''Psiloceras'' is an extinct genus of

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

. ''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, including ''Psiloceras'' were wiped out at the Triassic–Jurassic extinction event (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 (ICS) has assigned the First Appearance Datum of the ''Psiloceras spela''-group as the defining biological marker for the start of the

Hettangian The Hettangian is the earliest age and lowest stage of the Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (My ...

, 201.3 ± 0.2 million years ago, the earliest stage of the Jurassic, with the Global Boundary Stratotype Section and Point (GSSP) located at Kuhjoch pass,

Karwendel Mountains The Karwendel is the largest mountain range of the Northern Limestone Alps. The major part belongs to the Austrian federal state of Tyrol, while the adjacent area in the north is part of Bavaria, Germany. Four chains stretch from west to east; in ...

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

References

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