Introduction
Among prehistoric marine creatures, few rival the legacy and allure of the ammonites. These ancient mollusks flourished for hundreds of millions of years until their extinction with the dinosaurs. Prized for their diversity and intricate shell designs, ammonite fossils offer glimpses into lost Mesozoic oceans. This blog explores what makes these spiral-shelled fossils so significant, how to identify real specimens, and how ammonites lived long before the modern seas formed.
Origins and Anatomy
Ammonites belonged to the Phylum Mollusca, which includes modern Cephalopods like squid and octopus. They first appeared around 400 million years ago in the Devonian and diversified extensively by the Mesozoic Era. Ammonites possessed a hard external spiral shell divided into chambers, with a soft-bodied animal occupying the outermost chamber. Like their cousins, the nautiluses, they could regulate buoyancy via gas in these shell chambers.
The shell morphology varied greatly between ammonite groups. Ribbing, keels, tubercles, and intricate sutures adorned the shells. Uncoiled variants like baculites also existed. Most ammonites were under 12 inches wide, but some giant Cretaceous species reached 6 feet in diameter. Ammonites had tentacles and jaws like beaks for grasping prey. Their rapid evolution and speciation make ammonites extremely useful index fossils for correlating and dating Mesozoic sediments.
Rarity and Diversity of Specimens
Intact ammonite fossils are rare due to their fragile aragonite shells dissolving easily. The most common finds are solitary casts and molds displaying exterior shell contours. Exceptional localities like the Santana Formation in Brazil yield ammonites with mother-of-pearl interiors preserved. These iridescent specimens showcase the hidden beauty concealed within the shells.
The two main ammonite orders are Goniatites from the Paleozoic and Ammonites from the Mesozoic. Ammonites display intricate suture patterns along the interior shell walls, while Goniatites have simpler rounded sutures. Hundreds of ammonite genera existed, often with short species durations. Their rapid evolution and speciation make ammonites extremely useful index fossils for correlating sediments and dating rocks throughout the Mesozoic.
Noted Historic Finds
Some monumental ammonite discoveries profoundly influenced paleontology. A cliff near Whitby, England contains a boulder zone with thousands of complete ammonites accumulated after a catastrophic 1839 storm. These specimens fueled debate on gradualism versus catastrophism in the 1800s.
William Smith's 1815 geological map utilized ammonite succession to date rocks and correlate far-flung outcrops, proving the validity of faunal sequence dating. The unique Boreal ammonite fauna of Svalbard led Wilhelm Bölsche to propose the concept of ancient polar climates distinct from low-latitude areas. Such revolutionary insights stemmed from the scrutiny of ammonites.
Ammonites in Ancient Cultures
Ammonites have been admired and revered by diverse human cultures since prehistory. Uncoiled variants inspired South Asian cosmological symbols like the legend of churning the ocean of milk. Ammonite fossils found inside colonial-era Christian shrines in northern Mexico indicate indigenous people likely viewed them as sacred objects.
Ancient Greeks and Romans called ammonites "ammonite stones" after the Egyptian god Amun due to their coiled shape evoking rams' horns. The luminescent sheen of certain specimens added to their mystique. From Neolithic jewelry to Algonquian pipes, ammonites have long captivated our ancestors.
Where to Find Ammonites Today
The best places to find ammonites are fossil-rich Cretaceous and Jurassic marine strata in Europe, the Americas, North Africa, and Asia. The gault clay of southern England yields abundant pyrite-preserved specimens. Ammonites are common in Madagascar's Tsaratanana Formation and the prairies of Alberta, Canada. Museums display exceptional complete fossils like the many-chambered Parapuzosia Seppenradensis. For collectors, authentic ammonites are sold at fossil shows, shops, auction houses, and occasionally by universities.
Modern Successors
While ammonites perished long ago, their legacy endures in the chambered nautilus, which still inhabits tropical Pacific and Indian Ocean waters. These living fossils resemble what a typical ammonite would have looked like. Nautiluses move through water via jet propulsion and hunt small fish and crustaceans. Female nautiluses attach eggs to rocks and exhibit remarkable homing abilities. Though once more widespread, nautiluses today are rare and protected due to overfishing. They represent a vital evolutionary link to the ammonites of ages past.
Causes of Extinction
Nevertheless, the exact reasons for the ammonite's extinction during the Cretaceous mass extinction remain unknown. Some possible factors include:
- The recent emergence of modern competitors such as squid and belemnites contributes to rough times for this Jurassic animal species.
- Watersheds, where seaways would be shallower, could no longer be the home of many marine species.
- The bolide impact processes that led to the disaster were catastrophes.
- The overall diminished fitness observed from intermittent larval reproduction caused by environmental change likely explained the eventual extinction.
- Ocean acidification is working against the ability of shells which will be pushed to the brink.
The extinction of dinosaurs was the offspring of a comprehensive process that began in the Devonian Period 350 million years ago. Today, this lineage is survived only by nautiluses, which are widely considered to be living representatives of a much older ancestor. Ammonites might serve as an illustration that any species, despite its success in the world, faces the difficulty and uncertainty of survival.
The chest of the spiral shell seeks the dichotomy between the two actualities of the modernizing world through the interpretive dance of pieta and the sinew-skinned muscles.
Conclusion
Ammonites may not be proven to be the most successful creatures in oceanic history, having sojourned on earth for over 350 million years, whereas nobody knows the reason for this disappearance. Their ranks feature spectacular shells that explain the luxury of a contrasting past where the seas had a different character than today. At each fracture we have made, a legend is either written or revealed in the ink of that mineralized refugee who escaped the early time depths. The spiraled form of ammonite fossils will always captivate and sparkle the human mind, telling the tale of things that used to be like that, like the wonders hidden thousands of years ago under the wavy waters of faraway times.
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