Asteroceras Calcite Ammonite Fossil Jurassic Coast Charmouth Lyme Regis Dorset UK Specimen Authentic

Description

Genuine Asteroceras Calcite Ammonite from the Jurassic Coast

This authentic Asteroceras fossil ammonite preserved in calcite originates from the Black Ven Marls of the Lower Lias along the Jurassic Coast at Stonebarrow Cliff, Charmouth, near Lyme Regis in Dorset, UK. The Jurassic Coast is internationally recognised as a UNESCO World Heritage Site and is famous for producing some of the finest marine fossils from the Early Jurassic period.

The specimen has been carefully selected for its preservation and calcite mineralisation, which highlights the internal chamber structure of the ammonite. Calcite replacement often creates striking crystalline textures within the fossil, adding visual interest while preserving the original biological structure of the shell.

The photographs in this listing show the exact fossil specimen you will receive, allowing collectors to clearly view the fossil’s spiral form, rib structure, and mineralisation before purchase. Full sizing can be seen in the listing images.

This fossil is a genuine specimen and includes a fully hand-signed Premium Certificate of Authenticity, confirming its authenticity and geological origin.

Fossil Identification and Scientific Classification

The fossil belongs to the genus Asteroceras, a distinctive group of Early Jurassic ammonites known for their strongly ribbed shells and symmetrical spiral coiling.

Scientific classification includes:

• Kingdom: Animalia
• Phylum: Mollusca
• Class: Cephalopoda
• Subclass: Ammonoidea
• Order: Ammonitida
• Superfamily: Psiloceratoidea
• Family: Asteroceratidae
• Genus: Asteroceras

The genus Asteroceras was formally described by the palaeontologist James Sowerby, whose work on Jurassic fossils from southern England helped establish the early scientific study of ammonites.

Ammonites were marine cephalopods related to modern squid, octopus, and nautilus, but unlike modern cephalopods they possessed an external coiled shell.

Geological Age and Stratigraphic Context

This fossil originates from the Lower Lias deposits of the Early Jurassic Period, approximately 201 to 174 million years ago. The Lower Lias is well known for its fossil-rich marine sediments that record the early stages of Jurassic marine ecosystems.

The specimen comes specifically from the Black Ven Marls, a series of dark marine clay and marl deposits exposed along the cliffs between Charmouth and Lyme Regis. These sediments accumulated in a shallow marine environment within the Jurassic seas that once covered southern Britain.

The Lower Lias is divided into several ammonite biozones used by geologists for stratigraphic dating, and species of Asteroceras are commonly associated with the Obtusum Zone, a well-known ammonite biozone of the Sinemurian Stage.

Morphology and Shell Structure

Asteroceras ammonites are recognised for their distinctive shell morphology. The shell forms a tightly coiled spiral known as a planispiral shell, where each whorl grows outward from the previous one.

Key morphological features include:

• Strong, evenly spaced ribbing across the shell surface
• Rounded whorl cross-sections
• Deeply incised rib patterns extending across the shell
• Central umbilicus where inner coils are visible

The ribs provided structural reinforcement for the shell and are one of the most recognisable features used in ammonite identification.

Internally, the shell was divided into a series of chambers separated by walls called septa. The living animal occupied the outermost chamber while the inner chambers were used for buoyancy control.

Calcite Mineralisation

In this specimen, the ammonite chambers have been replaced or infilled with calcite, a common mineralisation process within the Black Ven Marls. Calcite preservation often produces crystalline textures that reflect light differently from the surrounding matrix, highlighting the internal structure of the fossil.

This type of preservation occurs when mineral-rich groundwater enters the empty shell chambers after burial. Over time, dissolved calcium carbonate crystallises within the chambers, forming calcite crystals while preserving the original shell shape.

Depositional Environment of the Black Ven Marls

During the Early Jurassic, the region that is now Dorset formed part of a warm shallow sea covering much of southern Britain. The seabed accumulated fine clay and marl sediments where marine organisms were buried and fossilised.

The environment included:

• Shallow marine shelf waters
• Calm sediment deposition
• Soft muddy seabed conditions
• Abundant marine life including ammonites, bivalves, and marine reptiles

The Lower Lias deposits of the Jurassic Coast are famous for preserving a wide range of marine fossils including:

• Ammonites such as Asteroceras
• Belemnites
• Marine bivalves
• Marine reptiles including ichthyosaurs and plesiosaurs

Fossilisation and Preservation

When the ammonite died, the shell sank to the seabed and became buried within fine sediment. Over millions of years, mineralisation processes preserved the shell and internal chamber structure. In many cases, the shell itself dissolves while mineral deposits such as calcite preserve the form of the original organism.

The cliffs at Charmouth and Lyme Regis continue to erode naturally, exposing new fossils from these ancient marine sediments.

Collecting and Scientific Importance

Ammonites are among the most important index fossils used in geological dating, allowing scientists to identify and correlate sedimentary rock layers across wide regions. Asteroceras is particularly significant within Early Jurassic stratigraphy.

Collectors value ammonites from the Jurassic Coast for several reasons:

• Classic spiral shell morphology
• Distinctive ribbed shell structure
• Historic fossil locality recognised worldwide
• Association with the Early Jurassic marine ecosystem

Specimens from the Charmouth and Lyme Regis area are especially prized due to the region’s long history of fossil discovery and scientific study.

Authentic Fossil Specimen

This specimen is a genuine Asteroceras calcite ammonite fossil from the Black Ven Marls of the Lower Lias on the Jurassic Coast at Stonebarrow Cliff, Charmouth, near Lyme Regis, Dorset, UK.

Key details include:

• Authentic Asteroceras ammonite fossil
• Mineralisation: Calcite chamber preservation
• Formation: Black Ven Marls
• Stratigraphy: Lower Lias, Sinemurian Stage
• Geological Age: Early Jurassic Period
• Locality: Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, UK
• UNESCO Jurassic Coast fossil locality
• Exact fossil shown in listing photographs
• Full sizing visible in listing images
• Includes fully hand-signed Premium Certificate of Authenticity

This fossil represents a remarkable example of Early Jurassic marine life preserved within the famous fossil-bearing cliffs of the Dorset Jurassic Coast, making it a highly desirable addition to fossil collections and natural history displays.