Description
Genuine Darellia recticostata Ammonite Fossil
This carefully chosen fossil is a genuine Darellia recticostata ammonite from Worm Bay, Dorset, UK. Dating from the Bajocian Stage of the Middle Jurassic, it represents an extinct marine cephalopod that inhabited the warm seas covering southern Britain approximately 170 million years ago.
Its compressed coiling and characteristically developed ribbing make this a fascinating example of British Jurassic ammonite diversity. The combination of a named species, Dorset provenance and distinctive shell morphology gives the specimen strong appeal for ammonite enthusiasts, geology students and collectors of classic British fossils.
The photograph shows the actual fossil specimen you will receive, allowing you to examine its individual coiling, ornamentation, preservation, colour, matrix and other natural geological characteristics before purchasing. Full sizing and proportions can be seen in the accompanying photograph.
This authentic fossil is supplied with a generic Certificate of Authenticity card carrying a lifetime guarantee.
Fossil Classification and Geological Details
Scientific name: Darellia recticostata
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 associated with the graphoceratid ammonites, a major group that flourished around the transition from the Aalenian into the Bajocian. Graphoceratids are often recognised by their laterally compressed shells, defined ventral structures and carefully developed rib patterns.
Ammonites from the Dorset Inferior Oolite succession are scientifically important because their rapid evolutionary changes allow individual layers of Middle Jurassic rock to be placed into detailed chronological order. Named species such as Darellia recticostata therefore have significance beyond their decorative appearance.
Recticostate Ribbing and Shell Morphology
The species name recticostata refers to the form of the ribs ornamenting the shell. The term combines Latin roots associated with straight or comparatively direct ribs, describing an important visual feature of the ammonite’s flanks.
The shell is planispirally coiled, meaning that its whorls developed in a single flat plane around a central umbilicus. Successive whorls partly overlapped the earlier growth, creating a streamlined and relatively compressed profile.
Ribs extend outwards from the inner part of the whorl and cross the flanks towards the outer margin. Compared with ammonites displaying strongly curved, divided or irregular ornamentation, the costae of Darellia recticostata can have a more direct radial appearance. The strength and spacing of these ribs may change as the shell increases in size.
The outermost edge of the shell is known as the venter. Graphoceratid ammonites commonly developed a narrow or keeled ventral region, contributing to an efficient hydrodynamic form. Shell compression, umbilical width, rib direction and the shape of the venter are all features used by palaeontologists when distinguishing related ammonite species.
The exact amount of visible ribbing and whorl detail depends upon the growth stage and preservation of the individual fossil. Any naturally incomplete areas, matrix attachments, mineral deposits, fractures or weathered surfaces are authentic geological characteristics.
Bajocian Geology of Dorset
Dorset is internationally renowned for its Jurassic rocks and diverse fossil record. Although the county is especially famous for the Lower Jurassic cliffs around Lyme Regis and Charmouth, it also preserves important Middle Jurassic strata containing distinctive ammonite assemblages.
During the Bajocian, the land now forming Dorset lay beneath an epicontinental sea connected to the broader marine basins of north-western Europe. Warm seawater covered a complex seabed on which limestone, carbonate sand, clay, marl and iron-rich sediment accumulated.
The fossil-bearing Inferior Oolite succession records repeated changes in sea level, sediment supply and seabed conditions. Some beds accumulated relatively slowly and became condensed, while others contain abundant remains of organisms that lived in the surrounding sea.
Ammonites occurred alongside belemnites, bivalves, brachiopods, gastropods, echinoids, crinoids and other marine invertebrates. Fish and marine reptiles occupied higher levels within the same ecosystem.
The detailed succession of ammonite species preserved in Dorset has allowed geologists to divide the Bajocian into zones, subzones and even narrower faunal horizons. This high-resolution biostratigraphy makes the region particularly important for understanding Middle Jurassic time.
The Ancient Marine Environment
The Bajocian sea around Dorset formed part of a marine landscape very different from the modern English coastline. Britain consisted of islands and shallow submerged platforms positioned between larger landmasses and deeper oceanic basins.
Carbonate-producing organisms contributed shell fragments and skeletal material to the seabed. Currents transported and reworked some of this sediment, while quieter periods allowed fine mud to settle through the water.
The presence of ammonites indicates an open connection with wider marine environments. These mobile animals could spread across extensive areas, allowing closely related faunas to occur in Britain and continental Europe.
Changes in sediment type and fossil content provide evidence of fluctuating water depth, current energy and oxygen conditions. The rock enclosing an ammonite therefore preserves part of the environment as well as the remains of the animal itself.
Life of a Middle Jurassic Ammonite
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.
The older coiled portion was divided into smaller chambers by internal walls called septa. A narrow tube known as the siphuncle connected these chambers and helped regulate their liquid and gas content. This provided buoyancy control and allowed the animal to 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 ammonite in the opposite direction. The compressed shell of Darellia recticostata may have reduced resistance as the animal moved through the sea.
A crown of tentacles surrounding the mouth was probably used to capture small prey, gather food and investigate the environment. Its possible diet may have included planktonic organisms, small crustaceans and other marine animals.
Natural Fossilisation and Preservation
After death, the ammonite shell descended to the Bajocian seabed. Its soft tissues decayed while sediment accumulated around and inside the empty shell. Burial helped protect the remains from complete destruction by waves, currents and scavengers.
Over millions of years, the surrounding sediment became compacted and cemented into rock. Mineral-rich groundwater altered the original aragonitic shell, which could recrystallise, dissolve or be replaced by other minerals.
Sediment filling the chambers hardened into an internal mould capable of preserving the overall coiling and impressions of the rib pattern. External moulds may also retain details of the original shell surface where the shell material itself has disappeared.
This Darellia recticostata ammonite is suitable for a fossil cabinet, office, classroom, study or natural-history collection. The photograph shows the exact carefully chosen 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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