Placenticeras sp. - fragment of a fossil ammonite with ammolite from the Cretaceous of Canada. (polished cabochon; ~1.7 centimeters across at its widest)
Ammolite is biogenic gem material from Alberta, Canada. It has stunningly intense, iridescent rainbow colors. Ammolite is fossil shell material from Placenticeras ammonites. Ammonites are an extinct group of swimming squid-like organisms with planispirally coiled shells (the chambered nautilus in modern oceans is a distant relative of ammonites, but has a similar body plan). Ammonite shells were originally nacreous aragonite (“mother of pearl”) (CaCO3). Geologic studies have shown that ammolite gem material formed from slight diagenetic alteration of the original ammonite's nacreous aragonite shell. Diagenesis has significantly intensified and brightened the play of colors.
Ammolite is mined, polished, and treated by resin- or epoxy-impregnation to stabilize it. Very rarely, complete specimens of Placenticeras ammonite shells preserved in ammolite are recovered - such specimens are exceedingly valuable (for example, see figure 2 of Mychaluk et al., 2001).
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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 persited 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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Name & classification: Placenticeras meeki or Placenticeras intercalare (Animalia, Mollusca, Cephalopoda, Ammonoidea, Ammonitina)
Stratigraphy: Bearpaw Formation, Campanian Stage, upper Upper Cretaceous, ~70-75 Ma
Locality: mine in the St. Mary River Valley west or northwest of Welling and south-southwest of Lethbridge, southern Alberta, southwestern Canada
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Reference cited:
Mychaluk, K.A., A.A. Levinson & R.L. Hall. 2001. Ammolite: iridescent fossilized ammonite from southern Alberta, Canada. Gems & Gemology 37(1): 4-25.
