Ammonites, ancient marine creatures with distinctive spiral shells, vary in their fossil rarity. While many ammonite fossils are common and widely distributed, certain types and states of preservation are considered rare and highly sought after by collectors and scientists alike.
Understanding Ammonites
Ammonites were a diverse group of marine mollusks, belonging to the class Cephalopoda, which includes modern-day squids, octopuses, and cuttlefish. They flourished throughout the Mesozoic Era, encompassing the Jurassic and Cretaceous periods, from approximately 201 to 66 million years ago. These creatures were characterized by their chambered, external shells, typically coiled in a flat spiral, though some evolved into more unusual shapes. The ammonite animal itself lived in the outermost chamber, using earlier chambers filled with gas or fluid to regulate its buoyancy, much like a submarine.
The General Abundance of Ammonite Fossils
Ammonite fossils are among the most common and geographically widespread fossils found globally. Their widespread presence stems from their abundance in ancient oceans, where they were prolific breeders. Their hard, mineralized shells, primarily composed of aragonite, were highly conducive to fossilization.
When an ammonite died, its shell often sank to the seafloor and was quickly buried by sediment, protecting it from decay and scavenging. Rapid burial in marine environments, well-suited for preservation, significantly increased their fossilization chances. Their rapid evolution also means that different species existed for relatively short geological periods, making them exceptional “index fossils” used by geologists to accurately date rock layers worldwide.
Factors Influencing Ammonite Rarity
Despite their general abundance, several specific factors contribute to the rarity of certain ammonite fossils, elevating their scientific and monetary value. Some ammonite species were less abundant in ancient ecosystems or had restricted geographical ranges, meaning fewer individuals existed to become fossilized. For example, some species from particular geological formations are known to be far less common than others.
Exceptional Preservation
Exceptional preservation is a significant factor determining rarity. While shells are common, finding ammonites with preserved soft tissues, such as ink sacs or beak-like jaws, is exceedingly rare. Certain fossilization processes can create specimens with stunning iridescent or opalized shells, known as ammolite, which are highly prized. The primary source for gem-quality ammolite is the Bearpaw Formation in Southern Alberta, Canada, making these vibrant specimens regionally unique. Complete, unbroken specimens, especially large ones, are also less common than fragments.
Unusual Size or Form
Unusual size or form also contributes to rarity. Most ammonites were relatively small, but some species grew to immense sizes, such as Parapuzosia seppenradensis, which could reach diameters of 2.5 to 3.5 meters. Additionally, while most ammonites had coiled shells, a group known as heteromorphs evolved highly irregular or uncoiled shapes, appearing as straight, helical, or even knot-like forms. These heteromorphic ammonites, like Nipponites mirabilis or Baculites, are rarer due to their unusual morphology and often more fragile structures.
Geological Context and Accessibility
The geological context and accessibility of fossil sites also play a role in perceived rarity. Fossils from specific rock formations or geological ages might be less common due to environmental conditions that were less favorable for preservation during those times. Fossils from difficult-to-access or protected sites, where collecting is restricted, also become rarer due to limited availability.
Discovering Ammonite Fossils
Ammonite fossils are frequently found in marine sedimentary rock formations across the globe, including coastal cliffs, quarries, and riverbeds. Regions like North America’s Great Plains and the Jurassic Coast of England are well-known for their abundant ammonite deposits. Collectors often search for them in areas where erosion exposes new rock layers, such as beach environments after storms.
Understanding local geology and the types of rocks likely to contain ammonites is helpful when searching for fossils. Responsible collecting practices include obtaining permission for private land access, adhering to regulations in protected areas, and ensuring the long-term preservation of discoveries. While many finds will be common examples, the possibility of uncovering a rare, exceptionally preserved, or unusually shaped ammonite adds to the enduring appeal of fossil hunting.