Arnioceras Calcite Multi Block Ammonite Fossils Lower Lias Black Ven Lyme Regis Dorset UK

Description

Magnificent Arnioceras Calcite and Multi-Ammonite Fossil Block from the Jurassic Coast

This stunning Arnioceras Calcite and multi block fossil ammonite originates from the world-renowned Black Ven Marls, part of the Lower Lias Formation on the Jurassic Coast of Dorset, UK, a UNESCO World Heritage Site known for its exceptional fossil record. The specimen was discovered by our team members, Alister and Alison, on 07 October 2025, and has been meticulously cleaned, prepared, and treated by Alison to museum-quality standards. The photograph shows the actual specimen you will receive, ensuring authenticity and transparency.

Scale cube = 1 cm. Please see the photo for full sizing details.

All of our fossils are 100% genuine specimens, and each comes with a Certificate of Authenticity verifying its origin and preparation.

Geological Context and Formation

This ammonite fossil block was collected from the Black Ven Marls Member of the Charmouth Mudstone Formation, which dates to the Lower Jurassic (Sinemurian Stage, approximately 199–190 million years ago). These layers were deposited in a calm, shallow marine environment where fine-grained silts and clays accumulated on a sea floor with low oxygen levels. Such conditions prevented decay and scavenging, allowing marine life to be preserved in exceptional detail.

Over millions of years, the ammonites underwent calcite mineralisation, in which calcium carbonate replaced the original shell material, forming the gleaming crystalline structure seen today. This process not only enhanced the visual beauty of the fossils but also helped stabilise them for long-term preservation.

Scientific Classification

• Genus: Arnioceras
• Order: Ammonitida
• Superfamily: Psiloceratoidea
• Family: Arnioceratidae
• Formation: Lower Lias (Black Ven Marls, Charmouth Mudstone Formation)
• Geological Stage: Sinemurian
• Locality: Black Ven, Lyme Regis, Dorset, United Kingdom

Arnioceras ammonites are key index fossils of the Lower Jurassic period. They are characteristic of the Arnioceras semicostatum biozone, a major stratigraphic interval used by geologists to correlate rock layers across Europe. This genus represents one of the earliest and most distinctive ammonite lineages to flourish following the Triassic–Jurassic mass extinction, offering valuable insights into evolutionary recovery and marine biodiversity during the early Jurassic seas.

Morphological Description and Features

The Arnioceras ammonite is notable for its moderately evolute shell, displaying bold, well-defined ribs that radiate from the umbilicus to the venter. The ribs are evenly spaced and strong, giving the shell an elegant and balanced appearance. The whorls are slightly compressed, with a gently rounded venter and clear growth lines.

In this multi-ammonite fossil block, several Arnioceras individuals are preserved together in their natural matrix, offering a fascinating glimpse into early Jurassic marine life. The calcite mineralisation produces a subtle golden or cream lustre, accentuating the ribbing and chamber structure of each ammonite. The fine preservation allows even the smallest details, such as suture lines and rib terminations, to be seen clearly under natural light.

This specimen captures both the artistic beauty and scientific value of the Arnioceras genus — a perfect harmony of nature’s design and geological history.

Depositional Environment and Palaeontological Significance

The Black Ven Marls were deposited in a low-energy offshore setting, part of a shallow epicontinental sea that once covered much of southern England. The finely laminated marls and shales indicate slow sedimentation rates and minimal disturbance, ideal conditions for fossil preservation. Periodic anoxic events on the sea floor ensured the survival of delicate shells and skeletal structures.

Ammonites such as Arnioceras were free-swimming cephalopods, closely related to modern squids and nautiluses. Their chambered shells provided buoyancy control, enabling them to move through the water column and hunt small planktonic prey. Their abundance and rapid evolution make them excellent index fossils, used by palaeontologists to date and correlate rock layers with remarkable precision.

The presence of multiple ammonites within a single block reflects the rich marine ecosystem of the Sinemurian seas and highlights the geological significance of the Black Ven Marls as a world-class fossil locality.

Collector’s and Display Value

This Arnioceras Calcite and multi block ammonite fossil from Lyme Regis combines scientific importance with natural elegance. Its striking rib patterns, crystalline calcite finish, and multi-fossil composition make it ideal for collectors, educators, and display enthusiasts alike. The fossil has been professionally stabilised and preserved, ensuring it will remain in excellent condition for years to come.

Discovered and prepared entirely by our own team, this specimen represents both a piece of geological history and a work of natural art.

A truly exceptional Lower Jurassic Arnioceras Calcite multi-ammonite fossil from the Black Ven Marls, showcasing the timeless beauty of Dorset’s Jurassic Coast and the legacy of early life in the ancient seas.