Description
Genuine Stephanoceras cf. nodosum Ammonite Fossil
This carefully chosen fossil is a genuine Stephanoceras cf. nodosum ammonite from Hildene Brickworks at Hildene in Dorset, UK. It originates from the Inferior Oolite Group and dates from the Middle Bajocian Stage of the Middle Jurassic, within the Humphriesianum Subzone.
This authentic British ammonite represents an extinct marine cephalopod that inhabited the warm seas covering southern England approximately 168 to 170 million years ago. Its robust coiling, prominent ribbing and nodose shell ornamentation give it the bold sculptured appearance for which Stephanoceras ammonites are well known.
The photograph shows the actual fossil specimen you will receive, allowing its individual shape, ribbing, preservation, colour, matrix and natural geological characteristics to be examined before purchase. Full sizing and proportions can be seen in the accompanying photograph.
A generic Certificate of Authenticity card with a lifetime guarantee is included.
Fossil Identification and Classification
Scientific identification: Stephanoceras cf. nodosum
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Stephanoceratoidea
Family: Stephanoceratidae
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Middle Bajocian
Biozone: Humphriesianum Subzone
Rock unit: Inferior Oolite Group
Locality: Hildene Brickworks, Hildene, Dorset, UK
The abbreviation “cf.” is derived from the Latin word confer, meaning “compare with”. It indicates that the preserved features of this specimen compare closely with Stephanoceras nodosum, although the identification is presented cautiously rather than claiming an entirely definitive species assignment.
This form of identification is commonly used in palaeontology when a fossil displays the principal characteristics of a named species but is incomplete, naturally worn or preserved in a way that conceals some diagnostic features.
Robust Stephanoceras Shell Morphology
Stephanoceras ammonites are recognised for their strongly ornamented, moderately evolute shells. In an evolute ammonite, successive whorls overlap the earlier growth only slightly, leaving a broad central umbilicus and allowing much of the juvenile coiling to remain visible.
Primary ribs arise near the umbilical margin and travel outwards across the flanks. These ribs commonly terminate in raised lateral tubercles or nodes before dividing into several finer secondary ribs. The secondary ribs then continue across the outer edge of the shell, known as the venter.
The species name nodosum refers to the presence of conspicuous nodes or nodose ornamentation. These raised structures contribute to the heavily sculptured appearance of the shell and provide important characteristics for comparing related stephanoceratid ammonites.
The whorls are generally substantial and rounded rather than extremely compressed. This robust construction distinguishes Stephanoceras from many more streamlined Jurassic ammonites with narrow shells and sharply keeled venters.
The exact strength of the ribs, visibility of the nodes and completeness of the whorls depend upon the specimen’s growth stage and preservation. Natural fractures, incomplete areas, matrix attachments, mineral deposits and surface wear are authentic geological features formed during fossilisation and later exposure.
Humphriesianum Subzone and Bajocian Biostratigraphy
The Bajocian is the second major stage of the Middle Jurassic, following the Aalenian and preceding the Bathonian. It records an important interval of ammonite diversification within the shallow and moderately deep marine environments of western Europe.
The Humphriesianum interval takes its name from ammonites historically associated with Stephanoceras humphriesianum. Fossils from this part of the Bajocian are especially important for biostratigraphy, the scientific practice of using fossils to identify and correlate layers of sedimentary rock.
Ammonites are highly effective index fossils because their shell forms evolved relatively rapidly and many groups were geographically widespread. Changes in ribbing, whorl proportions, tuberculation and adult shell form allow palaeontologists to distinguish successive ammonite assemblages.
The recorded Humphriesianum Subzone therefore gives this specimen a much more precise geological context than the broader designation of Middle Jurassic alone. It connects the fossil with a recognisable stage in the evolution of European stephanoceratid ammonites.
Inferior Oolite Group of Dorset
The Inferior Oolite Group is a varied succession of Middle Jurassic marine rocks exposed across Dorset, Somerset and neighbouring parts of southern England. Despite its traditional name, the succession contains much more than oolitic limestone. It includes fossiliferous limestone, marl, sandstone, clay-rich sediment, shell beds and locally iron-rich deposits.
During the Middle Bajocian, the area now forming Dorset lay beneath a warm epicontinental sea connected to the wider marine basins of north-western Europe. Britain consisted of islands, submerged platforms and marine channels rather than the continuous landmass seen today.
Carbonate grains, mud, sand and broken shell material accumulated across the seabed. Changes in sea level, current strength, water depth and sediment supply produced a complex series of fossil-bearing layers.
Some beds accumulated slowly and became condensed, concentrating durable fossils and mineral grains within comparatively thin horizons. Other layers preserve evidence of erosion, reworking and the movement of shells before final burial.
The surrounding marine community included ammonites, belemnites, nautiloids, brachiopods, bivalves, gastropods, echinoids and crinoids. Fish and marine reptiles occupied higher levels within the ecosystem, while worms and other invertebrates lived on or within the seabed.
Hildene Brickworks Fossil Locality
Hildene Brickworks represents an inland Dorset locality where quarrying and brickmaking exposed fossil-bearing Jurassic strata. Such historic workings are important because they provided access to geological layers that may now be overgrown, infilled or otherwise difficult to examine.
Ammonites collected from accurately recorded quarry horizons are especially useful because they retain information about both the animal and the layer of rock in which it was preserved. This allows specimens to be placed within established ammonite zones and compared with fossils from other Dorset localities.
The Hildene specimen therefore records part of the industrial and geological history of the region as well as the ancient marine life of the Bajocian sea.
Life in the Middle Jurassic Sea
Ammonites were extinct marine molluscs related to modern squid, cuttlefish, octopuses and nautiluses. The living animal occupied the final and largest section of its shell, known as the body chamber. Its head, eyes and tentacle-bearing soft body extended through the aperture.
The older coiled section of the shell was divided into chambers by curved internal walls called septa. A narrow tube known as the siphuncle connected these chambers and enabled the ammonite to regulate their liquid and gas content. This provided buoyancy control and helped the animal maintain its position within the water column.
Movement was achieved through jet propulsion. Water was drawn into the mantle cavity and expelled through a muscular funnel, pushing the animal in the opposite direction. Tentacles surrounding the mouth were probably used to capture small crustaceans, planktonic organisms and other marine prey.
The strongly ribbed and nodose shell of Stephanoceras cf. nodosum provided structural reinforcement while recording the ammonite’s growth. New shell material was added around the aperture as the animal matured, with additional chambers formed behind the expanding body chamber.
Natural Fossilisation and Authentic Preservation
After death, the ammonite shell settled onto the Bajocian seabed. Its soft tissues decayed while sediment entered and accumulated around the empty shell. Burial helped protect the remains from complete destruction by currents, scavengers and chemical dissolution.
Over millions of years, the surrounding sediment became compacted and cemented into rock. Mineral-rich groundwater could dissolve, recrystallise or replace the original aragonitic shell. Sediment filling the chambers hardened into an internal mould that preserved the coiled form and impressions of the original ornamentation.
Natural fractures, weathered areas, mineral staining, attached matrix and incomplete sections record the specimen’s individual geological history. The photograph shows the exact carefully selected Stephanoceras cf. nodosum ammonite fossil you will receive. Full sizing is shown in the photo. A generic Certificate of Authenticity card with a lifetime guarantee is included.
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