Description
Asteroceras obtusum Calcite Ammonite from the Jurassic Coast of Dorset
This genuine Asteroceras obtusum calcite ammonite fossil was discovered within the Black Ven Marls of the Lower Lias Formation at Stonebarrow Cliff near Charmouth, Lyme Regis, Dorset, England. Lyme Regis sits along the world-famous Jurassic Coast UNESCO World Heritage Site, a coastline internationally recognised for its exceptional Early Jurassic fossil record.
The species Asteroceras obtusum lived during the Early Jurassic Period, approximately 196–190 million years ago, within the Sinemurian Stage. This ammonite species is particularly important in palaeontology because it defines the Obtusum Biozone, a widely used ammonite biozone that helps geologists date and correlate Early Jurassic marine sediments across Europe.
This specimen displays the distinctive ribbed shell morphology typical of the species and has undergone natural calcite mineralisation, where calcite crystals form within the shell chambers during fossilisation. This mineralisation enhances the internal structure and adds striking visual detail to the fossil.
The ammonite has been carefully selected for quality and preservation, making it a desirable display specimen for collectors and natural history enthusiasts. The photographs in the listing 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 provenance.
Scientific Classification and Identification
Ammonites belong to an extinct group of marine cephalopods that were closely related to modern squid and octopus. They were among the most abundant marine animals throughout the Mesozoic Era.
Scientific classification of this specimen includes:
- Kingdom: Animalia
- Phylum: Mollusca
- Class: Cephalopoda
- Subclass: Ammonoidea
- Order: Ammonitida
- Superfamily: Psiloceratoidea
- Family: Asteroceratidae
- Genus: Asteroceras
- Species: Asteroceras obtusum
The genus Asteroceras is characteristic of the Early Jurassic and is frequently used by palaeontologists for biostratigraphic correlation, especially within Sinemurian rock sequences.
Morphology and Shell Structure
Asteroceras ammonites are known for their distinctive shell form and ornamental ribbing.
Typical morphological features include:
- Planispiral shell coiling
- Moderately involute whorls
- Strong radial ribbing across the shell surface
- Rounded whorl cross-section
- Ribbing that may bifurcate toward the outer whorl
The shell was internally divided by curved partitions known as septa, which created a series of chambers called camerae. The ammonite animal lived in the outermost chamber, while the inner chambers functioned as a buoyancy system.
Gas and fluid within these chambers were regulated through a tube-like structure called the siphuncle, allowing the animal to maintain balance and move vertically within the water column.
Geological Formation and Age
This ammonite fossil originates from the Black Ven Marls, a member of the Lower Lias Formation, deposited during the Early Jurassic.
Key geological details include:
- 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 primarily of dark marine clay, marl, and thin limestone bands, deposited in relatively calm marine conditions. These sediments are particularly well known for preserving marine fossils in remarkable detail.
Early Jurassic Marine Environment of Southern England
During the Early Jurassic, southern Britain was covered by a warm shallow sea connected to the Tethys Ocean. This marine environment supported a wide variety of marine life.
Fossils commonly found in the Lower Lias include:
- Ammonites such as Asteroceras
- Belemnites
- Marine reptiles including ichthyosaurs and plesiosaurs
- Early marine fish
- Bivalves and brachiopods
Fine mud settled slowly onto the seabed, burying shells and skeletal remains. Over millions of years these remains became fossilised within the sediment layers.
Calcite Mineralisation and Fossil Preservation
The internal chambers of many Lower Lias ammonites became filled with calcite crystals during fossilisation. This mineralisation process occurred when groundwater rich in dissolved calcium carbonate entered the empty shell chambers.
As the minerals crystallised, they formed:
- Calcite crystal infill within the chambers
- Enhanced preservation of shell structure
- Attractive mineral textures visible on the fossil surface
Calcite ammonites from the Dorset coast are highly regarded by collectors due to their combination of palaeontological significance and natural mineral beauty.
Fossil Discoveries at Stonebarrow Cliff
Stonebarrow Cliff near Charmouth is one of the most productive fossil locations along the Dorset coast. Erosion of the cliffs continually exposes fresh fossil-bearing layers within the Lower Lias.
The Lyme Regis region has played a major role in the history of palaeontology since the early nineteenth century, when numerous marine reptile and ammonite discoveries helped scientists understand ancient marine ecosystems.
Today the Jurassic Coast remains one of the most important fossil localities in the world.
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, United Kingdom.
Key details include:
- Authentic Asteroceras obtusum ammonite fossil
- Calcite mineralised shell chambers
- 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, UK
- 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 beautifully preserved marine cephalopod from the Early Jurassic seas that once covered southern England, making it an exceptional specimen for fossil collectors, geology enthusiasts, and natural history displays.
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