RARE Promicroceras Xipheroceras planicosta Ammonite Dimorphic Pair Fossil Lyme Regis

Description

Rare Promicroceras (Xipheroceras) planicosta Dimorphic Pairing Ammonite Fossil

This is a rare and genuine Promicroceras (Xipheroceras) planicosta ammonite fossil, displaying a dimorphic pairing from the Black Ven Marls, Lower Lias Formation, found at Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, UK. The specimen originates from the world-famous Jurassic Coast World Heritage Site, renowned for its exceptional Early Jurassic ammonites. Carefully prepared and selected, this fossil shows both microconch and macroconch forms preserved together, providing a fascinating glimpse into Jurassic cephalopod biology. The actual specimen you see in the photo is the exact one you will receive, and a Certificate of Authenticity is included.

Geological Context

• Geological Period: Jurassic
• Stage: Lower Lias, Early Jurassic (~200–190 million years ago)
• Formation: Black Ven Marls
• Location: Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, United Kingdom

The Lower Lias deposits at Lyme Regis were laid down in shallow epicontinental seas. Fine muds, rich in organic material, accumulated on the sea floor, preserving the remains of ammonites, belemnites, marine reptiles, and other fauna in exceptional detail. Fossils from this location are highly sought after due to their quality of preservation and their scientific importance in understanding the Early Jurassic marine ecosystem.

Taxonomic Classification

• Kingdom: Animalia
• Phylum: Mollusca
• Class: Cephalopoda
• Order: Ammonitida
• Superfamily: Psiloceratoidea
• Family: Promicroceratidae
• Genus: Promicroceras
• Subgenus: Xipheroceras
• Species: Promicroceras (Xipheroceras) planicosta

This species is notable for its dimorphism, where two distinct shell morphologies (microconch and macroconch) represent male and female forms of the same species. Such dimorphic pairs provide rare and valuable insights into ammonite life history and evolutionary adaptations.

Morphological Features

This fossilised pair shows the following characteristics:

• Planispiral coiling with finely ribbed ornamentation typical of Promicroceras.
• Microconch and macroconch dimorphism, highlighting size and rib variation between forms.
• Sharp, continuous ribs that extend across the venter, giving the ammonites a striking appearance.
• Well-preserved whorls with distinct suture lines visible under close inspection.

The preservation of dimorphic pairs is uncommon, making this specimen a scientifically significant and highly desirable collector’s piece.

Scientific and Collector Importance

Promicroceras (Xipheroceras) planicosta is a biostratigraphic marker species for the Lower Lias and an important fossil for dating Early Jurassic deposits. Dimorphic specimens such as this are extremely rare and of great interest to both collectors and palaeontologists. They shed light on ammonite reproduction, ecology, and evolutionary biology.

For collectors, this fossil represents more than a beautiful display piece. It is a window into the life of Jurassic seas and an example of natural history that is both educational and aesthetically appealing. Its provenance from the legendary Lyme Regis coast adds additional value, as this area is historically linked to fossil hunting pioneers such as Mary Anning.

Key Details

• Species: Promicroceras (Xipheroceras) planicosta
• Type: Dimorphic ammonite pairing (microconch and macroconch)
• Age: Lower Lias, Early Jurassic (~200–190 million years old)
• Location: Stonebarrow Cliff, Charmouth, Lyme Regis, Dorset, UK
• Formation: Black Ven Marls
• Authenticity: 100% genuine, supplied with Certificate of Authenticity

Authenticity and Collectability

This specimen is a rare dimorphic ammonite fossil from Lyme Regis, offering both scientific and display value. With its detailed ribbing, excellent preservation, and pairing of two forms in one specimen, it is a standout addition to any fossil collection. All of our fossils are authentic, carefully chosen, and come with a Certificate of Authenticity for your assurance.

This Promicroceras (Xipheroceras) planicosta ammonite fossil embodies the richness of the Jurassic seas and represents a unique opportunity to own a genuine, rare piece of Earth’s history from one of the most famous fossil localities in the world.