Description
Genuine Graphoceras stigmosum Ammonite Fossil
This carefully chosen fossil is a genuine Graphoceras stigmosum ammonite from Cock Road Farm near Beaminster in Dorset, UK. It originates from the Lower Inferior Oolite Formation and dates from the Lower Bajocian Stage of the Middle Jurassic, specifically the Discites Zone.
This British ammonite represents an extinct marine cephalopod that inhabited the seas covering southern England approximately 170 million years ago. Its recognisable coiling, graphoceratid ornamentation and precisely recorded stratigraphic position make it an appealing specimen for collectors of Dorset fossils, British ammonites and Middle Jurassic natural history.
The photograph shows the actual fossil specimen you will receive, allowing its individual shape, ribbing, preservation, colour, matrix and natural geological characteristics to be examined before purchase. Full sizing and proportions can be seen in the accompanying photograph.
This authentic fossil includes a generic Certificate of Authenticity card with a lifetime guarantee.
Fossil Classification and Geological Details
Scientific name: Graphoceras stigmosum
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Hammatoceratoidea
Family: Graphoceratidae
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Lower Bajocian
Biozone: Discites Zone
Rock unit: Lower Inferior Oolite Formation
Locality: Cock Road Farm, Beaminster, Dorset, UK
Graphoceras belongs to the Graphoceratidae, an important family of compressed, keeled ammonites that became especially diverse during the Aalenian and earliest Bajocian. Their rapid evolutionary development makes graphoceratid ammonites valuable for identifying and correlating individual layers of Middle Jurassic rock.
The genus was established by the British palaeontologist Samuel S. Buckman, whose extensive studies of Jurassic ammonites and their stratigraphic distribution helped develop the detailed system of ammonite biohorizons used in British geology.
Graphoceras Shell Shape and Ornamentation
Graphoceras stigmosum possessed a planispirally coiled shell, meaning that its whorls developed within a single flat plane around a central umbilicus. The shell is generally compressed from side to side, producing a relatively slender and streamlined profile.
Successive whorls overlap much of the earlier growth, while retaining a visible central umbilical region. The flanks are ornamented with curved or gently sinuous ribs that extend from the inner part of the whorl towards the outer margin. These ribs may vary in strength, spacing and curvature as the ammonite increases in size.
The outer edge of an ammonite shell is known as the venter. In Graphoceras, this region commonly carries a pronounced median keel forming a narrow raised ridge around the shell’s circumference. The combination of compressed whorls, flexuous ribbing and a keeled venter created the elegant shell profile associated with graphoceratid ammonites.
Shell form and ornamentation could change through growth. Inner whorls may display finer, more closely spaced sculpture, while later portions can develop broader or more widely separated ribs. These variations provide palaeontologists with useful characteristics for distinguishing related species.
The exact detail visible on this fossil depends upon its individual preservation. Natural fractures, incomplete sections, mineral staining, attached matrix and worn areas are authentic geological features acquired during burial, fossilisation and eventual exposure.
Lower Bajocian Discites Zone
The Bajocian is the second stage of the Middle Jurassic, following the Aalenian and preceding the Bathonian. The Discites Zone forms the earliest major ammonite zone of the Lower Bajocian and marks an important transition in the development of European ammonite faunas.
The boundary between the Aalenian and Bajocian is recognised through changes among graphoceratid ammonites. Late Aalenian species of Graphoceras were succeeded by closely related early Bajocian forms, including ammonites traditionally classified within Hyperlioceras. This evolutionary succession enables palaeontologists to recognise the base of the Bajocian with considerable precision.
A biozone is an interval of sedimentary rock identified through the occurrence of a characteristic fossil species or assemblage. Because ammonites evolved rapidly and were widely distributed through Jurassic seas, they are among the most effective fossils for dividing marine rocks into detailed chronological units.
The Discites Zone attribution therefore gives this specimen a more precise geological context than the broader Middle Jurassic designation alone. It places the fossil close to the beginning of Bajocian time and within an important phase of graphoceratid evolution.
Inferior Oolite Geology of Dorset
The Inferior Oolite Group is a varied succession of Middle Jurassic marine rocks exposed across Dorset, Somerset and neighbouring counties. Despite its name, not every layer consists of true oolitic limestone. The succession includes fossiliferous limestone, marl, sandstone, iron-rich beds, shell debris and locally condensed deposits.
During the Lower Bajocian, southern Britain lay beneath a warm epicontinental sea connected to marine basins across north-western Europe. Britain consisted of islands, shallow platforms and submerged marine channels rather than the continuous landmass visible today.
Carbonate grains, mud, sand and biological debris accumulated across the seabed. Some beds formed slowly and became condensed, allowing durable fossils and mineral grains to become concentrated within relatively thin layers. Periods of erosion and reworking could expose older sediment before it was covered by younger deposits.
The surrounding marine ecosystem supported ammonites alongside belemnites, nautiloids, bivalves, brachiopods, gastropods, echinoids and crinoids. Fish and marine reptiles occupied higher levels within the food web, while worms and other invertebrates lived within the seabed sediment.
Cock Road Farm Fossil Locality
Cock Road Farm near Beaminster lies within an area of west Dorset celebrated for its Middle Jurassic fossil localities. Inland exposures and former quarry workings around Beaminster have revealed parts of the Inferior Oolite succession that may be difficult to examine along the coast.
Fossils from accurately recorded beds at localities such as Cock Road Farm are especially useful because their geological context can be connected with established ammonite zones. This permits comparison with other classic Dorset sites, including Horn Park, Bradford Abbas and Burton Bradstock.
The rocks preserve evidence of changing sea levels, sedimentation rates and marine conditions during the early Bajocian. Their ammonite assemblages provide a detailed record of evolutionary change and enable individual fossil-bearing horizons to be matched across southern England.
Life of a Middle Jurassic Ammonite
Ammonites were extinct marine molluscs related to modern squid, cuttlefish, octopuses and nautiluses. The living animal occupied the final and largest section of its shell, known as the body chamber. Its head, eyes and tentacle-bearing soft body projected through the aperture.
The older coiled section was divided into smaller chambers by curved internal walls called septa. A narrow tube known as the siphuncle connected these chambers and allowed the ammonite to regulate their liquid and gas content. This system provided buoyancy control and helped the animal maintain its position within the water column.
Movement was achieved using jet propulsion. Water was drawn into the mantle cavity and expelled through a muscular funnel, pushing the animal in the opposite direction. The compressed and keeled shell of Graphoceras stigmosum provided a streamlined profile suited to active movement through open marine water.
Tentacles surrounding the mouth were probably used to capture small crustaceans, planktonic organisms and other prey. As the ammonite grew, new shell material was added around the aperture and additional chambers formed behind the living animal.
Natural Fossilisation and Authentic Preservation
After death, the ammonite shell settled onto the Lower Bajocian seabed. Its soft tissues decayed while sediment accumulated around and within the empty shell. Burial helped protect the remains from complete destruction by currents, scavengers and chemical dissolution.
Over millions of years, the surrounding sediment became compacted and cemented into rock. Mineral-rich groundwater could recrystallise, replace or dissolve the original aragonitic shell. Sediment filling the chambers hardened into an internal mould capable of preserving the coiled form and impressions of the original ornamentation.
Natural fractures, matrix attachments, mineral deposits, surface wear and incomplete areas record the specimen’s individual geological history. The photograph shows the exact carefully selected Graphoceras stigmosum ammonite you will receive. Full sizing is shown in the photo. A generic Certificate of Authenticity card with a lifetime guarantee is included.
Reviews
There are no reviews yet.