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 renowned Green Ammonite Beds at Stonebarrow, near Charmouth and Lyme Regis, Dorset. This carefully chosen specimen presents a naturally rounded pebble form created entirely by coastal processes, while still clearly preserving the ammonite within its original matrix. The calcite-replaced shell contrasts attractively with the characteristic green-grey tones of the surrounding rock, producing a tactile and visually appealing fossil that remains completely natural and unaltered. The photographs show the exact specimen you will receive, with full sizing details visible. As with all of our fossils, this piece is 100% genuine and supplied with a Certificate of Authenticity.

Geological Formation & Provenance

The fossil originates from Stonebarrow, one of the most important and well-documented sections of the Jurassic Coast, a UNESCO World Heritage Site of global geological significance. The Green Ammonite Beds form part of the Lower Lias Group, deposited during the Early Jurassic Period, approximately 196–193 million years ago. These sediments accumulated in a shallow marine environment on a continental shelf, where variations in sediment chemistry produced the distinctive greenish matrix associated with this horizon. Natural erosion along the Dorset coastline has released and gently rounded some fossils into pebble forms, creating specimens such as this while preserving their 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 abundant and diverse organisms in Jurassic seas. Oistoceras is a characteristic Lower Lias ammonite, valued for its stratigraphic significance and its role in defining Early Jurassic marine faunas within the Green Ammonite Beds.

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 strengthened the fossil and enhanced contrast between the shell and surrounding matrix, allowing the ammonite to remain clearly identifiable. The pebble form is entirely the result of natural marine and coastal erosion, not artificial polishing, making this an authentic example of how fossils are shaped and revealed along the Jurassic Coast.

Depositional Environment & Natural Formation

During the Early Jurassic, the area 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 within marl and limestone. Much later, coastal erosion at Stonebarrow exposed these fossils, and some were naturally rounded into pebble forms while retaining their internal fossil detail.

Scientific & Collecting Significance

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

Display, Care & Authenticity

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

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