Description
Genuine Homeoplanulites difficilis Microconch Ammonite
This carefully chosen fossil is a genuine Homeoplanulites difficilis microconch ammonite from Lower Callovian deposits in Deux-Sèvres, Nouvelle-Aquitaine, France. It represents an extinct marine cephalopod that inhabited the warm seas of western Europe during the early part of the Callovian Stage of the Middle Jurassic.
The specimen is especially interesting because it is identified as a microconch, the smaller shell form within a sexually dimorphic ammonite pairing. Microconchs commonly possess distinctive adult features and provide important evidence for understanding ammonite growth, reproduction and variation.
The listing photograph shows the actual fossil you will receive rather than a stock image or representative example. Its individual shell form, ribbing, colour, matrix and natural preservation can therefore be examined before purchase. Full sizing is shown in the accompanying photograph.
Fossil Classification and Geological Information
Scientific name: Homeoplanulites difficilis
Fossil type: Microconch ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Perisphinctoidea
Family: Perisphinctidae
Genus: Homeoplanulites
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Callovian
Stratigraphic position: Lower Callovian
Locality: Deux-Sèvres, Nouvelle-Aquitaine, France
Homeoplanulites belongs to the perisphinctid ammonites, a major Jurassic group recognised for evolute coiling and prominent radial ornamentation. These ammonites are valuable to palaeontologists because changes in their shell form and ribbing help distinguish successive Jurassic faunas.
Microconch Ammonite Dimorphism
Many ammonite species are represented by two adult shell forms traditionally called macroconchs and microconchs. Macroconchs are larger and are generally interpreted as female forms, while microconchs are smaller and are usually interpreted as males.
A microconch is not simply an immature ammonite. It is an adult shell form that completed its growth at a smaller diameter and may possess specialised features around the aperture. These can include extended projections known as lappets, which developed on either side of the shell opening. Depending on preservation, the delicate aperture and lappets may be complete, partial or absent.
The identification of this specimen as a microconch gives it particular scientific and collectable interest. It represents the smaller adult form within an ammonite population and illustrates how individuals assigned to the same biological species could develop noticeably different mature shells.
Distinctive Homeoplanulites Morphology
The shell grew in a flat spiral known as planispiral coiling. In an evolute ammonite, successive whorls overlap only slightly, leaving much of the earlier coiling visible around a broad central umbilicus.
Homeoplanulites ammonites typically display rounded or gently compressed whorls with strong radial ribs. Primary ribs begin near the umbilical margin and extend across the flanks, commonly dividing into two or more finer secondary ribs towards the outer shell region. This branching pattern is known as bifurcation or polyfurcation and is characteristic of many perisphinctid ammonites.
The ribs may be straight, gently curved or inclined forward as they approach the venter. Their spacing and strength can change during growth, so the inner juvenile whorls may display a different pattern from the mature outer whorl. Small constrictions or interruptions in the ornament may also occur and represent pauses or changes in shell development.
These features give Homeoplanulites difficilis an attractive textured appearance and allow the fossil to be studied as both a natural display specimen and an example of Middle Jurassic ammonite morphology.
Chambered Shell and Buoyancy
The living ammonite occupied the final and largest section of its shell, known as the body chamber. The older inner portion was divided into numerous sealed chambers by internal walls called septa.
A slender tube known as the siphuncle passed through these chambers. By controlling the quantities of liquid and gas within the chambered part of the shell, the animal could regulate buoyancy and maintain its position within the water column.
Where the septa met the outer shell wall, they formed complex suture lines. These patterns may become visible where the original shell surface has weathered away or has been exposed during fossil preparation. Ammonitic sutures often display elaborate branching lobes and saddles and are important features in fossil cephalopod classification.
The soft-bodied animal would have possessed eyes, a head and a group of grasping appendages surrounding the mouth. Movement was likely achieved through jet propulsion, with water expelled through a muscular funnel.
Lower Callovian Middle Jurassic Age
The Callovian is the final stage of the Middle Jurassic Epoch, following the Bathonian and preceding the Oxfordian. The Lower Callovian represents its earliest subdivision and dates to approximately 166 million years ago.
During this interval, shallow marine environments extended across large areas of Europe. Ammonites were abundant and evolved rapidly, producing distinctive assemblages that allow sedimentary rocks to be divided into detailed biozones and correlated between separate localities.
Their widespread distribution makes ammonites particularly useful index fossils. A specimen with recorded stratigraphic and locality information therefore preserves more than the remains of an extinct animal; it also provides evidence for the relative age and marine origin of the rock in which it was found.
Jurassic Marine Environment of Western France
During the Lower Callovian, the region now forming Deux-Sèvres was influenced by shallow seas connected to the wider western European marine system and the Tethys Ocean. The modern landscape had not yet developed, and areas of western France alternated between submerged carbonate platforms, marine channels and low-lying land.
Lime-rich mud, clay, sand and shell debris accumulated across the seabed. These environments supported ammonites, belemnites, bivalves, brachiopods, gastropods, echinoderms, crustaceans and fish, together with larger marine reptiles.
After death, an ammonite shell could drift briefly or descend directly to the seafloor. Burial beneath sediment protected it from complete destruction. As further layers accumulated, pressure compacted the sediment and mineral-rich groundwater entered the shell or the empty cavity left by its dissolution.
Over millions of years, the shell could be preserved as mineralised material, an internal mould or a combination of both. Later uplift, erosion and excavation exposed the fossil-bearing Jurassic strata of Nouvelle-Aquitaine.
Actual Fossil and Certificate of Authenticity
This is a genuine fossil specimen rather than a replica, cast or modern reproduction. It has been individually selected for its natural character, geological interest and collectable appearance. Variations in colour, mineralisation, matrix, completeness and surface texture are authentic results of burial, fossilisation and natural weathering.
The listing photograph shows the exact Homeoplanulites difficilis microconch ammonite that will be supplied. Full sizing is provided in the photograph so its dimensions can be assessed before purchase.
The specimen includes a generic Certificate of Authenticity card carrying a lifetime guarantee of authenticity. It is an excellent addition to a Jurassic ammonite collection, French fossil display, geological teaching collection or natural-history cabinet.
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