Description
Polished Arnioceras Ammonite Fossil – Whitby Mudstone Formation, East Yorkshire
Presented here is a genuine Arnioceras ammonite fossil that has been carefully polished to reveal the internal chamber structure, collected from the Whitby Mudstone Formation along the Holderness Coast in East Yorkshire, United Kingdom. This fossil originates from the Lower Jurassic Period during the Sinemurian Stage, approximately 199–190 million years ago, when warm shallow seas covered much of what is now northern England.
Polished ammonites are particularly attractive display specimens because the cutting and polishing process exposes the internal chamber walls and mineralised structures, allowing the intricate internal architecture of the shell to be clearly visible.
The specimen offered here has been carefully selected for display quality, and the photographs show the exact fossil you will receive. The fossil includes a Certificate of Authenticity lifetime guarantee generic card, confirming that the specimen is genuine.
Full sizing and scale can be seen in the listing photographs.
Arnioceras – A Classic Early Jurassic Ammonite
The genus Arnioceras is a well-known ammonite from the Early Jurassic and is commonly found in Lower Lias deposits across northern Europe, including England, France, and Germany.
Ammonites belong to the class Cephalopoda, a group of marine molluscs that also includes modern squid, octopus, and cuttlefish. Unlike modern cephalopods, ammonites possessed an external coiled shell divided internally into multiple chambers.
Taxonomic classification includes:
• Class: Cephalopoda
• Subclass: Ammonoidea
• Order: Ammonitida
• Superfamily: Psiloceratoidea
• Family: Arietitidae
• Genus: Arnioceras
Arnioceras species are recognised by their broad whorls and strong ribbing, giving the shell a distinctive robust appearance. These ammonites were active swimmers that used jet propulsion, drawing water into their mantle cavity and expelling it to move through the water column.
Shell Morphology and Internal Structure
Ammonite shells consisted of a coiled outer shell divided into internal compartments known as camerae or chambers. As the animal grew, it sealed off older chambers and lived in the newest chamber at the opening of the shell.
Key morphological features of Arnioceras ammonites include:
• Strong radial ribbing across the shell whorls
• Rounded whorl profile
• Wide umbilicus exposing inner shell coils
• Complex suture lines where chamber walls meet the outer shell
• Gas-filled chambers connected by a siphuncle used to regulate buoyancy
In polished specimens, these internal chambers become visible, often filled with mineral deposits that contrast beautifully with the surrounding rock matrix.
The polished surface reveals the geometric chamber pattern, highlighting the natural engineering that allowed ammonites to control buoyancy and remain suspended in the water column.
Whitby Mudstone Formation – Lower Jurassic Deposits
This fossil originates from the Whitby Mudstone Formation, a major Lower Jurassic geological unit exposed along the Yorkshire coast, including the Holderness Coast and the famous Whitby cliffs.
The formation dates primarily to the Sinemurian and Pliensbachian stages of the Early Jurassic, approximately 199–183 million years ago.
These rocks were deposited within a shallow marine basin connected to the ancient Tethys Ocean. Fine marine sediments accumulated on the seabed and later lithified into dark mudstones and shales that now preserve abundant marine fossils.
The Whitby Mudstone Formation is renowned for preserving:
• Ammonites used for stratigraphic dating
• Belemnites and other cephalopods
• Marine reptiles such as ichthyosaurs and plesiosaurs
• Bivalves and gastropods
• Marine fish and crustaceans
The abundance and diversity of ammonites in these sediments allow geologists to divide the rock layers into precise ammonite biozones, making the Yorkshire coast an internationally important reference for Early Jurassic stratigraphy.
Jurassic Marine Environment of Northern England
During the Early Jurassic, northern England lay beneath a warm shallow sea rich in marine life. Ammonites such as Arnioceras were among the most abundant organisms in these waters.
These animals played an important role in the marine food chain, serving both as predators of small marine organisms and as prey for larger animals such as marine reptiles and fish.
Their rapid evolutionary changes and wide distribution make ammonites some of the most useful fossils for understanding Jurassic geological history and marine ecosystems.
Fossilisation and Polishing
After the ammonite died, its shell settled onto the seabed where it became buried within fine marine sediments. Over millions of years, mineral-rich groundwater replaced the original shell material or filled the internal chambers with calcite or other minerals.
Polishing the fossil exposes the internal chamber structure and mineral infill, transforming the specimen into a visually striking display piece while still preserving the original fossil structure.
The result is a specimen that beautifully displays both the external ribbing and internal chamber architecture of the ammonite shell.
Authenticity and Collector Information
• Genuine fossil specimen – not a cast or replica
• Fossil Type: Arnioceras sp. Ammonite (Polished)
• Class: Cephalopoda
• Subclass: Ammonoidea
• Family: Arietitidae
• Geological Formation: Whitby Mudstone Formation
• Age: Lower Jurassic, Sinemurian Stage (~199–190 million years old)
• Locality: Holderness Coast, East Yorkshire, United Kingdom
• Includes Certificate of Authenticity lifetime guarantee generic card
• The exact fossil shown in the photographs is the specimen you will receive
This genuine polished Arnioceras ammonite from the Whitby Mudstone Formation of Yorkshire makes an outstanding addition to any Jurassic fossil collection, ammonite display, or natural history cabinet, showcasing the remarkable beauty and complexity of Early Jurassic marine life preserved for nearly 200 million years.
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