Description
Rare Olcostephanus cf. infundibulatum Ammonite Fossil
This rare and genuine ammonite fossil is identified as Olcostephanus cf. infundibulatum and originates from Drôme in the Auvergne-Rhône-Alpes region of south-eastern France. It dates from the Hauterivian Stage of the Lower Cretaceous, preserving evidence of marine life from approximately 133 to 129 million years ago.
The abbreviation “cf.” comes from the Latin confer, meaning “compare with.” It indicates that the specimen closely resembles material attributed to Olcostephanus infundibulatum, although its preserved characteristics do not permit an entirely certain species-level identification. This careful terminology is widely used in palaeontology when a fossil displays the principal features of a species but is incomplete, weathered or affected by natural variation.
The photographs show the exact fossil you will receive. This carefully chosen piece has been selected for its distinctive ammonite morphology, rarity and documented Lower Cretaceous provenance. Full sizing and scale information are provided in the listing photographs.
Olcostephanus Classification and Geological Importance
Olcostephanus was established as an ammonite genus by the Austrian palaeontologist Melchior Neumayr in 1875. It is the type genus of the family Olcostephanidae, an important group of Early Cretaceous ammonites particularly characteristic of Valanginian and early Hauterivian marine deposits.
Its broad scientific classification includes:
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Suborder: Ammonitina
Superfamily: Perisphinctoidea
Family: Olcostephanidae
Genus: Olcostephanus
Comparable species: Olcostephanus infundibulatum
Genus authority: Neumayr, 1875
Ammonites were extinct marine cephalopods related to living squid, cuttlefish, octopuses and nautiluses. Their rapid evolution and widespread distribution make them highly valuable index fossils for dating and correlating Mesozoic marine rocks.
Distinctive Olcostephanid Morphology
Ammonites belonging to Olcostephanus commonly possess strongly coiled, comparatively robust shells with rounded or inflated whorl sections. Successive whorls may overlap considerably, producing a compact shell with a relatively restricted central umbilicus.
Their ornamentation typically consists of prominent primary ribs arising near the umbilical margin. These may begin at swollen points or small umbilical tubercles before dividing into several finer secondary ribs across the flanks. The secondary ribs continue towards and across the venter, or outer edge of the shell, creating the characteristic bundled rib pattern associated with many olcostephanid ammonites.
Rib strength, rib density, whorl inflation and umbilical width could change as the ammonite matured. These growth-related differences are important when comparing individual specimens and distinguishing closely related species. Depending on preservation, this fossil may display external shell ornamentation, a sediment-filled internal mould or a combination of both.
The naturally sculpted ribbing gave the shell additional structural strength without requiring excessive shell thickness. Its rounded form and chambered construction also assisted the living animal in maintaining buoyancy within the water column.
Chambered Shell and Ammonite Life
The shell of this ammonite was divided internally by curved partitions called septa. These created a sequence of chambers connected by a narrow tube known as the siphuncle. The animal occupied the final and largest chamber, while the abandoned earlier chambers formed the buoyancy apparatus of the shell.
By controlling the balance of fluid and gas within these chambers, the ammonite could regulate its position in the water. The intersections between the septa and the outer shell wall formed intricate suture lines, which are important features in ammonite classification.
The living animal would have extended its head, eyes and muscular arms from the shell aperture. It probably fed on small marine animals, planktonic organisms or organic material captured within the water. Although its precise behaviour cannot be observed directly, the shell form indicates a fully marine cephalopod adapted to life in the Early Cretaceous sea.
Hauterivian Lower Cretaceous Age
The Hauterivian Stage followed the Valanginian and preceded the Barremian within the Lower Cretaceous. This interval witnessed continuing diversification among ammonites, marine reptiles, fish, bivalves, gastropods, brachiopods and numerous microscopic organisms.
Olcostephanus is especially associated with the later Valanginian and earlier Hauterivian. Its relatively restricted geological range makes the genus useful in biostratigraphy, the scientific practice of dating and correlating sedimentary rocks through their fossil content.
The recorded Hauterivian age gives this specimen valuable geological context. Rather than being simply described as a general Cretaceous ammonite, it can be placed within a defined interval of Early Cretaceous marine history.
Ancient Tethyan Sea of South-Eastern France
During the Hauterivian, the area now forming Drôme lay within the French Subalpine Basin along the northern margin of the ancient Tethys Ocean. Much of south-eastern France was submerged beneath a marine environment in which thick sequences of limestone and marl accumulated.
These sediments were deposited in offshore settings influenced by changing sea levels, currents, water depth, oxygen availability and the supply of fine material from nearby land. The resulting rocks preserve diverse ammonite assemblages alongside belemnites, nautiloids, bivalves, gastropods, brachiopods, echinoids and other marine fossils.
Fine carbonate mud and clay settled slowly across the seabed. Alternating conditions produced the layered limestone and marl formations now exposed throughout the mountains, valleys and eroded slopes of the Auvergne-Rhône-Alpes region.
The widespread marine connections of the Tethyan realm allowed ammonites to disperse between different basins. Their fossils are therefore valuable for comparing Lower Cretaceous successions across France and other parts of Europe.
Natural Fossilisation and Preservation
After this ammonite died, its shell descended to the seabed and was buried beneath marine sediment. The soft tissues decomposed, while mud and mineral-rich material entered the empty shell chambers and surrounded the exterior.
As additional layers accumulated, the sediment was compacted and gradually transformed into rock. The original aragonite shell could be altered, dissolved or replaced by other minerals. Sediment hardened within the chambers, preserving the ammonite as an internal mould, while portions of the original exterior could retain the ribs and other shell features.
Natural fractures, incomplete edges, matrix deposits, colour variations, mineral markings and weathered areas are authentic results of burial, fossilisation and later geological exposure. These features document the specimen’s individual history and ensure that it is completely unique.
Drôme Fossil Provenance
Drôme is internationally recognised for its fossil-rich Lower Cretaceous marine strata. The region forms part of the historically important Vocontian and French Subalpine geological areas, where extensive natural exposures have contributed significantly to the study of ammonite evolution and Cretaceous biostratigraphy.
The limestones and marls of this region have produced numerous scientifically important ammonite groups. Specimens with recorded regional and stratigraphic provenance are particularly desirable because their original geological setting remains documented.
This Olcostephanus cf. infundibulatum therefore combines distinctive shell ornamentation with a classic French fossil locality and a defined Hauterivian age, making it suitable for specialist and general fossil collections alike.
Actual Specimen and Certificate of Authenticity
This is a genuine fossil specimen and not a cast, replica or artificial reproduction. The listing photographs show the actual Olcostephanus cf. infundibulatum ammonite that will be supplied, allowing you to examine its natural shape, ribbing, colour, matrix and individual preservation before purchasing.
Full sizing is shown in the photographs.
The fossil includes a generic Certificate of Authenticity card carrying a lifetime guarantee. Its rare olcostephanid identity, robust ribbed morphology, Lower Cretaceous age and documented Drôme provenance make it an excellent addition to an ammonite collection, French fossil display, natural-history cabinet or educational geological collection.
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