Description
Genuine Orthogarantiana densicostata Ammonite Fossil
This carefully chosen fossil is a genuine macroconch specimen of Orthogarantiana densicostata, an ammonite from the Bajocian Stage of the Middle Jurassic. It originates from Calvados in Normandy, France, an internationally important region for Jurassic geology and ammonite biostratigraphy.
The photograph shows the actual fossil specimen you will receive, allowing you to inspect its individual shape, ribbing, preservation, colour, matrix and natural geological features before purchasing. 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 Age
Scientific name: Orthogarantiana densicostata
Fossil type: Ammonite
Growth form: Macroconch
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Stephanoceratoidea
Family: Stephanoceratidae
Subfamily: Garantianinae
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Bajocian
Locality: Calvados, Normandy, France
The species was originally described by the German palaeontologist Friedrich August von Quenstedt during the nineteenth century. It has subsequently been placed within the genus Orthogarantiana, although some classifications treat the genus as a subgenus of Garantiana.
Orthogarantiana densicostata is associated with the Upper Bajocian Niortense Zone, particularly the Polygyralis Subzone and the Densicostata Horizon. These finely divided ammonite biozones allow palaeontologists to identify and correlate narrow intervals of Jurassic geological time.
Macroconch Ammonite Form
The term macroconch refers to the larger member of a sexually dimorphic ammonite pair. Many ammonite groups are interpreted as having two adult shell forms: a relatively large macroconch and a smaller microconch. These forms are generally regarded as representing females and males respectively, although direct confirmation is impossible because the soft-bodied animals are extinct.
The macroconch of Orthogarantiana densicostata developed a larger mature shell and a more extended adult body chamber. Its associated microconch form has traditionally been linked with ammonites classified within Strenoceras.
This macroconch identification adds particular scientific and collectable interest to the specimen because it reflects the complex growth patterns and probable sexual dimorphism present among Jurassic ammonites.
Shell Shape and Dense Ribbing
The species name densicostata refers to the densely ribbed ornamentation characteristic of this ammonite. The shell is generally evolute, with a broad central umbilicus that leaves much of the earlier coiling visible.
Numerous closely spaced primary ribs arise near the umbilical margin and travel across the flanks. These commonly divide into secondary ribs before passing over the outer edge of the shell. The resulting pattern creates the strongly sculptured appearance associated with the Garantianinae.
Depending on preservation, the specimen may display areas of clear ribbing, exposed whorls, natural matrix, mineral replacement or surface weathering. Variations in texture, colour and completeness are authentic results of fossilisation and make every ammonite individually distinctive.
Bajocian Geology of Calvados
Calvados contains some of Normandy’s most historically significant Middle Jurassic deposits. The Bajocian Stage itself takes its name from the Latin form of Bayeux, a town in the Calvados department. The stage was formally introduced by the French palaeontologist Alcide d’Orbigny during the nineteenth century.
During the later Bajocian, the area now occupied by Normandy was covered by a shallow sea connected to the wider Jurassic marine basins of north-western Europe. Sediment accumulated across the seabed as layers of carbonate mud, limestone, sand, clay and locally iron-rich oolitic material.
The fossil-bearing Oolithe Ferrugineuse de Bayeux, or Bayeux Iron Oolite, is especially well known for its detailed succession of Upper Bajocian ammonites. Important exposures and quarry sections in Calvados have yielded diverse members of the Garantianinae and helped palaeontologists establish a refined regional biostratigraphy.
These deposits preserve evidence of changing sea levels, sedimentation rates and marine habitats. Ammonites occur alongside belemnites, bivalves, brachiopods, gastropods, echinoids and other organisms that inhabited the Jurassic seabed and surrounding water column.
Life in the Middle Jurassic Sea
Ammonites were marine cephalopods related to living squid, cuttlefish, octopuses and nautiluses. The animal occupied the final and largest section of its coiled shell, known as the body chamber.
The inner shell was divided by curved walls called septa into a series of chambers. A narrow tube known as the siphuncle connected these chambers and enabled the animal to regulate the balance of liquid and gas within them. This system provided buoyancy control and helped the ammonite maintain its position in the sea.
Movement was achieved through jet propulsion. Water was drawn into the mantle cavity and expelled through a muscular funnel. Tentacles surrounding the mouth were probably used to capture small prey, gather food and interact with the surrounding environment.
The strongly ribbed shell of Orthogarantiana densicostata was not merely decorative. Ribbing could strengthen the shell against water pressure and physical damage while reflecting the animal’s growth throughout its life.
Natural Fossilisation and Preservation
After the ammonite died, its shell settled onto the Bajocian seabed. Soft tissues decayed, while the shell was gradually covered by marine sediment. Burial protected part of the shell from erosion and scavenging.
Over millions of years, compaction and mineral-rich groundwater transformed the surrounding sediment into solid rock. The original aragonitic shell could be replaced by minerals, preserved as a thin layer or dissolved away. Sediment filling the empty shell produced an internal mould that retained the overall coiling and rib pattern.
Natural fractures, worn areas, matrix attachments, mineral deposits and small incomplete sections are normal characteristics of an ancient fossil. They record the specimen’s long history of burial, mineralisation, geological movement and eventual exposure.
Authentic Jurassic Display Specimen
This Orthogarantiana densicostata macroconch is an appealing specimen for ammonite collectors, fossil enthusiasts, geology students and anyone interested in Jurassic palaeontology. Its named species, diagnostic dense ribbing, macroconch form and Calvados provenance give it considerable educational and natural-history interest.
It is suitable for display in a fossil cabinet, study, office, classroom or private collection. It also makes a distinctive gift for a collector or anyone fascinated by prehistoric marine life and the geology of Normandy.
The photograph shows the exact carefully chosen fossil specimen you will receive. Full sizing can be seen in the photo. A generic Certificate of Authenticity card with a lifetime guarantee is included.
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