Abstract
Abstract. The Christian Malford lagerstätte in the Oxford Clay Formation of Wiltshire contains exceptionally well-preserved squid-like cephalopods, including Belemnotheutis antiquus (Pearce). Some of these fossils preserve muscle tissue, contents of ink sacks and other soft parts of the squid, including arms with hooks in situ and the head area with statoliths (ear bones) present in life position. The preservation of soft-tissue material is usually taken as an indication of anoxic or dysaerobic conditions on the sea floor and within the enclosing sediments. Interestingly, in the prepared residues of all these sediments there are both statoliths and arm hooks as well as abundant, species-rich, assemblages of both foraminifera and ostracods. Such occurrences appear to be incompatible with an interpretation of potential sea floor anoxia. The mudstones of the Oxford Clay Formation may have been compacted by 70 %–80 % during de-watering and burial, and in such a fine-grained lithology samples collected for microfossil examination probably represent several thousand years and, therefore, a significant number of foraminiferal life cycles. Such samples (even if only 1–2 cm thick) could, potentially, include several oxic–anoxic cycles and, if coupled with compaction, generate the apparent coincidence of well-preserved, soft-bodied, cephalopods and diverse assemblages of benthic foraminifera.
Highlights
During the construction of the Great Western Railway west of Swindon in the 1840s “borrow pits” were excavated to provide material for the adjacent railway embankment
As these mudstones contain species-rich and abundant assemblages of microfossils that appear to indicate a normal, oxygenated sea floor this would look to be incompatible with soft-bodied preservation, which is often taken to indicate a lack of sea floor oxygen and rapid burial
In the Oxford Clay Formation (Callovian–Oxfordian) of southern England there is a species-rich, abundant assemblage of foraminifera and ostracods, though the latter have not been fully investigated. These mudstones are highly compacted as many macrofossils are completely flattened
Summary
During the construction of the Great Western Railway west of Swindon in the 1840s “borrow pits” were excavated to provide material for the adjacent railway embankment. The mudstones of the Oxford Clay Formation (Callovian, Jurassic) yielded a large number of exceptionally well-preserved coleoid fossils (Pearce, 1841; Owen, 1844; Mantell, 1848), many of which have been redescribed by Donovan (1983), Page (1991), Page and Doyle (1991), and Donovan and Crane (1992) The majority of these specimens can be attributed to the Phaeinum Subchronozone (Athleta Chronozone, upper Callovian, Middle Jurassic), and many contain fossilized soft tissues, muscle fibres and the cell content of their ink sacks (Wilby et al, 2004, 2008; Hart et al, 2016a). This research explores this apparent contradiction and looks at the issue of taphonomy and the impacts of sediment compaction
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