Description
Iridescent Caloceras johnstoni Fossil Ammonite from the Jurassic of Somerset
This striking Iridescent Caloceras johnstoni fossil ammonite comes from the renowned Blue Lias Formation of Watchet, Somerset, United Kingdom, one of Britain’s classic Jurassic fossil localities. The specimen preserves the beautifully coiled shell of an extinct marine cephalopod that lived approximately 201–199 million years ago during the Early Jurassic Period.
The ammonite displays attractive natural iridescence, a shimmering effect produced by the mineralised shell layers reflecting light, often seen in exceptionally preserved specimens from the Blue Lias sediments. This visual effect enhances the intricate ribbed shell structure typical of early Jurassic ammonites.
The fossil shown in the photographs is the exact specimen you will receive, carefully selected for its preservation and aesthetic appeal. Every fossil sold includes a Certificate of Authenticity with a lifetime guarantee, confirming the specimen is a genuine natural fossil.
Please refer to the photographs for full specimen sizing and scale.
About Caloceras johnstoni – Early Jurassic Marine Cephalopod
Caloceras johnstoni is an ammonite species belonging to a group of fast-evolving marine molluscs that flourished in the world’s oceans throughout the Jurassic Period. Ammonites were related to modern squid, cuttlefish, and octopus, but possessed an external coiled shell divided internally into chambers.
Taxonomic classification:
- Phylum: Mollusca
- Class: Cephalopoda
- Subclass: Ammonoidea
- Order: Ammonitida
- Superfamily: Psiloceratoidea
- Family: Psiloceratidae
- Genus: Caloceras
- Species: Caloceras johnstoni
Members of the Psiloceratidae family represent some of the earliest ammonites to appear following the end-Triassic mass extinction, making them important index fossils used by geologists to identify very early Jurassic strata.
Caloceras johnstoni was formally described in the 19th century by the British palaeontologist James Sowerby, whose pioneering work helped document many of the United Kingdom’s Jurassic fossils.
Geological Context – The Blue Lias Formation
This fossil originates from the Blue Lias Formation, a famous sequence of alternating limestone and shale layers deposited during the Hettangian Stage of the Early Jurassic. The formation is widely exposed along parts of the Somerset coastline, particularly around Watchet, where coastal erosion frequently reveals marine fossils.
During this period, southern Britain was submerged beneath a shallow epicontinental sea. Fine sediments settled on the seabed in relatively calm waters, creating ideal conditions for preserving marine organisms. Periodic changes in sediment supply and oxygen levels produced the alternating limestone and shale beds characteristic of the Blue Lias.
The sediments preserve a rich fossil fauna including:
- Ammonites
- Belemnites
- Marine reptiles
- Bivalves and gastropods
- Crinoids and other echinoderms
Because ammonites evolved rapidly and had wide geographic distribution, species such as Caloceras johnstoni are important biostratigraphic markers, helping geologists date early Jurassic rocks with remarkable precision.
Morphology and Notable Features of the Ammonite Shell
The shell of Caloceras johnstoni forms a tightly coiled spiral known as a planispiral shell, divided internally into chambers called camerae. The animal lived in the outermost chamber while the internal chambers were filled with gas or liquid to regulate buoyancy, allowing the ammonite to move vertically in the water column.
This species typically exhibits:
- Fine radial ribbing across the shell surface
- A rounded whorl profile
- A moderately evolute coil where earlier whorls remain partially visible
- Subtle growth lines marking stages of shell expansion
The natural iridescent sheen seen on some Blue Lias ammonites results from the preservation of the shell’s original aragonitic nacre layers, which reflect light to produce shimmering colours.
A Genuine British Fossil with Certificate of Authenticity
This authentic Jurassic ammonite fossil is an excellent example of a classic British fossil species and makes a superb addition to any fossil collection, geological display, or educational set.
Key features of this specimen include:
- Genuine Caloceras johnstoni fossil ammonite
- Natural iridescent shell preservation
- Geological formation: Blue Lias
- Age: Early Jurassic, Hettangian Stage
- Locality: Watchet, Somerset, United Kingdom
- Beautiful spiral ammonite morphology
- The exact fossil shown in the photographs
- Includes a Certificate of Authenticity with lifetime guarantee
Specimens from the Blue Lias of Somerset are among the most iconic Jurassic fossils in Britain, offering a direct connection to the ancient seas that covered the region over 200 million years ago. This ammonite preserves the elegant spiral geometry and natural beauty that has fascinated fossil collectors and palaeontologists for generations.
Reviews
There are no reviews yet.