Ammonites
Extinct coiled cephalopods that dominated the world's oceans for 350 million years — among the most commonly collected and scientifically important fossils in the world.
Overview
Ammonoids were chambered, coiled cephalopod molluscs — swimming relatives of modern squid and octopus that carried their buoyant shells through over 300 million years of ocean history. Rapid evolution and worldwide distribution made them the definitive index fossils of the Mesozoic, and their beautiful spiral shells are among the most collected fossils on Earth.
Quick Facts
Key Characteristics
- •Planispirally coiled (or uncoiled 'heteromorph') chambered shell
- •Shell divided by septa; suture lines where septa meet the wall evolved from goniatitic → ceratitic → ammonitic complexity
- •Buoyancy regulated by adjusting gas/liquid in the phragmocone chambers
- •Aptychi — paired jaw plates — in many Mesozoic forms
- •Sexual dimorphism (macroconchs and microconchs) documented in many genera
Fossil Record
Ammonite shells were originally aragonite and fossilise readily in limestones, shales and chalks. Complete specimens are common; some faunas are pyritised (Yorkshire, Germany), others preserve spectacular iridescent ammolite (Pierre Shale, Alberta). Key fossil-bearing units include the Blue Lias and Whitby Mudstone (England), the Muschelkalk (Germany), the Solnhofen Limestone, and the Pierre Shale of the Western Interior Seaway.
Global Distribution
Global — every continent including Antarctica. Classic regions include England, Germany, France, Morocco, Madagascar, the North American Great Plains, Russia, India and Japan.
Evolutionary & Scientific Importance
Ammonites are the premier Mesozoic index fossils: their rapid turnover defines biozones used to correlate marine rocks worldwide, and their final disappearance marks the K-Pg boundary. Suture complexity, heteromorph experiments and dimorphism make them a textbook study of evolution in the fossil record.