Oistoceras Calcite Ammonite Fossil Pebble Jurassic Coast Stonebarrow Charmouth Dorset UK

Description

Specimen Overview

Offered here is a genuine Oistoceras calcite ammonite fossil pebble collected from the classic Green Ammonite Beds at Stonebarrow, near Charmouth and Lyme Regis, Dorset. This specimen has been carefully chosen for its natural presentation as a rounded pebble while still clearly preserving the ammonite within. Calcite mineralisation enhances the shell form, creating an attractive contrast against the characteristic green-grey matrix. The photographs show the exact fossil you will receive, with full sizing details clearly visible. As with all of our fossils, this piece is 100% genuine and is supplied with a Certificate of Authenticity.

Geological Formation & Provenance

The fossil originates from Stonebarrow, one of the most important and well-studied sections of the Jurassic Coast, a UNESCO World Heritage Site. The Green Ammonite Beds form part of the Lower Lias Group, deposited during the Early Jurassic, approximately 196–193 million years ago. These sediments were laid down in a shallow marine environment on a continental shelf, where periodic changes in sedimentation and seawater chemistry produced the distinctive green-toned matrix for which this horizon is known. Pebble forms such as this developed through natural coastal erosion and reworking, giving the fossil a smooth, tactile finish while retaining its geological integrity.

Fossil Type & Taxonomic Context

Oistoceras is an extinct genus of ammonite belonging to the Class Cephalopoda, Subclass Ammonoidea, and Order Ammonitida. Ammonites were free-swimming marine molluscs related to modern squid and cuttlefish and were among the most diverse and abundant animals in Jurassic seas. Oistoceras is a characteristic Lower Lias ammonite, valued for its stratigraphic importance and distinctive shell form within Early Jurassic ammonite assemblages.

Morphology & Calcite Preservation

This specimen preserves the recognisable coiled shell morphology of Oistoceras, with whorl outlines visible despite the naturally smoothed pebble exterior. Calcite replacement has preserved the original shell geometry while enhancing definition and durability. The pebble form results from natural coastal processes rather than artificial shaping, making this an authentic example of how fossils are weathered and revealed along the Dorset coast. The result is a visually pleasing and highly tactile fossil that remains firmly rooted in its geological context.

Depositional Environment & Formation

During the Early Jurassic, the region now forming Dorset lay beneath a warm, shallow sea rich in marine life. Ammonites such as Oistoceras thrived in these waters, swimming actively in search of prey. After death, shells settled onto the seabed and were buried by fine sediments. Over millions of years, compaction and mineralisation preserved the shells, and later coastal erosion exposed and naturally rounded some specimens into pebble forms, such as this example from Stonebarrow.

Scientific & Collecting Significance

Ammonites from Stonebarrow, Charmouth, and Lyme Regis have been collected and studied for over two centuries and played a fundamental role in the development of geological and stratigraphic science. The Green Ammonite Beds are especially renowned for producing well-preserved Lower Lias ammonites with reliable provenance. Pebble specimens are particularly appealing to collectors as they combine scientific authenticity with a naturally finished, display-friendly form.

Display, Care & Authenticity

This fossil is ideal for display in a private collection, educational setting, or natural history cabinet, and is equally suited to handling and close examination. The specimen has been carefully cleaned to reveal natural features, with no artificial restoration or reconstruction. You will receive the exact fossil shown, accompanied by a Certificate of Authenticity, confirming its origin and genuine nature.

A classic and visually appealing ammonite from one of the most famous fossil horizons on the Jurassic Coast, this Oistoceras calcite ammonite pebble from the Green Ammonite Beds of Stonebarrow represents an authentic piece of Early Jurassic marine history from Dorset, UK.