Rare Protogrammoceras geometricum Ammonite Fossil from Upper Aalenian Jurassic Deux Sevres France

Description

Geological Context

This Protogrammoceras geometricum ammonite fossil originates from the Upper Aalenian Stage of the Jurassic Period, approximately 174 to 170 million years ago. It was collected in Deux-Sèvres, France, a locality well known for its richly fossiliferous deposits from the Middle Jurassic. The Aalenian marks the beginning of the Middle Jurassic and is an interval of significant evolutionary radiation for ammonites. The rocks of this stage are often characterised by marine sedimentation, capturing the life of shallow seas connected to the ancient Tethys Ocean.

Taxonomic Classification

• Kingdom: Animalia
• Phylum: Mollusca
• Class: Cephalopoda
• Order: Ammonitida
• Superfamily: Hildoceratoidea
• Family: Hildoceratidae
• Genus: Protogrammoceras
• Species: Protogrammoceras geometricum

As a member of the Hildoceratidae family, Protogrammoceras exhibits many of the hallmarks of this lineage, including compressed shell morphology and distinct ribbing patterns that make these fossils both scientifically important and visually appealing to collectors.

Morphological Features

This ammonite fossil showcases the diagnostic traits of Protogrammoceras geometricum:

• Planispiral coiling with a relatively compressed and involute whorl profile.
• Geometric ribbing patterns, giving the species its name, with ribs extending across the flanks and often bifurcating near the ventrolateral margins.
• The suture lines are clearly visible in places, displaying the intricate folded pattern typical of ammonites in this family.
• The shell surface is preserved in excellent detail, highlighting growth lines and ornamentation that add both scientific and aesthetic value.

These features not only aid in reliable identification but also provide insight into how the species adapted to its marine environment.

Palaeoenvironment and Depositional Setting

During the Aalenian, the region of present-day Deux-Sèvres was part of a shallow epicontinental sea, dominated by fine silts and carbonate muds. These calm marine conditions were ideal for ammonite populations, as well as other invertebrates such as belemnites, bivalves, and gastropods. Ammonites like Protogrammoceras geometricum were likely active swimmers, preying on plankton, small crustaceans, and other nektonic organisms. Their chambered shells allowed precise buoyancy control, enabling them to occupy various ecological niches in the Jurassic seas.

Scientific and Collector Importance

• Stratigraphic Marker: Protogrammoceras geometricum is a key ammonite for correlating Upper Aalenian deposits across Europe.
• Evolutionary Significance: This genus helps illustrate the diversification of the Hildoceratidae and the emergence of more specialised forms in the Middle Jurassic.
• Collector’s Appeal: Its geometric ribbing and well-defined shell make it a sought-after specimen for display.
• Educational Value: Perfect for teaching about ammonite morphology, Jurassic marine ecosystems, and the importance of index fossils.

Authenticity and Presentation

• This specimen is 100% genuine and includes a Certificate of Authenticity.
• The photographs provided show the exact fossil you will receive.
• A 1 cm scale cube in the images ensures accurate sizing reference.

This Protogrammoceras geometricum ammonite fossil from the Upper Aalenian of Deux-Sèvres, France is an exceptional piece for collectors, educators, or anyone interested in the fascinating history of Jurassic marine life. Its combination of rarity, scientific importance, and aesthetic preservation make it a standout specimen for any fossil collection.