Ammonites are an extinct group of marine animals belonging to the cephalopod subclass Ammonoidea and are excellent index fossils. They are utilized by geologists and paleontologists to link the rock layer in which they are found to specific geological time periods. The closest living relative of the ammonite, is not the modern Nautilus, which they somewhat outwardly resemble, but rather the octopus, squid and cuttlefish. Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as "heteromorphs"). They ranged from the lower Jurassic (200Ma) to the Upper Cretaceous (65.5Ma).
Ammonites provided by Panther Creek Minerals, San Antonio, Texas.
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