Oxynoticeras

Oxynoticeras Pyrite Ammonites: Golden Relics of Charmouth’s Jurassic Coast

The Jurassic Coast of southern England is world-renowned for its incredible fossil heritage, with Charmouth being one of the prime locations for fossil hunting. Among the many ammonite species discovered here, the Oxynoticeras genus holds a special allure, especially when found preserved in pyrite, often called “fool’s gold.” These strikingly golden ammonites offer both scientific insight and aesthetic appeal, making them highly coveted by fossil enthusiasts and collectors. Oxynoticeras pyrite ammonites are more than just visually stunning; they represent a fascinating chapter in Earth’s history, dating back over 190 million years to the Early Jurassic period. This article explores the significance of Oxynoticeras, its unique preservation in pyrite, and its importance within the fossil-rich landscapes of Charmouth.

Overview of Oxynoticeras Ammonites
Ammonites were marine cephalopods, related to modern squids and octopuses, that lived for millions of years in Earth’s ancient oceans. Characterized by their coiled, chambered shells, ammonites were a highly diverse and successful group, evolving into many species before their extinction at the end of the Cretaceous period, approximately 66 million years ago. Ammonite fossils are abundant in many marine sedimentary rocks, making them key fossils for paleontological study.

Oxynoticeras is a genus of ammonite that lived during the Early Jurassic period, specifically in the Sinemurian stage, around 199 to 190 million years ago. Oxynoticeras is notable for its thin, discoidal shell that is compressed and streamlined, with a flattened profile that gives it a somewhat sharp or blade-like edge, rather than the more rounded appearance typical of many other ammonites. The whorls of the shell are tightly coiled, and the ribs are often smooth or only faintly developed, giving the ammonite a clean, sleek appearance.

This smooth and thin shell design suggests that Oxynoticeras ammonites were fast-moving, open-water swimmers, likely capable of rapid movement to avoid predators or catch prey. These ammonites were well-adapted to a pelagic lifestyle, living in the upper levels of the ocean where oxygen levels were high and food was abundant.

The Pyrite Preservation Phenomenon
One of the most striking and rare occurrences in fossil preservation is the replacement of original shell material with pyrite, a metallic mineral composed of iron sulfide (FeS₂), commonly known as “fool’s gold” because of its shiny, golden appearance. The formation of pyrite fossils occurs under specific conditions, usually in anoxic (low-oxygen) environments where decay processes are limited, allowing the pyrite to precipitate and replace the organic material or the original shell structure.

Oxynoticeras ammonites found in Charmouth are sometimes preserved in pyrite, resulting in fossils that have a brilliant, golden sheen. These fossils are particularly striking and highly valued due to their rarity and unique appearance. The pyrite coating not only enhances the aesthetic beauty of the fossil but also helps preserve the fine details of the ammonite’s shell, such as its smooth whorls and subtle ribbing, in stunning clarity.

The preservation of ammonites in pyrite is often seen in areas with shale or mudstone deposits, where the low oxygen levels beneath the sea floor prevented the complete decay of the ammonite shells. Over millions of years, the shell was replaced by pyrite, creating a fossil that is as much a mineral specimen as it is a record of ancient life.

Geological Context of Oxynoticeras at Charmouth
Charmouth is part of the larger Jurassic Coast, which stretches over 95 miles along the southern coast of England. The cliffs and beaches around Charmouth are primarily composed of Blue Lias, a sedimentary rock formation that was deposited during the Early Jurassic. This formation consists of alternating layers of limestone and shale, which were laid down in a shallow, oxygen-poor sea. The conditions in this ancient sea were ideal for the preservation of marine fossils, including ammonites.

The Blue Lias formation is famous for producing beautifully preserved ammonites, including Oxynoticeras, particularly in the form of pyrite fossils. After storms or heavy rain, erosion of the cliffs releases fossils from the rock layers, which can then be found washed up on the beaches or embedded in limestone nodules that have fallen from the cliffs. These nodules sometimes crack open to reveal exquisitely preserved pyritized ammonites, including species of Oxynoticeras.

Fossil Hunting for Oxynoticeras at Charmouth
Fossil hunting at Charmouth is a popular activity, and the chance of finding a pyrite ammonite, such as Oxynoticeras, adds a special excitement to the experience. The best time for fossil hunting is after storms or during low tide when the waves have eroded the cliffs and exposed new fossils. While many fossils are found simply by walking along the beach and searching among the shingle, the most stunning pyrite fossils are often discovered inside nodules.

Oxynoticeras fossils can be distinguished by their thin, flattened shells and smooth or faintly ribbed surfaces. When preserved in pyrite, these ammonites take on a golden color that makes them stand out from other fossils. Although finding a perfectly preserved pyrite Oxynoticeras is a rare occurrence, the thrill of discovering one is an unforgettable experience for fossil hunters.

For beginners and those new to fossil hunting, Charmouth offers guided fossil walks through the Charmouth Heritage Coast Centre, where experienced guides help identify different types of fossils and teach participants how to search for and handle delicate specimens, including pyrite ammonites.

Scientific Significance of Oxynoticeras
Oxynoticeras is an important genus for paleontologists studying the Early Jurassic because it provides valuable information about marine life and environments during this period. As with many ammonites, Oxynoticeras fossils serve as index fossils, helping geologists date the rock layers in which they are found. The presence of Oxynoticeras fossils indicates that the rock was deposited during the Sinemurian stage, around 199 to 190 million years ago, allowing geologists to correlate rock formations across different regions.

The streamlined shape and thin shell of Oxynoticeras suggest that it was a fast-moving, open-water ammonite, likely adapted to life in the upper levels of the ocean. This contrasts with other ammonites that may have been more suited to nearshore or deeper water environments. By studying the distribution of Oxynoticeras fossils, paleontologists can reconstruct the ecological niches that different ammonite species occupied and gain a better understanding of the biodiversity and complexity of Early Jurassic marine ecosystems.

Pyrite Fossils in Collections
Pyrite ammonites are particularly prized by collectors due to their striking appearance and rarity. The golden sheen of pyrite fossils makes them stand out in any collection, and when combined with the delicate and intricate structure of Oxynoticeras shells, these fossils become true showpieces. Museums along the Jurassic Coast, such as the Lyme Regis Museum and the Charmouth Heritage Coast Centre, often display pyrite ammonites to educate the public about the fossilization process and the geological history of the region.

For private collectors, finding a pyrite-preserved Oxynoticeras ammonite is considered a significant achievement. These fossils not only represent a unique geological phenomenon but also offer a glimpse into the ancient seas that once covered southern England.

Conclusion
Oxynoticeras pyrite ammonites from Charmouth are among the most visually stunning and scientifically fascinating fossils found along the Jurassic Coast. With their sleek, streamlined shells and golden pyrite preservation, these ammonites offer a unique window into the marine environments of the Early Jurassic period. The combination of their rarity, aesthetic appeal, and paleontological significance makes Oxynoticeras pyrite ammonites highly coveted by fossil hunters, collectors, and scientists alike.

For those visiting Charmouth, the opportunity to discover an Oxynoticeras ammonite, particularly one preserved in pyrite, is a thrilling experience that connects them to a world that existed over 190 million years ago. Whether admired for their beauty or studied for their scientific value, these golden fossils continue to captivate and inspire, preserving a piece of Earth’s ancient history for future generations to explore and appreciate.