Description
Genuine Cammontisphinctes nodatus Ammonite Fossil
This carefully chosen fossil is an authentic Cammontisphinctes (C.) nodatus ammonite from the Inferior Oolite Group at 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, providing the specimen with detailed and valuable stratigraphical provenance.
The abbreviated taxonomic form Cammontisphinctes (C.) nodatus records both the genus and nominated subgenus. This level of identification makes the specimen particularly appealing to collectors of scientifically documented British ammonites, Middle Jurassic fossils and Inferior Oolite material from classic Dorset localities.
Shell Form and Ammonite Morphology
Cammontisphinctes was a coiled marine cephalopod with a planispiral shell divided internally into a succession of chambers. Ammonites of this general group typically display a relatively open umbilicus, allowing several earlier whorls to remain visible within the centre of the shell. Their whorls commonly carry pronounced radial ribbing, which may branch or divide as it extends towards the outer margin.
The species name nodatus refers to a nodose or prominently ornamented appearance, reflecting the strengthened points, swellings or tubercle-like features that can occur where elements of the shell ornament develop. The precise details visible on this individual fossil depend upon its preservation, the portion exposed from the matrix and the extent to which the original shell or its internal mould survives.
The shell’s ornamentation was not merely decorative. Ribbing and strengthened shell features may have contributed to structural support while also reflecting the evolutionary characteristics used by palaeontologists to distinguish closely related ammonite groups. Whorl proportions, rib density, rib division, umbilical width and the shape of the outer margin are all important features when identifying Jurassic ammonites.
During life, the animal occupied the final and largest chamber of the shell, known as the body chamber. The older chambers formed the phragmocone and were separated by curved walls called septa. These chambers were connected by a narrow tube called the siphuncle, through which the ammonite regulated gases and fluids to assist with buoyancy and movement through the water.
Polygyralis Subzone Biostratigraphy
This specimen comes from the Polygyralis Subzone, a finely defined ammonite-based division within the Bajocian Stage. Geological subzones represent narrower intervals than stages and zones, allowing fossil-bearing strata to be dated and correlated with greater precision.
Ammonites are particularly useful for biostratigraphy because they evolved rapidly, were abundant in Jurassic seas and were distributed across broad geographical areas. Changes in their shell form and species succession allow palaeontologists to compare rock layers from separate localities and recognise deposits formed during equivalent intervals of geological time.
The recorded Polygyralis Subzone provenance therefore adds considerable scientific value to this fossil. It places the specimen within a specific part of the Bajocian ammonite succession and links it directly with the changing marine communities preserved in the Inferior Oolite rocks of southern England.
Inferior Oolite Group Geology
The Inferior Oolite Group is an important succession of Middle Jurassic sedimentary rocks extending through parts of Dorset, Somerset, Gloucestershire and the Midlands. Despite its name, the group contains a varied range of marine deposits rather than being composed entirely of oolitic limestone.
Its rocks include fossil-rich limestone, sandy limestone, marl, iron-rich beds and layers containing rounded carbonate grains known as ooids. Ooids formed when calcium carbonate accumulated in concentric coatings around small nuclei, such as sand grains or shell fragments, while being rolled repeatedly by waves and currents in shallow seawater.
The changing character of the Inferior Oolite records variations in water depth, current strength, sediment supply, seabed stability and marine ecology. At different times, the Jurassic seabed supported diverse communities of ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids and other organisms.
After death, ammonite shells settled onto the seafloor. Some were buried rapidly enough to escape complete destruction by scavenging, currents and dissolution. Over millions of years, the surrounding sediment hardened into rock while the shell was replaced, mineralised or preserved as an internal mould.
Oborne Wood Quarry, Dorset
Oborne Wood Quarry, near Sherborne in Dorset, is a recognised British locality for fossils from the Inferior Oolite Group. Its Bajocian strata have yielded a varied ammonite fauna that helps palaeontologists interpret the detailed succession of Middle Jurassic marine deposits in southern England.
Specimens from documented quarry horizons are particularly desirable because the associated geological information provides far more context than a fossil without a known locality or stratigraphical position. This ammonite’s recorded origin at Oborne Wood Quarry and placement within the Polygyralis Subzone make it suitable for a specialist British ammonite collection, a regional Dorset fossil display or an educational collection illustrating Jurassic biostratigraphy.
Middle Jurassic Marine Environment
During the Bajocian, approximately 170 million years ago, the area that would eventually become Dorset lay beneath a warm sea. The region formed part of a broad marine environment in which carbonate sediments accumulated across shallow seabeds.
Ammonites were active swimming predators or opportunistic feeders within this ecosystem. Their soft-bodied anatomy was broadly related to that of modern squid, cuttlefish, octopuses and nautiluses, although ammonites formed their own highly successful and now-extinct cephalopod lineage.
The fossil record preserved within the Inferior Oolite provides evidence of an evolving marine ecosystem. Differences between ammonite assemblages from successive beds show how species appeared, diversified and disappeared through time, making these fossils valuable both as collectable natural objects and as tools for geological research.
Authentic British Jurassic Fossil
This genuine Cammontisphinctes (C.) nodatus ammonite fossil is an excellent addition to a collection of British fossils, Dorset ammonites, Inferior Oolite specimens or Middle Jurassic marine life. Natural matrix, mineral deposits, worn areas, incomplete shell sections and surface irregularities are authentic features produced by fossilisation and its long geological history.
This is a carefully selected individual specimen, and the photograph shows the actual fossil you will receive, rather than a representative example. 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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