Description
Genuine Caumontisphinctes aplous Ammonite Fossil
This carefully chosen specimen is a genuine Caumontisphinctes aplous ammonite fossil from the Inferior Oolite Group at Oborne Wood Quarry, Dorset, UK. Dating from the Bajocian Stage of the Middle Jurassic Period, it is assigned more precisely to the Polygyralis Subzone, giving the fossil detailed geological, geographical and biostratigraphical provenance.
The combination of an identified ammonite species, a recognised ammonite subzone and a classic Dorset locality makes this an appealing specimen for collectors of British fossils, Middle Jurassic cephalopods and stratigraphically documented Inferior Oolite ammonites.
Caumontisphinctes Shell Form and Ornamentation
Caumontisphinctes was an extinct ammonite genus characterised by a planispirally coiled, chambered shell. Typical members of the genus possess a relatively evolute form, meaning that successive whorls overlap only slightly and leave a broad central umbilicus in which several earlier whorls remain visible.
The whorls are generally compressed to moderately inflated, with a rounded or ovoid cross-section. Strong radial ribs extend outwards from the umbilical region and may divide into secondary ribs across the outer part of the whorl. Tubercles or strengthened points can develop where the ribs bifurcate and around the ventrolateral area. The venter, forming the outermost edge of the coil, may be comparatively smooth or carry a shallow groove.
This combination of open coiling, sharp ribbing and locally developed tuberculation gives Caumontisphinctes a distinctive sculptural appearance. The exact features visible on this individual specimen depend upon its growth stage, preservation and the amount of shell exposed from the surrounding rock.
The species name aplous distinguishes this form within the wider Caumontisphinctes group. Palaeontologists compare features such as whorl proportions, umbilical width, rib density, points of rib division, tuberculation and ventral structure when separating closely related Jurassic ammonites.
Chambered Shell and Ammonite Biology
During life, the ammonite occupied the final and largest section of its shell, known as the body chamber. The older portion of the shell formed the phragmocone, which was divided into a succession of smaller chambers by internal walls called septa.
A narrow tube called the siphuncle connected the chambers and allowed the animal to regulate their liquid and gas content. This provided a buoyancy-control system that helped the ammonite maintain its position and move through the water.
Ammonites were marine molluscs belonging to the cephalopod lineage and were distant relatives of modern squid, cuttlefish, octopuses and nautiluses. Their shells developed continuously throughout life, preserving changes in whorl shape and ornamentation as the animal matured.
Bajocian Polygyralis Subzone
This fossil comes from the Polygyralis Subzone, an ammonite-defined interval within the Bajocian succession. Ammonite subzones are comparatively narrow divisions of geological time recognised through the appearance, abundance and succession of characteristic ammonite groups.
In the standard Western European Bajocian succession, the Polygyralis Subzone is associated with the Niortense Zone. Such detailed biostratigraphical information allows fossil-bearing beds from different locations to be compared and correlated with far greater precision than would be possible from lithology alone.
Ammonites are particularly useful index fossils because many species evolved rapidly and were distributed widely through Jurassic seas. Their changing shell forms create a recognisable sequence through the rocks, enabling palaeontologists to distinguish individual intervals within the broader Middle Jurassic timescale.
The Polygyralis Subzone provenance therefore gives this specimen added scientific and collecting interest. It connects the ammonite directly with a specific phase in the evolution of the marine fauna preserved within the Inferior Oolite succession.
Inferior Oolite Group Geology
The Inferior Oolite Group is a varied sequence of Middle Jurassic marine sedimentary rocks found across southern and central England. In the Dorset and Somerset region, it includes bioclastic, peloidal, sandy, ferruginous, argillaceous and extensively bioturbated limestones, together with subordinate ooidal limestone, sandstone, limestone conglomerate, lime-mudstone and mudstone.
The word “oolite” refers to limestone containing ooids, which are small rounded carbonate grains formed as calcium carbonate accumulated in layers around tiny nuclei such as sand grains or shell fragments. Repeated movement by waves and currents helped produce their rounded form.
However, the Inferior Oolite is not a single uniform limestone. Its changing rock types record variations in water depth, sediment supply, current strength, oxygenation and seabed stability. Ferruginous horizons contain iron-rich grains or coatings, while bioclastic beds incorporate abundant broken shells and other skeletal debris.
Burrows and other evidence of bioturbation show that many seabeds were inhabited by organisms that repeatedly disturbed and reworked the sediment. Ammonites lived in the surrounding water alongside belemnites, while bivalves, brachiopods, gastropods, echinoids and numerous burrowing animals occupied the seabed.
Fossilisation in a Jurassic Sea
When this ammonite died, its shell descended to the seafloor and became incorporated into the marine sediment. The soft tissues decayed, while sediment could enter the empty body chamber and any damaged internal chambers.
Continued burial compacted the surrounding deposits. Mineral-rich groundwater circulated through the sediment as it hardened into limestone, allowing the original shell to be mineralised, replaced or dissolved. Where sediment hardened inside the shell before its removal, a natural internal mould was produced, retaining the ammonite’s overall shape and external ornamentation.
Natural matrix, mineral staining, worn ribs, incomplete whorls, shell loss and surface irregularities are genuine products of this fossilisation process and the specimen’s long geological history.
Oborne Wood Quarry, Dorset
Oborne Wood Quarry, in the Sherborne area of Dorset, is a recognised locality for fossiliferous Inferior Oolite rocks. The succession around Oborne has produced important ammonite assemblages from multiple Bajocian intervals and has contributed to detailed studies of Middle Jurassic stratigraphy in southern England.
Ammonite faunas from beds at Oborne Wood have been compared with equivalent Subfurcatum and associated zonal deposits elsewhere in southern Britain. This demonstrates the value of the locality for correlating fossiliferous horizons across Dorset and Somerset.
Fossils retaining both their collecting locality and ammonite-subzone information are especially desirable because they preserve meaningful scientific context. This specimen is therefore suitable for a specialist British ammonite collection, a regional Dorset fossil display or an educational collection illustrating Jurassic biostratigraphy.
Authentic British Jurassic Fossil for Display
This genuine Caumontisphinctes aplous ammonite is an excellent addition to a collection of British fossils, Dorset ammonites, Inferior Oolite specimens or Middle Jurassic marine life. Its strongly ornamented ammonite form, Oborne Wood Quarry provenance and Polygyralis Subzone age give it considerable geological, educational and display interest.
This is a carefully selected individual specimen, and the photograph shows the actual fossil you will receive, rather than a representative example. Natural colour variation, surrounding matrix, mineral deposits, weathered areas and incomplete sections are authentic characteristics of the fossil.
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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