Description
Genuine Lissoceras oolithicum Ammonite Fossil
This carefully chosen specimen is a genuine Lissoceras oolithicum ammonite fossil from the Inferior Oolite Group at Oborne Wood Quarry, North Dorset, UK. Dating from the Bajocian Stage of the Middle Jurassic Period, it is assigned more precisely to the Polygyralis Subzone, giving the fossil excellent geological and stratigraphical provenance.
Lissoceras oolithicum was originally described under the name Ammonites oolithicus by the French palaeontologist Alcide d’Orbigny in 1842. The accepted combination places the species within the genus Lissoceras, a distinctive group of predominantly smooth-shelled Jurassic ammonites.
Smooth-Shelled Lissoceras Morphology
Unlike the strongly ribbed and prominently ornamented ammonites commonly associated with the Inferior Oolite, Lissoceras is recognised by its comparatively smooth and streamlined shell. Its shell is typically involute, meaning that each successive whorl overlaps much of the preceding one. This creates a relatively narrow central umbilicus and gives the ammonite a compact, neatly coiled appearance.
The whorls are generally laterally compressed, with smooth flanks or only very fine ornamentation. The venter, or outer edge of the coiled shell, is characteristically rounded to blunt and lacks a pronounced keel. These understated features make Lissoceras readily distinguishable from many of the heavily ribbed stephanoceratid and perisphinctid ammonites found in Bajocian rocks.
The exact features visible on this specimen depend upon its natural preservation and the portion exposed from the surrounding matrix. It may retain areas of original shell, impressions of the shell surface or an internal mould formed when sediment entered the empty chambers and later hardened.
Lissoceras is placed within the extinct cephalopod order Ammonitida and the superfamily Haploceratoidea. Modern taxonomic treatments associate it with the lissoceratid lineage of smooth and compressed Jurassic ammonites.
Polygyralis Subzone Provenance
The Polygyralis Subzone is a finely defined ammonite-based interval within the Subfurcatum Zone of the Bajocian Stage. Ammonite subzones divide geological time more precisely than broader stages and zones, using the appearance and succession of characteristic ammonite assemblages.
Material from Oborne Wood has played an important role in recognising this interval in southern England. Published palaeontological research records ammonites from an Oborne Wood horizon equivalent to the Polygyralis Subzone of the Subfurcatum Zone.
Ammonites are particularly useful index fossils because they evolved rapidly and were widely distributed through Jurassic seas. Changes in their shell form and species composition allow geologists to correlate fossil-bearing beds between separate localities. The recorded Polygyralis Subzone provenance therefore gives this specimen considerably more scientific context than an ammonite without a known horizon.
Inferior Oolite Group Geology
The Inferior Oolite Group is an important Middle Jurassic rock succession extending across parts of southern and eastern England. In Dorset and Somerset, it consists of a varied series of bioclastic, sandy, ferruginous, peloidal and bioturbated limestones, accompanied by subordinate ooidal limestone, sandstone, limestone conglomerate, lime-mudstone and mudstone.
The term “oolite” refers to limestone containing ooids, which are small rounded carbonate grains. These formed as calcium carbonate accumulated in concentric layers around tiny nuclei such as sand grains or shell fragments. Repeated movement by currents and waves helped develop their rounded form.
However, the Inferior Oolite is not composed entirely of ooidal rock. Its changing sediment types record variations in water depth, current strength, seabed stability, sediment supply and marine conditions. Fossils may occur within bioclastic limestone composed partly of broken skeletal material, ferruginous beds enriched with iron minerals or sediments extensively disturbed by burrowing organisms.
Middle Jurassic Marine Environment
During the Bajocian, approximately 170 million years ago, the region that would eventually become Dorset was covered by a warm sea. These waters supported diverse communities of ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids and other marine organisms.
Ammonites were cephalopods related to modern squid, cuttlefish, octopuses and nautiluses. Their coiled shells were divided internally by walls called septa, creating a succession of chambers. The animal occupied the final and largest body chamber, while the older chambers formed the phragmocone.
A narrow tube called the siphuncle connected these chambers and helped regulate the shell’s buoyancy. Following death, some shells settled onto the Jurassic seabed and were buried by marine sediment. Over millions of years, the sediment hardened into rock while the shell was mineralised, replaced or preserved as a natural internal mould.
Oborne Wood Quarry, North Dorset
Oborne Wood Quarry, near Sherborne in North Dorset, is a recognised British locality for Middle Jurassic fossils from the Inferior Oolite. Its fossil-bearing deposits have yielded varied ammonite assemblages that help document the changing marine fauna of the Bajocian.
The locality is especially valued by collectors because ammonites can be associated with carefully defined stratigraphical horizons. Fossils retaining both locality and subzone information offer a direct connection to the detailed geological history of southern England.
Authentic British Fossil for Collection and Display
This genuine Lissoceras oolithicum ammonite is an excellent addition to a collection of British fossils, Dorset ammonites, Middle Jurassic specimens or Inferior Oolite marine life. Its smooth shell form provides an attractive contrast to the strongly ribbed ammonites frequently encountered in Bajocian deposits.
Natural matrix, mineral deposits, weathered areas, incomplete shell sections and surface irregularities are authentic features of fossilisation and its long geological history. This is a carefully selected individual fossil, and the photograph shows the actual specimen you will receive, rather than a representative example.
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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