Seatown Ammonites

Lots of serious fossil hunters go to Seatown in Dorset to find fossil ammonites that have fallen to the beach from the cliffs. The cliffs for the most part are composed of Green Ammonite Member which is part of the Charmouth Mudstone Formation laid down in the Jurassic Period. The ammonites that are most commonly found in this type of rock are Aegoceras, Androgynoceras, Liparoceras, and Oistoceras. I haven’t found any decent fossils of the type I could pick up and take home, but there are plenty of fossils and ammonite impressions to be seen lying in pieces of rock on the shingle beach where people with hammers have broken them open. These pictures show some of the specimens that I found on my last visit. I am not sure which species they represent but maybe some local geologist may be able to look at these images and tell me what they are.

Belemnites at Seatown

Belemnite fossil in situ in Belemnite Marl at Seatown, Dorset, England.

The most common fossils at Seatown on the Dorset coast are belemnites. These are bullet-shaped internal hard parts of a type of extinct cephalopod (think cuttlefish, squid and octopus). For a great deal of the length of the beach, the rock strata are hidden by debris falling down from layers above. There are lots of minor mudslides and landslips. However, as you get nearer to the western extremity of the beach, approaching Golden Cap, a continuous kerb-like, harder, and more calcareous stone layer makes an appearance. This is the Belemnite Stone that has been raised to view by a small anticlinal flexure. Below it are many layers of Belemnite Marl that can be seen in cross-section in the vertical face at the base of the cliff; and also extending out horizontally beneath the gravelly beach and exposed at low tide. They alternate light and dark layers. Fossils are abundant with belemnites predominating but ammonites are also common. The huge numbers of belemnites are thought to have resulted from mass die-offs following mating frenzies.

Fossil Worm Tubes at Winspit

Worm tube fossils in a boulder at Winspit in Dorset

Boulders on the back of the quarried ledge at Winspit contain fossils and trace fossils. A recent discovery of mine when I last visited were numerous very small worm tubes, frequently amassed in discrete areas or layers, around and below black chert nodules in the Portland Stone Cherty series rocks. It is difficult to be certain of the identification of these worm tubes (maybe someone can look at these pictures and tell me) but it is known that serpulid worms called Glomerula gordialis are found in this particuar geological and geographical location, and I am assuming for the time being that these are the same species.

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 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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Fossils of the Trigonia clavellata Formation at Ringstead

Jurassic fossil seashells embedded in rock pavement at Ringstead Bay

A gallery of photographs of the fossil-rich limestone pavement at the western end of Ringstead Bay in Dorset, England. The more-or-less horizontal strata form a stepped rock pavement that extends from the cliff at the top of the shore out to a ledge which projects seawards like a peninsula at low tide, between Bran Ledge and Perry Ledge. The fossils are almost all seashells, mostly bivalves, and predominantly Myophorella clavellata. This type of shell used to be known as Trigonia clavellata and it is this old name by which the particular rock beds are known – Trigonia clavellata Formation or Beds; or simply Clavellata Beds.

The Clavellata Formation comprises a series of layers: Sandy Block, Chief Shell Beds, The Clay Band, and Red Beds – each with a different composition but all in the upper part of the larger Corallian Formation belonging to the Upper Jurassic Period dating from over 155 million years ago.

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Fossil wood at Lyme Regis

Fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (1a)

As you walk along Monmouth Beach at Lyme Regis in Dorset, hopping from one boulder to the next, you might see strange brown markings on some of the larger stones. Although not as spectacular as some of the finds recorded from this beach, a closer look will reveal that, in some cases,  these are the fossilised remains of pieces of wood. Their presence demonstrates that the rock was formed in seas that were not too far from land. Some pieces have been discovered with the fossilised remains of marine invertebrate animals still attached to the undersurface – showing that the wood was free floating for a while before settling in the sediments.

In this post, three such boulders of blue-grey Lias limestone containing this dark brown textured petrified wood (or lignite) are shown – together with close-up images of the peculiar texture and pattern of the preserved timber.

For more information about the other incredible types of fossils that have been found on this beach, see Ian West’s website at http://www.soton.ac.uk/~imw/liasfos.htm.

Detail of fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (1b)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (1c)

Fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (2a)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (2b)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (2c)

Fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (3a)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (3b)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (3c)

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Rhynchonella inconstans & Lopha gregarea

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 1

Two Jurassic Coast fossils from Ringstead Bay in Dorset, UK. Although there are superficial similarities between these marine shell fossils, they are in fact the preserved remains of two very different kinds of organism. One is a marine bivalved Mollusc but the other is a Brachiopod. The two intact specimens are stuck together in a stony matrix which contains other smaller and fragmentary shell fossils. The clump of fossils was found in the Ringstead Bay area which is part of  the Jurassic Coast World Heritage Site.

Externally, they both look fairly similar with their possession of two shell valves, and distinct natural sculpturing of sharply angled ribs radiating out over their surface. However, the two valves in a Mollusc represent the coverings for the left and right sides of the animal contained within. In Brachiopods, on the other hand, the shell halves represent the external protection for the dorsal (upper) and ventral (lower) parts of the animal’s body. The animals in the two separate kinds of organism are therefore always orientated in different ways within their shells – although the shells are formed by much the same sort of biological mechanism.

Brachiopods are, strangely enough, related to the minute, colonial animals known as Bryozoa (Sea Mats or Moss Animals). Bryozoa and Brachiopods uniquely possess a structure called the lophophore with which the animals filter food particles from the sea water flowing through their shells. In Brachiopods, the lophore is supported by long internal extensions of the hard shell – and therefore Brachipods can be said to possess an internal as well as and external skeleton, compared with the Bivalves with just an external shell.

We are much more familiar with the many species of bivalve Molluscs (like clams, cockles, and mussels) than with Brachiopods. Brachiopods still exist today but there are only a couple of hundred species of these ‘lamp shells’ now remaining – compared with the many thousands of now extinct species from the long distant past.

The Brachiopod is Rhynchonella inconstans – seen on the left in the picture above and from other perspectives in the photographs below. It has other synonyms. It is also called Torquirhynchia inconstans and Rhactorhynchia inconstans. It is found in Lower Kimmeridge Clay from Upper Jurassic deposits (beneath the Kimmeridge Clays from which Liostrea delta is frequently recovered). The Jurassic Period lasted from about 195 to 136 million years ago.

The most easily recognised feature of R. inconstans is the way that one half of the shell is stepped down from the other in a peculiar form of asymmetry which is thought to be an adaptation to life in tidal environments.

The bivalved mollusc is probably Lopha gregarea – seen on the right in the top picture (with other views in the lower photographs). It is also known as Ostrea gregarea, Ostrea gregaria, and Alectryonia gregarea. I say ‘probably’ because it would perhaps be more likely to be Lopha marshi from its association with R. inconstans but it just does not look like it to me. Maybe someone out there could advise me?

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 2

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 3

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 4

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Ammonites at Chapmans Pool

Ammonite fossil: Pavlovia rotunda ammonite fossil in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, exposed on the flat beds of rock beneath the shingle at Chapmans Pool. Dorset, UK, on the Jurassic Coast World Heritage Site (1)

There is one type of fossil that you are more likely to find than any other at Chapmans Pool in Dorset, UK – and that is the ammonite – possibly Pavlovia rotunda or maybe even Pavlovia pallasiodes. These are fossil cephalopods dating from the Jurassic Period. The shelly remains of many thousands of these were buried in sediments that were later compressed into rocks – the so called Rotunda Shales – which are part of the series of marls, shales and clays known as the Kimmeridge Clay that was formed during the Upper Jurassic Period over 135 million years ago. 

The fossil shells (which are generally fragile and flakey) occur at the western side of the bay, at the foot of Houns Tout. Most frequently, they are found where they have been squashed between the rock layers or strata. It would be very difficult to remove the fossils from the bedrock – although the ammonite impressions are sometimes found in large pebbles or cobbles. Usually, these ammonites are seen on the surface of the shale rock pavement that extends seawards from the base of the cliff, often temporarily concealed by a layer of gravel or pebbles. They can also be found on large sharp pieces of shale that have fallen down from the cliff in piles of scree. The third place they can be seen is embedded in the vertical cliff face where they are usually visible in cross-section in a distinct sloping band that traverses the lower part of the cliff.

Chapmans Pool ammonite: Pavlovia rotunda ammonite fossil in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, exposed on the flat beds of rock beneath the shingle at Chapmans Pool. Dorset, UK, on the Jurassic Coast World Heritage Site (2)

Dorset ammonite: Pavlovia rotunda ammonite fossil in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, exposed on the flat beds of rock beneath the shingle at Chapmans Pool. Dorset, UK, on the Jurassic Coast World Heritage Site (3)

Fossil ammonite cross-section: Pavlovia rotunda ammonite fossil in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, exposed in cross-section within the vertical cliff-face at Chapmans Pool. Dorset, UK, on the Jurassic Coast World Heritage Site (4)

Ammonite fossil Pavlovia rotunda: Pavlovia rotunda ammonite fossil in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, compressed fossil found on a piece of shale in a pile of scree at the base of the cliff, at Chapmans Pool, Dorset, UK, on the Jurassic Coast World Heritage Site (5)

Chapmans Pool fossil: Pavlovia rotunda ammonite fossil in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, compressed fossil found on a piece of shale in a pile of scree at the base of the cliff, at Chapmans Pool, Dorset, UK, on the Jurassic Coast World Heritage Site (6)

Small ammonite fossils on the beach at Chapmans Pool: Pavlovia rotunda ammonite fossil in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, compressed fossils found on flat bed of shale extending seawards from the base of the cliff, at Chapmans Pool, Dorset, UK, on the Jurassic Coast World Heritage Site (7)

Chapmans Pool ammonite: Compressed fossil ammonite, Pavlovia rotunda, in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, found on flat bed of shale extending seawards from the base of the cliff, at Chapmans Pool, Dorset, UK, on the Jurassic Coast World Heritage Site (8)

Compressed fossil ammonite, Pavlovia rotunda, in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, found on flat bed of shale extending seawards from the base of the cliff, at Chapmans Pool, Dorset, UK, on the Jurassic Coast World Heritage Site (9)

Ammonite fossil at Chapmans Pool: Compressed fossil ammonite, Pavlovia rotunda, in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, found on flat bed of shale extending seawards from the base of the cliff, at Chapmans Pool, Dorset, UK, on the Jurassic Coast World Heritage Site (10)

Jurassic Coast fossil ammonite: Compressed fossil ammonite, Pavlovia rotunda, in situ in the Rotunda Shales of the Kimmeridge Clay Series of Jurassic rock strata, found on flat bed of shale extending seawards from the base of the cliff, at Chapmans Pool, Dorset, UK, on the Jurassic Coast World Heritage Site (11) 

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