An Ancient Lineage: 344 Million Years of Oceanic Dominance.

Greetings. Ammonoids are extinct spiral shelled cephalopods that existed in the world's seas and oceans for over 344 million years. They are more closely related to living coleoids; octopuses, squid, and cuttlefish, than they are to shelled nautiloids, including the living Nautilus. The earliest ammonoids appeared during the Devonian Period, with the last species vanishing during or soon after the Cretaceous/Paleogene extinction event. They are commonly referred to as ammonites, a name which is most frequently used for members of the order Ammonitida, the last group of ammonoids from the Jurassic Period until their unfortunate extinction.

Ammonites are excellent index fossils, linking the rock layer which a particular genus or species is found to specific geologic periods of time. The name 'ammonite,' from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resembles tightly coiled rams' horns.

Because ammonites and their close relatives are extinct, their way of life is relatively unknown. Their soft body parts are only very rarely preserved for paleontologists to examine, however, a great deal of information has been worked out by examining fossilized ammonoid shells and by using models of these shells in water tanks. Many ammonoids thrived in the open waters of ancient seas and oceans, rather than at the sea bottom, because their fossils are often found in rocks laid down under conditions where no bottom-dwelling life is found. In general, they appear to have inhabited aquatic habitats within 250 meters of the surface. Many ammonoids are believed to have been good swimmers, with flattened, streamlined shells, although some ammonoids were far less effective swimmers and as a result were likely to have been bottom-dwelling by necessity. Mesozoic species likely avoided predation by squirting ink, much like modern cephalopods, ink being occasionally preserved in fossil specimens from the time frame.

The soft body of the creatures occupied the largest segments of the shell at the end of the coil. The smaller earlier segments were walled off and the animals could maintain their buoyancy by filling them with gas. Thus, the smaller sections of the animal's coil would have always floated above the larger sections. Many ammonite shells have been found with round holes once interpreted as a result of limpets attaching themselves to the shells. However, the triangular formation of the holes, their size and shape, and their presence on both sides of the shells, corresponding to upper and lower jaw structures, is strong evidence of the bites of mosasaurs that often preyed upon ammonites.  

The smallest ammonoids existed during the Late Carboniferous Period, about 290 million years ago. Adult specimens of the time reached only 10 millimeters in shell diameter. The majority of ammonite species from the later Triassic and Jurassic periods attained sizes exceeding 23 centimeters in diameter. Far larger species are found in the rocks of the Late Jurassic and the Cretaceous, such as members of the genus Titanites, species of which grew to 54 centimeters in diameter. The species Parapuzosia seppenradensis of the Cretaceous Period of what is now Northern Europe, is one of the largest-known ammonites, sometimes reaching an impressive 2 meters in diameter. 

There is fossil evidence that a few ammonite species made it through the Cretaceous/Paleogene extinction event, but only survived for a few hundred thousand years beyond that. Even the tough and durable ammonites could not withstand the environmental stresses of the Chicxulub impactor, and that is a shame. It would be fascinating to see live ammonites in today's oceans, what a wondrous sight indeed. 

Thank you for your time and consideration.