Description
Genuine Garantiana cf. subgaranti Ammonite Fossil
This carefully selected specimen is a genuine Garantiana (Garantiana) cf. subgaranti ammonite fossil from the Inferior Oolite Group at Horn Park Quarry, near Beaminster in Dorset, England. It comes from the Subfurcatum Zone of the Bajocian Stage and represents an extinct marine cephalopod that inhabited the shallow seas covering southern Britain during the Middle Jurassic.
The identification includes the scientific abbreviation “cf.”, meaning “compare with”. This indicates that the fossil displays characteristics closely comparable to Garantiana subgaranti, while retaining an appropriate degree of caution over definitive species-level attribution. This terminology is widely used for fossils that strongly resemble a named species but do not preserve every feature required for completely certain identification.
The photograph shows the actual fossil you will receive rather than a stock image or representative example. Its individual shell form, ribbing, preservation, matrix and natural surface details can therefore be examined before purchase. Full sizing is shown in the accompanying photograph.
Fossil Classification and Geological Details
Scientific identification: Garantiana (Garantiana) cf. subgaranti
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Stephanoceratoidea
Family: Stephanoceratidae
Subfamily: Garantianinae
Genus: Garantiana
Subgenus: Garantiana
Comparable species: Garantiana subgaranti
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Bajocian
Ammonite biozone: Subfurcatum Zone
Rock unit: Inferior Oolite Group
Locality: Horn Park Quarry, Beaminster, Dorset, UK
The Subfurcatum Zone belongs to the later part of the Bajocian ammonite succession. The Inferior Oolite Group in Dorset spans strata of Aalenian to earliest Bathonian age and comprises a varied sequence of marine limestones, including bioclastic, peloidal, sandy, ferruginous and locally ooidal beds.
Strongly Ribbed Garantiana Morphology
Garantiana ammonites are recognised for their prominently ornamented, generally evolute shells. Evolute coiling means that successive whorls overlap relatively little, leaving the earlier whorls visible within a broad central umbilicus. This open spiral form gives the shell a strongly defined and visually appealing appearance.
Primary ribs typically begin near the umbilical margin and extend across the flanks. Many divide into two secondary ribs around the middle or outer part of the whorl, a feature known as bifurcation. These secondary ribs continue towards and across the ventral region. Depending on growth stage and preservation, the points of division may be slightly raised or accentuated.
The ribs can be straight, gently curved or inclined forwards as they approach the outer edge of the shell. Their strength, spacing and direction may change during growth, allowing the inner juvenile whorls to differ subtly from the mature outer section. Garantiana is characteristically strongly ribbed, with exposed inner whorls and bifurcating ornament among its most recognisable features.
Natural variations in rib definition, completeness, mineralisation and attached matrix are authentic aspects of the fossil. They reflect the animal’s original shell development together with the effects of burial, compression, mineral replacement and later weathering.
Chambered Shell and Ammonite Anatomy
The living ammonite occupied the final and largest section of the shell, called the body chamber. The older inner portion was divided into a sequence of sealed chambers by curved internal walls known as septa.
A narrow tube called the siphuncle connected these chambers. By regulating the balance of liquid and gas within them, the animal could control its buoyancy and maintain an effective position within the water column.
Where the septa joined the outer shell wall, they formed intricate suture lines. These may become visible where the original shell surface has weathered away or has been exposed through preparation. The branching lobes and saddles of ammonitic sutures are valuable features in the classification of extinct cephalopods.
The soft-bodied animal was related to other cephalopods and probably possessed well-developed eyes, grasping appendages and a muscular funnel used for jet-propelled movement. Its chambered external shell combined protection with a specialised buoyancy system.
Bajocian Subfurcatum Zone
The Bajocian is the second stage of the Middle Jurassic and occurred approximately 171 to 168 million years ago. It followed the Aalenian and preceded the Bathonian, during a period when ammonites were abundant and rapidly diversifying throughout European marine basins.
The Subfurcatum Zone is an established ammonite biozone within the Bajocian succession. Biozones are intervals of rock recognised by characteristic ammonite assemblages and the evolutionary appearance or disappearance of particular taxa.
Because ammonites evolved rapidly and occupied geographically extensive seas, they are among the most useful index fossils for Jurassic geology. Their successive faunas enable geologists to compare beds from different quarries and coastal exposures with considerable precision.
This specimen therefore preserves more than the remains of an individual marine animal. Its recorded association with the Subfurcatum Zone provides a valuable connection to a specific interval within the Middle Jurassic rock record.
Inferior Oolite Group Geology
The Inferior Oolite Group forms a prominent belt of Middle Jurassic sedimentary rock extending through parts of southern and central England. In Dorset and Somerset, it consists of a condensed and highly varied marine succession containing limestone, iron-rich beds, shell debris, peloids, sand and locally abundant ooids.
Ooids are small, rounded carbonate grains formed as layers of calcium carbonate accumulated around tiny particles in agitated shallow water. Although not every bed is strongly oolitic, these grains contributed to the historical name “Inferior Oolite”, distinguishing the unit from younger oolitic limestones higher in the Jurassic succession.
The changing limestone types, erosion surfaces, condensed horizons and fossil assemblages record repeated variations in water depth, sediment supply, current activity and seabed conditions. Ammonites preserved within these rocks are especially important for establishing the sequence and relative ages of individual beds.
Horn Park Quarry, Beaminster
Horn Park Quarry is a nationally important geological locality near Beaminster in west Dorset. Its preserved quarry faces expose an unusually detailed Inferior Oolite succession and have contributed greatly to the study of British Aalenian, Bajocian and earliest Bathonian stratigraphy.
The quarry was worked intermittently for limestone and was investigated by generations of geologists and fossil specialists. Its beds contain diverse ammonite faunas that allow the succession to be divided into remarkably precise biological intervals. Horn Park is now protected for its geological importance, making well-provenanced fossils from the locality particularly desirable as historical and scientific specimens.
During the Bajocian, the area now forming Dorset lay beneath a warm, shallow sea. Limestone, shell fragments, carbonate grains and other marine sediment accumulated across the seabed. The waters supported ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids, crinoids, fish and other marine organisms.
Fossilisation in a Jurassic Sea
After death, the ammonite shell may have drifted temporarily or descended directly to the seafloor. Burial beneath marine sediment helped protect it from complete destruction by currents, scavengers and chemical dissolution.
As additional layers accumulated, the sediment became compacted and cemented into limestone. Mineral-rich groundwater could fill the shell chambers, replace original shell material or produce an internal mould retaining the ammonite’s form. Later geological uplift, quarrying and careful preparation exposed the fossil preserved today.
Any natural fractures, worn areas, incomplete margins, colour variations or attached limestone are part of this geological process. They distinguish the specimen as an authentic fossil rather than a manufactured reproduction.
Actual Specimen and Certificate of Authenticity
This is a genuine fossil specimen and not a replica, cast or modern reproduction. It has been individually chosen for its natural character, geological interest and collectable appearance.
The listing photograph shows the exact Garantiana (Garantiana) cf. subgaranti ammonite fossil that will be supplied. Full sizing is provided in the photograph so that its dimensions can be assessed before purchase.
The fossil includes a generic Certificate of Authenticity card carrying a lifetime guarantee of authenticity. It is an excellent addition to a British ammonite collection, Middle Jurassic display, Dorset fossil collection, geological teaching set or natural-history cabinet.
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