Description
Genuine Chondroceras (Sphaeroceras) mansellei Ammonite Fossil
This carefully chosen fossil is a genuine Chondroceras (Sphaeroceras) mansellei ammonite from the Inferior Oolite Group at Sandford Lane Quarry, Sherborne, Dorset, UK. It dates from the Bajocian Stage of the Middle Jurassic and is recorded from the Sauzei Zone, an important ammonite biozone within the Lower Bajocian succession.
The specimen represents an extinct marine cephalopod that lived in the warm seas covering southern Britain approximately 170 million years ago. The photograph shows the actual fossil specimen you will receive, allowing its natural shell form, ribbing, matrix, colouration and individual preservation to be viewed before purchase.
Full sizing please see photo.
Lower Bajocian Sauzei Zone Fossil
The Sauzei Zone is a recognised biostratigraphic division of the Lower Bajocian. It is named after characteristic ammonites used by geologists to identify and correlate marine rock layers of this age across Britain and parts of continental Europe.
Ammonites are especially useful index fossils because many species evolved rapidly and occupied wide geographical ranges. Changes in shell shape, ribbing, coiling and ornamentation allow palaeontologists to distinguish successive ammonite faunas and divide Jurassic rock sequences into detailed zones and subzones.
A fossil recorded from the Sauzei Zone therefore carries valuable geological information in addition to its appeal as a collectable natural object. It belongs to a clearly defined interval within the early Middle Jurassic and provides evidence of the ammonite communities that inhabited the north-west European seas during the Bajocian.
Chondroceras Shell Form and Ornamentation
Chondroceras is a distinctive Bajocian ammonite genus known for its compact, inflated and strongly ornamented shell. The shell grew in a planispiral coil, with each successive whorl expanding around a central umbilicus in a single plane.
Members of the group commonly possess broad, rounded whorls and a comparatively deep umbilical region. Strong primary ribs arise near the umbilicus and extend across the flanks. These may divide into finer secondary ribs as they approach the outer part of the shell, creating a bold and textured appearance.
Raised swellings or tubercles may occur where the primary ribs branch. The outer edge of the shell, called the venter, is typically broad and rounded, with the secondary ribs continuing across it. This combination of compact coiling, inflated whorls and branching rib ornamentation gives Chondroceras its recognisable form.
The identification Chondroceras (Sphaeroceras) mansellei reflects the close morphological relationship between these robust sphaeroceratid ammonites. The shell proportions and ornamentation may change considerably during growth, with juvenile whorls often appearing more openly coiled and strongly ribbed before the shell becomes increasingly inflated.
Inferior Oolite Group Geology
The Inferior Oolite Group is one of southern England’s most important Middle Jurassic marine rock successions. It includes fossiliferous oolitic limestones, sandy limestones, marls, iron-rich beds and condensed horizons deposited under changing shallow-sea conditions.
Oolitic limestone contains numerous small rounded carbonate grains known as ooids. These formed as calcium carbonate accumulated in repeated coatings around tiny sediment particles or shell fragments moving through warm, agitated seawater. The ooids eventually settled onto the seabed, where compaction and natural cementation transformed the sediment into limestone.
Deposition was frequently interrupted by variations in sea level, current strength and sediment supply. During periods of very slow accumulation, shells could remain exposed on the seabed, become worn or reworked and later collect within concentrated fossil horizons.
These processes helped create the rich ammonite assemblages for which the Inferior Oolite Group is renowned. Fossils from the succession provide detailed evidence of changing marine environments, biological evolution and sedimentary conditions throughout the Bajocian.
Sandford Lane Quarry, Sherborne
Sandford Lane Quarry near Sherborne lies within a geologically important area of north-west Dorset. Inland quarries and former workings around Sherborne have exposed fossil-bearing Middle Jurassic strata and contributed valuable specimens to British geological and palaeontological collections.
During the Bajocian, the Sherborne area formed part of a shallow marine shelf connected to extensive seas across western Europe. The water supported a varied ecosystem containing ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids and other marine organisms.
A specimen with recorded provenance from Sandford Lane Quarry has added collectable and scientific interest. It provides a direct connection to a documented Dorset fossil locality and to the long history of Jurassic collecting in the Sherborne region.
Scientific Classification and Geological Details
Fossil type: Ammonite
Scientific identification: Chondroceras (Sphaeroceras) mansellei
Family: Sphaeroceratidae
Superfamily: Stephanoceratoidea
Order: Ammonitida
Geological period: Jurassic
Geological epoch: Middle Jurassic
Geological stage: Bajocian
Biozone: Sauzei Zone
Geological unit: Inferior Oolite Group
Locality: Sandford Lane Quarry, Sherborne, Dorset, United Kingdom
The family Sphaeroceratidae includes robust Middle Jurassic ammonites known for compact coiling, inflated whorls and strongly developed ribbing. Their rapidly changing shell forms make them valuable for studying the evolution and stratigraphy of Bajocian marine deposits.
Chambered Shell and Ammonite Biology
The living ammonite occupied the final and largest portion of the shell, known as the body chamber. Earlier sections were divided internally by curved walls called septa, producing a sequence of chambers collectively known as the phragmocone.
A narrow tube called the siphuncle connected these chambers and allowed the animal to regulate their liquid and gas content. This helped control buoyancy and maintain position within the Jurassic water column.
Where the septa met the inner shell wall, they formed patterned suture lines. These may become visible where the outer shell layer has been naturally weathered, removed or carefully prepared.
Ammonites belonged to the cephalopods, the same broad group of marine molluscs that includes modern squid, cuttlefish, octopuses and nautiluses. They were highly successful throughout much of the Mesozoic Era and developed an exceptional variety of shell forms before becoming extinct at the end of the Cretaceous Period.
Natural Fossilisation and Preservation
After the ammonite died, its shell settled onto the Jurassic seabed or was moved by currents before burial. Sediment gradually accumulated around and inside the shell, protecting portions of it from complete destruction.
Over millions of years, pressure and mineral-rich groundwater transformed the surrounding sediment into rock. The original shell may have been preserved, mineralised, replaced or dissolved, leaving a natural internal mould. Many ammonites retain a combination of altered shell material, sediment-filled chambers and surrounding matrix.
Any incomplete areas, worn ribs, minor fractures, mineral-filled sections, colour variations or attached rock are authentic geological features. These characteristics record the specimen’s burial, fossilisation, exposure and recovery and make every genuine ammonite unique.
Collectable Dorset Jurassic Ammonite
This Chondroceras (Sphaeroceras) mansellei ammonite fossil would make an excellent addition to a collection of British fossils, Dorset ammonites, Sauzei Zone fossils, Bajocian specimens or Inferior Oolite fossils. Its recorded identification, biozone and quarry locality also make it suitable for a fossil cabinet, geological study collection, educational display or distinctive gift for a Jurassic fossil enthusiast.
This is a 100% genuine fossil specimen and includes a Certificate of Authenticity generic card with a lifetime guarantee. The fossil has been individually selected for its collectable qualities, and the photograph shows the exact specimen that will be supplied.
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