Ammonites were a diverse group of extinct marine mollusks, related to modern cephalopods. These ancient creatures inhabited Earth’s oceans for over 300 million years, from the Devonian period until their extinction alongside the dinosaurs at the end of the Cretaceous period, 66 million years ago. Their distinctive, often spiral-shaped shells make them sought-after by collectors and important as “index fossils” for dating rock layers. Their intricate suture lines add aesthetic appeal and aid scientific classification.
Ancient Environments and Fossil Preservation
Ammonites lived primarily in shallow ancient seas, and their preservation as fossils required specific geological conditions. After an ammonite died, its shell typically sank to the seabed. Rapid burial by sediment was needed to prevent decay or scavenging. This rapid burial separated the remains from destructive biological and physical processes. Over millions of years, layers of mud, silt, and sand accumulated, compacting and eventually transforming into marine sedimentary rocks like shale, limestone, and mudstone.
Permineralization was a common fossilization method. Minerals dissolved in groundwater, such as calcite or silica, infiltrated the shell’s porous spaces, replacing organic material and preserving its structure. Ancient seafloors eventually uplifted and eroded, exposing these fossil-rich rock layers globally.
Top Locations for Ammonite Hunting
Ammonite fossils are found where Jurassic and Cretaceous marine sedimentary rocks are exposed. Coastal cliffs, riverbeds, quarries, road cuts, and exposed badlands are common sites for these finds. The Jurassic Coast in the United Kingdom, a UNESCO World Heritage site, is renowned for abundant ammonite fossils, with Lyme Regis and Charmouth in Dorset being prime hunting grounds. Monmouth Beach, near Lyme Regis, features a natural “Ammonite Pavement” with numerous embedded specimens. Other notable UK sites include Robin Hood’s Bay in North Yorkshire and Helmsdale in Scotland.
In North America, ammonites are widely distributed, especially across the Great Plains, where ancient inland seaways existed. South Dakota’s Pierre Shale formation is known for its extensive ammonite deposits. Other promising areas include Lake Texoma (Oklahoma and Texas) and Ammonite Canyon in Nevada, named for its rich fossil finds. Maryland’s Calvert Cliffs also yield marine fossils, including some ammonites, though they are more famous for shark teeth.
Tips for Finding Ammonite Fossils
Preparation and observation are key when hunting for ammonite fossils. Essential tools include a geological hammer and chisel for splitting rock, and safety glasses for eye protection. Sturdy, closed-toe footwear is advisable for uneven terrain. Look for distinct shapes, textures, or colors in the rock indicating a fossil; ammonites often appear as circular or spiral forms, or texture variations.
Scanning exposed rock surfaces and splitting shale or mudstone layers can reveal hidden specimens. Always prioritize safety: check tide times on beaches to avoid being cut off by water. Maintain a safe distance from cliff bases due to rockfall risk, and inform someone of your location and expected return.
Collecting and Preserving Your Finds
After finding an ammonite, basic identification involves recognizing its characteristic coiled shell, though some rarer forms are straight or irregularly coiled. For initial cleaning, a soft brush and water can remove loose dirt. Avoid harsh chemicals or aggressive scrubbing, which could damage the fossil. Transport your finds carefully, wrapping them to prevent breakage.
Ethical collecting practices are important for responsible fossil hunting. Always secure permission before collecting on private land. Collecting in national parks or other protected areas is generally prohibited, and regulations vary by region and land ownership. Some public lands allow casual collection of common invertebrate fossils for personal use, but commercial collecting is often restricted or requires permits. Depositing scientifically significant finds in a museum or research institution can contribute to scientific knowledge.