|The Subclass Ammonoidea is an entirely extinct group. Ammonites are
among the most abundant and well known of all fossils. They lived from
the Devonian to the Cretaceous, some 330 million years in all. The shell
is tightly coiled in a planispiral fashion with the siphuncle
situated near the outer margin. Many ammonite shells are ornamented and
frequently ribbed, and these ribs may bear knobs and spines.|
Because the hard outer shell encases almost all of the organism, little is known about the soft tissues, and it is assumed that they were basically similar to those of modern nautiloids. However, unlike nautiloids, ammonoids possessed ink sacs, and their jaws bore a closer resemblance to those of octopuses.
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