Description
Genuine Darellia semicostata Ammonite Fossil
This carefully chosen fossil is a genuine Darellia semicostata ammonite from Worm Bay, Dorset, UK. Dating from the Bajocian Stage of the Middle Jurassic, it represents an extinct marine cephalopod that inhabited the ancient seas covering southern Britain approximately 170 million years ago.
British Darellia ammonites are attractive and scientifically interesting examples of the diverse ammonite faunas that evolved during the early part of the Bajocian. Their compressed, carefully coiled shells and defined ornamentation make them desirable specimens for collectors of Dorset fossils, British ammonites and Middle Jurassic palaeontology.
The photograph shows the actual fossil specimen you will receive, allowing its individual coiling, ribbing, preservation, colour, matrix and natural geological characteristics to be examined before purchase. Full sizing and proportions can be seen in the accompanying photograph.
This authentic fossil includes a generic Certificate of Authenticity card with a lifetime guarantee.
Fossil Classification and Geological Details
Scientific name: Darellia semicostata
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Hammatoceratoidea
Family: Graphoceratidae
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Bajocian
Locality: Worm Bay, Dorset, UK
Darellia is closely associated with the graphoceratid ammonites and has also been treated within or alongside the genus Hyperlioceras in ammonite classifications. These ammonites form part of an important evolutionary succession around the beginning of the Bajocian Stage. Graphoceratid ammonites from southern England have been studied extensively because their rapidly changing shell forms allow individual fossil-bearing horizons to be distinguished with considerable precision.
Distinctive Darellia Shell Morphology
Darellia semicostata possessed a planispirally coiled shell, meaning that its whorls developed in a flat spiral around a central umbilicus. The shell was generally compressed from side to side, producing a relatively slim profile rather than the broad, inflated form displayed by some other Jurassic ammonites.
Successive whorls partly enclosed the earlier growth, while the central umbilical region remained visible. The flanks could display curved or gently flexuous ribs, with the strength and spacing of the ornamentation changing as the ammonite matured.
The outer edge of the shell, known as the venter, may have carried a defined keel. A keel is a narrow raised structure running around the outer circumference of the shell and is a characteristic feature of many streamlined graphoceratid ammonites.
The name semicostata refers to the specimen’s costation or ribbing. Depending upon growth stage and preservation, portions of the shell may appear more clearly ribbed than others. The combination of compressed whorls, ribbed flanks, visible coiling and a potentially keeled venter created an efficient hydrodynamic shell form.
The exact appearance of these features varies naturally between individual specimens. Surface wear, incomplete areas, attached matrix, mineral staining and natural fractures are authentic geological characteristics resulting from fossilisation and later exposure.
Bajocian Geology of Dorset
Dorset is internationally renowned for its Jurassic geology and exceptionally varied fossil record. During the Bajocian, the region formed part of a marine basin connected to the wider seas covering much of north-western Europe.
Sediment accumulated across the seabed as layers of limestone, marl, clay, sand and locally iron-rich material. Differences in sea level, current strength, water depth and sediment supply produced a complex succession of fossil-bearing beds.
These ancient marine environments supported ammonites alongside belemnites, nautiloids, bivalves, brachiopods, gastropods, echinoids and other invertebrates. Predatory fish and marine reptiles also formed part of the broader Jurassic ecosystem.
Ammonites are particularly important in the study of these rocks because they evolved rapidly and were widely distributed. Distinctive ammonite assemblages allow geologists to correlate individual beds between coastal exposures, inland quarries and other European localities.
British Geological Survey resources describe ammonites as extinct marine relatives of living cephalopods and emphasise their importance throughout Jurassic and Cretaceous geological history.
Life in the Middle Jurassic Sea
The living ammonite occupied the final and largest section of its shell, known as the body chamber. Its head, eyes and tentacle-bearing soft body projected from the aperture at the front of this chamber.
The older coiled portion of the shell was divided internally by curved walls called septa. These created a series of chambers connected by a narrow tube known as the siphuncle. By regulating the proportions of gas and liquid within these chambers, the ammonite could control its buoyancy and maintain its position in 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. The compressed and potentially keeled shell of Darellia suggests an ammonite adapted to active movement through open marine water.
Tentacles surrounding the mouth were probably used to capture small prey, collect food and investigate the surrounding environment. Although ammonites are often compared with modern nautiluses, much of their soft-bodied anatomy is reconstructed through comparisons with living cephalopods because direct preservation is exceptionally rare.
Natural Fossilisation and Preservation
After the ammonite died, its shell settled onto the Bajocian seabed. The soft tissues decayed while sediment entered and accumulated around the empty shell. Burial helped protect the remains from currents, scavenging and complete physical destruction.
Over millions of years, pressure compacted the surrounding deposits into sedimentary rock. Mineral-rich groundwater altered the original aragonitic shell, which could recrystallise, dissolve or be replaced by other minerals.
Sediment filling the shell hardened into an internal mould capable of preserving the coiled form and impressions of the original ornamentation. Where shell material was removed, an external mould could retain details of the ribs, growth lines and other surface structures.
Natural cracks, weathered areas, mineral deposits, matrix attachments and incomplete sections form part of the fossil’s history. Each feature reflects the processes of burial, mineralisation, geological movement and erosion experienced by the specimen over an immense period of time.
Authentic British Jurassic Collectable
This Darellia semicostata ammonite is suitable for fossil collectors, ammonite enthusiasts, geology students and educational natural-history displays. Its identifiable species, Dorset provenance and Bajocian geological age give it strong scientific, decorative and collectable appeal.
It is ideal for display in a fossil cabinet, study, classroom, office or private collection. It also makes a distinctive natural-history gift for anyone interested in British fossils, Jurassic marine life, palaeontology or the geology of Dorset.
The photograph shows the exact carefully selected fossil specimen 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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