Description
Genuine Graphoceras concavum Ammonite Fossil
This carefully chosen fossil is a genuine Graphoceras (Lioceras) concavum ammonite from Burton Bradstock in Dorset, UK. This characteristic species belongs to the Upper Aalenian Stage of the Middle Jurassic and is associated with the Graphoceras concavum Zone. It represents an extinct marine cephalopod that inhabited the seas covering southern Britain approximately 171 million years ago.
The specimen combines an identifiable ammonite species with a classic Dorset locality and an important position within Middle Jurassic biostratigraphy. Its compressed shell, distinctive ribbing and keeled outer margin make it an attractive addition to a British fossil collection, ammonite display or educational geological cabinet.
The photograph shows the actual fossil specimen you will receive. Its individual coiling, ornamentation, preservation, colour, matrix and natural geological features can therefore be examined before purchase. Full sizing and proportions can be seen in the accompanying photograph.
This genuine fossil includes a generic Certificate of Authenticity card with a lifetime guarantee.
Fossil Classification and Geological Details
Scientific name: Graphoceras (Lioceras) concavum
Fossil type: Ammonite
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Superfamily: Hammatoceratoidea
Family: Graphoceratidae
Geological period: Jurassic
Epoch: Middle Jurassic
Stage: Upper Aalenian
Biozone: Graphoceras concavum Zone
Locality: Burton Bradstock, Dorset, UK
The species was originally described by James Sowerby in the early nineteenth century. It has appeared in palaeontological literature under different generic combinations, including Graphoceras concavum and Lioceras concavum, reflecting changes in the classification of closely related graphoceratid ammonites.
The genus Graphoceras is scientifically important because its species form part of a rapidly evolving ammonite lineage used to identify and correlate Aalenian marine rocks. The Concavum Zone marks the final major ammonite zone of the Aalenian immediately before the beginning of the Bajocian Stage.
Compressed Shell and Sinuous Ribbing
Graphoceras concavum possessed a planispirally coiled shell, with each whorl developing in a single flat plane around a central umbilicus. The shell is generally compressed from side to side, producing a narrow and relatively streamlined profile.
Successive whorls partly overlap the earlier growth, leaving a clearly defined central umbilical area. The flanks are ornamented with curved or sinuous ribs that sweep across the shell towards the outer margin. These ribs may change in strength and spacing as the ammonite grows, creating a sculptured pattern that records successive stages of shell development.
The outer edge of the shell, known as the venter, typically carries a distinct keel. This raised ridge extends around the outer circumference and gives the ammonite a sharp, hydrodynamic outline. The combination of compressed whorls, flexuous ribbing and a keeled venter is characteristic of many graphoceratid ammonites.
The name concavum refers to a concave or hollowed aspect associated with the shell form. Whorl proportions, umbilical shape, rib curvature and ventral structure are all important features used when distinguishing this species from related Aalenian ammonites.
The exact amount of visible ribbing, keel and whorl detail depends upon the growth stage and preservation of the individual fossil. Natural fractures, incomplete areas, mineral deposits, surface wear and attached matrix are authentic geological characteristics.
Upper Aalenian Concavum Zone
The Aalenian is the earliest stage of the Middle Jurassic, following the Toarcian and preceding the Bajocian. It represents an important period of evolutionary change among European ammonites, particularly within the Graphoceratidae.
The Graphoceras concavum Zone occupies the uppermost part of the Aalenian. It is named after this species and provides a precise biostratigraphic context for fossil-bearing rocks deposited near the close of the stage.
A biozone is an interval of sedimentary rock identified by a characteristic fossil species or assemblage. Ammonites are especially useful for this purpose because they evolved rapidly and were distributed through extensive marine environments. Their successive appearances allow geologists to divide the Jurassic rock record into zones, subzones and narrower faunal horizons.
The zonal significance of Graphoceras concavum gives this specimen additional scientific interest. It represents a recognisable point in the changing ammonite succession shortly before new groups became prominent during the Bajocian.
Jurassic Geology of Burton Bradstock
Burton Bradstock lies on the Dorset coast within one of Britain’s most celebrated Jurassic landscapes. The coastal succession includes the Bridport Sand Formation and overlying fossil-bearing rocks of the Inferior Oolite Group.
At Burton Bradstock, Graphoceras concavum is particularly associated with the upper part of the Bridport Sand Formation. These rocks originated as marine sands deposited across the seabed during the Aalenian. The sediments were later compacted and cemented into the distinctive sandstone visible in the cliffs.
During this time, southern Britain lay beneath a warm sea connected to the broader marine basins of north-western Europe. Britain consisted of islands, submerged platforms and marine channels rather than the continuous landmass visible today.
The sea supported ammonites alongside belemnites, bivalves, brachiopods, gastropods, echinoids and other marine organisms. Fish and marine reptiles occupied higher levels within the ecosystem, while burrowing animals lived within the sandy seabed.
Changes in sea level, current strength, water depth and sediment supply influenced the deposits formed at Burton Bradstock. Some shells were transported or worn before burial, while others were covered quickly enough to retain recognisable ornamentation.
Life of a Middle Jurassic Ammonite
Ammonites were marine molluscs related to living squid, cuttlefish, octopuses and nautiluses. The living animal occupied the final and largest section of its shell, known as the body chamber.
The older coiled part of the shell was divided into smaller 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 provided buoyancy control and allowed the ammonite 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. The compressed, keeled shell of Graphoceras concavum provided a streamlined profile suited to movement through open marine water.
Tentacles surrounding the mouth were probably used to capture small prey, gather food and investigate the surrounding environment. As the animal grew, new shell material was added around the aperture and additional chambers formed behind the expanding body chamber.
Natural Fossilisation and Authenticity
After death, the ammonite shell settled onto the sandy Aalenian seabed. Its soft tissues decayed while sediment entered and accumulated around the empty shell. Continued 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 coiling and impressions of the original ornamentation.
Natural weathering, mineral staining, fractures, incomplete areas and attached matrix record the specimen’s individual geological history. These features ensure that every fossil is unique and provide visible evidence of its great age and natural origin.
This Graphoceras (Lioceras) concavum ammonite is suitable 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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