Description
Genuine Garantiana aff. transiti Ammonite Fossil
This carefully selected specimen is a genuine Garantiana (Garantiana) aff. transiti ammonite fossil from the Inferior Oolite Group at Horn Park Quarry, near Beaminster in Dorset, England, UK. It originates from the Lower Bajocian Subfurcatum Zone and represents an extinct marine cephalopod that inhabited the warm, shallow seas covering southern Britain during the Middle Jurassic.
The scientific abbreviation “aff.” comes from the Latin affinis, meaning related or allied to. In palaeontological identification, it indicates that the fossil has a close morphological affinity with Garantiana transiti but differs slightly from typical examples or does not preserve every feature needed for an entirely definitive species-level assignment. This recognised scientific notation provides a precise and appropriately cautious identification.
The photograph shows the actual ammonite fossil you will receive rather than a stock image or representative example. Its individual coiling, ribbing, limestone matrix, colour and natural preservation can therefore be examined before purchase. Full sizing is shown in the accompanying photograph.
Fossil Classification and Geological Information
Scientific identification: Garantiana (Garantiana) aff. transiti
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Stephanoceratoidea
Family: Stephanoceratidae
Subfamily: Garantianinae
Genus: Garantiana
Subgenus: Garantiana
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Bajocian
Stratigraphic interval: Lower Bajocian
Ammonite biozone: Subfurcatum Zone
Rock unit: Inferior Oolite Group
Locality: Horn Park Quarry, Beaminster, Dorset, UK
Garantiana is a distinctive Middle Jurassic ammonite genus recognised for its comparatively open coiling and strongly developed ribbing. Members of the genus form an important part of the Bajocian ammonite succession and help palaeontologists distinguish and correlate individual fossil-bearing horizons within European marine rocks.
Strongly Ribbed Garantiana Morphology
The shell grew in a flat spiral known as planispiral coiling. Each new whorl increased in diameter as the animal matured, forming the familiar circular outline associated with ammonites. The central exposed region is called the umbilicus, the sides of the shell are known as the flanks, and the outer margin is called the venter.
Garantiana ammonites usually possess an evolute or moderately evolute shell. In evolute coiling, successive whorls overlap only slightly, leaving much of the earlier spiral visible around a relatively broad umbilicus. This open form allows the development of the inner whorls and their ornamentation to remain clearly displayed.
Strong primary ribs generally arise close to the umbilical margin and extend across the flanks. These ribs commonly divide into two secondary branches towards the outer part of the whorl, a feature known as bifurcation. Occasional additional or intercalated ribs may appear between the principal rib bundles.
The branching points can be slightly raised, while the secondary ribs may curve or incline forwards as they approach the venter. A shallow ventral groove or interruption may separate the ribs where they reach the shell’s outer margin. Rib strength, spacing and direction can change throughout growth, meaning that the juvenile inner whorls may differ from the mature outer section.
This pronounced ornamentation gives Garantiana fossils their characteristic textured appearance. Natural variations in rib definition, shell completeness and attached matrix are authentic features created by the animal’s growth and its subsequent fossilisation.
Chambered Shell and Ammonite Anatomy
The living ammonite occupied the final and largest section of its shell, called the body chamber. The older coiled portion was divided into numerous sealed compartments by curved internal walls known as septa.
A narrow tube called the siphuncle passed through these chambers near the shell’s outer edge. By adjusting the quantities of liquid and gas within the chambered section, the ammonite could regulate its buoyancy and maintain an effective position within the water column.
Where the septa joined the outer shell wall, they created complex suture lines. These patterns consist of finely divided lobes and saddles and may become visible where the original exterior shell layer has weathered away or has been exposed during fossil preparation.
The animal itself was a soft-bodied marine mollusc related to other cephalopods. It probably possessed well-developed eyes, grasping appendages and a muscular funnel through which water could be expelled for jet-propelled movement. Its external shell provided protection while simultaneously functioning as a specialised buoyancy system.
Lower Bajocian Subfurcatum Zone
The Bajocian is the second stage of the Middle Jurassic, following the Aalenian and preceding the Bathonian. It records an interval when extensive shallow seas covered substantial areas of western Europe and ammonites diversified into many distinctive forms.
This specimen is recorded from the Subfurcatum Zone, a recognised ammonite biozone within the Bajocian succession. Biozones are divisions of sedimentary rock defined by characteristic fossil assemblages and the appearance, development or disappearance of particular ammonite groups.
Ammonites are highly valuable as index fossils because they evolved rapidly, were abundant and occupied geographically widespread marine environments. Their successive species and genera allow geologists to compare individual beds between separate quarries, coastal exposures and regions.
The recorded Subfurcatum Zone provenance therefore adds significant geological context to this specimen. It links the fossil to a specific interval of Middle Jurassic time rather than identifying it simply as a general Jurassic ammonite.
Inferior Oolite Group Geology
The Inferior Oolite Group is a varied succession of Middle Jurassic marine sedimentary rocks extending through parts of southern and central England. In Dorset, it contains a complex sequence of limestones formed from carbonate mud, shell fragments, ooids, peloids, sand grains and iron-rich minerals.
Ooids are small, rounded carbonate grains produced when calcium carbonate accumulated in concentric layers around tiny particles in warm, agitated seawater. Although these grains contributed to the name “Inferior Oolite”, the group includes many different limestone types, and not every individual bed is strongly oolitic.
The changing rock textures and fossil contents record repeated variations in water depth, current activity, sediment supply and seabed stability. Some horizons accumulated very slowly and contain condensed concentrations of fossils, while others represent more sustained periods of carbonate deposition.
Ammonites preserved within these limestones are particularly important because their changing forms provide the detailed biostratigraphic framework used to establish the sequence and relative ages of the individual beds.
Horn Park Quarry, Beaminster
Horn Park Quarry is an internationally important geological locality near Beaminster in west Dorset. Its surviving quarry faces preserve a highly detailed Inferior Oolite succession containing fossil-rich horizons that have been studied extensively by generations of palaeontologists.
Historic limestone working exposed a condensed series of Middle Jurassic beds with diverse ammonite faunas. These fossils have contributed significantly to understanding Bajocian ammonite evolution and to refining the stratigraphic correlation of the British Inferior Oolite Group.
The locality is protected for its geological importance. A fossil with recorded Horn Park Quarry provenance therefore carries additional historical and scientific interest, connecting the specimen to one of Britain’s most notable Middle Jurassic reference localities.
Middle Jurassic Marine Environment
During the Bajocian, the area now forming Dorset lay beneath a warm, shallow sea. Limestone-forming carbonate mud, ooids, shell fragments and other marine debris accumulated across the seabed under changing current and water-depth conditions.
Ammonites shared these waters with belemnites, nautiloids, bivalves, brachiopods, gastropods, echinoids, crinoids, crustaceans and fish. Larger marine reptiles occupied the surrounding water column, while burrowing, grazing and encrusting organisms lived across the seabed.
After death, the ammonite shell could drift temporarily or descend directly to the seafloor. Burial beneath sediment helped protect it from complete destruction by scavengers, currents and chemical dissolution.
As further layers accumulated, the sediment became compacted and cemented into limestone. Mineral-rich groundwater could fill the chambers, replace the original shell material or create a solid internal mould preserving the ammonite’s shape. Later uplift, erosion and quarrying exposed the fossil-bearing rocks from which this specimen was recovered.
Actual Fossil 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 appearance, geological interest and collectable quality. Variations in colour, texture, matrix, mineralisation, completeness and surface preservation are authentic results of burial, fossilisation and natural weathering.
The listing photograph shows the exact Garantiana (Garantiana) aff. transiti ammonite fossil that will be supplied. Full sizing is provided in the photograph so 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 fossil display, Dorset geological collection, educational teaching set or natural-history cabinet.
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