Description
Genuine Fontannesia grammoceroides Ammonite Fossil
This carefully chosen specimen is a genuine Fontannesia grammoceroides ammonite fossil from France, dating from the Middle Jurassic Period. Fontannesia is an uncommon and distinctive ammonite genus associated with the early Bajocian marine faunas of Europe, making this an appealing specimen for collectors of French fossils, Jurassic ammonites and scientifically identified prehistoric marine life.
The species is recorded as Fontannesia grammoceroides (Haug, 1887). The author’s name and date placed within brackets indicate that the species was originally described in a different taxonomic combination before later being transferred to the genus Fontannesia. The specimen’s recognised species identification adds considerable palaeontological and educational interest.
Fontannesia Shell Form and Ornamentation
Fontannesia grammoceroides possessed a planispirally coiled shell, meaning that its whorls developed within a single plane. The shell was generally evolute, with successive whorls overlapping only moderately and leaving a clearly visible central umbilicus. This open, balanced coiling gives the ammonite a particularly attractive and recognisable profile.
Fontannesia is characterised by a small to medium-sized, discoidal shell with ornamented flanks and a single median keel running along the venter, or outermost edge of the coil. The flanks commonly carry curved ribs that sweep across the shell towards the ventral region. Unlike some more heavily armoured Jurassic ammonites, the genus is not normally characterised by prominent tubercles.
The precise morphological details visible on this individual specimen depend upon its growth stage and natural preservation. Features may include the coiled whorl profile, central umbilicus, curved ribbing, ventral keel, growth lines and areas of mineralised shell. Where the original shell has been lost, a natural internal mould may preserve the overall shape and surface ornamentation.
Ammonite Classification
Fontannesia is generally classified within the ammonite family Sonniniidae, superfamily Hammatoceratoidea and extinct order Ammonitida. Sonniniid ammonites were an important and varied component of marine cephalopod faunas around the transition into the Bajocian.
The genus is known from several parts of the world, although European material is particularly significant for studying early Bajocian ammonite succession and dispersal. Fossil records show that related Fontannesia species formed part of ammonite assemblages used to compare Lower Bajocian deposits across widely separated regions.
The specific name grammoceroides means resembling Grammoceras, referring to similarities in the general shell form and ornamentation of these ammonite groups. Such names reflect the detailed comparison of whorl shape, ribbing, coiling and ventral structure used by palaeontologists to distinguish fossil cephalopods.
Chambered Shell and Ammonite Biology
The ammonite’s shell was divided internally into numerous chambers separated by curved walls called septa. The animal occupied the final and largest section, known as the body chamber, while the older chambered portion formed the phragmocone.
A slender tube called the siphuncle connected the chambers. By regulating the balance of liquid and gas within them, the ammonite could control its buoyancy and maintain its position in the water. As the animal grew, it added new shell material around the aperture and periodically constructed another septum behind its body.
Although their coiled shells resemble those of living nautiluses, ammonites formed a separate cephalopod lineage. They were related more broadly to modern squid, cuttlefish, octopuses and nautiluses and became among the most abundant and diverse marine animals of the Mesozoic Era.
Middle Jurassic Bajocian Seas
Fontannesia grammoceroides is associated with the Bajocian Stage of the Middle Jurassic, an interval when ammonite faunas were rapidly evolving and becoming increasingly diverse. Recorded examples have been assigned to the earliest Lower Bajocian ammonite succession, demonstrating the species’ usefulness in recognising finely divided intervals of Jurassic time.
During the Bajocian, much of western Europe was covered by warm marine waters. France occupied an important position between north-west European shelf seas and the broader Tethyan marine realm to the south. Connections between these waters allowed ammonites and other marine animals to spread across large geographical areas.
These seas supported ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids and numerous other organisms. Shell debris and carbonate mud accumulated on the seabed, eventually forming fossiliferous limestone and marl. Differences in water depth, currents, oxygen levels and sediment supply created a varied range of marine habitats.
Fossilisation and Natural Preservation
After the ammonite died, its shell descended to the Jurassic seabed. The soft parts decayed, while sediment entered the body chamber and any internal chambers opened by damage. Continued deposition buried the shell beneath further layers of marine sediment.
The original shell was composed largely of aragonite, a crystalline form of calcium carbonate that can dissolve or recrystallise after burial. Sediment that had entered the shell could harden into a natural internal mould, preserving its coiling, ribs and other external features even after the original shell material disappeared.
Mineral-rich groundwater may also have replaced or coated parts of the shell. Natural matrix, mineral staining, small fractures, weathered ribs, incomplete whorls and irregular edges are authentic results of fossilisation and prolonged geological exposure. These characteristics form part of the fossil’s history rather than being modern manufacturing defects.
French Jurassic Fossil for Collection and Display
This genuine Fontannesia grammoceroides ammonite is an excellent addition to a collection of French fossils, European ammonites, Bajocian marine life or Middle Jurassic cephalopods. Its evolute coiling, ribbed flanks, ventral keel and recognised species identification 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, shape, matrix, mineralisation 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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