Visitors to Lyme Regis are always amazed by the wonderful large ammonites embedded in the beach rocks and rock pavement, and are often frantic in their search for take-away fossils as mementos for their day out…. but they frequently overlook the less spectacular but nonetheless fascinating myriads of trace fossils underfoot. Trace fossils are also known as ichnofossils and they are the evidence, preserved in stone, for the activities of organisms rather than the petrified remains of the animals themselves. They tell us about the way animals behaved and interacted with their environment in the past.
The patterns in the rocks shown in these photographs are the traces of burrows and tunnels that were made in the sediments before they became rock. They were made by marine invertebrate creatures like crabs and worms. The intriguing natural patterns are found in the Blue Lias Limestone from the Jurassic Period. The Jurassic system is exposed along the Dorset coast between Lyme Regis and Swanage with very few breaks. Melville and Freshney (1982) say that trace fossils are abundant in rocks of this area and there are four common types:
Diplocraterion with U-shaped burrows at right angles to the bedding planes, measuring 21 by 4 cm with the diameter of the tubes up to 0.7 cm. These were the permanent domicile of worms or crustaceans that angled or swept for food suspended in water.
Chondrites with regularly branching, vertical to horizontal burrows spreading down like the roots of a tree, and 2mm or more in diameter. The work of sediment-eating animals, perhaps sipunculoid worms.
Thalassinoides Ramifying, Y-shaped branching networks of plain horizontal and vertical tubes, 1 to 5 cm in diameter. Feeding burrows of decapod crustaceans.
Rhizocorallium Horizontal to oblique U-tubes, each arm 1 cm or more in diameter with arms several centimetres apart with reworked sediment between them. Tubes short or long depending on whether the animal was in a suspension-feeding or deposit-feeding phase of activity.
The pictures show several burrow types intermingled.
Melville R. V. and Freshney E, C. (1982) British Regional Geology: The Hampshire Basin and adjoining areas, Institute of Geological Sciences, NERC, HMSO, pp 7 – 8, ISBN 0 11 884203 X.
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