Brasilia decipiens Fossil Ammonite Aalenian Jurassic Horn Park Beaminster Dorset UK – Alice Purnell Collection with COA

Brasilia decipiens Fossil Ammonite – Aalenian, Jurassic, Horn Park, Beaminster, Dorset, United Kingdom

This outstanding Brasilia decipiens fossil ammonite is an authentic and beautifully preserved specimen from the Aalenian Stage of the Middle Jurassic, discovered at the famous Horn Park Quarry near Beaminster, Dorset, England. It originates from the esteemed Alice Purnell Collection, renowned for scientifically valuable and well-curated British Jurassic fossils. The photo shows the exact specimen you will receive, and the scale cube equals 1cm for precise sizing. This specimen perfectly captures the natural beauty and geological importance of Britain’s Jurassic heritage.

Geological Context and Provenance

The Horn Park Quarry, located in Beaminster, Dorset, is one of Britain’s most important Jurassic fossil localities and was the first Geological Nature Reserve designated in the UK (1954). It is world-famous for its rich and diverse Aalenian ammonite fauna and has provided key insights into the early Middle Jurassic marine ecosystems of southern England.

The specimen was collected from the Inferior Oolite Group, a formation deposited approximately 174 to 170 million years ago in a shallow, warm, marine shelf environment. The fine-grained limestones and calcareous muds of this deposit were laid down in clear, tropical waters that covered much of southern Britain during the Aalenian Age. These conditions created ideal settings for the fossilisation of ammonites, belemnites, brachiopods, and other marine organisms.

Scientific Classification

• Kingdom: Animalia

• Phylum: Mollusca

• Class: Cephalopoda

• Order: Ammonitida

• Superfamily: Hammatocerataceae

• Family: Graphoceratidae

• Genus: Brasilia

• Species: Brasilia decipiens (Buckman, 1892)

• Geological Stage: Aalenian (Middle Jurassic)

• Locality: Horn Park Quarry, Beaminster, Dorset, England, UK

The species Brasilia decipiens was first described by S.S. Buckman, one of Britain’s most prominent Jurassic ammonite specialists. His extensive research on Inferior Oolite ammonites established a detailed stratigraphic framework that is still used by palaeontologists today. Specimens of Brasilia are significant biostratigraphic markers used to identify and correlate Aalenian ammonite zones, particularly within the Opalinum Subzone of the Murchisonae Zone.

Morphology and Notable Features

This specimen displays the characteristic features of Brasilia decipiens — a tightly coiled, involute shell with narrow, slightly compressed whorls and a small umbilicus. The flanks are adorned with fine, regular ribbing that bifurcates toward the mid-flank and curves gently forward as it approaches the venter. The ribs are moderately dense, creating a striking yet refined ornamentation pattern typical of this genus.

The venter (outer margin) is smooth and gently rounded, often with faint keel development. The overall symmetry and balanced rib spacing reflect the elegant hydrodynamic design of this nektonic cephalopod, which would have swum freely in open marine environments. The high-quality preservation of this specimen reveals fine suture details, capturing the complexity of the ammonite’s chambered shell structure.

These ammonites were fast-moving marine predators, feeding on small crustaceans and planktonic organisms, propelling themselves through the water via jet propulsion. The combination of streamlined shell form and light construction made them agile and efficient hunters.

Depositional Environment and Biozone

The Aalenian represents a transitional stage between the Toarcian and Bajocian epochs, marked by climatic stability and widespread carbonate sedimentation across western Europe. The Horn Park Quarry ammonites are primarily associated with the Murchisonae Zone, a stratigraphic interval notable for its diversity of Graphoceratid ammonites, including Graphoceras concavum, Leioceras opalinum, and Brasilia decipiens.

The depositional environment of Horn Park was a shallow marine shelf, with well-oxygenated waters and periodic influxes of fine carbonate material. This setting allowed exceptional fossil preservation, particularly of ammonite shells that were rapidly buried by sediment after death. Fossils from this site provide valuable insight into early Middle Jurassic palaeogeography and cephalopod evolution.

Provenance – The Alice Purnell Collection

This specimen comes from the highly respected Alice Purnell Collection, known for its precisely documented British Jurassic material. Each specimen in this collection was carefully prepared, catalogued, and conserved, maintaining both its scientific and aesthetic integrity. Fossils from this provenance are prized among collectors and researchers for their authenticity, clarity of locality, and excellent preservation.

Authenticity and Presentation

All of our fossils are 100% genuine specimens and come with a Certificate of Authenticity, confirming the fossil’s identification, age, and provenance. The photo shows the actual specimen you will receive, and the scale cube equals 1cm for accurate reference. Every piece is carefully handled and selected to meet the highest standards for collectors, educators, and museums.

A Classic British Jurassic Ammonite

This Brasilia decipiens fossil ammonite from Horn Park, Beaminster, Dorset, is a quintessential example of early Aalenian ammonite fauna from one of the UK’s most scientifically important Jurassic sites. With its distinct ribbing, fine preservation, and provenance from the Alice Purnell Collection, this specimen represents a rare opportunity to own a genuine piece of Britain’s geological and palaeontological history — a timeless fossil from the ancient seas of the Jurassic world.