Description
Genuine Stephanoceras cf. planosum Ammonite Fossil
This carefully selected fossil is a genuine Stephanoceras cf. planosum ammonite from the Inferior Oolite Group at Burstock, Dorset, UK. It dates from the Bajocian Stage of the Middle Jurassic, making it approximately 170–168 million years old.
The abbreviation “cf.”, meaning “compare with,” indicates that the specimen displays features consistent with Stephanoceras planosum, while acknowledging the natural limitations that can arise when identifying an incomplete or partially preserved fossil. This is standard scientific terminology used for specimens that closely resemble a recognised species but cannot be assigned with absolute certainty.
The listing photographs show the actual fossil specimen you will receive, including its natural preservation, matrix, colouration and individual surface features. Full sizing and scale information can be seen in the accompanying photographs.
Bajocian Geology of Burstock, Dorset
Burstock lies within an important fossil-bearing region of west Dorset where rocks belonging to the Inferior Oolite Group preserve evidence of the warm marine environments that covered southern Britain during the Middle Jurassic.
The Inferior Oolite Group consists of a varied sequence of fossiliferous limestones, iron-rich oolitic beds, marls and condensed horizons. These sediments accumulated on a shallow marine shelf influenced by changing sea levels, currents and periods of reduced sedimentation. In some layers, fossils became concentrated following natural erosion and reworking of the seabed, producing deposits particularly rich in ammonites, belemnites, bivalves, brachiopods and other marine remains.
Oolitic limestone formed when small carbonate grains developed repeated mineral coatings as they were moved by currents in warm, shallow seawater. The presence of oolitic and shell-rich sediments indicates an active marine environment with well-oxygenated water and periods of relatively strong current movement.
Ammonites from these rocks are scientifically important because their rapidly changing shell forms allow palaeontologists and geologists to divide the Jurassic succession into detailed zones and subzones. Specimens such as this help document the changing ammonite communities that inhabited the European seas during the Bajocian.
Stephanoceras Ammonite Morphology
Stephanoceras is one of the best-known ammonite genera associated with the Bajocian Stage. These ammonites are typically recognised by their robust, strongly ornamented shells and relatively open central umbilicus.
The shell is planispirally coiled, with each whorl wrapped around the preceding growth stage. Stephanoceratid ammonites commonly display prominent primary ribs that extend outward from the umbilical region. These ribs may develop into raised tubercles or nodes along the flanks before dividing into finer secondary ribs that continue across the outer part of the shell.
This branching rib pattern produced a strong and distinctive sculptural appearance. The whorl section is often rounded or depressed rather than extremely narrow, giving many Stephanoceras specimens a broad and substantial form. Ornamentation can vary according to the animal’s growth stage, with juvenile whorls sometimes displaying different rib strength or spacing from the mature shell.
The living ammonite occupied the final body chamber. Earlier chambers formed the phragmocone, which contained gas and liquid and helped the animal regulate its buoyancy. Like modern nautiluses, ammonites possessed chambered shells, although they were more closely related within the wider cephalopod group to squid, cuttlefish and octopuses.
Scientific Classification and Geological Details
Fossil type: Ammonite
Scientific identification: Stephanoceras cf. planosum
Family: Stephanoceratidae
Superfamily: Stephanoceratoidea
Order: Ammonitida
Geological group: Inferior Oolite Group
Geological period: Jurassic
Geological epoch: Middle Jurassic
Geological stage: Bajocian
Locality: Burstock, Dorset, United Kingdom
The family Stephanoceratidae contains a diverse group of strongly ornamented Middle Jurassic ammonites. Their distinctive ribs, tubercles and whorl proportions make them highly collectable, while their stratigraphic distribution gives them considerable value in the study and correlation of Bajocian rock layers.
Natural Preservation and Individual Character
This fossil has undergone a long geological journey involving burial, compaction, mineral replacement and eventual exposure within the Dorset landscape. Any attached matrix, small fractures, worn ribbing, incomplete areas, mineral-filled chambers or colour variations are natural features of a genuine Jurassic specimen.
These characteristics record the effects of fossilisation and distinguish the piece as an individual natural object. No two ammonites preserve in exactly the same way, giving every specimen its own combination of shell detail, matrix and geological texture.
Collectable British Jurassic Fossil
This Stephanoceras cf. planosum ammonite would make an excellent addition to a collection of British fossils, Dorset ammonites, Middle Jurassic fossils, Bajocian fossils or Inferior Oolite specimens. It is suitable for display in a fossil cabinet, study collection, natural history display, office or educational setting.
The fossil is supplied as a 100% genuine specimen and includes a Certificate of Authenticity generic card with a lifetime guarantee. It has been individually selected for its collectable qualities, and the photograph shows the exact specimen that will be supplied.
Full sizing please see photo.
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