The shells, which are either straight or coiled, served as protective and supportive structures as well as hydrostatic devices, enabling the animal to compensate for varying water depths.
Ammonoids are characterized and distinguished from index fossils because of their wide geographic distribution in shallow marine waters, rapid evolution, and easily recognizable features.
Hutton, a Scottish geologist, first proposed formally the fundamental principle used to classify rocks according to their relative ages.
He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time.
Overview of Methods Superposition Stratigraphy Dendrochronology Radiocarbon C14 Radiometric Dating Methods Obsidian Hydration Dating Paleomagnetic/Archaeomagnetic Luminescence Dating Methods Amino Acid Racemization Fission-track Dating Ice Cores Varves Pollens Corals Cation Ratio Fluorine Dating Patination Oxidizable Carbon Ratio Electron Spin Resonance Cosmic-ray Exposure Dating This is an excellent overview of dating methodologies, and is a chapter in a textbook on Archaeology.
You may find it useful for the clear definitions, and for excellent links on a variety of topic.
Three groups of ammonoids succeeded one another through time, each group having a more complex suture pattern.
Most ammonoid genera became extinct at the end of that period, but a few survived and evolved into many diverse forms during the Cretaceous Period.
They hypothesize that the sudden decline of plankton during the K–T extinction at the end of the Cretaceous brought about the demise of the remaining ammonoid groups.
The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances.
The ammonoids were shelled forms, and nearly all are thought to have been predacious in habit.
There is evidence that these animals consumed zooplankton, crustaceans, and other ammonoids.