Temporal range: Hettangian
Psiloceras is an extinct genus of cephalopod belonging to the ammonite subclass. Psiloceras fossils are commonly found at Watchet, Somerset, England. Here smooth-shelled Psiloceras planorbis (along with other species) are to be found as usually flattened fossils in the Blue Lias. Unlike most earlier ammonites, which had complex shell shapes and ornamentation, Psiloceras had a smooth shell.
All Ammonites, with the sole exemption of the genus Psiloceras were wiped out at the Triassic–Jurassic extinction event (201.3 million years ago). Hence all ammonites that lived during the Jurassic and Cretaceous are descendants of Psiloceras.
The International Commission on Stratigraphy (ICS) has assigned the First Appearance Datum of the Psiloceras spela-group as the defining biological marker for the start of the Hettangian, 201.3 ± 0.2 million years ago, the earliest stage Jurassic.
Jurassic of Argentina, Austria, Canada, China, France, Germany, New Zealand, Spain, the United Kingdom, United States 
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