Eternity Series - Ripples of Time

Aileen David

Hawthorn, Australia

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This is an ammonite fossil found at the Australian Museum. It was a special request from my son.

Ammonites
As they pertain specifically to the order Ammonitida, are an extinct group of marine animals belonging to the cephalopod subclass Ammonoidea. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.
The closest living relative of the Ammonitida, is not the modern Nautilus which they somewhat outwardly resemble, but rather the subclass Coleoidea (octopus, squid, and cuttlefish).
Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as “heteromorphs”). Their name came from their spiral shape as their fossilized shells somewhat resemble tightly-coiled rams’ horns. Pliny the Elder (d. 79 A.D. near Pompeii) called fossils of these animals ammonis cornua (“horns of Ammon”) because the Egyptian god Ammon (Amun) was typically depicted wearing ram’s horns.1 Often the name of an ammonite genus ends in -ceras, which is Greek (κέρας) for “horn” (for instance, Pleuroceras).
The majority of ammonite species feature a shell that is a planispiral flat coil, but other species feature a shell that is nearly straight (as in baculites). Still other species’ shells are coiled helically, superficially like that of a large gastropod (as in Turrilites and Bostrychoceras). Some species’ shells are even initially uncoiled, then partially coiled, and finally straight at maturity (as in Australiceras). These partially uncoiled and totally uncoiled forms began to diversify mainly during the early part of the Cretaceous and are known as heteromorphs.
Perhaps the most extreme and bizarre looking example of a heteromorph is Nipponites, which appears to be a tangle of irregular whorls lacking any obvious symmetrical coiling. However, upon closer inspection the shell proves to be a three-dimensional network of connected “U” shapes. Nipponites occurs in rocks of the upper part of the Cretaceous in Japan and the USA.
Ammonites vary greatly in the ornamentation (surface relief) of their shells. Some may be smooth and relatively featureless, except for growth lines, and resemble that of the modern Nautilus. In others various patterns of spiral ridges and ribs or even spines are shown. This type of ornamentation of the shell is especially evident in the later ammonites of the Cretaceous.

from: Wikipedia

Artwork Comments

  • greenstone
  • Aileen David
  • greenstone
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  • Vanessa Barklay
  • Vanessa Barklay
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  • Mike Oxley
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  • JUSTART
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  • stephaniek
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  • wahumom
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