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Jurassic Ammonite Zones

Ammonite species evolved rapidly but only survived for approximately 250 k years before sufficiently evolving into a new species. As ammonites were marine species and had a wide, global distribution they are perfect marker species to correlate rock units across the world.

 

Therefore, sedimentary rock layers can be dated based on the presence of specific ammonites. These time periods are known as ammonite zones. For example, Quenstedoceras lamberti, defines the Lamberti Zone at the very top of the Corallian rock sequence.

Psiloceras planorbis
201.3 ± 0.2 million years.  

Hettangian Stage, Jurassic Period.


Psiloceras is an extinct genus of ammonite. Psiloceras is among the earliest known Jurassic ammonites, and the appearance of the earliest Psilocerasspecies 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.

 

The base (The first  appearance datum) of the Hettangian and therefore the Jurassic in Britain was historically defined by the first appearance of the ammonite Psiloceras planorbis.

Psiloceras fossils are commonly found at Watchet, Somerset, England.

Psiloceras planorbis (BGS N. Ireland)

Psiloceras planorbis
201.3 ± 0.2 million years.  

Sinemurian Stage, Jurassic Period.


Psiloceras is an extinct genus of ammonite. Psiloceras is among the earliest known Jurassic ammonites, and the appearance of the earliest Psilocerasspecies 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.

 

The base (The first  appearance datum) of the Hettangian and therefore the Jurassic in Britain was historically defined by the first appearance of the ammonite Psiloceras planorbis.

Psiloceras fossils are commonly found at Watchet, Somerset, England.

Psiloceras planorbis (BGS N. Ireland)