Ammonite

Kingdom: Animalia

Phylum: Mollusca

Class: Cephalopoda

Subclass: Ammonoidea

More closely related to squid and octopuses then to nautiloids (having had convergent evolution) they are a good index fossil having a wide range in both time span and area. Ammonites first appeared during the Devonian period and did not go extinct until the end of the Cretaceous, the later species started to develop more and more complex sutures. There are three type of suture patterns found in ammonites Goniatitic which had undivided lobes this is found in earlier species, Ceratitic which has lobes that are subdivided and rounded undivided saddles this is found in ammonites of the early Mesozoic, and Ammonitic which has lobes and saddles that are both subdivided and this is found in species of the late Mesozoic.

It is supposed that most species lived in open oceans (having only found there fossils in rock layers with no benthic species) cat scans have shown plankton in parts of the buccal cavity hinting that plankton was there main food source. The presence of ink is also found in specimens at times so ammonites may have used ink to avoid predation like modern day squids.

The main difference between the Ammonite and the Nautiloids is that the siphuncle of ammonites runs along the ventral periphery of the septa while in nautiloids it runs through the center.

Ammonites may have had a jaw apparatus called aptychus in Mesozoic ammonites it would have been a pair or calcified aptychus.

Do to the long history of ammonites and their great diversity their size ranges from 23 cm to up to 2.3 meters depending on the species.