Fossil Oysters 2

Fossil Oysters - Deltoideum delta from Dorset's jurassic Coast

This is the second in a series of Postings about fossil oysters. In this Post there are detailed photographs, taken from all angles, of a fossil oyster found with the two halves still in position, on the beach at Ringstead Bay, Dorset, UK. It’s a specimen of Deltoideum delta, and in common with many species of present day oysters such as Ostrea edulis, the shells of this extinct species can exhibit a wide variety of variations – in shape, size, and many other characteristics.  The most noticeable feature of this particular specimen is the attachment of six other oyster shells – four to the outer surface of the left valve and two to the exterior of the right valve.

The left or lower valve would have been the original surface of attachment for the spat oyster as it settled after a brief larval life. Old oyster shells are a preferred settlement substrate. It is not possible to say whether the oyster shells on which it settled were empty or still occupied by the living animals. Eventually the oyster would have overgrown and incorporated the other oyster shells as it developed.

On the other hand, the oyster shells attached to the outer surface of the right or uppermost valve would have settled on it only when the oyster had itself reached maturity.

Additionally, the ligament – which is the horny proteinaceous (conchyiolin) structure that connects the two halves of the oyster at the dorsal margins of the shells in life – seems to have been fossilised along with the calcareous shell valves. The shells have a brittle texture and an almost porcelain-like appearance. There are multiple cracks and fractures, as is common with the oyster fossils from this locality, and a certain flattening of the valves seems to have occurred so that there is now very little space indeed between the the two halves of the oyster.

[At the bottom of this posting is a slide show of the oyster shells photographed with a reference scale to indicate size. These pictures have a minimum of labelling. If anyone should want more information about these fossil oysters, please do not hesitate to get in touch.]

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

Fossil Oysters - Deltoideum delta

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Posted in FOSSIL OYSTER SHELLS, Nature, OYSTER SHELL VARIATIONS, PALAEONTOLOGY | Tagged , , , , , , , , , , , , , , , , , , | 3 Comments

Wild Oysters on the Queensland Coast Part 2

Jessica's Nature Blog

Row of wild oysters growing on barnacle-covered rock.

The oysters that I found on the rocks at the northern end of Three Mile Beach in Port Douglas were so different from the ones I had seen at Cape Tribulation that I wondered if they were oysters at all.

The identification of Rock Oysters of the Saccostrea Group in the Indo-West Pacific is a fairly hot topic and some very interesting work was completed a few years ago to try and sort out what is what. See the work of Katherine Lam and Brian Morton.

On the basis of shell morphology, I think the oysters illustrated in this post are Saccostrea mordax which are distinct from the other Saccostrea species in having regularly-spaced grooves radiating from the umbone to the ventral margin of the right valve, the triangular shell shape, and finely plicated valve margin (with regular m-shapes). The left valve is completely attached as in the other…

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Wild Oysters on the Queensland Coast Part 1

Jessica's Nature Blog

Rock Oysters growing at Cape Tribulation, Queensland

I have written a lot about the natural variations in oyster shells belonging to the British Native, Flat, or European Oyster, Ostrea edulis Linnaeus. However variable these shells may be, it is always possible to identify the shells as belonging to that species, and to distinguish them from other species.

In Australia and the Far East, the oysters that grow wild and naturally on the tropical shores include several species of Saccostrea which can be difficult to differentiate from one another because of the diversity of their outward appearance. The morphologies of Saccostrea glomerata, Saccostrea cucullata, Saccostrea kegaki, and Saccostrea mordax, are so variable and overlapping that is not always possible to tell them apart by eye. As with so many other groups of organism currently being investigated (marine algae for example), it is only by use of mitochondrial-DNA analysis that true identities and relationships can…

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Black Oyster Shells on the Beach 3

Black-stained oyster shell right valve inner surface (Ostrea edulis Linnaeus)

A most unusual form of very thick-shelled burial-blackened oyster shell right valve. It looks almost like a set of nesting bowls. It is clearly an old shell – it lived a long time – but it failed to grow much in diameter. All the energies were diverted into increasing the thickness of the shell rather than widening it. This kind of oyster is termed a ‘stunter’. When found in catches from commercially-fished natural or wild oyster beds, this undersized but mature oyster would typically, be returned to the sea bed to grow on. However, they never do achieve the minimum legal size to catch and market. This means that they can breed and potentially produce more specimens of ‘stunters’ should this growth anomaly be inheritable.

Black-stained oyster shell right valve outer surface (Ostrea edulis Linnaeus)

Black-stained oyster shell right valve inner surface (Ostrea edulis Linnaeus)

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Posted in MODERN OYSTER SHELLS, OYSTER SHELL VARIATIONS | Tagged , , , , , , , , , , | Leave a comment

Black Oyster Shells on the Beach 2

Black staining from burial is one of a multiplicity of variations in the appearance of an oyster shell.

These Black Oyster Postings are about the multiplicity of variations in shells of the British Native Oyster, also known as the Flat or European Oyster, Ostrea edulis Linnaeus. Sometimes, as in this Posting, the shells are photographed just as found, in situ, on the beach. In other posts, the shells have been collected for more detailed examination and have been photographed indoors. By understanding the range and variety in macroscopic differences, and the causes of the variations, it is possible to interpret more accurately the meaning of variations in archaeological and palaeontological specimens of oyster shell. These posts can therefore be considered as a database for oyster variation – and a starting point for discussions and understanding.

Black staining from burial is one of a multiplicity of variations in the appearance of an oyster shell.

Black staining from burial is one of a multiplicity of variations in the appearance of an oyster shell.

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Posted in MODERN OYSTER SHELLS, Nature, OYSTER SHELL VARIATIONS | Tagged , , , , , , , , , | Leave a comment

Black Oyster Shells on the Beach 1

Right valve outer surface Flat European Oyster shell, Ostrea edulis Linnaeus, as found on the beach with black staining colouration.

An example of a black-stained oyster shell, Native or Flat European Oyster (Ostrea edulis Linnaeus), washed ashore at Rhossili Bay on the Gower Peninsula. The colour is due to prolonged burial in anoxic sediments.

For more information about this phenomenon see:

Colour variation in oyster shells 1

and Black Oysters at Rhossili Bay.

Right valve inner surface Flat European Oyster shell, Ostrea edulis Linnaeus, washed up on the beach with black staining colouration.

Right valve outer surface Flat European Oyster shell, Ostrea edulis Linnaeus, as found on the beach with black staining colouration.

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Fossil Oysters 1

Fossil oyster shell, Deltoideum (Liostrea) delta, paired valves with right valve uppermost.

This Post features one of many fossil oysters that I have picked up on the beach over the years because they were in some way unusual and because of my interest in the variation in oyster shells from both present day populations and from samples of oyster shell from archaeological excavations.

I am particularly interested in the range of sizes, shapes, and the evidence for both encrusting or infesting epibiont organisms. These features reflect the variability in a species which can be a response to the immediate natural environment of the oyster during life and the effect of taphonomic processes (things that happen to the shell after the death of the animal) on the appearance of the shell, whether it be fossil, archaeological or recent in origin.

The paired shells of the oyster in this specimen from Ringstead Bay in Dorset, UK, were still together in situ. There was very little internal space between the right and left valve – as is typical for the species which is Deltoideum (Liostrea) delta. It is a common find in the Kimmeridge Clay of the Jurassic Period that outcrops on the beach. The left valve had another broken oyster shell attached to it externally. This would have been the substrate on which the spat oyster had originally settled.

On opening and separating the two valves, it was possible to examine the hinge or ligament area. The ‘scar’ where the flexible ligament had conected the two shells showed many growth lines indicating some age had been achieved prior to death. Petrified material attached to the ligament scar, fragments on both valves but more noticeable on the left valve, could be the fossilised remains of the ligament itself.

Fossil oyster shell, Deltoideum (Liostrea) delta, paired valves with left valve uppermost.

Fossil oyster shell, Deltoideum (Liostrea) delta, paired shells separated to display the outer surfaces of the right and left valves.

Fossil oyster shell, Deltoideum (Liostrea) delta, paired valves with left and right valves separated to show the inner surfaces.

Fossil oyster shell, Deltoideum (Liostrea) delta, inner surface of left valve displayed.

Fossil oyster shell, Deltoideum (Liostrea) delta, inner surface of left valve displayed with scale.

Fossil oyster shell, Deltoideum (Liostrea) delta, inner surface of left valve showing detail of the ligament scar with growth lines and ?fossilised remnants of ligament itself.

Fossil oyster shell, Deltoideum (Liostrea) delta, inner surface of right valve displayed.

Fossil oyster shell, Deltoideum (Liostrea) delta, inner surface of right valve displayed with scale.

Fossil oyster shell, Deltoideum (Liostrea) delta, inner surface of right valve showing detail of the ligament scar with growth lines and ?fossilised remnants of ligament itself.

Fossil oyster shell, Deltoideum (Liostrea) delta, side view of paired valves with right valve uppermost.

Fossil oyster shell, Deltoideum (Liostrea) delta, side view of paired valves with left valve uppermost.

Fossil oyster shell, Deltoideum (Liostrea) delta, close-up detail of surface texture right valve

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Posted in FOSSIL OYSTER SHELLS, Nature, PALAEONTOLOGY | Tagged , , , , , , , , , , , , , , , , | 3 Comments