Ammonoidea

-Marine mollusc with straight, curved or coiled shell.

-Soft bodied "foot" is modified to ring of tentacles around mouth.

-Shell is composed mostly of aragonite and is filed with gas to govern buoyancy.

-Group evolved from the Nautiloidea in Silurian times with an increase in complexity of the suture line, and migration of the siphuncle to the outer margin of the shell.

-Represented by three orders: the Goniatitida, Prolecanitida and Cerititida.

-Goniatitida were the bulk of Permian ammonoids, but were suffering decline throughout the Permian.
-Suffered abupt extinction of remaining genera at P-T boundary.
-One goniatite genus seen in basal Triassic which dies out very quickly.

-Prolecanitida were small order which was declining throughout Permian.
-Only just survived across P-T boundary, before dying out completely in the first Triassic epoch.

-Cerititida evolved and diversified from the Prolecanitida as they were declining in the Late Permian.
-The order quickly diversified before being decimated by the extinction event.
-Reappear as Lazarus taxa in the Early Triassic, again rapidly radiating to become dominant ammonoid order. -Triassic cerititid "newcomers" first started to evolve before the Mesozoic.

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