4 Inch Androgynoceras Fossil Ammonite Lower Lias Stonebarrow Cliff Charmouth Lyme Regis Dorset

Description

Exceptional 4″ Androgynoceras Fossil Ammonite from the Jurassic Coast

This magnificent 4-inch Androgynoceras fossil ammonite originates from the Green Ammonite Beds of the Lower Lias Formation, collected from Stonebarrow Cliff, Charmouth, near Lyme Regis, Dorset, UK — a world-renowned site along the Jurassic Coast, designated as a UNESCO World Heritage Site. The specimen was discovered by our team members, Alister and Alison, on 06 October 2025, and has been professionally cleaned, prepped, and treated by Alison to the highest standards. The accompanying photograph shows the actual specimen you will receive, ensuring total transparency and authenticity.

Scale cube = 1 cm. Please see photos for full sizing information.

All of our fossils are 100% genuine specimens and are accompanied by a Certificate of Authenticity verifying their provenance and preparation.

Geological and Stratigraphic Context

This fossil comes from the Green Ammonite Beds, a unit within the Charmouth Mudstone Formation of the Lower Jurassic (Upper Sinemurian to Lower Pliensbachian Stage, approximately 197–190 million years ago). These beds are famous for their olive-grey to greenish claystone layers enriched with pyrite and calcite, producing beautifully preserved ammonites and marine fossils. The sediments were deposited in a calm, shallow sea that once covered much of southern England, forming part of the extensive Tethyan seaway.

Within the Stonebarrow Cliff section, the Green Ammonite Beds are highly fossiliferous, and Androgynoceras species are among the most sought-after ammonites from this locality. These beds are closely associated with the Androgynoceras lataecosta biozone, which is widely used as a stratigraphic marker in European Jurassic successions.

Scientific Classification

• Genus: Androgynoceras
• Order: Ammonitida
• Superfamily: Eoderoceratoidea
• Family: Eoderoceratidae
• Formation: Lower Lias (Green Ammonite Beds, Charmouth Mudstone Formation)
• Geological Stage: Sinemurian–Pliensbachian
• Locality: Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, United Kingdom

The genus Androgynoceras is an early Jurassic ammonite known for its ornate ribbing and distinctive shell structure. Its fossil record provides valuable insights into ammonite evolution during the transition from the Sinemurian to the Pliensbachian stages.

Morphological Characteristics

The Androgynoceras ammonite displays a planispiral shell with strong, widely spaced radial ribs that bifurcate or trifurcate along the flanks and cross the venter with little interruption. The ribs often exhibit gentle curvature, creating a visually striking pattern. This specimen exhibits fine preservation of rib detail and a clean, natural matrix, highlighting the contrast between the ammonite and the surrounding rock.

The 4-inch diameter makes it a substantial and visually impressive piece, large enough to reveal intricate surface textures and chamber outlines. Some Androgynoceras specimens from this bed show traces of original aragonite or secondary calcite infill within the chambers, contributing to the fossil’s natural luster and mineral character.

These morphological features reflect the ammonite’s adaptation to a nektonic lifestyle, enabling it to swim freely in open marine environments while maintaining buoyancy through its chambered shell.

Depositional Environment and Palaeontological Significance

The Green Ammonite Beds were deposited under low-energy, offshore conditions where fine-grained sediments settled slowly on the sea floor. Intervals of limited oxygenation helped preserve organic remains and mineralised shells. The presence of iron-rich clays and pyrite nodules gives these layers their characteristic greenish hue.

Ammonites such as Androgynoceras were fast-evolving, free-swimming cephalopods, and their shells serve as key biostratigraphic tools for correlating Jurassic strata worldwide. The abundance of well-preserved specimens from Charmouth and Lyme Regis has allowed geologists to refine the Lower Jurassic ammonite zonation with remarkable precision. Fossils from this region continue to be studied for their insights into palaeoecology, evolution, and environmental change during the early Jurassic seas.

Collector’s and Display Appeal

This 4″ Androgynoceras ammonite from Stonebarrow Cliff, Charmouth, is an outstanding collector’s specimen, combining scientific importance with exceptional aesthetic appeal. Its bold ribbing, well-defined whorls, and classic preservation make it a perfect centrepiece for any natural history display or personal collection.

Prepared with care and expertise by Alison, this specimen captures the essence of the early Jurassic marine world in both form and history. The Green Ammonite Beds of Dorset are among the most famous fossil localities in the world, explored since the time of Mary Anning, and this piece continues that legacy of British palaeontological heritage.

A truly remarkable example of an authentic Androgynoceras ammonite fossil, preserved in its original matrix, offering both natural beauty and a direct connection to life from nearly 200 million years ago.