Trace fossil burrows in a limestone boulder, made by marine invertebrate creatures in ancient times, at Winspit, Dorset, UK - part of the Jurassic Coast (1)

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.

General view of the trace fossil bearing boulders where the valley meets the sea at Winspit, Dorset, UK - part of the Jurassic Coast (2) 

Trace fossils in a limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (3)

Close-up of trace fossils shown in Photos 2 & 3, limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (4) 

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.

Limestone boulder with trace fossils at Winspit, Dorset, UK - part of the Jurassic Coast (5) 

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

Close-up of trace fossils in limestone boulder (shown in photo 5) at Winspit, Dorset, UK - part of the Jurassic Coast (6) 

Large Titanites giganteus ammonite fossil in a limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (7) 

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. 

Trace fossils on the surface of a large limestone boulder (with walking pole for scale), Winspit, Dorset, UK - part of the Jurassic Coast (8) 

Detail of trace fossils on limestone boulder (photo 8) at Winspit, Dorset, UK - part of Jurassic Coast (9) 

Detail of trace fossils on limestone boulder (photo 8) at Winspit, Dorset, UK - part of Jurassic Coast (10) 

Trace fossils on a limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (11) 

Detail of trace fossils in a limestone boulder (photo 11) at Winspit, Dorset, UK - part of the Jurassic Coast (12) 

Detail of trace fossils in a limestone boulder (photo 11) at Winspit, Dorset, UK - part of the Jurassic Coast (13) 

Limestone boulder with surface covered in trace fossils at Winspit, Dorset, UK - part of the Jurassic Coast (14) 

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.

Detail of trace fossils on a limestone boulder (photo 14) at Winspit, Dorset, UK - part of the Jurassic Coast (15) 

Revision of a post first published 9 December 2009

COPYRIGHT JESSICA WINDER 2011

All Rights Reserved

5 Replies to “Trace fossils in Winspit rocks”

  1. Since this Jurassic Coast shoreline has been designated a World Heritage site (on account of its fantastic geology and fossils), it has attracted a lot more visitors – which is good. Many come with the hope of finding fossils. Unfortunately, due to media coverage, they sometimes have unrealistic expectations. It may be OK to pick up things lying loose on the beach but digging them out of cliffs is just plain dangerous. And trying to remove large ‘set pieces’ that are part of the attraction for every visitor results in inevitable failure and permanent damage to the fossils. This spoils the enjoyment for other fossil-hunting visitors. I have noticed the same problem at Lyme Regis further down the coast where attempts have been made recently to remove specimens of large water-worn ammonites from the rock pavements .

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