''Psiloceras'' is an extinct genus of ammonite. ''Psiloceras'' is among the earliest known Jurassic ammonites, and the appearance of the earliest ''Psiloceras'' species form the definition for the base of the Jurassic. Unlike most earlier ammonites, which had complex shell shapes and ornamentation, ''Psiloceras'' had a smooth shell.

Taxonomy

Almost all ammonites, with the sole exemption of a few members of the family

Psiloceratidae Psiloceratidae is an extinct family of cephalopods belonging to the ammonite subclass. Description The Psiloceratidae are evolute, smooth or with blunt primary ribbing. The venter is rounded and generally smooth, in some feebly keeled. Sutures a ...

, including ''Psiloceras'' were wiped out at the

Triassic–Jurassic extinction event The Triassic–Jurassic (Tr-J) extinction event, often called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, , and is one of the top five major extinction events of the Phanerozoic eon, profoundly affect ...

(201.3 million years ago). Most authors assume that ''Psiloceras'' descended from the Phyllocerataceae. ''P. spelae'' is probably the earliest species of ''Psiloceras''.

Biostratigraphic significance

The

International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigr ...

(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 of the

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

, with the

Global Boundary Stratotype Section and Point A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. The effort to define GSSPs is conducted b ...

(GSSP) located at Kuhjoch pass, Karwendel Mountains,

Northern Calcareous Alps The Northern Limestone Alps (german: Nördliche Kalkalpen), also called the Northern Calcareous Alps, are the ranges of the Eastern Alps north of the Central Eastern Alps located in Austria and the adjacent Bavarian lands of southeastern Germany ...

, Austria, which was ratified in 2010. The base of the Jurassic in Britain was historically defined by the first appearance of the ammonite ''Psiloceras planorbis.''

Distribution

Jurassic of Argentina, Austria, Canada, China, France, Germany, New Zealand, Spain, the United Kingdom, United States ''Psiloceras'' fossils are commonly found at

Watchet Watchet is a harbour town, civil parish and electoral ward in the county of Somerset, England, with a population in 2011 of 3,785. It is situated west of Bridgwater, north-west of Taunton, and east of Minehead. The town lies at the mouth of ...

, Somerset, England. Here smooth-shelled ''Psiloceras planorbis'' (along with other species) are to be found as usually flattened fossils in the

Blue Lias The Blue Lias is a geological formation in southern, eastern and western England and parts of South Wales, part of the Lias Group. The Blue Lias consists of a sequence of limestone and shale layers, laid down in latest Triassic and early Jurassi ...

References

Jurassic ammonites Index fossils Ammonites of North America Hettangian life Psiloceratidae Ammonitida genera {{Ammonitina-stub