Description
Genuine Polyplectus Ammonite Fossil from Dorset
This carefully chosen specimen is a genuine Polyplectus sp. ammonite fossil from the Charmouth Mudstone Formation of Dorset, England, UK. Preserved within rocks of Lower Jurassic age, it represents an extinct marine cephalopod that lived in the prehistoric seas covering southern Britain many millions of years ago.
The fossil has been identified to the genus Polyplectus. The abbreviation “sp.” indicates that it is attributed to this distinctive ammonite genus without assigning it to a particular species. This is standard scientific terminology for specimens whose observable characteristics support identification at genus level.
The listing photograph shows the actual fossil you will receive, not a stock image or representative example. Its individual preservation, shape, colour, matrix and natural surface features can therefore be assessed before purchase. Full sizing is shown in the accompanying photograph.
Fossil Classification and Geological Information
Scientific identification: Polyplectus sp.
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Hildoceratoidea
Family: Hildoceratidae
Genus: Polyplectus
Geological period: Jurassic
Epoch: Early Jurassic
Rock unit: Charmouth Mudstone Formation
Locality: Dorset, England, UK
Polyplectus belongs to the Hildoceratidae, an important family of Early Jurassic ammonites. Hildoceratid ammonites developed a wide variety of compressed and hydrodynamic shell forms and are particularly significant in the study and correlation of Lower Jurassic marine strata.
The genus Polyplectus was established by the German palaeontologist Alpheus Hyatt during the nineteenth century. Its name refers to the intricate folding visible in the ammonite’s suture pattern, one of the most complex and scientifically informative structures preserved in ammonite shells.
Distinctive Polyplectus Shell Morphology
Ammonite shells grew in a flat spiral known as planispiral coiling. Each successive whorl increased in size and partially enclosed the preceding whorl, producing the familiar circular form. The central exposed area is called the umbilicus, while the sides of the shell are known as the flanks and the outermost margin is called the venter.
Polyplectus ammonites are generally recognised by their compressed, relatively streamlined shells. The flanks may be flattened or gently convex, leading towards a narrow outer region. Depending on preservation and growth stage, the shell may display fine curved ribs, growth lines or traces of a ventral keel. These features contributed to a hydrodynamic form suited to movement through open marine waters.
One of the genus’s most notable characteristics is its highly elaborate suture pattern. Sutures mark the points where internal chamber walls met the outer shell. In advanced ammonites these lines developed complex arrangements of finely divided lobes and saddles. When exposed by natural weathering or fossil preparation, they may appear as intricate leaf-like or branching patterns across the shell.
The living animal occupied the final body chamber at the open end of the shell. Earlier parts were divided into sealed buoyancy chambers by internal walls called septa. A narrow organic tube, the siphuncle, passed through these chambers and allowed the animal to regulate their fluid and gas content, helping it maintain buoyancy within the water column.
Charmouth Mudstone Formation
The Charmouth Mudstone Formation forms part of the Lower Jurassic Lias Group and is one of the most celebrated fossil-bearing geological units in Britain. It is prominently exposed along the Dorset coast around Lyme Regis, Charmouth, Black Ven, Stonebarrow and Golden Cap.
The formation consists predominantly of marine mudstone and shale, with beds and nodules of limestone together with occasional sandstone horizons. Its sediments accumulated on the floor of a shallow sea during the Early Jurassic. Traditionally described as part of the Lower Lias, the succession records changing seabed conditions, oxygen levels, sediment supply and marine communities.
Repeated cycles of soft mud deposition created favourable conditions for fossil preservation. Harder limestone beds and nodules developed where calcium carbonate became concentrated within the sediment. Ammonite shells preserved in these rocks may occur as mineralised shell material, compressed impressions, internal moulds or fossils contained partly within their original matrix.
The Charmouth Mudstone Formation is divided into several named units, including the Shales-with-Beef, Black Ven Marl, Belemnite Marl and Green Ammonite members. Together, these strata preserve an exceptionally detailed record of Lower Jurassic marine life.
Lower Jurassic Marine Environment
During the Early Jurassic, the area now forming Dorset lay beneath a warm sea and was positioned significantly farther south than it is today. The coastline, cliffs and rolling countryside familiar today did not yet exist. Instead, the region formed part of an extensive marine basin inhabited by ammonites, belemnites, bivalves, brachiopods, gastropods, crustaceans, fish and marine reptiles.
Ammonites were active cephalopods related to modern squid, cuttlefish and nautiluses. They probably used jet propulsion for movement and possessed tentacles for capturing prey. Their chambered shells provided buoyancy while also protecting the soft body housed within the outer chamber.
After death, the shell could float temporarily or sink directly to the seabed. Burial beneath fine marine sediment protected it from scavenging and erosion. As additional layers accumulated, the sediment was compacted and transformed into rock. Mineral-rich water circulating through the buried shell could replace original material or fill its internal spaces, producing the fossil preserved today.
British Jurassic Fossil Collectable
Dorset is internationally famous for its Lower Jurassic fossils and its association with the early development of palaeontology. Fossil-bearing cliffs around Lyme Regis and Charmouth were explored by pioneering collectors including Mary Anning, whose discoveries transformed scientific understanding of prehistoric marine life.
A recorded specimen from the Charmouth Mudstone Formation provides a tangible connection to this important geological region. Its scientific identification and locality information make it suitable for an ammonite collection, British fossil display, natural-history cabinet or educational geology collection.
Actual Specimen and Certificate of Authenticity
This is a genuine fossil and not a replica, cast or reproduction. It has been individually selected for its natural character, collectable appearance and geological interest. Differences in colour, texture, mineralisation, matrix, completeness and surface preservation are authentic results of burial, fossilisation and natural weathering.
The photographs form an important part of the listing and show the exact Polyplectus sp. ammonite fossil that will be supplied. Full sizing is provided in the photograph.
The specimen includes a generic Certificate of Authenticity card with a lifetime guarantee of authenticity, providing lasting assurance that it is a genuine fossil. It is an excellent choice for collectors of British ammonites, Lower Jurassic fossils, Dorset natural history and prehistoric marine life.
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