Governance

Oxford Geology Group logo

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)

Jurassic Zone Ammonites #02 Quenstedoceras lamberti

Ammonites are common & conspicuous fossils in Mesozoic marine sedimentary rocks. Ammonites are an extinct group of cephalopods - they’re basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group.

 

The origin of the name Ammonite is from the Ancient Greek. The coiled shell shaped of an ammonite is said to be reminiscent of a ram’s horn. The ancient Egyptian god Amun (Ammon in Greek) was often depicted with a ram’s head & horns.

 

Pliny’s Natural History, written in the 70s A.D., refers to these fossils as Hammonis cornu (the horn of Ammon), and mentions that people living in north eastern Africa revered them as sacred. Pliny also observed that ammonites were often pyritised.

At present, the 11 Jurassic stages (representing approximately 70 million years) can be divided into 145 ammonite-based zones or subzones. Quenstedoceras Lamberti defines the Lamberti Zone at the very top of the Callovian stage circa 163.5 ± 4.0 Ma. Image ca 3.5 cm width.

ammonite_edited_edited.jpg

Ammonites are common & conspicuous fossils in Mesozoic marine sedimentary rocks. Ammonites are an extinct group of cephalopods - they’re basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group.

 

The origin of the name Ammonite is from the Ancient Greek. The coiled shell shaped of an ammonite is said to be reminiscent of a ram’s horn. The ancient Egyptian god Amun (Ammon in Greek) was often depicted with a ram’s head & horns.

 

Pliny’s Natural History, written in the 70s A.D., refers to these fossils as Hammonis cornu (the horn of Ammon), and mentions that people living in north eastern Africa revered them as sacred. Pliny also observed that ammonites were often pyritised.

At present, the 11 Jurassic stages (representing approximately 70 million years) can be divided into 145 ammonite-based zones or subzones. Quenstedoceras Lamberti defines the Lamberti Zone at the very top of the Callovian stage circa 163.5 ± 4.0 Ma. Image ca 3.5 cm width.

ammonite_edited_edited.jpg