Description
Genuine Orthogarantiana densicostata Ammonite Fossil
This carefully chosen specimen is a genuine Orthogarantiana densicostata ammonite fossil from Calvados in Normandy, France. It originates from the Poitou Formation and dates to the Upper Bajocian interval of the Middle Jurassic, representing an extinct marine cephalopod that inhabited the ancient seas of western Europe approximately 168–170 million years ago.
Its named taxonomic identification, documented geological formation, precise Jurassic interval and recorded French locality give this fossil excellent scientific, educational and collectable interest. It is particularly suitable for collectors of ammonites, French fossils, Jurassic marine life and historically significant European geological specimens.
The photograph shows the exact fossil you will receive. Full sizing and scale information can be seen in the accompanying photograph.
Upper Bajocian Middle Jurassic Geology
The Bajocian is a stage of the Middle Jurassic Period named after Bayeux in Normandy. During the Upper Bajocian, much of north-western France was covered by warm marine waters connected to the extensive Jurassic seas that spread across large areas of Europe.
Fine carbonate mud, mineral particles, broken shells and other marine remains accumulated across the seabed. Through burial, compaction and natural cementation, these sediments eventually formed limestone, marl and related fossil-bearing rocks. The resulting deposits preserve evidence of a diverse marine ecosystem containing ammonites, belemnites, nautiloids, brachiopods, bivalves, gastropods, echinoids and other invertebrates.
When this ammonite died, its shell settled onto the Jurassic seabed. Burial beneath sediment protected part of its form from destruction by currents, scavengers and chemical dissolution. During fossilisation, the original shell could be mineralised, replaced or dissolved, leaving a natural mould or stone-filled representation of the chambered interior.
The colour, texture, matrix, mineral deposits, weathered areas and completeness of the specimen are authentic features created by its long geological history. These natural characteristics ensure that every ammonite fossil is individually distinctive.
Orthogarantiana densicostata Shell Characteristics
Orthogarantiana is an extinct genus of ammonites associated with the Upper Bajocian. The genus is placed within the Garantianinae, a group of ornamented Middle Jurassic ammonites commonly classified within the family Stephanoceratidae, although some taxonomic treatments have placed related forms within Parkinsoniidae.
The shell developed in a planispiral coil, with successive whorls growing around a central umbilical region. Garantianine ammonites commonly display moderately evolute shells, meaning that the inner whorls remain visible rather than being completely concealed by later growth. This creates an open umbilicus and a clearly defined coiled appearance.
The ornamentation is characterised by closely arranged radial ribs extending across the flanks. Some ribs may divide as they approach the outer portion of the whorl, while the ventral area may display an interruption, weakening or change in the rib pattern. Rib density, curvature, branching, whorl proportions and ventral structure are among the features used by palaeontologists when comparing related ammonite species.
Depending on its individual preservation, this specimen may retain sections of the inner and outer whorls, densely ribbed flanks, umbilical region, ventral margin, internal chambers or surrounding Jurassic matrix. Its surviving details record both the growth of the living ammonite and the geological processes that affected its shell after burial.
Extinct Jurassic Marine Cephalopod
Ammonites were marine molluscs related to living squid, cuttlefish, octopuses and nautiluses. Unlike most modern cephalopods, they possessed a hard external shell divided internally into a series of chambers.
The animal occupied the final and largest section of the shell, known as the body chamber. The older chambers formed the phragmocone, which contributed to buoyancy control. A narrow connecting structure called the siphuncle passed through the chambers and allowed the ammonite to regulate the proportions of liquid and gas inside them.
As the animal grew, it added progressively larger chambers around the outside of its shell. The partitions separating the chambers met the shell wall along intricate suture lines. These sutures, together with external ribbing, shell proportions, coiling and whorl shape, provide important evidence for ammonite classification.
Because ammonites evolved rapidly and were widely distributed through Jurassic marine environments, their fossils are valuable biostratigraphical tools. Particular ammonite assemblages can help geologists identify and correlate rock layers of similar age across different localities.
Poitou Formation Fossil from Normandy
The Poitou Formation preserves sedimentary rocks deposited within the Middle Jurassic seas of France. These strata provide evidence of changing water depth, sediment supply, marine circulation and biological communities during the Bajocian.
Calvados holds exceptional importance in the history of Jurassic palaeontology. The Bajocian Stage takes its name from the region around Bayeux, and ammonites collected from Normandy have played an important role in studying and subdividing Middle Jurassic rock successions.
A specimen retaining its species name, formation, geological interval and locality offers greater scientific and collectable value than a fossil without recorded provenance. This Orthogarantiana densicostata would make a distinctive addition to an ammonite collection, geological study cabinet, educational display or natural-history collection.
Actual Specimen with Certificate of Authenticity
This is a genuine fossil specimen and not a cast, replica or reproduction. It has been individually selected for its natural appearance, geological character and collectable appeal.
The photograph shows the actual Orthogarantiana densicostata ammonite fossil you will receive. Full sizing is shown in the photograph.
A generic Certificate of Authenticity card is included with a lifetime guarantee of authenticity, providing lasting confidence that this is a genuine Middle Jurassic fossil specimen.
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