Description
Asteroceras obtusum Calcite Ammonite Fossil from the Jurassic Coast
This genuine Asteroceras obtusum calcite ammonite fossil originates from the Black Ven Marls of the Lower Lias Formation exposed at Stonebarrow Cliff near Charmouth, close to Lyme Regis in Dorset, UK. This area forms part of the world-famous Jurassic Coast UNESCO World Heritage Site, renowned for its exceptional fossil record spanning the Triassic, Jurassic, and Cretaceous periods.
The specimen displays the classic spiral shell form of an Early Jurassic ammonite and has undergone calcite mineralisation, where the internal chambers of the shell have been infilled with crystalline calcite. This mineralisation often highlights the internal structure of the ammonite and creates a visually striking fossil while preserving the original biological form.
The fossil has been carefully selected for its preservation and fossil detail, and the photographs in this listing show the exact specimen you will receive. 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.
Fossil Identification and Scientific Classification
The fossil belongs to the species Asteroceras obtusum, an ammonite widely known from the Early Jurassic marine deposits of southern England.
Scientific classification includes:
- Kingdom: Animalia
- Phylum: Mollusca
- Class: Cephalopoda
- Subclass: Ammonoidea
- Order: Ammonitida
- Superfamily: Psiloceratoidea
- Family: Asteroceratidae
- Genus: Asteroceras
- Species: Asteroceras obtusum
The genus Asteroceras was formally described by the naturalist James Sowerby, who studied many of the ammonite fossils from the Lower Jurassic deposits of England.
Ammonites were marine cephalopods related to modern squid, octopus, and nautilus. Unlike their modern relatives, ammonites possessed a coiled external shell divided into internal chambers that helped regulate buoyancy in the water.
Geological Formation and Age
This fossil originates from the Black Ven Marls, part of the Lower Lias Formation, which was deposited during the Early Jurassic Period approximately 199 to 190 million years ago.
The Lower Lias sediments were deposited in a shallow marine basin that covered much of southern Britain following the end-Triassic extinction event.
The Black Ven Marls consist primarily of:
- Clay-rich marine marl
- Dark shale layers
- Occasional limestone bands
- Fossil-rich marine sediments
Ammonites such as Asteroceras obtusum are particularly important because they serve as index fossils used in Jurassic stratigraphy. The species is closely associated with the Obtusum Biozone of the Sinemurian Stage, making it useful for dating specific layers within the Lower Lias sequence.
Morphology of Asteroceras obtusum
Asteroceras ammonites are recognised for their distinctive shell structure and ribbing patterns.
Key morphological features include:
- Planispiral coiled shell
- Moderately wide whorls
- Strong rounded ribs extending across the shell surface
- Broad rib spacing with occasional bifurcation
- Visible central umbilicus exposing inner whorls
The shell was internally divided by thin walls called septa, creating a series of chambers known as camerae. The living animal occupied the outermost chamber while the inner chambers helped control buoyancy.
The ribs and shell form also strengthened the shell structure, helping it withstand pressure in the marine environment.
Calcite Mineralisation
In this specimen, the internal chambers of the ammonite have been infilled with calcite crystals, a common mineralisation process within the Black Ven Marls.
This occurs when mineral-rich groundwater enters the empty chambers of the shell after burial. Calcium carbonate crystallises over time, forming calcite that preserves the internal chamber structure.
Calcite preservation often highlights the ammonite’s internal geometry, producing reflective crystalline textures that contrast with the surrounding rock matrix.
Marine Environment of the Early Jurassic Dorset Coast
During the Early Jurassic, southern England lay beneath a warm shallow epicontinental sea connected to the wider Tethys Ocean.
The marine ecosystem supported a diverse range of organisms including:
- Ammonites such as Asteroceras
- Belemnites
- Bivalves and gastropods
- Marine fish
- Marine reptiles including ichthyosaurs and plesiosaurs
The fine mud that accumulated on the seabed created ideal conditions for fossil preservation, allowing shells and skeletal remains to be buried rapidly within sediment.
Fossil Locality of Stonebarrow Cliff and Charmouth
Stonebarrow Cliff near Charmouth is one of the most productive fossil localities along the Dorset coast. The cliffs expose extensive Lower Lias sediments and are well known among collectors and researchers for producing ammonites and marine reptile fossils.
Natural erosion along the coastline continually reveals new fossils from these Jurassic deposits, contributing to the long history of fossil discoveries in the region.
Lyme Regis and Charmouth became internationally famous for fossil discoveries during the nineteenth century, helping establish the scientific study of prehistoric life.
Authentic Fossil Specimen
This specimen is a genuine Asteroceras obtusum calcite ammonite fossil from the Black Ven Marls of the Lower Lias at Stonebarrow Cliff near Charmouth, Lyme Regis, Dorset, UK.
Key details include:
- Authentic Asteroceras obtusum ammonite fossil
- Calcite mineralised internal chambers
- Geological Formation: Black Ven Marls
- Stratigraphy: Lower Lias Formation
- Geological Age: Early Jurassic Period
- 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 fossil represents a well-preserved Early Jurassic ammonite from the ancient seas that once covered southern Britain, making it an outstanding addition to fossil collections, geological displays, and natural history collections.
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