Description
Genuine Cylioceras undatum Ammonite Fossil
This carefully chosen specimen is a genuine Cylioceras undatum ammonite fossil from Burton Bradstock, Dorset, UK. It originates from the fossiliferous Middle Jurassic succession associated with the Inferior Oolite Group and the Opalinum Zone, giving the specimen valuable locality and biostratigraphical information.
The species was named by the British palaeontologist Sydney Savory Buckman in 1899, and the accepted spelling is Cylioceras undatum. Specimens of this uncommon British ammonite are particularly associated with the early Middle Jurassic rocks exposed around Burton Bradstock.
Although sometimes commercially described as Bajocian, the Opalinum Zone belongs to the Lower Aalenian Stage, immediately preceding the Bajocian. The fossil therefore dates from the early part of the Middle Jurassic, approximately 174 million years ago. Burton Bradstock exposes the Bridport Sand Formation beneath a condensed Inferior Oolite succession, preserving fossil horizons from both the Aalenian and Bajocian stages.
Distinctive Cylioceras Shell Morphology
Cylioceras undatum was a small marine cephalopod with a planispirally coiled and internally chambered shell. Its shell form is comparatively evolute, meaning the later whorls do not completely conceal the earlier growth stages. This leaves a visible central umbilicus and gives the fossil an attractively open, clearly coiled appearance.
The shell is recognised by its curved or gently undulating ornamentation. Fine ribs or growth lines may sweep across the flanks, producing the wavy surface pattern reflected in the species name undatum, meaning waved or undulating. The whorls are generally compressed rather than strongly inflated, and the outer edge of the shell may appear narrow and rounded.
The exact features visible on this individual specimen depend upon its growth stage, natural preservation and the amount exposed from the surrounding Jurassic matrix. Areas of original shell, rib impressions, whorl outlines, mineral deposits and sections of natural internal mould may all contribute to its appearance.
Chambered Shell and Ammonite Biology
During life, the ammonite occupied the final and largest section of its shell, known as the body chamber. Behind this was the phragmocone, a sequence of progressively smaller chambers divided by internal walls called septa.
A narrow tube known as the siphuncle passed through these chambers. By regulating their liquid and gas content, the ammonite could control buoyancy and maintain its position in the water. New shell material was added around the aperture as the animal grew, producing the familiar spiral form.
Ammonites were extinct marine molluscs belonging to the cephalopod lineage. They were more closely related to modern squid, cuttlefish and octopuses than to the externally similar chambered nautilus. Their rapid evolution and widespread distribution make them among the most useful fossils for dating and correlating Jurassic marine rocks.
Opalinum Zone Biostratigraphy
The Opalinum Zone marks the earliest standard ammonite zone of the Aalenian Stage. It takes its name from Leioceras opalinum, another characteristic ammonite frequently recorded from the lower Middle Jurassic succession at Burton Bradstock.
Ammonite zones are recognised through the appearance and succession of distinctive species and genera. Because ammonites evolved relatively quickly and inhabited broad marine areas, geologists can use them to compare rock layers from widely separated localities.
The recorded Opalinum Zone provenance therefore provides this specimen with more scientific context than an ammonite known only from a general Jurassic age. It links the fossil with a specific interval near the beginning of the Middle Jurassic and with an important stage in the development of later Jurassic ammonite faunas.
Inferior Oolite and Burton Bradstock Geology
The cliffs at Burton Bradstock consist mainly of the Bridport Sand Formation, capped by a comparatively thin and highly condensed succession of the Inferior Oolite Group. The local rocks record changing shallow-marine conditions, including variations in water depth, current strength, sediment supply and seabed stability.
The Inferior Oolite contains marine limestone, including beds rich in shell fragments and rounded carbonate particles called ooids. Ooids formed as calcium carbonate accumulated in layers around tiny nuclei, such as sand grains or fragments of shell, while being moved repeatedly by waves and currents.
At Burton Bradstock, individual fossiliferous beds may be relatively thin yet represent substantial periods of geological time. Slow sedimentation, erosion and breaks in deposition helped concentrate shells and other marine remains within particular horizons. The underlying Bridport Sand Formation is also well known for Aalenian ammonites, including fossils associated with the Opalinum and Scissum intervals.
Aalenian Marine Environment
During the early Aalenian, the region that would eventually become Dorset lay beneath a warm sea. Ammonites moved through the water alongside belemnites and other cephalopods, while bivalves, gastropods, brachiopods, echinoids and burrowing organisms occupied the seabed.
After death, the shell of this Cylioceras descended to the seafloor. Soft tissues decayed, and sediment entered the body chamber or any broken internal chambers. Continued burial gradually transformed the surrounding sediment into rock.
The original aragonitic shell could subsequently dissolve or be replaced by minerals. Sediment hardened within the shell to form a natural internal mould, preserving its coiling and surface ornamentation even where the original shell material was no longer present.
Authentic Dorset Jurassic Fossil
This genuine Cylioceras undatum ammonite is an excellent addition to a collection of British fossils, Dorset ammonites, Aalenian cephalopods or Inferior Oolite specimens. Its uncommon species identification, Opalinum Zone provenance and association with the classic Burton Bradstock locality provide strong geological, educational and display interest.
Natural matrix, mineral staining, weathered areas, incomplete whorls, shell loss and surface irregularities are authentic characteristics created during fossilisation and more than 170 million years of geological history.
This is a carefully selected individual specimen, and the photograph shows the actual fossil 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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