Description
Genuine Sphaeroceras brongniartii Ammonite Fossil
This carefully chosen fossil is a genuine Sphaeroceras brongniartii ammonite from the Inferior Oolite Group at Frogden Quarry, Sherborne, Dorset, UK. It dates from the Bajocian Stage of the Middle Jurassic, representing an extinct marine cephalopod that inhabited the warm seas covering southern Britain approximately 170–168 million years ago.
The photograph shows the actual fossil specimen you will receive, allowing its natural shell shape, ribbing, mineralisation, colour variations, preservation and surrounding matrix to be examined before purchase. This is an individually selected fossil rather than a representative example, so the specimen pictured is the exact piece that will be supplied.
Sphaeroceras Ammonite Morphology
Sphaeroceras is recognised for its highly distinctive, compact shell form. The genus belongs to a group of ammonites whose mature shells commonly developed broad, inflated whorls and a rounded, almost spherical appearance. This characteristic morphology gives sphaeroceratid ammonites a very different profile from the flatter and more openly coiled forms found among many other Jurassic groups.
The shell grew planispirally, with successive whorls developing around a central umbilicus within a single plane. During the earlier growth stages, the inner whorls may have been relatively open and prominently ornamented. As the ammonite matured, later whorls could become increasingly inflated and overlap more of the juvenile shell.
Ribbing commonly begins near the umbilical region and extends across the flanks. Primary ribs may divide into finer secondary ribs towards the broad outer surface, or venter. Ornamentation can vary considerably through growth, and mature portions of the shell may appear smoother or more widely ribbed than the inner whorls.
The specific name brongniartii is associated with a compact and inflated sphaeroceratid form. The visible proportions and shell sculpture of an individual specimen will depend on its growth stage, the amount of shell preserved and the effects of natural weathering and fossilisation.
Bajocian Middle Jurassic Marine Life
The Bajocian Stage forms part of the Middle Jurassic and records an important period in the evolution and diversification of ammonites. During this interval, the region now occupied by Dorset lay beneath a shallow marine shelf connected to extensive seas across north-western Europe.
These warm waters supported varied communities of ammonites, belemnites, brachiopods, bivalves, gastropods, echinoids and other marine organisms. Ammonites occupied an important position within these ecosystems and evolved into numerous forms with different shell proportions, rib patterns and styles of ornamentation.
Their rapid evolutionary changes and broad distribution make ammonites especially valuable to geologists. Distinctive assemblages can be used to identify the relative age of fossil-bearing beds and correlate Jurassic rock sequences between separate localities.
Inferior Oolite Group Geology
The Inferior Oolite Group is one of southern England’s most significant fossil-bearing Middle Jurassic rock successions. It contains oolitic limestone, sandy limestone, marl, iron-rich deposits and condensed fossil horizons formed under changing shallow-marine conditions.
Oolitic limestone contains small rounded carbonate grains known as ooids. These developed when calcium carbonate accumulated in repeated coatings around tiny shell fragments or sediment particles moved by waves and currents in warm seawater. The grains eventually settled onto the seabed, where compaction and natural cementation transformed the sediment into limestone.
Deposition was frequently interrupted by changes in sea level, current strength and sediment supply. During periods of slow sediment accumulation, shells could remain exposed on the sea floor and become weathered, transported or concentrated before final burial. This produced fossil-rich horizons containing ammonites and other marine remains from the ancient Jurassic ecosystem.
Frogden Quarry, Sherborne
Frogden Quarry, near Sherborne in Dorset, is an important inland Jurassic fossil locality known for exposures of the Inferior Oolite succession. Its sedimentary rocks preserve a detailed record of Bajocian marine deposition and changing ammonite faunas.
The quarry’s deposits were formed under varying sea-floor conditions, including intervals of carbonate sedimentation, erosion and reworking. These geological processes created a complex succession in which fossils may occur within limestone, marl or condensed shell-rich horizons.
A specimen with recorded provenance from Frogden Quarry has strong scientific and collectable interest. It connects the fossil directly with a documented British locality and with the long tradition of geological research and fossil collecting in the Sherborne area.
Scientific Classification and Geological Details
Fossil type: Ammonite
Scientific name: Sphaeroceras brongniartii
Family: Sphaeroceratidae
Superfamily: Stephanoceratoidea
Order: Ammonitida
Geological period: Jurassic
Geological epoch: Middle Jurassic
Geological stage: Bajocian
Geological unit: Inferior Oolite Group
Locality: Frogden Quarry, Sherborne, Dorset, United Kingdom
The Sphaeroceratidae are notable for their compact coiling, inflated whorls and significant changes in shell form during growth. These distinctive characteristics make members of the family visually recognisable and valuable for studying the evolution of Bajocian ammonites.
Chambered Shell and Marine Lifestyle
The living ammonite occupied the final and largest section of its shell, called the body chamber. Earlier parts were divided internally by curved walls known as septa, creating a series of chambers collectively called the phragmocone.
A narrow connecting tube called the siphuncle enabled the animal to regulate the proportions of liquid and gas within these chambers. This system helped control buoyancy and maintain position within the Jurassic water column.
Where the septa met the internal shell wall, they produced complex suture lines. These may become visible where the outer shell material has been naturally removed, dissolved during fossilisation or exposed through careful preparation.
Ammonites belonged to the cephalopods, the wider group of marine molluscs that includes modern squid, cuttlefish, octopuses and nautiluses. They were highly successful throughout much of the Mesozoic Era 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 transported by currents before burial. Sediment gradually entered and surrounded the shell, protecting portions from complete destruction.
Over millions of years, geological pressure and mineral-rich groundwater transformed the surrounding sediment into rock. The original shell material may have been preserved, mineralised, replaced or dissolved to leave a natural internal mould. Some fossils retain a combination of altered shell, sediment-filled chambers and attached matrix.
Any incomplete areas, naturally worn ribs, small fractures, mineral-filled sections, colour variations or surrounding rock are authentic geological characteristics. These features record the specimen’s burial, fossilisation, exposure and eventual recovery, ensuring that every genuine ammonite has an individual appearance.
Collectable Dorset Jurassic Fossil
This Sphaeroceras brongniartii ammonite fossil would make an excellent addition to a collection of British fossils, Dorset ammonites, Bajocian fossils, Frogden Quarry specimens or Inferior Oolite fossils. Its distinctive inflated morphology, documented geological unit and recorded locality also make it suitable for a fossil cabinet, educational collection, natural history 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 carefully selected for its collectable qualities, and the photograph shows the exact specimen that will be supplied.
Full sizing please see photo.
Reviews
There are no reviews yet.