Iridescent Caloceras johnstoni Ammonite Fossil Jurassic Watchet Somerset UK Specimen

Description

Iridescent Caloceras johnstoni Ammonite Fossil from Doniford Bay, Somerset

This exceptional fossil specimen is an iridescent ammonite identified as Caloceras johnstoni, recovered from the famous Blue Lias Formation at Doniford Bay near Watchet, Somerset, United Kingdom. Dating to the Early Jurassic Period, this ammonite represents one of the earliest members of the Jurassic ammonite faunas that flourished shortly after the end-Triassic extinction event.

The specimen was discovered by our own team members, Alister and Alison, on 28 February 2026 along the Jurassic coastline at Doniford Bay. Following recovery, the fossil was carefully cleaned, prepared, and treated by Alison, ensuring that the delicate shell structure and natural iridescent surface were preserved.

The photographs in this listing show the exact specimen you will receive, a carefully chosen fossil with excellent natural character. This fossil is a genuine specimen and includes a Certificate of Authenticity with a lifetime guarantee card.

Full sizing of the fossil can be seen in the listing photographs.

Geological Origin – Blue Lias Formation

This ammonite originates from the Blue Lias Formation, a sequence of alternating limestones and shales that form part of the classic Lower Jurassic stratigraphy of Somerset. These rocks were deposited approximately 201–190 million years ago during the earliest stages of the Jurassic Period.

The Blue Lias formed in a shallow epicontinental sea that covered much of southern Britain at the time. Fine marine sediments accumulated on the seabed, preserving marine organisms with remarkable detail.

Doniford Bay near Watchet is a well-known fossil locality where coastal erosion continually reveals new fossils from the Blue Lias cliffs and foreshore. The area yields a wide range of Early Jurassic fossils including:

• Ammonites
• Belemnites
• Bivalves
• Marine reptiles
• Crinoids and other echinoderms

These deposits are particularly significant for preserving early Jurassic ammonite species that are important for biostratigraphic zonation.

Caloceras johnstoni – Early Jurassic Ammonite

The genus Caloceras belongs to the ammonite order Ammonitida, a group of extinct marine cephalopods closely related to modern squid and octopus.

Taxonomic classification:

• Phylum: Mollusca
• Class: Cephalopoda
• Subclass: Ammonoidea
• Order: Ammonitida
• Superfamily: Psiloceratoidea
• Family: Psiloceratidae
• Genus: Caloceras
• Species: Caloceras johnstoni

Members of the Psiloceratidae are among the earliest ammonites to appear in the Jurassic fossil record following the end-Triassic extinction event. Species such as Caloceras johnstoni are therefore important index fossils used by geologists to identify specific Early Jurassic rock layers.

Morphology and Iridescent Shell Preservation

Ammonites possessed a coiled external shell divided into internal chambers known as camerae, which were connected by a tube called the siphuncle. This structure allowed the animal to regulate buoyancy as it moved through the water.

Characteristic features of Caloceras johnstoni include:

• A tightly planispiral coiled shell
• Smooth to gently ornamented whorl surfaces
• A relatively compressed shell profile
• Fine growth lines preserved on the outer shell
• Simple suture patterns typical of early Jurassic ammonites

This specimen displays natural iridescence, a shimmering colour effect caused by the preservation of the original shell layers. When the microscopic crystal structure of the shell is preserved, light refracts through the layers and produces vivid metallic colours.

Iridescent ammonites from the Blue Lias are highly sought after because this preservation of the original shell material is relatively rare.

Early Jurassic Marine Environment

During the Early Jurassic, the seas surrounding Somerset supported a diverse marine ecosystem. The warm shallow waters were inhabited by a wide range of organisms including:

• Ammonites such as Caloceras
• Belemnites
• Bivalves and gastropods
• Marine reptiles including ichthyosaurs and plesiosaurs
• Crinoids and other echinoderms

Ammonites were active swimming predators, using jet propulsion to move through the water while hunting small marine organisms such as crustaceans and plankton.

Their rapid evolutionary changes and widespread distribution make them some of the most important fossils for dating Jurassic rocks.

Authentic Fossil from the Jurassic Coastline of Somerset

This specimen represents a genuine ammonite fossil discovered directly from the fossil-rich coastline of Somerset. Fossils collected by experienced fossil hunters from well-known British localities are particularly valued by collectors.

Key features of this specimen include:

• Genuine Caloceras johnstoni ammonite fossil
• Naturally iridescent shell preservation
• From the Blue Lias Formation
• Early Jurassic Period
• Collected at Doniford Bay, Watchet, Somerset, UK
• Discovered by Alister and Alison on 28 February 2026
• Cleaned, prepared, and treated by Alison
• Photographs show the exact fossil specimen you will receive
• Supplied with a Certificate of Authenticity and lifetime guarantee

This remarkable ammonite makes an excellent addition to Jurassic fossil collections, ammonite displays, geology collections, and natural history exhibits, offering a beautiful and scientifically important example of marine life from the earliest Jurassic seas.