Psiloceras

Psiloceras Temporal range: Hettangian PreЄ Є O S D C P T J K Pg N ↓
File:Psiloceras Planorbis.jpg
Psiloceras planorbis
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Ammonoidea
Family:	Psiloceratidae
Genus:	Psiloceras Hyatt, 1867
Species
P. planorbis (J.C. Sowerby, 1824) (type) synonym Ammonites planorbis, A. sampsoni, P. sampsoni P. becki P. brevicellatum P. calliphylloides P. calliphyllum Neumayr, 1879 synonym Aegoceras calliphyllum P. costosum P. distinctum P. erugatum P. hagenowi P. marcouxi Guex, Taylor, Rakus & Bucher, 1998 P. minillaensis von Hillebrandt, 2000 P. naumanni P. pacificum Guex, 1980 P. polymorphum Guex, 1980 P. plicatulum (Quenstedt, 1883) synonym Ammonites plicatulus P. primocostatum von Hillebrandt, 2000 P. psilonotum P. sampsoni P. spelae Guex, Taylor, Rakus & Bucher, 1998 P. spelae tirolicum von Hillebrandt & Krystyn, 2009 P. tenerum P. tibeticum Yin et al., 2007 P. tilmanni Lange, 1925

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.

Taxonomy

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.

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, 201.3 ± 0.2 million years ago, the earliest stage Jurassic.

Distribution

Jurassic of Argentina, Austria, Canada, China, France, Germany, New Zealand, Spain, the United Kingdom, United States