Arnioceras Multi Block Fossil Ammonites Positive and Negative Jurassic Coast Dorset UK

Description

Arnioceras Multi Block Fossil Ammonites

This listing is for a rare Arnioceras multi block fossil ammonite slab, showing both positive and negative impressions, from the Black Ven Marls, Lower Lias Formation, Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, UK. Dating to the Early Jurassic (Sinemurian stage, approximately 199–196 million years ago), this specimen is a striking and scientifically important piece that captures multiple ammonites preserved together. It was discovered by our own team members, Alister and Alison, on the 14th September 2025, and was carefully cleaned, prepped, and treated by Alison to ensure its natural features were beautifully revealed.

Geological Context – Black Ven Marls, Jurassic Coast

The Black Ven Marls are internationally recognised as one of the richest fossil-bearing deposits in the world, forming part of the UNESCO World Heritage Jurassic Coast. These Lower Lias sediments were deposited in a shallow marine basin, where fine-grained muds and marls accumulated, preserving marine life in extraordinary detail. Ammonites from this region are renowned for their abundance, variety, and importance to paleontology. Fossils preserved in positive and negative relief, such as this multi block, provide a unique perspective on how ancient shells were embedded and compacted within the sediment.

Species Profile – Arnioceras

The genus Arnioceras belongs to the Order Ammonitida, Superfamily Psiloceratoidea, and the Family Arietitidae. These ammonites are distinguished by their strongly ribbed, evolute shells, where all whorls are clearly exposed. The ribs are bold and radiate across the whorls, often bifurcating as they reach the outer shell, creating a striking visual ornamentation. These features make Arnioceras one of the most distinctive ammonite genera of the Sinemurian stage. As free-swimming predators, Arnioceras ammonites thrived in Early Jurassic seas, preying upon smaller marine invertebrates.

Morphology and Notable Features

This specimen is particularly notable for preserving multiple ammonites in positive and negative relief, showing both the raised fossil impressions and the natural counter-impressions within the matrix. Such fossils provide an excellent teaching tool for understanding the process of fossilisation and sediment compaction. The ammonites exhibit the classic evolute coiling and bold ribbing of Arnioceras, making them visually impressive as well as scientifically significant. A multi block like this not only offers aesthetic appeal but also captures the natural abundance of ammonites in the Jurassic seas.

Scientific and Stratigraphic Importance

Arnioceras is an important ammonite genus for biostratigraphy, defining part of the Arnioceras semicostatum biozone within the Lower Sinemurian stage. Their rapid evolutionary turnover and widespread distribution make them key index fossils, enabling geologists to correlate Lower Jurassic strata across Europe and beyond. Fossil blocks containing multiple specimens further highlight the dense populations that once inhabited the seas of the Early Jurassic, providing insight into both biodiversity and paleoecology.

Ethical Discovery and Preparation

This fossil was responsibly collected by our team from Stonebarrow Cliff and meticulously prepared by Alison, who used specialist techniques to reveal the ammonites while preserving both positive and negative impressions. Such careful preparation ensures the fossil retains its educational and display value, while ethical collecting practices guarantee its provenance.

Authenticity and Presentation

All our fossils are 100% genuine specimens and come with a Certificate of Authenticity. The photographs show the exact specimen you will receive, not a stock image. For accurate sizing, a 1 cm scale cube is included in the images. This multi block fossil has been selected for its striking preservation and its rare combination of positive and negative impressions, making it an exceptional addition to any fossil collection, museum display, or educational resource.

This Arnioceras multi block fossil ammonite slab from the Black Ven Marls, Jurassic Coast, Charmouth, Dorset, UK, is an outstanding specimen that beautifully captures both positive and negative ammonite impressions, offering scientific value and natural artistry in one piece.