Description
Asteroceras obtusum Calcite Ammonite from the Jurassic Coast
This authentic Asteroceras obtusum calcite ammonite fossil comes from the Black Ven Marls of the Lower Lias Formation at Stonebarrow Cliff near Charmouth, Lyme Regis, Dorset, United Kingdom. The Lyme Regis area forms part of the internationally famous Jurassic Coast UNESCO World Heritage Site, renowned for producing exceptional Early Jurassic marine fossils.
The specimen represents the species Asteroceras obtusum, a well-known Early Jurassic ammonite that lived approximately 196 to 190 million years ago during the Sinemurian Stage of the Lower Jurassic. Fossils from this species are particularly associated with the Obtusum Biozone, an important ammonite biozone used by palaeontologists to date Early Jurassic rock layers.
This fossil displays the characteristic ribbed shell morphology of the species and has undergone natural mineralisation that produced calcite crystallisation within the shell, enhancing both preservation and visual appeal. The specimen has been carefully selected for its display quality, making it a superb addition to fossil collections, geological displays, or educational collections.
The photographs show the exact fossil specimen you will receive, and full sizing can be viewed in the listing images.
This fossil is a genuine specimen and includes a Certificate of Authenticity lifetime guarantee generic card, confirming its authenticity and geological origin.
Scientific Classification and Fossil Identification
Asteroceras belongs to the extinct group of marine cephalopods known as ammonites, which were among the most successful marine animals of the Mesozoic Era.
Scientific classification includes:
- Kingdom: Animalia
- Phylum: Mollusca
- Class: Cephalopoda
- Subclass: Ammonoidea
- Order: Ammonitida
- Superfamily: Psiloceratoidea
- Family: Asteroceratidae
- Genus: Asteroceras
- Species: Asteroceras obtusum
The species Asteroceras obtusum is a classic Early Jurassic ammonite used extensively in biostratigraphy, particularly for identifying the Obtusum Zone of the Sinemurian Stage.
Distinctive Morphology of Asteroceras obtusum
Asteroceras ammonites are recognised by their distinctive shell structure and ribbed ornamentation.
Key morphological features include:
- Moderately involute shell coiling
- Strong radial ribbing extending across the whorls
- Broad rounded whorl cross-sections
- Ribbing that may bifurcate toward the outer whorl
- Prominent growth lines along the shell surface
The ammonite shell consisted of multiple internal chambers separated by curved partitions known as septa. The animal lived in the outer chamber while the inner chambers were used to regulate buoyancy through a structure called the siphuncle.
These buoyancy adaptations allowed ammonites to move vertically through the water column and maintain balance while swimming.
Geological Formation and Age
The fossil originates from the Black Ven Marls, part of the Lower Lias Formation, a sequence of marine sediments deposited during the Early Jurassic Period.
Key geological information includes:
- Formation: Black Ven Marls
- Stratigraphic Group: Lower Lias
- Geological Period: Jurassic
- Stage: Sinemurian
- Biozone: Obtusum Zone
- Approximate Age: 196–190 million years
The Black Ven Marls consist of alternating layers of clay-rich marl, shale, and limestone bands, deposited in a relatively calm marine basin.
These sediments are famous for preserving a wide range of marine fossils including ammonites, marine reptiles, and fish.
Early Jurassic Marine Environment of Dorset
During the Early Jurassic, much of southern Britain was submerged beneath a warm shallow sea connected to the Tethys Ocean. This marine basin supported a diverse ecosystem of marine life.
Common organisms found within these deposits include:
- Ammonites such as Asteroceras
- Belemnites
- Marine reptiles including ichthyosaurs and plesiosaurs
- Early marine fish
- Bivalves and brachiopods
Fine sediment accumulated on the seabed, gradually burying shells and skeletal remains which later fossilised within the sediment layers.
Calcite Mineralisation and Fossil Preservation
Many ammonites from the Lower Lias are preserved with calcite mineralisation, which occurs when calcium carbonate crystallises within the shell cavities during fossilisation.
This process often produces:
- Bright calcite crystal infilling
- Enhanced structural preservation
- Visible internal chamber details
Calcite preservation is highly prized among collectors because it highlights the internal architecture of the ammonite shell while adding natural mineral beauty to the specimen.
Fossil Collecting at Stonebarrow Cliff
Stonebarrow Cliff near Charmouth is one of the most productive fossil localities along the Dorset coastline. Erosion of the cliffs continually exposes new fossil-bearing layers of the Lower Lias, revealing marine fossils that have remained buried for nearly 200 million years.
The region has played a crucial role in the development of palaeontology since the nineteenth century, when fossil discoveries from Lyme Regis helped scientists understand prehistoric marine ecosystems.
Authentic Fossil Specimen
This specimen represents a genuine Asteroceras obtusum calcite ammonite fossil from the Black Ven Marls of the Lower Lias Formation at Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, UK.
Key details include:
- Authentic Asteroceras obtusum ammonite fossil
- Calcite mineralised shell structure
- Geological Formation: Black Ven Marls
- Stratigraphy: Lower Lias Formation
- Geological Age: Early Jurassic
- Stage: Sinemurian
- Biozone: Obtusum Zone
- Locality: Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, United Kingdom
- Fossil from the Jurassic Coast UNESCO World Heritage Site
- Exact specimen shown in listing photographs
- Full sizing visible in listing images
- Includes Certificate of Authenticity lifetime guarantee generic card
This ammonite fossil represents a preserved marine cephalopod from the Early Jurassic seas that once covered southern Britain, offering an exceptional specimen for fossil collectors, geological enthusiasts, and natural history displays.
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