MICROFOSSIL EVIDENCE FOR A MID-JURASSIC SQUID EGG-LAYING AREA IN ASSOCIATION WITH THE CHRISTIAN MALFORD LAGERSTATTE

In the 1840s, during construction of the railway line west of Swindon, a number of beautifully preserved coleoids (belemnites and squid-like cephalopods) were found. These famous specimens of Belemnoteuthis and Mastigophora, as well as a number of fish, are described as a fossil lagerstätte under the name of the ‘Christian Malford Squid Bed’. Many of these specimens, which come from the Phaeinum Subzone (Athleta Zone, Callovian) of the Oxford Clay Formation, contain soft tissue, muscle fibres and the content of their ink sacs.

In 2007, the British Geological Survey excavated a site, some ~100 m from the original borrow pits alongside the railway, as well as drilling a number of borehole cores. Our work on core No. 10 has recovered exceptionally large numbers of statoliths, otoliths (fish ‘ear’ bones), squid hooks and foraminifera. Statoliths are the small, paired, aragonitic stones found in the fluid-filled cavities (or statocysts) within the cartilaginous heads of all modern and probably all fossil coleoids. Jurassic statoliths have yet to be described in any detail as there are only a few references to them in the literature. The exceptional abundance of statoliths and squid hooks recorded in the samples from the core may represent a Jurassic squid-breeding ground which existed for an extended interval of late Callovian time. The annual spawning of female squid massively enlarges their ovaries and this breaks down the body wall leaving spent individuals to die. The highest numbers of statoliths occur over a 3 m thickness of strata with the greatest abundance ~1 m below the Christian Malford Squid Bed. The numbers recorded in this part of the Phaeinum Subzone are well above background levels in the rest of the Jurassic in the UK (Malcolm Clarke, pers.com.) where one has to wash several kg of sediment to recover <200 statoliths. In our samples 1kg of clay has yielded >400 statoliths.

The occurrence of abundant, though low diversity, foraminiferal assemblages (especially Epistomina spp.) in the same samples point to an oxic, though possibly stressed, environment. The significant proportion of deformed foraminifera in the assemblages appears to confirm that the environment was less than optimal but still able to support a relatively diverse and abundant population.