Patterns made by trace fossils at Lyme Regis

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.

REFERENCE

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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Jurassic Coral Fossils at Ringstead

These photographs all show fossil corals found in boulders on the beach at Ringstead Bay in Dorset, England. They date from the Jurassic Period and are embedded, along with numerous other fossils, in the Ringstead Coral Bed which is the topmost layer of the Ringstead Formation (that in turn is at the top of the larger Corallian Formation).

Whilst I cannot say for certain exactly which fossils each image portrays, I can say that there are four corals known from this rock bed. These are Thecosmila annularis, Thamnasteria concinna, Thamnasteria arachnoides, and Protoseris waltoni. It is highly likely that the pictures show details of the colonial Thamnasteria genus and I think both species are represented here. I will being going to my local museum to check the identifications with their reference collections. Thecosmila is a larger solitary coral so can be excluded as a possibility for these particular fossils..

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Tube-worm Holes in Ringstead Chalk Boulders

Close-up of tube-worm holes in a chalk boulder

Pebbles and beach stones riddled with small holes are a common beachcomber’s find. These small burrows and borings in the stone are frequently made by marine worms. The worms themselves, and the mud and sand tubes in which they live within the burrows, are usually absent. However, on the water’s edge in many coastal locations, if you know where to look, it is possible to spot the burrows still occupied by the worms; this is usually in large and mainly immovable boulders, or in the bedrock of the beach platform, or the base of a cliff face. The worm itself is almost impossible to see because at low tide, when the rock is exposed to the air, it retreats into the tube and burrow. Though sometimes, apparently, its two palps or feelers can be seen protruding from the hole and waving around vigorously. I haven’t observed that myself so far.

Without microscopically examining the actual worms, it isn’t possible to say with a 100 per cent certainty what these worms are. Nevertheless, there are enough characters available to say that these are most likely to be marine polychaetes of the Spionidae, and probably one of the Polydora group, maybe Polydora ciliata (Johnston).

All the Polydora species make a U-shaped tube from small particles of mud, or whitish calcareous matter if they have been burrowing into calcareous algae, shell, or limey stone; all this is stuck together with secreted mucus. The tube is normally embedded in the burrow that it has excavated. There are two holes in the mud tube, one at the front and one at the back end – but they lie side by side because the tube and burrow are U-shaped. In the examples photographed here, many worm tubes are packed together, and there are instances where the chalk burrows have joined together and broadened out into deeper, less well-defined, depressions.

The method by which the worms create the burrows is thought to be an almost incidental process. The worms initially settle and manufacture their mud tubes in the shelter of slight cracks and crevices in rock or shell surfaces, or between sessile barnacles, or amongst soft algae in rock depressions, and other such places on the seashore where it always remains damp at low tide. The metabolism of the living worm leads to the production of slightly acidic waste. Over time, the seepage of these waste products gradually eats into and dissolves the rock or shell on which the worm tube lies, enabling the worm to retreat further and further into the safety of the substrate. The burrow formed like this reflects the shape of the U-shaped mud tube.

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Irish Moss at Ringstead

Irish Moss is the common name for the red seaweed Chondrus crispus Stackhouse. It is an extremely variable, commercially exploited, seaweed and it can differ in shape, size, and colour depending on the physical environment in which it is living. Recent findings seem to indicate that another type of variation is occurring at the moment due global changing conditions. The species itself is currently the subject of extensive projects worldwide – from personal records of observations to full-scale academically rigourous research.

The shape of Irish Moss that I most frequently see on Dorset beaches is the dichotomously branched, flat bladed form with a short stipe or stalk. This is basically a red form but can be every shade of pink in the palette with varying degrees of green or greeny-yellow colour – especially at the tips of the fronds. This form doesn’t actually look anything like moss despite the name. This is the form that washed up on the beach at Ringstead by the boulders at the foot of the White Nothe cliff, and is shown in photographs 1 – 3.

However, on the same trip, I saw a seaweed that really did look like moss. It was a bright golden green which caught my eye. Made up of many fine, branched, rounded cross-sectional elements reaching no more than a centimetre or so high, it was growing epiphytically on brown fucoid seaweeds (such as Bladder Wrack, and Toothed Wrack) attached to inter-tidal boulders. I am curious to know what it really is. I have searched in texts and on-line for possibilities without finding illustrations of something identical. However, from what I have learned so far, I conclude for the moment that it is indeed another known form of Irish Moss. This is shown in photographs 6 – 8.

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A Masked Crab at Studland

A Masked Crab waiting for the tide to come in

I saw a little Masked Crab (Corystes cassivelaunus Pennant) on Knoll Beach at Studland the other day. It was an unusual sighting for that location. The crab was alive – but lucky to be so. It had buried itself in the wet sand to survive the rigours of exposure at low tide. There are not many other places for an animal to hide on this part of the beach.

The small crab, only a couple of inches long, would probably have stayed out of view until the tide came in again – except that this was the afternoon that several schools decided that it was just the right moment for the students to run on the beach while the sun was shining. The youngsters pounded their way along the shore and one of them stepped on the very spot where the crab was sheltering. Being disturbed by this close encounter, it surfaced, all covered in wet sand, as I walked past it and eastwards in the direction of Shell Bay.

I was surprised to see this same little seashore creature again as I made my way back along the water’s edge going westwards. I know it was the same crab because it was almost the only live thing I found, and certainly the most interesting. It was one of those days when there was not much at all newly washed ashore: a few fresh clumps of spindly red seaweed, some brown Sea Oak and strands of kelp, a few pieces of translucent green Sea Lettuce, and some clusters of Slipper Limpets. Lots of empty bivalve shells.

On this second meeting with the Masked Crab, the creature was more active and had got rid of the sand which had been covering it before. It was waiting for the waves. I have seen this activity previously in Masked Crabs on Rhossili Beach on the Gower Peninsula. The animal sits facing the sea, using its legs to brace itself against the oncoming water. Its two fringed antennae can be joined together to form a single tube and this was projecting forwards and upwards – looking very much like an angler holding a fishing rod. It was fascinating to watch the way to crab parted and then joined the antennae, moving them side to side as if using them to gauge the speed and timing of the next wave. The antennae form a breathing tube when the crab is buried.

I took a few photographs of the Masked Crab and some short video clips which you can see below. I hope that you will appreciate that it was a bit difficult to film the crab in action because of its small size and the necessity for recording it in such a low position – plus the imminent drenching of both the crab, the camera, and the photographer.

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Goose Barnacles at Ringstead Bay

I always like to find Goose Barnacles (Lepas anatifera) on flotsam at the beach. These strange creatures live attached to items that free-float around the oceans of the world; and we only see them when they wash ashore, as they did yesterday at Ringstead Bay in Dorset, England. Thousands of these strange marine creatures were clustered onto a tree trunk and its branches that lay freshly beached on the shingle. All the pebbles here seem to have returned now – it was only a week or so ago that they had all more or less disappeared following stormy weather.

Click here for more information about Stranded Goose Barnacles and Goose Barnacles on Rhossili Beach in Jessica’s Nature Blog.

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Ringstead Bay Fossil Bivalve – Ctenostreon proboscideum

Most of the examples of this fossil bivalve, Ctenostreon proboscideum, were partial specimens embedded in the rocks at Ringstead Bay in Dorset, England. However, the large strongly-ribbed shell is unmistakable and easily recognised in the many boulders on the beach at the west end of the bay – at least they were easily seen when the pebbles had all been washed away after the storms. The photographs in the gallery above show Ctenostreon shells as they were found on the beach last week. The boulders had fallen from the Ringstead Coral Bed which is a narrow layer,  packed with fossils, of no more than 30 centimetres depth, and which can be seen in short lengths in the vertical section through the strata at the top of the beach.

The almost complete fossil specimen shown with the blue background (photographed at home) was found many years ago after similar severe weather. You can see that the two valves are still together and the space between them filled with marly limestone material, indicating that the original animal was already dead, with the two shells gaping open, when it was buried under new sediments.

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Ringstead Bay Rock Textures & Patterns – Part 3

Another gallery of close-up photographs showing natural colour, textures and patterns in an assortment of rocks on the beach at Ringstead in Dorset, England, illustrating the great variety in the geology of this stretch of World Heritage Coast.

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Ringstead Bay Rock Textures & Patterns – Part 2

This is a gallery of some of the rock textures and patterns found in material brought down to the seashore by land-slips on the eastern half of Ringstead Bay in Dorset, England. Some might say that the geology of this half of the bay is much more complex and interesting than the western half. In addition to a natural progression of strata from older rocks in the west to younger rocks in the east, there are faults and land-slips that result in much of this variable material appearing in mudslides at the top of the shore, and as boulders on the beach.

The pictures show details of the textures and patterns in some of the rocks and sediments that were laid down after the Ringstead Formation rocks that were illustrated in earlier posts. These later strata include those from the Jurassic Period Kimmeridge Formation (shales and clays), Portland Limestone Formation (Portland Freestone, Portland Cherty Series, Portland Sand), and the basal part of the Purbeck Formation. The layers can be seen in exposures west of Holywell House on top of the ‘cliff’; and boulders from them often end up rolling down to the beach below.

A bit further east the geologically more recent Cretaceous Period strata are exposed. These include the Gault, Greensand, and Lower, Middle, and Upper Chalk. All of these rock exposures are subject the slippage and land slide so that boulders frequently end up on the beach. I haven’t yet reached a complete understanding of the geology in this location. I have attached a description to each image that you can see if you click to enlarge the pictures. However, I cannot say with absolute certainty the identity of each of the rocks I photographed – but I am working on it with the help of the references listed below; and I’m hoping to visit the Dorset County Museum to look at their rock collections soon.

REFERENCES

West, I.M. 2013. Ringstead Bay to White Nothe: Geology of the Wessex Coast (Jurassic Coast, Dorset and East Devon World Heritage Site). Internet field guide. By Dr. Ian West, Romsey, Hampshire and Visiting Scientist at Southampton University. http://www.southampton.ac.uk/~imw/Ringstead-White-Nothe.htm. Version: 19th December 2013

British Geological Survey (2011) Geology of south Dorset and south-east Devon and its World Heritage Coast, Compiled by M. A. Woods, Special Memoir for 1:50 000 geological sheets 328 Dorchester, 341/342 West Fleet and Weymouth and 342/343 Swanage etc, NERC, ISBN 978 85272 654 9.

Melville, R. V. and Freshney, E. C. (1982) British Regional Geology: The Hampshire Basin and adjoining areas, 4th edition, Institute of Geological Sciences, NERC, HMSO, ISBN 0 11 884203 X.

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Ringstead Bay Fossil Oyster – Deltoideum delta

Cluster of attached fossil oysters from Ringstead Bay on the Jurassic Coast

Deposits containing this large fossil oyster, Deltoideum delta or Liostrea delta, were clearly visible on the shore at Ringstead Bay in Dorset after winter storms had rearranged and largely removed the normal thick layer of pebbles. They are recorded as being characteristic of several late Jurassic Period strata. I observed them in situ in the Ringstead Clay Member waxy clays at the top of the shore in the western half of the beach. I also noticed them in rocky outcrops on the water line of the lower shore where the different composition of the matrix makes me think the exposed strata were probably from the Sandsfoot Formation which lies beneath the Ringstead Formation and pre-dates it.

In the eastern half of Ringstead Bay, the part which was inaccessible on that particular visit of 1st March 2014, I had previously seen this species of fossil oyster shell in deposits of Kimmeridge Clay from the Kimmeridge Formation which were laid down after the Ringstead Formation layers. So this particular species of oyster was around for a long time, geologically speaking. Its appearance and disappearance in the various strata is due to the changing and cyclical nature of the environment in this location – meaning that very specific conditions were required for the species to thrive but changes in water depth, salinity, and temperature made the environment more or less suitable for their existence at different successive times.

There are more posts about Deltoideum delta in both Jessica’s Nature Blog and Oysters etc.

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