Trace fossils are geological records of biological activity. The rocks at Winspit in Dorset have preserved evidence of the burrowing habits of invertebrate animals that once lived in the soft wet sediments on the seabed and seashore of shallow seas over 135 million years ago in the Upper Jurassic period.
These trace fossils include burrows and tunnels made by crabs and worms. In some instances, the holes made by the creatures remain visible – as pictured in the first three photographs below. They look remarkably similar to the holes made by boring bivalved molluscs and by mud-tube dwelling marine polychaetes that occur in present day calcareous stones and mollusc shells (see the earlier posts on these subjects). As I am not a palaeontologist, I concede that this may not actually be a parallel causation – just a coincidence.
In most of the examples illustrated by the photographs below, the spaces enclosed by the burrows have been infilled by other sediments so that instead of the original hollow tubes and tunnels remaining in the hardened rock, the burrows have become roughly cylindrical solids. The infill substance seems to be harder than the matrix surrounding it. This means that the softer rock has weathered more readily and the burrows stand out like cords.
A pile of large angular boulders, between one and two metres across, rests on the rocky ledge at the foot of the cliffs where the valley meets the sea at Winspit. On the flat surfaces of some of the rocks are intricate patterns made up of horizontally branching networks of solid tubes about a centimetres or so in diameter. These are similar to the burrows made by decapod crustaceans (crabs) called Thalassinoides.
Large fossilised remains are found in these strata as well as the trace fossils. One of the best known, because of its large size, is the ammonite Titanites giganteus as shown in the picture above. It is about 40 centimetres in diameter. Although well embedded in the massive boulder, and a feature of the location for as long as anyone remembers, someone has recently tried unsuccessfully to hack it out of the rock. It is a shame because it has left the wonderful fossil damaged and defaced.
Some of the burrows, particularly the larger ones that may have been made by crabs, seem from these trace fossils to have passed through a series of layers in the sediments. Sediments from different layers are mixed up, and older sediments brought to the surface and deposited above more recent ones, as animals excavate their tunnels. This process is called bioturbation. Geologists, palaeontologists, and archaeologists are all aware of the implications of bioturbation for the interpretation of results from their research and excavations.
Another name for trace fossil is ichnofossil. To read some more about ichnofossils click here.
Revision of a post first published 9 December 2009
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