Description
Genuine Dorsetia semicostata Ammonite Fossil
This carefully chosen fossil is a genuine Dorsetia semicostata ammonite from Dorset, UK. It was collected from the Inferior Oolite Group and dates to the Lower Bajocian stage of the Middle Jurassic. The photographs show the actual specimen you will receive, including its natural form, preservation, colour, matrix and visible detail. Please use the sizing shown in the photographs for the specimen’s full dimensions.
Middle Jurassic Geology
The Bajocian is an early stage of the Middle Jurassic Period, dating to roughly 168 to 170 million years ago. During this interval, southern Britain lay beneath warm, shallow seas. Sediment accumulated across an extensive marine shelf and later hardened into the limestones, marls and other rocks now grouped within the Inferior Oolite. The name oolite refers to rock containing small, rounded carbonate grains called ooids, which commonly formed where warm, agitated seawater moved across shallow seabeds. Local conditions varied, and the succession records changing water depths, currents, sediment supply and marine communities.
Dorset and the Inferior Oolite
Dorset is internationally recognised for its Jurassic geology and fossil heritage. Although the county is especially famous for coastal exposures, Middle Jurassic rocks also preserve a rich record of marine life. Ammonites from the Inferior Oolite Group are scientifically valuable because their rapid evolution and broad geographical distribution allow rock layers to be compared across different localities. Individual species can help palaeontologists refine the relative age of strata and reconstruct the sequence of Jurassic marine environments.
Ammonite Species and Morphology
Dorsetia is an extinct genus of ammonite within the order Ammonitida. Ammonites were marine cephalopods related to living squid, octopus and nautilus. Their shells grew in a coiled form and were divided internally into chambers. The living animal occupied the outer body chamber, while older chambers contributed to buoyancy control. Dorsetia semicostata is associated with Lower Bajocian deposits and is a characteristic component of Middle Jurassic ammonite assemblages.
Diagnostic study of an ammonite considers features such as whorl shape, degree of coiling, umbilical width, rib style and the development of the shell’s outer margin. On fossil specimens, some of these features may be preserved as original shell material, mineral replacement, an impression or an internal mould produced when sediment filled the shell. Natural wear, matrix, small fractures and irregular surfaces are evidence of the specimen’s long geological history and are not manufactured defects.
A Real Fossil for Study or Display
This fossil offers a direct connection with a marine ecosystem that existed in Dorset more than 160 million years ago. Ammonites were active animals within Jurassic seas and formed part of complex food webs alongside bivalves, brachiopods, belemnites, marine reptiles and many other organisms. After death, burial in seabed sediment protected part of this shell long enough for fossilisation to take place.
The exact specimen pictured will be supplied. Its individual preservation makes it suitable for an established fossil collection, a geology teaching display, a natural-history cabinet or a distinctive gift for someone interested in palaeontology and British fossils.
Certificate of Authenticity
The specimen includes a generic Certificate of Authenticity card with a lifetime guarantee of authenticity. To preserve its natural surfaces, handle it carefully and avoid harsh cleaning chemicals, soaking, excessive moisture and sudden temperature changes. Display it securely, supported in a way that does not place pressure on fragile edges or exposed shell detail.
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