Mollusca		Neocephalopoda
CYRTOSOMA		Ammonoidea

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Ammonoidea

Cephalopoda
|--Nautiloidea
`--NEOCEPHALOPODA  
   |--ORTHOCERIDA  
   `--+--ASCOCERIDA  
      `--BACTRITIDA  
         |--AMMONOIDEA
         `--Coleoidea

Introduction

Like nautiloids, ammonoids were cephalopod animals possessing an external shell, but they differ in many details.  The sutures are generally complicated wavy lines, consisting of saddles "peaks" and lobes or "valleys".  The septal necks are retrochoanitic in early whorls but prochoanitic (directed forward toward aperture) in later whorls.  The protoconch (first chamber) is bulbous to egg-shaped (as opposed to Nautiloidea).  The siphuncle is usually ventral and marginal in position.

Three basic types of sutures occur in the Ammonoidea:

Goniatitic - most of the lobes are undivided (Devonian to Permian).
Ceratitic - lobes are serrate (Carboniferous to Triassic)
Ammonitic - both lobes and saddles are finely subdivided (Permian to Cretaceous).  

Links

Links

Ammonoidea - basic introduction - some nice drawings of  juvenile planktonic forms (Hamites) on this page

Ammonites - photos and some basic info of a number of species from the Cretaceous (mostly Campanian and Maastrichtian) of North America, and some specimen from Europe  - by the Worldwide Museum of Natural History