Arnioceras Fossil Ammonite Black Ven Marls Lower Lias Jurassic Coast Black Ven Lyme Regis Dorset UK

Authentic Arnioceras Fossil Ammonite

This exceptional Arnioceras Fossil Ammonite is a genuine specimen from the Black Ven Marls, Lower Lias Formation, discovered along the renowned Jurassic Coast at Black Ven, Lyme Regis, Dorset, UK. Dating from the Early Jurassic Period (approximately 195–200 million years ago), this ammonite is an outstanding example of Arnioceras semicostatum, a species celebrated for its distinctive ribbed shell and historical significance in British palaeontology.

Your specimen was discovered by our team members, Alister and Alison, on 13 September 2025, and has been expertly cleaned, prepared, and treated by Alison to preserve its natural form and detail. The photograph shows the exact specimen you will receive, making this a truly unique and authentic collector’s piece.

Geological Context and Origin

This fossil originates from the Black Ven Marls Member of the Charmouth Mudstone Formation, part of the Lower Lias Group that makes up the famous cliffs between Lyme Regis and Charmouth. These deposits belong to the Sinemurian Stage of the Early Jurassic and are part of the Arnioceras semicostatum Biozone, named after the very species this fossil represents.

The sediments were deposited in a low-energy, offshore marine environment, composed of fine muds and silts that settled slowly on the seabed. These conditions, often anoxic (oxygen-poor), prevented scavengers and slowed decomposition, allowing for exceptional preservation of marine life including ammonites, belemnites, and ichthyosaurs.

Over time, layers of sediment compressed into rock, encapsulating the remains of these ancient sea creatures. The ammonite shell was gradually replaced by minerals, preserving the detailed ribbing and whorl structure that define this species. The Black Ven Marls, in particular, are renowned for producing well-preserved examples with fine detail and excellent fossil contrast.

Scientific Classification

• Kingdom: Animalia
• Phylum: Mollusca
• Class: Cephalopoda
• Subclass: Ammonoidea
• Order: Ammonitida
• Superfamily: Psiloceratoidea
• Family: Arietitidae
• Genus: Arnioceras
• Age: Early Jurassic (Sinemurian Stage)
• Formation: Black Ven Marls, Lower Lias
• Locality: Black Ven, Lyme Regis, Dorset, UK

The genus Arnioceras was first formally described in the early 19th century and plays an important role in stratigraphy as an index fossil. Its presence marks a specific geological timeframe — the Arnioceras semicostatum Zone — making it a cornerstone for correlating Early Jurassic strata across Europe.

Morphological Features and Preservation

This Arnioceras Ammonite Fossil displays the characteristic features of the genus: a planispiral, evolute shell with prominent straight radial ribs that cross the flanks and extend to the venter without interruption. The whorls are wide and evenly coiled, forming an elegant spiral with a broad umbilicus typical of the species.

Under careful preparation, the fossil’s fine ribbing and subtle mineralisation are beautifully revealed, showing its distinctive texture against the marlstone matrix. The ammonite is preserved naturally with a combination of subtle pyritic and calcitic infill, giving a gentle metallic or crystalline sheen under light. These features make it both scientifically valuable and aesthetically striking.

The scale cube (1 cm) in the photograph provides accurate size reference, and full dimensions can be found in the accompanying images. The fossil has been professionally stabilised and treated to ensure durability while maintaining its original geological integrity.

Palaeobiological and Scientific Significance

Ammonites like Arnioceras were marine cephalopods, distant relatives of modern squids and nautiluses. They were active predators, using jet propulsion to move through the water as they hunted smaller marine organisms. Their coiled shells contained internal chambers that allowed for buoyancy control, a sophisticated adaptation that made them highly efficient swimmers in the open sea.

The Arnioceras semicostatum species represents one of the earliest and most characteristic ammonites of the Sinemurian, marking a critical phase in the evolutionary recovery following the Triassic–Jurassic extinction event. Fossils like this provide key insights into the adaptation and diversification of early Jurassic marine fauna.

The detailed preservation of ribbing and shell structure in this specimen makes it particularly valuable for both collectors and researchers. Each Arnioceras fossil is not just a geological relic but a direct record of ancient life — an ammonite that once thrived in the tropical seas that covered what is now southern England nearly 200 million years ago.

Authenticity and Provenance

All of our fossils are 100% genuine specimens and are accompanied by a Certificate of Authenticity. Each piece is responsibly collected, professionally prepared, and ethically sourced. The fossil shown in the photographs is the actual specimen you will receive — no replicas, casts, or artificial enhancements are ever used.

This Arnioceras Fossil Ammonite from Black Ven, Lyme Regis, Dorset is a beautifully preserved example from the world-renowned Jurassic Coast — a site of both scientific importance and natural beauty. It is a timeless collector’s piece, offering a tangible connection to the deep history of life on Earth.