Description
Genuine Plectrothyris fimbria Brachiopod Fossil
This carefully chosen fossil is a genuine specimen of Plectrothyris fimbria, an extinct marine brachiopod from the Lower Inferior Oolite of Dorset, UK. Dating from the Bajocian Stage of the Middle Jurassic, it represents an animal that lived in the warm seas covering southern Britain approximately 168 to 170 million years ago.
The listing photographs show the actual fossil specimen you will receive. This is not a stock image or a representative example. Please examine the photographs carefully for the fossil’s precise shape, natural colour, shell preservation, surface ornamentation, condition and any remaining limestone matrix. For full sizing and scale, please refer to the photographs.
Fossil Identification and Geological Details
Scientific name: Plectrothyris fimbria
Fossil type: Articulate brachiopod
Kingdom: Animalia
Phylum: Brachiopoda
Class: Rhynchonellata
Order: Rhynchonellida
Genus: Plectrothyris
Species: Plectrothyris fimbria
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Bajocian
Stratigraphic unit: Lower Inferior Oolite
Locality: Dorset, United Kingdom
Brachiopods are marine animals enclosed within two mineralised valves. Although they may superficially resemble bivalve molluscs, they belong to a completely separate animal phylum and possess a very different internal anatomy, shell symmetry and mode of life.
Plectrothyris fimbria Shell Morphology
Plectrothyris fimbria is recognised as a rhynchonellid brachiopod with a compact, strongly convex shell. Its two valves are unequal in shape, with one generally more inflated than the other. Unlike most bivalves, where each individual valve is commonly asymmetrical, brachiopod valves are usually bilaterally symmetrical along a line running from the beak to the front margin.
The shell commonly displays strong radiating ribs extending from the beak towards the anterior edge. These ribs may become more pronounced as the animal grew and can create a distinctly folded or corrugated appearance around the shell margin. The species name fimbria refers to a fringe-like quality, reflecting the strongly ornamented character associated with the shell.
A fold may be developed on one valve and a corresponding sulcus or groove on the opposing valve. When the shell closed, the ribbed margins fitted together closely, helping protect the soft body while allowing seawater to enter the shell cavity for feeding and respiration.
Depending on preservation and orientation, the specimen may display the beak, radiating costae, growth lines, valve curvature, fold-and-sulcus arrangement and parts of the interlocking anterior margin. Natural matrix attachment, worn ribs, minor chips, fractures, mineral staining and incomplete areas are normal characteristics of genuine Jurassic fossils.
Brachiopod Anatomy and Lifestyle
During life, Plectrothyris fimbria occupied the seabed and fed by filtering microscopic organic particles from seawater. Inside the shell was a specialised feeding organ called a lophophore, formed from coiled or curved structures bearing numerous tentacles. These tentacles generated water currents and captured suspended food.
Many articulate brachiopods were anchored to the substrate by a muscular stalk called a pedicle, which emerged through an opening near the shell’s beak. This attachment helped stabilise the animal on shell debris, firm sediment or other hard surfaces.
The shell valves were joined by a hinge containing interlocking teeth and sockets. Muscles opened and closed the shell, allowing the animal to regulate water flow and withdraw its feeding structures when disturbed. Its ribbed, strongly convex shell provided structural strength and helped it remain stable in an active shallow-marine environment.
Lower Inferior Oolite Geology
The Inferior Oolite is a major Middle Jurassic limestone succession exposed across parts of southern England, particularly in Dorset, Somerset and the Cotswolds. Despite its traditional name, “inferior” refers to its stratigraphic position beneath the younger Great Oolite rather than to the quality or importance of the rock.
The Lower Inferior Oolite was deposited in a warm, shallow sea during the Bajocian. Its rocks include fossil-rich limestones, iron-rich beds, shell debris, oolitic layers and locally condensed horizons. The term oolite describes limestone containing numerous small, rounded carbonate grains called ooids.
Ooids formed when calcium carbonate accumulated in concentric layers around tiny shell fragments, mineral grains or other nuclei. Constant movement by waves and currents rolled these particles across the seabed, producing their rounded form. Their presence indicates warm, agitated, relatively shallow water where carbonate precipitation was active.
The Inferior Oolite succession often varies considerably over short distances. Sedimentation was influenced by changing sea level, currents, seabed topography, erosion and periods when little new sediment accumulated. Some horizons contain dense concentrations of fossils representing communities that lived on the seabed or shells reworked by currents and storms.
Bajocian Marine Environment of Dorset
During the Bajocian Stage, Dorset formed part of a broad marine shelf bordering the ancient land areas of Jurassic Europe. The climate was warm, and the shallow sea supported a diverse ecosystem of brachiopods, bivalves, gastropods, ammonites, belemnites, echinoids, corals, bryozoans and crinoids.
Brachiopods such as Plectrothyris fimbria lived among carbonate sediment and accumulated shell material on the seafloor. Clear marine water provided the circulating currents required for filter feeding, while firm patches of seabed offered suitable attachment surfaces.
Storms and stronger currents could disturb these communities, transporting shells and mixing them with broken skeletal debris. Quieter intervals allowed fine carbonate sediment to settle around the remains. Repeated burial, erosion and redeposition contributed to the fossil-rich character of many Inferior Oolite beds.
Fossilisation and Preservation
After the brachiopod died, its soft tissues decayed while the mineralised shell remained on the seabed. The two valves could stay joined or become separated through decay, wave action or disturbance by other organisms.
Carbonate sediment accumulated around and inside the shell. Continued burial compacted the seabed and transformed the sediment into limestone. Mineral-rich groundwater could fill internal spaces, strengthen the shell, recrystallise original material or produce a natural internal mould.
This fossil preserves evidence of both the original Jurassic animal and the geological processes that affected it over millions of years. Variations in shell completeness, exposed ribbing, matrix coverage and mineral colour give every specimen its own individual appearance.
Collectable Dorset Jurassic Fossil
This genuine Plectrothyris fimbria fossil combines a recognised Middle Jurassic brachiopod species with the classic Lower Inferior Oolite geology of Dorset. It would make an excellent addition to a British fossil collection, Jurassic display, brachiopod collection, geological cabinet or educational resource.
Its strongly ribbed shell and compact form make it an attractive specimen for collectors interested in prehistoric marine animals. It is also suitable as a natural-history gift for a fossil enthusiast, geology student, teacher or anyone interested in Britain’s Jurassic past.
Actual Specimen and Full Sizing
The fossil shown in the listing photographs is the exact specimen you will receive. It has been carefully selected for its natural appearance, identifiable brachiopod features and geological interest.
Please refer to the photographs for full sizing. The images provide the most accurate representation of the specimen’s dimensions, shell condition, visible morphology, preservation and remaining Lower Inferior Oolite matrix.
Certificate of Authenticity and Lifetime Guarantee
This fossil is a genuine natural specimen and includes a generic Certificate of Authenticity card with a lifetime authenticity guarantee. The certificate confirms that the item is supplied as a real fossil rather than a modern cast, replica or artificial reproduction.
The specimen will be carefully and securely packaged to help protect its fossil surfaces and surrounding matrix during transit.
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