Description
Genuine Leptosphinctes cf. astenonensis Ammonite Fossil
This carefully selected specimen is a genuine Leptosphinctes cf. astenonensis ammonite fossil from the Inferior Oolite Group at Oborne Wood Quarry, North Dorset, UK. It dates from the Bajocian Stage of the Middle Jurassic Period and is assigned more precisely to the Polygyralis Subzone, providing the fossil with valuable geological, geographical and stratigraphical provenance.
The abbreviation “cf.” is derived from the Latin word confer, meaning “compare with”. In palaeontological identification, it indicates that the specimen closely resembles Leptosphinctes astenonensis, although one or more diagnostic features may not be sufficiently preserved for an entirely certain species-level determination. The fossil can therefore be confidently associated with the genus Leptosphinctes and compared closely with the named species.
Leptosphinctes Shell Form and Ornamentation
Leptosphinctes was an extinct marine cephalopod with a planispirally coiled and internally chambered shell. Ammonites of this genus are generally recognised by their evolute form, meaning that the successive whorls overlap only slightly and leave a relatively broad central umbilicus. This open coiling allows several earlier whorls to remain visible within the centre of the fossil.
The shell flanks are typically ornamented with pronounced radial ribs. These may begin as primary ribs near the umbilical margin before dividing into two or more secondary ribs across the outer part of the whorl. This branching ornamentation is an important feature used when comparing related Bajocian ammonites.
The exact features visible on this individual fossil depend upon its natural preservation and the portion exposed from the surrounding Inferior Oolite matrix. The specimen may show ribbing, whorl curvature, an umbilical region, impressions of the original shell or sections of the internal mould. Matrix, mineral deposits, weathered areas and incomplete shell sections are natural results of burial, fossilisation and later geological exposure.
During life, the ammonite occupied the final and largest section of its shell, known as the body chamber. The earlier chambers formed the phragmocone and were separated by internal walls called septa. A narrow structure known as the siphuncle connected these chambers and enabled the animal to regulate gases and fluids, helping control buoyancy within the Jurassic sea.
Bajocian Polygyralis Subzone
The Polygyralis Subzone is a finely resolved ammonite-based division of the Bajocian Stage. Subzones represent narrower intervals than geological stages or ammonite zones and allow fossil-bearing rock layers to be correlated with a high degree of precision.
Ammonites are exceptionally useful for this form of geological dating because they evolved rapidly, were abundant in marine environments and were distributed across wide geographical areas. Distinctive ammonite assemblages appeared and disappeared through time, enabling palaeontologists to recognise equivalent horizons at separate localities.
The recorded Polygyralis Subzone provenance adds significant scientific interest to this specimen. It places the ammonite within a specific part of the Middle Jurassic succession and connects it directly with the changing marine fauna preserved within the Inferior Oolite rocks of southern England.
Inferior Oolite Group Geology
The Inferior Oolite Group is a major succession of Middle Jurassic marine sedimentary rocks exposed across parts of Dorset, Somerset, Gloucestershire and central England. Despite its name, the group is not composed entirely of oolitic limestone. It includes fossil-rich limestone, sandy limestone, marl, iron-rich horizons and beds containing rounded carbonate grains known as ooids.
Ooids formed when calcium carbonate accumulated in thin concentric layers around tiny nuclei, such as sand grains or shell fragments. Continuous movement by currents and waves helped create their rounded form. These deposits record relatively shallow marine environments, although conditions varied considerably during the formation of the wider Inferior Oolite succession.
Changes in water depth, current strength, sediment supply and seabed stability created a complex sequence of fossil-bearing beds. These supported diverse marine communities including ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids and other invertebrates.
Following death, some ammonite shells settled onto the seabed and were covered by carbonate-rich sediment. Over millions of years, the sediment hardened into rock while the original shell could be mineralised, dissolved or replaced. In many specimens, sediment filling the shell chambers formed a durable internal mould preserving the ammonite’s characteristic shape and ornamentation.
Oborne Wood Quarry, North Dorset
Oborne Wood Quarry, near Sherborne in North Dorset, is a recognised British locality for Middle Jurassic fossils from the Inferior Oolite Group. Its Bajocian deposits have produced varied ammonite assemblages that help document the detailed succession of marine life during this part of the Jurassic Period.
Specimens from recorded quarry horizons are particularly desirable because their locality and stratigraphical information provide meaningful scientific context. This fossil’s association with Oborne Wood Quarry and the Polygyralis Subzone makes it suitable for a specialist collection of British ammonites, Dorset fossils or stratigraphically documented Inferior Oolite specimens.
Middle Jurassic Marine Life
During the Bajocian, approximately 170 million years ago, the region that later became North Dorset was submerged beneath a warm marine environment. Ammonites were an important component of these seas and belonged to the extinct cephalopod order Ammonitida.
They were related to modern squid, cuttlefish, octopuses and nautiluses, although they formed a distinct and highly successful evolutionary lineage. Their shell shapes, ribbing patterns and rates of evolutionary change make them among the most important fossils for studying Jurassic geology and palaeontology.
Authentic British Fossil for Collection and Display
This genuine Leptosphinctes cf. astenonensis ammonite fossil is an excellent addition to a collection of British fossils, Dorset ammonites, Inferior Oolite specimens or Middle Jurassic marine life. Its evolute shell form, distinctive ribbing, recorded quarry provenance and Polygyralis Subzone age give it strong geological, educational and display interest.
This is a carefully chosen individual specimen, and the photograph shows the actual fossil you will receive, rather than a representative example. Natural variations, surrounding matrix, mineral deposits, surface texture, worn areas and incomplete sections are authentic features of the fossil and its long geological history.
Full sizing is shown in the photographs.
This fossil is a 100% genuine specimen and includes a generic Certificate of Authenticity card with a lifetime guarantee.
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