Description
Genuine Harpoceras opalinoides Ammonite Fossil
This carefully chosen fossil is a genuine Harpoceras opalinoides ammonite from Burton Bradstock in Dorset, UK. Dating from the Aalenian Stage of the Middle Jurassic, it represents an extinct marine cephalopod that inhabited the ancient seas covering southern Britain approximately 170 to 174 million years ago.
With its classic coiled form, British coastal provenance and important position within the early Middle Jurassic, this specimen is an appealing addition to an ammonite collection, geological display or cabinet of Dorset fossils.
The photograph shows the actual fossil specimen you will receive. Its individual coiling, surface ornamentation, preservation, colour, matrix and natural geological characteristics can therefore be examined before purchase. Full sizing and proportions are shown in the accompanying photograph.
This authentic fossil includes a generic Certificate of Authenticity card with a lifetime guarantee.
Fossil Classification and Geological Details
Scientific name: Harpoceras opalinoides
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Hildoceratoidea
Family: Hildoceratidae
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Aalenian
Locality: Burton Bradstock, Dorset, UK
Ammonites are among the most useful fossils for dating Jurassic marine rocks. Their shell forms evolved comparatively rapidly, while many genera spread across broad areas of the ancient seas. Successive ammonite faunas allow geologists to compare sedimentary layers and arrange them within a detailed sequence of geological time.
The species name opalinoides refers to its resemblance to opalinid ammonite forms associated with the opening part of the Aalenian. Its identification provides an interesting connection with the major evolutionary transition between the latest Lower Jurassic ammonites and the diverse faunas of the Middle Jurassic.
Streamlined Ammonite Shell Morphology
Harpoceras opalinoides possessed a planispirally coiled shell, meaning that each whorl developed around the central umbilicus within a single flat plane. Successive whorls partly overlapped the earlier growth, creating a compact and streamlined profile.
Ammonites of this general form commonly display compressed whorls with gently convex flanks. Curved or falcate ribs may sweep forwards across the sides of the shell, reflecting repeated stages of shell growth. The strength and visibility of this ornamentation can vary across the inner whorls and later body chamber.
The outer edge of the shell is known as the venter. In streamlined hildoceratoid ammonites, this region may be narrow and defined by a keel running around the shell’s circumference. A keeled venter would have produced a sharper outline and may have reduced resistance as the animal moved through open water.
Shell compression, rib curvature, umbilical width and ventral form are all important characteristics used when comparing closely related ammonites. The exact detail visible on this specimen depends upon its growth stage and natural preservation.
Any fractures, incomplete sections, attached matrix, mineral deposits, surface wear or colour variations are authentic geological features. They record the fossil’s long history of burial, alteration and exposure rather than modern manufacture.
The Aalenian Stage of the Middle Jurassic
The Aalenian is the earliest stage of the Middle Jurassic. It follows the Toarcian Stage of the Lower Jurassic and precedes the Bajocian. This interval records major evolutionary changes among ammonites and other marine organisms.
During Aalenian times, much of western Europe was covered by interconnected shallow and moderately deep seas. Britain existed as a series of islands, submerged platforms and marine basins positioned between larger continental land areas.
Ammonites were abundant and diverse inhabitants of these waters. Their changing shell forms allow Aalenian rocks to be divided into zones and smaller biostratigraphic intervals. These fossil-based divisions provide a precise framework for comparing British strata with deposits elsewhere in Europe.
The Aalenian sea also supported belemnites, nautiloids, bivalves, brachiopods, gastropods, echinoids and other marine invertebrates. Fish and marine reptiles occupied higher levels within the food web, while burrowing organisms lived within the seabed sediment.
Jurassic Geology of Burton Bradstock
Burton Bradstock lies on the internationally renowned Jurassic Coast of Dorset. Its cliffs preserve a valuable succession of sedimentary rocks, including the Bridport Sand Formation and overlying Middle Jurassic deposits of the Inferior Oolite Group.
The Bridport Sand Formation consists largely of fine-grained marine sandstone deposited as sand accumulated across the Jurassic seabed. The characteristic golden cliffs visible around Burton Bradstock represent ancient sediment that was progressively buried, compacted and cemented into rock.
Above these sands, changing marine conditions led to the deposition of limestone, iron-rich sediment, shell debris and carbonate material. Variations in sea level, currents, water depth and sediment supply produced a complex sequence of fossil-bearing beds.
Some fossils were buried close to where the animals died, while others were transported, weathered or reworked by currents before final burial. Slowly accumulating and condensed horizons could concentrate ammonites and other durable marine remains within relatively thin layers.
The fossil succession at Burton Bradstock is important for understanding the transition from the Aalenian into the Bajocian. Ammonites recovered from these strata provide evidence for the age of individual beds and for changing marine connections across Jurassic Europe.
Life of a Jurassic Ammonite
Ammonites were marine molluscs related to modern squid, cuttlefish, octopuses and nautiluses. The living animal occupied the final and largest portion of its shell, known as the body chamber. Its head, eyes and tentacle-bearing soft body projected through the aperture.
The older coiled portion was divided into chambers by curved internal walls called septa. A narrow tube known as the siphuncle connected these chambers and helped regulate their liquid and gas content. This chambered structure provided buoyancy control and enabled the animal to maintain its position within the water column.
Movement was achieved through jet propulsion. Water was drawn into the mantle cavity and expelled through a muscular funnel, pushing the animal in the opposite direction. Tentacles surrounding the mouth were probably used to capture small prey, collect food and investigate the surrounding marine environment.
As the ammonite grew, new calcium carbonate material was added around the shell opening. The animal moved forwards into an enlarged body chamber and produced a new septum behind itself, gradually creating the familiar chambered spiral.
Natural Fossilisation and Authentic Preservation
After death, the ammonite shell settled onto the Aalenian seabed. Its soft tissues decayed while sediment entered and accumulated around the empty shell. Burial helped protect the remains from complete destruction by currents, scavengers and chemical dissolution.
Over millions of years, the surrounding sediment became compacted and cemented into rock. Mineral-rich groundwater could recrystallise, replace or dissolve the original aragonitic shell. Sediment filling the chambers hardened into an internal mould that preserved the ammonite’s coiled form and impressions of its original ornamentation.
Each fossil has experienced a unique combination of burial, mineral alteration, geological movement, erosion and exposure. Natural differences in texture, completeness, colour and attached matrix make every specimen individually distinctive.
This Harpoceras opalinoides ammonite is ideal for fossil collectors, ammonite enthusiasts, geology students and educational natural-history displays. The photograph shows the exact carefully selected specimen you will receive. Full sizing is shown in the photo. A generic Certificate of Authenticity card with a lifetime guarantee is included.
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