Description
Genuine Eriella polychiades sanzi Ammonite Fossil
This carefully chosen fossil is a genuine Eriella polychiades sanzi ammonite from Milborne Wick, Somerset, UK. It dates from the Bajocian Stage of the Middle Jurassic, representing an extinct marine cephalopod that inhabited the warm seas covering southern Britain approximately 170–168 million years ago.
The photograph shows the actual fossil specimen you will receive, allowing its natural coiling, ornamentation, mineralisation, colour variations, preservation and any surrounding matrix to be examined before purchase. This is an individually selected fossil rather than a representative stock example.
Full sizing please see photo.
Bajocian Middle Jurassic Ammonite
The Bajocian was an important interval in the evolutionary history of ammonites. During this part of the Middle Jurassic, much of southern England lay beneath a shallow marine shelf connected to extensive seas across north-western Europe.
These waters supported diverse communities of ammonites, belemnites, brachiopods, bivalves, gastropods, echinoids and other marine organisms. Carbonate sediment, clay, sand and broken shell material accumulated across the seabed under changing conditions of water depth, current strength and sediment supply.
Ammonites are especially valuable to palaeontology because their shell forms evolved comparatively rapidly and many groups were geographically widespread. Distinctive assemblages allow geologists to divide Jurassic strata into detailed biozones and correlate fossil-bearing beds between different localities.
The Milborne Wick section is recognised for a prolific, well-preserved Lower Bajocian ammonite fauna associated with the Romani Subzone of the Humphriesianum Zone, making the locality particularly significant for British Middle Jurassic stratigraphy.
Eriella Ammonite Form and Ornamentation
Eriella polychiades sanzi possessed the characteristic planispirally coiled shell of a Jurassic ammonite. Each successive whorl developed around the earlier growth stages within a single plane, creating the familiar spiral form centred on the umbilicus.
The shell flanks may preserve ribs extending outward from the umbilical area towards the outer margin. Rib strength, curvature and spacing can change as the animal grows, so the exposed inner whorls may display different ornamentation from the later shell.
Features such as the width of the umbilicus, degree of whorl overlap, whorl section, rib pattern and shape of the venter are important when comparing ammonite taxa. The precise appearance of this specimen will reflect its individual growth stage and the effects of fossilisation, weathering and preparation.
Any sections retained within the natural matrix may display a different level of detail from more exposed areas. This variation is expected in an authentic fossil and contributes to the specimen’s individual geological character.
Chambered Shell and Marine Lifestyle
The ammonite’s living body occupied the final and largest portion of the shell, known as the body chamber. Earlier sections formed the phragmocone and were divided internally by curved walls called septa into a sequence of chambers.
A narrow connecting structure called the siphuncle helped regulate liquid within these chambers, enabling the animal to control the buoyancy of its shell. The shell continued growing through the addition of new material at the aperture, recording progressive stages in the animal’s development.
Where the septa met the inner shell wall, they formed patterned suture lines. These may become visible where the external shell layer has been naturally weathered, dissolved during fossilisation or carefully exposed.
Ammonites belonged to the extinct subclass Ammonoidea within the cephalopod molluscs. Their distant living relatives include squid, cuttlefish, octopuses and nautiluses. Fossilised shells provide most of the evidence used to reconstruct their anatomy and way of life because preservation of ammonite soft tissues is exceptionally rare.
Milborne Wick Fossil Locality
Milborne Wick is an important inland fossil locality in Somerset, close to the Jurassic geology of the Sherborne district. Its sedimentary rocks preserve evidence of an ancient seabed and the changing ammonite communities that occupied the region during the Bajocian.
Periods of relatively slow sedimentation allowed shells to accumulate on the seabed before final burial. Currents could move, abrade or reorient remains, while changing sea levels and sediment supply created fossil-rich horizons within the marine succession.
A specimen with documented Milborne Wick provenance has strong scientific and collectable interest. It connects the fossil to a recognised British ammonite locality and provides considerably more geological context than an example without locality or age information.
Scientific and Geological Details
Fossil type: Ammonite
Scientific identification: Eriella polychiades sanzi
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Geological period: Jurassic
Geological epoch: Middle Jurassic
Geological stage: Bajocian
Locality: Milborne Wick, Somerset, United Kingdom
Natural Fossilisation and Preservation
Following the animal’s death, its shell settled onto the Jurassic seabed or was transported by currents before burial. Sediment gradually entered and surrounded the shell, protecting portions from complete destruction.
Over millions of years, compaction and mineral-rich groundwater transformed the surrounding sediment into rock. The original aragonitic shell may have been altered, replaced or dissolved, leaving mineralised shell material, a natural internal mould or a combination of both.
Any incomplete areas, natural fractures, worn ribs, mineral-filled chambers, attached matrix or variations in colour are authentic geological characteristics. These features record the specimen’s burial, mineralisation, exposure and eventual recovery and are not manufacturing defects.
Collectable Somerset Jurassic Fossil
This Eriella polychiades sanzi ammonite fossil would make an excellent addition to a collection of British fossils, Somerset ammonites, Milborne Wick fossils, Bajocian specimens or Middle Jurassic marine life. Its named identification, documented age and recorded locality also make it suitable for a fossil cabinet, geological study collection, educational display or distinctive gift for an ammonite enthusiast.
This fossil is a 100% genuine specimen and includes a Certificate of Authenticity generic card with a lifetime guarantee. It has been carefully selected for its collectable qualities, and the photograph shows the exact specimen that will be supplied.
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