Description
Genuine Parkinsonia cf. interrupta Ammonite Fossil
This carefully selected specimen is a genuine Parkinsonia cf. interrupta ammonite fossil from the Inferior Oolite Group at Horn Park Quarry, near Beaminster in Dorset, England. It is recorded from the Lower Bajocian Subfurcatum Zone and represents an extinct marine cephalopod that lived in the warm seas covering southern Britain during the Middle Jurassic.
The scientific abbreviation “cf.” means “compare with”. It indicates that the fossil displays characteristics comparable to Parkinsonia interrupta, while the identification is retained with appropriate caution. This notation is commonly applied when the visible morphology strongly resembles a particular species but not every diagnostic feature is sufficiently preserved for an entirely certain species-level determination.
The listing photograph shows the actual fossil you will receive, not a stock photograph or representative example. Its individual shell shape, ribbing, colour, matrix and natural preservation can therefore be examined before purchase. Full sizing is shown in the accompanying photograph.
Fossil Classification and Geological Details
Scientific identification: Parkinsonia cf. interrupta
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Stephanoceratoidea
Family: Parkinsoniidae
Genus: Parkinsonia
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
Parkinsonia is an important Middle Jurassic ammonite genus recognised for its compressed, prominently ribbed shell. The genus belongs to the Parkinsoniidae, a family of ammonites particularly associated with Bajocian and Bathonian marine deposits.
Characteristic Parkinsonia Shell Morphology
The shell grew in a flat spiral known as planispiral coiling. Each new whorl increased in size as the ammonite matured, creating the familiar circular outline. The central exposed area is called the umbilicus, the lateral surfaces are the flanks, and the outermost edge is known as the venter.
Parkinsonia ammonites are generally characterised by an evolute to moderately evolute shell, meaning that successive whorls overlap relatively little and leave much of the earlier coiling visible. This produces a broad, clearly defined umbilical area and allows the ornamentation of the inner whorls to remain exposed.
Strong primary ribs arise near the umbilical margin and extend across the flanks. These ribs commonly divide into two or more secondary branches towards the outer part of the whorl. The point at which a rib divides is known as the bifurcation point. Rib strength, spacing and direction may change as the animal grows, so the inner juvenile whorls can display a different pattern from the mature outer whorl.
A characteristic interruption or smooth groove may occur along the ventral region, separating the opposing ends of the ribs. This feature gives many Parkinsonia shells their distinctive appearance. Depending on the fossil’s preservation, the specimen may also display growth lines, whorl boundaries, fragments of original shell material or sections of the chambered interior.
Natural areas of wear, attached limestone, incomplete margins and mineralised fractures are genuine parts of the fossil’s geological history rather than manufactured imperfections.
Chambered Shell and Ammonite Anatomy
The living ammonite occupied the final and largest part of the shell, known as the body chamber. The older inner portion was divided into numerous sealed compartments by curved internal walls called septa.
A slender tube known as the siphuncle passed through these chambers. By regulating the quantities of liquid and gas inside them, the ammonite could control its buoyancy and maintain an effective position within the water column.
Where each septum joined the outer shell wall, it created a complex line called a suture. Ammonitic sutures consist of elaborately divided lobes and saddles and may become visible where the original 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 would have possessed a head, well-developed eyes, grasping appendages and a muscular funnel used to expel water for jet-propelled movement. Its chambered external shell combined physical protection with an efficient buoyancy system.
Lower Bajocian Subfurcatum Zone
The Bajocian is the second stage of the Middle Jurassic, following the Aalenian and preceding the Bathonian. It represents an interval when shallow marine environments extended across substantial areas of western Europe and ammonites underwent rapid evolutionary diversification.
This specimen is recorded from the Subfurcatum Zone, an ammonite biozone within the Bajocian succession. Biozones are intervals of sedimentary rock identified by characteristic ammonite assemblages and the appearance or disappearance of particular forms.
Because ammonites evolved rapidly, were abundant and occupied widespread seas, they are extremely useful index fossils. Their successive faunas enable geologists to compare rocks exposed in different quarries and regions and place them within a detailed relative chronological sequence.
The zonal information accompanying this fossil therefore adds considerably to its geological interest. It connects the specimen to a defined part of the Middle Jurassic rock record rather than identifying it only 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 includes limestones containing different proportions of carbonate grains, shell fragments, sand, iron minerals and peloids.
The name “oolite” refers to the presence of ooids, small rounded carbonate grains formed when calcium carbonate accumulated in layers around tiny particles in warm, moving seawater. However, the Inferior Oolite Group contains many different limestone types and not every bed is strongly oolitic.
The succession records repeated changes in water depth, sediment supply, current strength and seabed conditions. Some horizons accumulated slowly and contain condensed concentrations of fossils, while others represent episodes of more rapid carbonate deposition.
Ammonites preserved within these beds are particularly important because they provide the detailed biostratigraphic evidence required to establish the order and relative ages of individual limestone horizons.
Horn Park Quarry, Beaminster
Horn Park Quarry is an internationally significant Middle Jurassic geological locality near Beaminster in west Dorset. Its surviving quarry faces preserve an exceptionally detailed succession of Aalenian and earliest Lower Bajocian strata, together with rich ammonite faunas used in high-resolution Jurassic stratigraphy.
The locality is protected as a National Nature Reserve and is regarded as an important reference site for the study of the Inferior Oolite Group. Its ammonites have been examined by generations of palaeontologists, making accurately provenanced examples from Horn Park especially desirable to collectors of British fossils and historically important geological specimens.
Middle Jurassic Marine Environment
During the Bajocian, the area now forming Dorset lay beneath a warm, relatively shallow sea. Limestone, shell debris, carbonate grains and fine sediment accumulated across a seabed inhabited by a diverse community of marine organisms.
Ammonites shared these waters with belemnites, nautiloids, bivalves, brachiopods, gastropods, echinoids, crinoids, crustaceans and fish. Larger marine reptiles occupied the surrounding water, while burrowing and encrusting organisms lived on or within the seabed.
After death, the ammonite shell could drift temporarily or descend directly to the seafloor. Burial beneath sediment protected it from complete destruction by currents, scavengers and chemical dissolution. Continued sedimentation, pressure and mineral-rich groundwater gradually preserved the shell as fossil material, an internal mould or a combination of both.
Later uplift and erosion brought the Middle Jurassic rocks closer to the surface. Quarrying exposed the limestone succession and allowed scientifically important fossils such as this specimen to be recovered.
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. Variations in colour, texture, mineralisation, matrix, completeness and surface preservation are authentic results of burial, fossilisation and natural weathering.
The listing photograph shows the exact Parkinsonia cf. interrupta ammonite fossil that will be supplied. Full sizing is provided in the photograph so that the specimen’s 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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