Description
Genuine Leptosphinctes Ammonite from Dorset
This carefully chosen fossil is an authentic Leptosphinctes aff. festonensis ammonite from the Inferior Oolite Group of Oborne Wood Quarry, Dorset, UK. It dates from the Bajocian Stage of the Middle Jurassic Period and is assigned more precisely to the Polygyralis Subzone, giving the specimen valuable geological and stratigraphical provenance.
The designation “aff.”, meaning affinis or “having affinity with”, indicates that the ammonite closely resembles the species Leptosphinctes festonensis but has been identified with a degree of taxonomic caution. This terminology is commonly used in palaeontology when a fossil displays the principal characteristics of a named species but preservation, natural variation or incomplete diagnostic features prevent an entirely definitive species-level determination.
Ammonite Form and Ornamentation
Leptosphinctes ammonites typically possess a coiled, planispiral shell with a relatively open central umbilicus, allowing several of the earlier whorls to remain visible. The whorls are generally compressed to moderately inflated and are decorated with distinct ribs that extend across the shell flanks. These ribs may divide or branch as they approach the outer part of the shell, producing the characteristic ornamented appearance associated with many Jurassic perisphinctid ammonites.
The exact shell features visible on this specimen depend upon its natural preservation and the portion exposed from the surrounding rock. Ribbing, whorl shape, umbilical structure, impressions of the original shell and sections of the body chamber may all contribute to its appearance. Natural matrix, mineral deposits, worn areas and incomplete shell sections are genuine features of fossilisation and its long geological history.
During life, the ammonite’s shell was divided internally into a series of chambers separated by walls called septa. The living animal occupied the final and largest chamber, while the older chambers formed the phragmocone. A narrow tube called the siphuncle connected the chambers and enabled the animal to regulate gases and fluids, assisting with buoyancy in the Jurassic sea.
Leptosphinctes belongs to the extinct cephalopod order Ammonitida and is associated with the broad group of strongly ribbed ammonites that became diverse during the Middle Jurassic. Its shell morphology provides an attractive contrast to the smoother and more involute ammonite forms also known from the Inferior Oolite succession.
Bajocian Polygyralis Subzone
The Polygyralis Subzone is a finely defined interval within the Bajocian Stage. Ammonite subzones represent comparatively narrow divisions of geological time established through the appearance and succession of particular ammonite assemblages. Because ammonites evolved rapidly and were distributed widely through Jurassic seas, they are exceptionally useful index fossils for identifying and correlating rock layers.
Detailed subzonal information greatly enhances the scientific interest of a fossil. It connects the specimen with a specific interval in the development of the Middle Jurassic marine ecosystem and enables comparison with fossils recovered from equivalent strata at other British and European localities.
The Bajocian Stage lasted during the early part of the Middle Jurassic, when southern Britain was covered by warm, shallow seas. These waters supported diverse communities of ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids and other marine organisms. After death, ammonite shells settled onto the seabed, where some were buried by carbonate-rich sediment and preserved within the rocks now assigned to the Inferior Oolite Group.
Inferior Oolite Group Geology
The Inferior Oolite Group is an important succession of Middle Jurassic sedimentary rocks extending through parts of southern and central England. It contains fossiliferous limestone, sandy limestone, iron-rich beds, marl and deposits incorporating rounded carbonate particles called ooids.
Ooids formed when calcium carbonate accumulated in concentric layers around tiny nuclei, such as sand grains or shell fragments, while being moved repeatedly by waves and currents. Their presence records shallow, agitated marine conditions. However, the Inferior Oolite is geologically varied, reflecting repeated changes in water depth, sediment supply, seabed conditions and marine life.
Ammonites are among the most scientifically important fossils found within this group. Their rapid evolutionary succession allows individual horizons to be dated and correlated with a level of precision that is rarely possible using many other fossil groups.
Oborne Wood Quarry, Dorset
Oborne Wood Quarry is a recognised Middle Jurassic fossil locality near Sherborne in Dorset. Its Inferior Oolite rocks have produced a varied assemblage of ammonites and other marine fossils, preserving evidence of the seas that covered the region during the Bajocian.
Fossils from carefully recorded quarry horizons are especially desirable because their location and stratigraphical position provide meaningful scientific context. This specimen’s association with the Polygyralis Subzone makes it suitable for a specialist British ammonite collection, an Inferior Oolite display or an educational collection illustrating Jurassic biostratigraphy.
Authentic Fossil for Collection and Display
This genuine Leptosphinctes aff. festonensis ammonite is an appealing addition to a collection of Dorset fossils, British ammonites, Middle Jurassic marine life or Inferior Oolite specimens. Its clearly recorded quarry provenance, Bajocian age and detailed subzonal information add significant geological interest.
The fossil is a carefully selected individual specimen. The photograph shows the actual fossil you will receive, rather than a representative example. Full sizing and proportions can be seen in the accompanying photographs.
This fossil is a 100% genuine specimen and includes a generic Certificate of Authenticity card with a lifetime guarantee.
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