Description
3 Inch Asteroceras obtusum Calcite Fossil Ammonite from the Jurassic Coast
This genuine 3 inch Asteroceras obtusum fossil ammonite originates from the Black Ven Marls of the Lower Lias Formation at Stonebarrow Cliff near Charmouth, Lyme Regis, Dorset, United Kingdom, part of the internationally renowned Jurassic Coast UNESCO World Heritage Site. The fossil dates to the Early Jurassic Period approximately 199–190 million years ago, when warm shallow seas covered much of southern Britain.
The specimen displays a beautifully preserved ammonite shell with calcite mineralisation, a common feature in fossils from the Lower Lias where the original shell structure has been replaced or infilled with crystalline calcite. This process often enhances the natural ribbing and spiral structure of the ammonite, creating an attractive and highly collectible fossil.
The ammonite shown in the photographs is the exact specimen you will receive, carefully selected for its preservation and aesthetic quality. This specimen includes a fully hand-signed Premium Certificate of Authenticity, confirming it is a genuine fossil.
Please refer to the photographs for full specimen sizing and scale.
About Asteroceras obtusum – Early Jurassic Ammonite Cephalopod
Asteroceras obtusum is a well-known species of ammonite from the Early Jurassic and is widely recognised as an important index fossil used by geologists to identify specific stratigraphic layers within the Lower Jurassic.
Taxonomic classification:
- Phylum: Mollusca
- Class: Cephalopoda
- Subclass: Ammonoidea
- Order: Ammonitida
- Superfamily: Psiloceratoidea
- Family: Asteroceratidae
- Genus: Asteroceras
- Species: Asteroceras obtusum
The species lends its name to the Obtusum Biozone, an important ammonite biozone within the Sinemurian Stage of the Lower Jurassic. Fossils of this species are widely used in stratigraphy to correlate rock layers across different regions.
The genus Asteroceras was first described by the Swiss palaeontologist Louis Agassiz, whose pioneering studies of fossil organisms greatly advanced early palaeontology.
Ammonites such as Asteroceras were active marine predators related to modern squid, octopus, and cuttlefish, although they possessed an external coiled shell divided internally into chambers.
Geological Context – The Black Ven Marls of the Lower Lias
This fossil originates from the Black Ven Marls, part of the Lower Lias Formation exposed along the cliffs between Charmouth and Lyme Regis. These rocks were deposited during the Early Jurassic Sinemurian Stage, when the region lay beneath a shallow tropical sea.
The Lower Lias consists of alternating layers of limestone, shale, and marl, which accumulated gradually on the seafloor. These fine sediments created ideal conditions for the preservation of marine fossils.
The marine ecosystem preserved in the Black Ven Marls included a diverse range of organisms such as:
- Ammonites including Asteroceras
- Belemnites
- Marine reptiles such as ichthyosaurs and plesiosaurs
- Bivalves and gastropods
- Fossil fish
- Crinoids and other echinoderms
Frequent coastal erosion along the Dorset coastline continues to expose new fossils from these Jurassic deposits.
Morphology and Calcite Preservation of the Ammonite
The shell of Asteroceras obtusum forms a classic planispiral coil, with successive whorls expanding outward from the centre. Internally the shell is divided into chambers known as camerae, separated by curved septa. The living animal occupied the outermost chamber while the internal chambers helped regulate buoyancy.
Characteristic features of Asteroceras obtusum include:
- Strong, well-defined radial ribbing across the shell
- A rounded whorl profile
- Moderately evolute coiling with visible inner whorls
- Distinct growth lines showing stages of shell development
In this specimen, the shell has undergone calcite mineralisation, where calcite crystals have replaced or filled the original shell structure. This mineralisation enhances the fossil’s visual appeal and often produces a subtle crystalline texture within the ammonite.
The ribbing and spiral geometry of the shell provided structural strength while maintaining a relatively lightweight shell suitable for buoyant movement in the water column.
A Genuine Jurassic Coast Fossil with Premium Certificate
This specimen represents a genuine ammonite from one of the most famous fossil localities in the world. Fossils from the Dorset coast have been studied since the early discoveries of marine reptiles and ammonites in the 19th century.
Key features of this fossil include:
- Genuine Asteroceras obtusum fossil ammonite
- Size approximately 3 inches
- Calcite-preserved shell structure
- Geological formation: Black Ven Marls, Lower Lias
- Age: Early Jurassic, Sinemurian Stage
- Locality: Stonebarrow Cliff, Charmouth near Lyme Regis, Dorset, United Kingdom
- From the Jurassic Coast UNESCO World Heritage Site
- The exact specimen shown in the photographs
- Includes a fully hand-signed Premium Certificate of Authenticity
This ammonite preserves the elegant spiral shell of a marine cephalopod that lived in the warm Jurassic seas nearly 200 million years ago, offering collectors a striking and authentic fossil from one of Britain’s most celebrated geological locations.
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