Description
Genuine Reynesella sp. Ammonite Fossil
This carefully chosen fossil is a genuine Reynesella sp. ammonite from Sherborne, Dorset, UK. It dates from the Bajocian Stage of the Middle Jurassic, representing an extinct marine cephalopod that lived in the warm seas covering southern Britain approximately 170–168 million years ago.
The abbreviation “sp.” indicates that the fossil has been identified confidently as belonging to the genus Reynesella, while a precise species has not been assigned. This is standard scientific terminology used for specimens that show the characteristic features of a recognised genus but may not preserve all the diagnostic details required for reliable identification at species level.
The photograph shows the actual fossil specimen you will receive, allowing its natural shell form, surface detail, colouration, preservation and any attached matrix to be examined before purchase. Full sizing and scale information can be seen in the accompanying photograph.
Bajocian Middle Jurassic Geology
The Bajocian Stage forms part of the Middle Jurassic Period and records an important interval in the evolution and diversification of ammonites. During this time, much of southern England lay beneath a broad marine shelf connected to the extensive north-west European seas.
These waters supported a varied ecosystem containing ammonites, belemnites, bivalves, brachiopods, gastropods, echinoids and other marine organisms. Fine mud, carbonate sediment, sand and shell debris accumulated across the seabed under changing conditions of water depth, current strength and sediment supply.
Some beds formed during periods of relatively continuous deposition, while others developed more slowly and were affected by erosion or reworking. Shells could remain exposed on the sea floor before burial, allowing them to become worn, broken, transported or concentrated into fossil-rich horizons.
Ammonites are especially important to Jurassic geology because they evolved comparatively rapidly and were widely distributed across ancient seas. Distinctive ammonite assemblages allow geologists to divide rock successions into detailed biozones and correlate strata between different localities.
Reynesella Ammonite Characteristics
Reynesella was a planispirally coiled ammonite, meaning that its shell grew in a single flat spiral around a central umbilicus. Each new whorl expanded around the preceding growth stage as the animal increased in size.
The visible shell form may show a relatively open central region, allowing portions of the earlier whorls to remain exposed. Ornamentation typically consists of ribs extending outward from the umbilical area across the flanks. These ribs may curve, strengthen or divide as they approach the outer edge of the shell.
Whorl proportions, rib strength, rib spacing and the form of the venter are among the features used when comparing ammonite genera. The exact appearance of an individual fossil depends on the growth stage of the ammonite and the effects of burial, mineralisation, weathering and preparation.
Juvenile whorls may preserve finer and more closely spaced ornamentation, while later growth can develop broader rib spacing and more substantial shell proportions. Areas remaining within the natural matrix may also preserve different levels of detail from more exposed sections.
Chambered Shell and Ammonite Biology
The living ammonite occupied the final and largest section of its shell, known as the body chamber. Earlier portions were divided internally by curved walls called septa, creating a sequence of chambers collectively known as the phragmocone.
The animal could regulate the proportions of gas and liquid within these chambers, helping it control buoyancy within the Jurassic water column. The intersections between the septa and the outer shell formed intricate suture lines, which may become visible where the shell surface has been naturally removed or weathered.
Ammonites belonged to the cephalopods, the same broad group of marine molluscs that includes modern squid, cuttlefish, octopuses and nautiluses. Although often compared with the chambered nautilus because of their coiled shells, ammonites formed a distinct and highly diverse evolutionary group.
Sherborne Fossil Locality
Sherborne lies within a geologically important area of Dorset known for its Jurassic sedimentary rocks and varied fossil record. The surrounding region preserves evidence of ancient marine environments that repeatedly covered southern Britain during the Jurassic Period.
Fossils from documented Dorset localities are particularly sought after because they combine natural visual appeal with reliable geographical and geological context. A fossil from Sherborne provides a direct connection to the prehistoric marine environments that once occupied this part of England.
Dorset has a long association with fossil discovery, scientific study and collecting. While the county is internationally known for its Jurassic Coast, inland areas around Sherborne also preserve significant Jurassic strata and fossil-bearing deposits.
Scientific and Geological Details
Fossil type: Ammonite
Scientific identification: Reynesella sp.
Order: Ammonitida
Geological period: Jurassic
Geological epoch: Middle Jurassic
Geological stage: Bajocian
Locality: Sherborne, Dorset, United Kingdom
The identification to genus level provides useful scientific information while accurately reflecting the preserved features of the specimen. Its Bajocian age places it within a period when ammonites were rapidly developing new shell shapes and ornamental patterns across the European marine basins.
Natural Fossilisation and Preservation
After the ammonite died, its shell settled onto the seabed or was transported by currents before burial. Sediment gradually accumulated around the remains, protecting parts of the shell from complete destruction.
Over millions of years, pressure and mineral-rich groundwater transformed the surrounding sediment into rock. The original shell material may have been preserved, altered, replaced by minerals or dissolved to leave an internal mould. Some ammonites retain a combination of mineralised shell, sediment-filled chambers and natural matrix.
Any incomplete areas, minor fractures, worn ribs, mineral deposits, attached rock or colour variations are genuine geological characteristics. These features record the specimen’s burial, fossilisation, exposure and eventual recovery and ensure that every authentic ammonite has its own individual appearance.
Collectable British Jurassic Fossil
This Reynesella sp. ammonite fossil would make an excellent addition to a collection of British fossils, Dorset ammonites, Sherborne fossils, Bajocian fossils or Middle Jurassic marine specimens. Its recorded genus, geological stage and locality also make it suitable for a fossil cabinet, natural history display, educational collection, office or distinctive gift for a fossil enthusiast.
This is a 100% genuine fossil specimen and includes a Certificate of Authenticity generic card with a lifetime guarantee. The fossil has been individually selected for its collectable qualities, and the photograph shows the exact specimen that will be supplied.
Full sizing please see photo.
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