Description
Genuine Cylicoceras undatum Ammonite Fossil
This carefully chosen specimen is a genuine Cylicoceras undatum ammonite fossil from France, dating from the Aalenian Stage of the Middle Jurassic Period. Its recognised species identification, European provenance and distinctive coiled shell form make it an appealing piece for collectors of French fossils, Jurassic ammonites and prehistoric marine life.
The species is recorded as Cylicoceras undatum S. Buckman, 1889, linking it with the extensive palaeontological work of British ammonite specialist Sydney Savory Buckman. The name is occasionally encountered under alternative spellings in older collections and commercial records, but Cylicoceras undatum is the taxonomic form used here.
This is an individual natural specimen. The photograph shows the actual fossil you will receive, allowing its preservation, colour, matrix and surface features to be examined before purchase.
Cylicoceras Shell Form and Ornamentation
Cylicoceras undatum possessed a planispirally coiled shell, meaning that its successive whorls developed within a single plane. The shell was relatively evolute, with the later whorls leaving much of the earlier growth visible around a broad central umbilicus. This creates an attractive open-coiled appearance in which the ammonite’s spiral growth pattern can be clearly appreciated.
The whorls were generally compressed rather than strongly inflated. Fine ribs or growth lines curved across the flanks and could form a gently waved or undulating pattern. This characteristic is reflected in the species name undatum, derived from a Latin term meaning waved or undulating.
The outer margin of the shell, known as the venter, may be comparatively narrow. Depending on preservation, it can display a subtle keel or defined ventral structure. Palaeontologists identify ammonites of this type by comparing the width of the umbilicus, degree of whorl overlap, whorl cross-section, rib curvature and development of the ventral region.
The exact features visible on this fossil depend upon its growth stage and natural preservation. It may retain sections of mineralised shell, impressions of the original surface, visible whorl boundaries or a natural internal mould formed when sediment filled the shell and later hardened.
Graphoceratid Ammonite Classification
Cylicoceras is associated with the ammonite family Graphoceratidae, an important group of Jurassic cephalopods particularly characteristic of the Aalenian Stage. It belongs within the extinct order Ammonitida and the broader cephalopod subclass Ammonoidea.
Graphoceratid ammonites evolved rapidly during the Aalenian and developed a variety of compressed, ribbed and keeled shell forms. Their progressive changes through successive rock layers make them especially valuable for dating and correlating Middle Jurassic marine deposits.
Cylicoceras undatum represents one of these distinctive Aalenian forms. Its relatively open coiling and gently curved ornamentation distinguish it from more involute ammonites in which the outer whorl conceals much of the earlier shell.
Aalenian Middle Jurassic Age
The Aalenian Stage forms the earliest division of the Middle Jurassic and dates to approximately 174 to 170 million years ago. It follows the Toarcian Stage of the Early Jurassic and precedes the Bajocian Stage.
This was a significant interval in ammonite evolution. Several older Early Jurassic lineages disappeared or changed, while graphoceratid ammonites became prominent components of marine faunas. Their rapid diversification produced a detailed fossil succession that now helps geologists divide Aalenian rocks into zones, subzones and smaller biohorizons.
Ammonites are highly effective index fossils because many species existed for comparatively limited periods while being distributed across extensive marine regions. Correctly identified examples can therefore help palaeontologists compare rocks of similar age from different parts of Europe.
Aalenian Seas of France
During the Aalenian, much of present-day France was covered by warm marine waters. The modern landscape had not yet formed, and the region included shallow continental shelves, carbonate platforms and deeper offshore basins connected with the wider Tethyan marine realm.
Sediments accumulating in these environments included limestone, marl, clay and calcareous mud. Their composition varied according to water depth, local currents, biological productivity and the amount of sediment entering the sea.
Ammonites lived within the water column alongside belemnites and other cephalopods. The seabed supported bivalves, gastropods, brachiopods, echinoids and numerous burrowing organisms. Shells and skeletal fragments accumulated with fine sediment, gradually creating the fossil-bearing rocks now exposed in different regions of France.
The widespread marine connections of the Jurassic allowed ammonite populations to disperse between areas that are now separate countries. French specimens can therefore be compared with related Aalenian ammonites found elsewhere in western and central Europe.
Chambered Shell and Buoyancy System
The ammonite shell was divided internally into a succession of chambers separated by walls known as septa. The living animal occupied the final and largest section, called the body chamber, while the older chambered portion formed the phragmocone.
A slender tube called the siphuncle connected the chambers near the outer edge of the shell. By regulating their fluid and gas content, the animal could adjust its buoyancy and maintain its position within the sea.
As the ammonite grew, new shell material was added around the aperture. The animal moved forward into a larger living space and constructed a new septum behind itself. This repeated process produced the distinctive spiral shell and preserved changes in ornamentation through different growth stages.
Ammonites were related to modern cephalopods such as squid, cuttlefish, octopuses and nautiluses, although they formed their own highly successful and now-extinct evolutionary lineage.
Natural Fossilisation and Preservation
After this ammonite died, its shell settled onto the Jurassic seabed. The soft tissues decayed, while sediment entered the body chamber and any internal chambers exposed through natural breakage.
Continued burial placed the shell beneath additional layers of marine sediment. Over millions of years, compaction and mineral-rich groundwater transformed the sediment into solid rock. The original shell, composed mainly of aragonite, could dissolve, recrystallise or be replaced by other minerals.
Sediment hardened inside the shell to create a natural internal mould capable of preserving its coiling and ornamentation even where the original shell material disappeared. Natural matrix, mineral staining, incomplete whorls, worn ornamentation, small fractures and irregular edges are authentic results of fossilisation and prolonged geological exposure.
Authentic French Ammonite for Collection and Display
This genuine Cylicoceras undatum ammonite is an excellent addition to a collection of French fossils, Aalenian ammonites, Middle Jurassic cephalopods or European natural-history specimens. Its recognised species identification, graphoceratid affinities and characteristic open-coiled form give it strong geological, educational and display interest.
The fossil is a carefully chosen individual piece, and the photograph shows the actual specimen you will receive, rather than a representative example. Natural variations in colour, mineralisation, matrix, shape and preservation make every fossil unique.
Full sizing is shown in the photographs.
This fossil is a 100% genuine specimen and includes a generic Certificate of Authenticity card with a lifetime guarantee.
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