Ammonites were a diverse and highly successful group of extinct marine invertebrate animals that belonged to the phylum Mollusca and class Cephalopoda. They proliferated in the oceans for about 140 million years, from the early part of the Jurassic Period through the end of the Cretaceous Period, when they went extinct along with the dinosaurs about 65 million years ago due to major extinction events.
The Mesozoic Era spans the time intervals of three major geological periods - the Triassic, Jurassic, and Cretaceous. The Jurassic Period lasted from 201 to 145 million years ago. This was followed by the Cretaceous Period, which lasted from 145 to 66 million years ago. Ammonites were prolific during both the Jurassic and Cretaceous periods.
With a total duration spanning 140 million years, ammonites displayed remarkable longevity as a group. They survived several mass extinctions before finally succumbing to the cataclysmic asteroid impact at the end of the Cretaceous that radically altered global environments.
Characteristics of Ammonite Fossils and Ammonite Shells
Ammonites possessed an external shell that was typically coiled in a planispiral shape, meaning the shell was a flat, open spiral, like a flattened nautilus shell. The ammonite shell was divided into chambers by calcite walls called septa. As the ammonite grew over its lifetime, newer and larger shell chambers were progressively added.
The exterior surface of the ammonite shell was ornamented with ribs, keels, knobs, ridges, crenulations, and other intricate ornamental designs that are very useful in classifying and distinguishing different ammonite genera and species. Compared to the shells of their closest living relatives, the modern cephalopods including nautilus, ammonites generally had more convoluted suture patterns on their internal septa as well as more elaborate sculpturing and flamboyant modifications to their external spiral ammonite shell morphology.
Like modern coleoid cephalopods including squid, cuttlefish, and octopuses, ammonites had soft body parts that occupied the outer shell chamber. Their ancestral relationship as mollusks placed them in the phylum Mollusca along with bivalves like clams and gastropods like snails. As cephalopods, they possessed tentacles and highly developed eyes and were able to actively swim through the water column by expelling fluid from their chambered ammonite shells.
Geological Significance
The extinction of ammonites occurred at the end of the Cretaceous Period around 65 million years ago, in tandem with the extinction of dinosaurs. This marked mass extinction is widely attributed to an asteroid impact that filled the atmosphere with debris, radically altering global environments and climate.
Due to their durable aragonite shells, ammonites fossilized prolifically. Fossilization occurred when shells were buried in marine sediments and the original aragonite was replaced by other minerals while retaining exquisite detail of the ammonite shell features.
Early stratigraphers like Friedrich Quenstedt, Albert Oppel, Francis Lee, Charlotte Murchison, and Roderick Murchison recognized the potential of using ammonites and close nautiloid relatives to correlate and relatively date rock sequences across different locations. This pioneering work laid the foundations for biostratigraphy as a discipline.
The rapid evolutionary turnover rates and short life spans of ammonites allowed the development of biozones characterized by restricted temporal ranges of different ammonite taxa. This enables ammonite species to function as precise index fossils and guide fossils to date relative geological time intervals.
Ammonites as Geological Tools
The sheer diversity among ammonites with over 10,000 species described, along with their rapid evolutionary turnover and short individual life spans on the order of decades, make them extremely useful biostratigraphic markers for relative dating of Mesozoic marine sediments on both local and global scales.
Since nearly all ammonites lived in open marine environments, their preserved shells are widespread and distributed globally within marine sedimentary rock units.
The broad geographic distribution patterns of most ammonite taxa mean their fossils can be correlated across widely separated marine depositional basins. This enables precise global correlation of rock units using biostratigraphic ammonite zonation schemes.
Use in Distinguishing Geological Time Intervals
The exceptional resolution in the relative dating and correlation of Mesozoic marine rock units is made possible by the detailed ammonite biostratigraphic framework. Ammonite biozones provide the sharpest distinction between geological time slices throughout the evolution of the group across the entirety of the Mesozoic Era.
For example, the extinction of major ammonite taxonomic groups like the Desmocerataceae at around 100 million years ago marks the transition from the Albian to Cenomanian stages within the Cretaceous Period. The disappearance of this family is a globally synchronous bioevent that can differentiate the relative ages of strata.
Visual Representation and Myths Surrounding Ammonites
In typical illustrated geological guides, ammonite fossils are traditionally depicted standing vertically, with the body chamber oriented upwards and the initial shell coils at the bottom. This matches the usual orientation of ammonite specimens in museum fossil collections but is opposite of the life orientation.
In life, the ammonite animal's soft body occupied the largest outer shell chamber with the body oriented downwards. The ammonite would swim through the water column in a horizontal position with its tentacles extended outward to capture prey.
Ancient Egyptian mythology interpreted ammonite fossils, especially ammonites of the genus Hamites, as resembling the horns or coils of the ram god Amun. Amun was one of the most powerful Egyptian gods and was associated with fertility and reproduction.
In medieval Europe, ammonite fossils were often interpreted as petrified snakes because their coil shapes resemble a coiled serpent. They were thought to hold mystical powers and protect against snake bites. Fossil ammonites found high in the Alps were associated with Saint Ceratus and thought to possess healing powers due to the saint's associations with snakes. Pilgrims would wear ammonites as amulets.
Ammonites are common and widespread fossils found in many areas throughout the world. Their abundance and accessibility resulted in their incorporation into early mythologies and folklore traditions throughout history in many cultures predating modern scientific understanding.
Conclusion
In conclusion, ammonites were a highly successful taxonomic group of swimming cephalopods that radiated extensively throughout the oceans during the Jurassic and Cretaceous periods of the Mesozoic Era.
Their sequential extinction at biozone-specific intervals during the Cretaceous provides exceptional biostratigraphic resolution that enables precise correlation of marine sedimentary rock units on both local and global geographic scales.
While ammonites have been extinct for the last 65 million years following the meteorite impact at the close of the Cretaceous period, they continue to play integral roles in deciphering details of geological history through their abundant fossil record and rapid evolutionary radiation across their 140 million years of existence. Their fossilized shells persist as invaluable tools for stratigraphy and reconstructing the environments of Earth's past.
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References
A useful introductory textbook covering additional details on ammonites from an evolutionary perspective is "Palaeobiology: A Synthesis" (2nd Edition) by Derek E. G. Briggs and Peter R. Crowther, published in 2001.
To explore related geological topics in more depth, two recommended textbooks are "Principles of Stratigraphy" by Sam Boggs Jr., published in 2006, and "Stratigraphy: A Modern Synthesis" by Andrew Miall, published in 2015. Both provide more details on biostratigraphic dating.