vertebrate fossils occur in the Peterborough Member, a unit of organic-rich rocks which represent the lowest part of the formation. Considerable commercial interests in the Peterborough Member date to the 1870s when excavation of its clays began for brick making. The high organic content of these clays meant that they fired quickly at low temperatures, allowing for production of high-quality bricks at low cost. The Oxford Clay brick pits are now mostly closed, but the tremendous economic interest in the Oxford Clay has ensured that multitudes of fossils were continually excavated from quarries on an industrial scale for nearly 100 years, permitting a detailed view of the Oxford Clay palaeobiota.
Palaeoenvironment and palaeoecology
The Oxford Clay sea was a warm (water column temperatures of 20°C) and shallow (tens of metres) marine environment, with a rich supply of nutrients from local land sources. The abundance of light and nutrients supported a rich and complex ecosystem (below). Planktonic organisms, including numerous types of algae and zooplankton, were abundant in the Oxford Clay sea and likely formed the basis of its food web. Plankton was the food source for small invertebrates and juvenile fish, which in turn were preyed on by the larger fish, ammonites, belemnites, squid and reptiles that comprise the majority of Oxford Clay fossils. Ammonites are particularly common components of the Oxford Clay, being represented by some 78 species. The community of bony fishes and sharks was almost as rich as that of the ammonites, with 32 species adapted to exist in a variety of ecological niches. The Oxford Clay fauna contains one of the most spectacular bony fish to ever evolve, the 12-15 m long pachycormid Leedichthys problematicus. This animal was not a predator however, but instead filtered plankton from the water column using enormous gill apparatus.
|Schematic reconstruction of the Peterborough Member fauna, palaeoenvironment and nutrient cycling. Animals are not to scale, unless you wish to invoke the Father Dougal sense of size. Based on data from Martill and Hudson (1991) and Martill et al. (1994).|
|Composition and abundances of the Peterborough Member reptile fauna. Based on Martill and Hudson 1991.|
|Micro- and macroconchs (male and female, respectively) of the ammonite Erymnoceras coronatum, hanging around the Oxford Clay seaway. The macroconch is 40-50 cm across, while the microconch, as is typical of ammonites, is about 20-25% of that size.|
Ammonites, nektonic cephalopods with chambered external shells, form the backbone of biostratigraphy for Mesozoic rocks. Ammonite faunas evolved rapidly enough to permit identification of one million year intervals of Mesozoic time, allowing for very precise dating of ammonite-bearing rocks. The Erymnoceras coronatum ammonites shown above are one of the index fossils for the Peterborough Member, placing it firmly in the middle Callovian stage of the Jurassic.
Oxford Clay ammonites provide key data on the evolution of ammonites, and were integral in identifying male (small, elaborately ornamented ‘microconchs’) and female (much larger, less ornamented ‘macroconchs’) morphs. Despite the abundance and familiarity of ammonite fossils however, many aspects of their anatomy and lifestyles remain mysterious. Questions such as what they ate, where they lived in the water column, their floating orientation, as well as the exact nature of the squid-like creature living within the shell, remain unanswered.
Bonus fun: the assembled board
As a way of signing off these two linked posts, I thought it might be fun to show off the entire display board itself, just so anyone interested can see how all the text and images here will hang together. The entire thing is well over 3 m long, so should look fairly imposing when it's finally printed.
|UoP's Oxford Clay and Ophthalmosaurus display text, coming soon to a display cabinet near me.|
- Martill, D. M. and Hudson, J. D. (1991). Fossils of the Oxford Clay (Field Guides to Fossils) (No. 4). The Palaeontological Association, London.
- Martill, D. M., Taylor, M. A., Duff, K. L., Riding, J. B., & Bown, P. R. (1994). The trophic structure of the biota of the Peterborough Member, Oxford Clay Formation (Jurassic), UK. Journal of the Geological Society, 151(1), 173-194.