Fossil Record

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The Subclass Ammonoidea is the only extinct group of cephalopod although in its time it was as a prodigious marine predator. This subclass came to dominance at the end of the Devonian after the extinction of many of the subclasses of Nautiloidea. This may be due to the ammonoids adapted mode of life, where their shells could resist implosion at depth. They were also ecological specialists that could adapt to a wider range of environments and ways of life than the nautiloids. Ammonoids almost became extinct at the end-Permian and end-Triassic mass extinctions, but recovered dramatically in the Jurassic (see table below) to become even more widespread.  

This diverse subclass of cephalopod is among the most abundant, useful and well known of all fossils groups.  The shell of the ammonoid is generally tightly coiled in a planispiral fashion with the siphunclesituated near the outer margin. Many ammonite shells are ornamented and ribbed, as well as often bear knobs and spines.  The hard outer shell would have encases most of the organism while little is known about the soft tissues, as soft part preservation is extreamely rare. It is generally assumed that their shell morphology was similar in appearance to that of modern nautiloids, but they are infact more closely related to coleodiea (octopus, squid and cuttle fish).The Ammonoitida finally became extinct at the Cretaceous - Tertiary mass extinction 65 million years ago, after roaming the seas for 330 million years.

Table above shows the time ranges of the first and last members from the subclasses of the three cephalopod classes, includes relative abundance by families. (Clarkson, 1998)

Cladogram showing the evolutionary relationship of shared characteristcs leading to the class Cephalopoda. (source: Wikipedia)

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