English: Placenticeras sp. - fossil ammonite from the Cretaceous of South Dakota, USA. (DMNH 35497, Denver Museum of Nature & Science, Denver, Colorado, USA)

Ammonites are common & conspicuous fossils in Mesozoic marine sedimentary rocks. Ammonites are an extinct group of cephalopods - they’re basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group.

Ammonites get their name from the coiled shell shape being reminiscent of a ram’s horn. The ancient Egyptian god Amun (“Ammon” in Greek) was often depicted with a ram’s head & horns. Pliny’s Natural History, book 37, written in the 70s A.D., refers to these fossils as “Hammonis cornu” (the horn of Ammon), and mentions that people living in northeastern Africa perceived them as sacred. Pliny also indicates that ammonites were often pyritized.

This Placenticeras ammonite has been prepared and polished to show the fractal-like ammonitic sutures.

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Ammonite info. from the Wyoming Geological Museum in Laramie, Wyoming:

Ammonites

Ammonites are extinct molluscs of the Class Cephalopoda, a group represented today by the octopus, squid, and shell-bearing Nautilus. Ammonites appeared midway through the Paleozoic Era (400 million years ago). They diversified many times over their 300 million year history, and persisted through three mass-extinction events. During the Mesozoic Era (from 250 to 65 million years ago), ammonites reached their greatest diversity, achieving many different shell forms and ways of life. At the end of the Mesozoic Era, ammonites became extinct, together with the dinosaurs, pterosaurs, and marine reptiles.

Ammonite Anatomy

Ammonites, like the modern Nautilus, possessed an external shell divided into a series of chambers by thin walls called septa. These chambers were connected by a flesh-bearing tube known as the siphuncle. By analogy with the living Nautilus, it served to regulate fluid and gas levels in each chamber, enabling ammonites to control their buoyancy. Although ammonites are common fossils, little is known about their soft parts. However, it is thought that their soft anatomy was similar to that of modern squid and octopi. They probably possessed eight to ten arms surrounding a beak-like mouth. Locomotion probably involved bringing water into a cavity, formed by the fleshy mantle, then expelling it by muscular contraction through a funnel-like opening called the hyponome, therby implementing a form of jet-propulsion.

Ammonite Ecology

Ammonites were common constituents of Cretaceous marine ecosystems and were represented in many habitats in the shallow seas that covered North America during the Mesozoic Era. Ammonites lived in both nearhsore and offshore settings in both benthic (seafloor) and pelagic (open ocean) habitats. Some species could probably even migrate between both types of habitats.

Feeding Habits

Most ammonites, like their modern cephalopod relatives, were probably carnivores, although some may have been passive planktivores. The carnivorous ammonites possesssed powerful jaws adapted for crushing prey, which included crustaceans, fish, clams, snails, and even other ammonites.

Reproduction and Growth

Ammonites, like their modern relatives the octopi and squids, hatched as tiny larvae in huge numbers and probably grew to maturity within a short span of time. Most adults were small, while those of some species were huge, reaching sizes greater than 6 feet (2 meters) in diameter. Aberrant ammonites that changed their shape during growth are thought to have changed their habitat as well.

Ammonite Sexes

Like modern cephalopods, ammonites showed distinct differences between sexes. Shells of female ammonites, known as macroconchs, are larger and possess little or no ornamentation. Males, known as microconchs, are smaller than females and commonly possess distinct ornamentation.

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Classification: Animalia, Mollusca, Cephalopoda, Ammonoidea, Ammonitida, Placenticeratidae

Stratigraphy: Pierre Shale, Upper Cretaceous

Locality: DMNH museum locality # 1320, Meade County, western South Dakota, USA

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See info. at: en.wikipedia.org/wiki/Placenticeras and

en.wikipedia.org/wiki/Ammonitida