cephalopods

• Phylum - Mollusca 

  Class - cephalopods 

  Sub classes - nautiloids and ammonoids 
• Nautiloids first appeared in the upper Cambrian. Early forms had a straight shell which may have caused balance problems, evolution of coiled nautiloids solved this problem.
• Siphuncle is used to adjust the proportion of gas and liquids within the gas chamber which controls the buoyancy. 
• The first chamber formed is called a protoconch. 
• Nautiloids are not used as zone fossils because they have remained unchanged for 400 million years. 
• Ammonoids are closely related to nautiloids, they became extinct at the end of the cretaceous, they're very common in the upper Palaeozoic and Mesozoic eras. 
• Ammonoids have a similar movement to nautiloids. Their tentacles were also used for slow swimming or pulling themselves along the sea floor. They also controlled their buoyancy by the gas in their chambers. 
• Other cephalons are predators so it is likely that ammonoids were too, although they may have scavenged for food because they were slow. \
• There are three different types of ammonoids: goniatites, Ceratites and ammonites. 
• They are distinguished by their suture lines where the septa fuses to the shell. 
• With time sutures became longer and more complex which are stronger and the animal can withstand greater pressure. 
• Goniatites were the earliest form of ammonoid, they were around from the Devonian to the end of the Permian. 

  
• Ceratites were around from the late Permian to the end of the Triassic. 

  
• Ammonites were around from the Jurassic to the end of the cretaceous. 

  
• The Jurassic period id divided into over 60 zones which represent time periods of less than 1 million year, each time division is based on changes in the suture lines, shell ornament, ribbing and style of coiling. 
• Ammonites were facies free because they were free swimmer so were preserved in many environments. They had hard parts and were abundant so were globally widespread. They also evolved rapidly through lots of short lived evolutionary stages.   
• Belemnites had no external shell but an internal bullet-shaped guard which was surrounded by flesh. The internal shell had chambers which kept it afloat. 
• Belemnites are good paleocurrent indicators, the pointed end  points in the direction of the current.