Description
Rare Kosmoceras transitionis Iridescent Fossil Ammonite from Russia
This listing offers a rare Kosmoceras (Spinikosmoceras) transitionis ammonite fossil from the Callovian stage of the Middle Jurassic, discovered in the world-renowned fossil beds of Ryazan, Russia. Formed around 165 – 161 million years ago, this specimen represents one of the more visually striking ammonites, displaying both exquisite detail and natural iridescence. Its surface shimmers with rainbow hues, the result of well-preserved nacre that has survived millions of years of burial and geological transformation.
Each fossil is unique, and the photos provided show the exact specimen you will receive. The scale cube of 1 cm ensures accurate sizing, and as with all of our fossils, it is supplied with a Certificate of Authenticity, confirming its genuine origin and geological heritage.
Geological Context
The Callovian stage belongs to the Middle Jurassic epoch, a time when vast shallow seas extended across large parts of Europe and Russia. The Ryazan region’s marine sediments—composed primarily of calcareous clays and marls—provided perfect conditions for the preservation of ammonites. These marine environments were low in oxygen at the seabed, which slowed decomposition and preserved fine details of the ammonite shells, including iridescent nacre.
The Kosmoceras genus is an important biozone indicator in Jurassic stratigraphy. Fossils like this specimen are not only visually stunning but also hold scientific value, contributing to our understanding of Jurassic sea life, sedimentary environments, and evolutionary history.
Scientific Classification
- Genus: Kosmoceras
- Subgenus: Spinikosmoceras
- Species: Kosmoceras transitionis
- Order: Ammonitida
- Superfamily: Stephanoceratoidea
- Family: Kosmoceratidae
Members of the family Kosmoceratidae are particularly renowned for their ornate shell morphology, characterized by ribbing, nodes, and tubercles. The subgenus Spinikosmoceras is especially noted for its pronounced spines and ribbed ornamentation, giving specimens a dramatic appearance even when partially preserved.
Morphological Features
This Kosmoceras transitionis specimen exhibits the following traits:
- Strongly ribbed shell with regularly spaced costae.
- Ornamentation with tubercles and spines, typical of the Spinikosmoceras subgenus.
- Well-developed whorl section with planispiral coiling.
- Areas of preserved iridescence, producing flashes of green, blue, and gold across the shell.
- Complex suture lines, a hallmark of ammonites from this family.
These morphological traits not only make Kosmoceras a key fossil for scientific study but also ensure its enduring popularity among collectors.
Paleoenvironment and Ecology
During the Callovian stage, Kosmoceras inhabited warm, shallow epicontinental seas. It was a nektonic carnivore, using its coiled shell for buoyancy while actively hunting prey such as small crustaceans, molluscs, and plankton. The spines and ornamentation may have served both hydrodynamic functions and as defensive adaptations against predators, including marine reptiles and larger cephalopods.
The fossil-bearing layers in Ryazan reflect a marine shelf environment where periodic changes in oxygenation and sedimentation created conditions for exceptional fossil preservation. The iridescence seen in this specimen is a rare feature, highlighting both the exceptional quality of preservation and the geological uniqueness of this locality.
Collector and Display Value
This Kosmoceras transitionis ammonite fossil is a prized piece for both serious collectors and enthusiasts of natural history. Its combination of rare preservation, detailed morphology, and stunning iridescence makes it suitable for display, study, or as a centrepiece in a curated fossil collection.
Because the specimen originates from the Ryazan region of Russia, a locality long celebrated for producing high-quality ammonites, it carries additional provenance value. Each detail of the shell tells a story of evolution, adaptation, and geological history spanning millions of years.
Key Features
- Species: Kosmoceras (Spinikosmoceras) transitionis
- Age: Middle Jurassic, Callovian stage (approx. 165 – 161 million years old)
- Provenance: Ryazan, Russia
- Family: Kosmoceratidae, Superfamily Stephanoceratoidea
- Distinctive ribbing, spines, and tuberculate ornamentation
- Natural iridescent nacre preserved on shell surface
- Supplied with a Certificate of Authenticity
- Exact specimen shown is the one you will receive
Owning this Kosmoceras transitionis ammonite is an opportunity to acquire an authentic, beautifully preserved piece of Jurassic history—an iridescent fossil that combines natural artistry with scientific importance.
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