Discover France and Explore the Fossils and Geodiversity of Oléron Island in Charente-Maritime
- Wayne Munday
- 8 minutes ago
- 6 min read
Sip back and discover France and explore the fossils and geodiversity of Oléron Island, off France’s Atlantic coast in Charente-Maritime, home to the scientifically significant Chassiron Bonebed, located at the island’s northern tip near to the striking black-and-white Phare de Chassiron lighthouse. The Late Jurassic deposits at Chassiron, preserved within Early Tithonian Purbeck Beds, capture an exceptionally diverse paralic ecosystem, including fish, amphibians, reptiles, crocodilians, turtles, invertebrates, plants and mammals. This fossil assemblage represents a rare Konzentrat-Lagerstätte, preserving entire communities in remarkable detail rather than soft tissues. This ancient coastal mosaic of lagoons, tidal flats and wetlands was shaped by fluctuating salinity, periodic marine incursions, and shifting sea levels, producing habitats for euryhaline sharks like Parvodus and Planohybodus, bony fishes such as Scheenstia mantelli and Ionoscopus, amphibians, turtles, and diverse crocodyliforms. Plant fossils, including charophytes, conifer fragments, ferns, and lycopsids, reveal extensive coastal forests supplying driftwood to the lagoons. Dinosaur trace fossils, including theropod, sauropod, and thyreophoran footprints, record activity on tidal flats, while Chassiron is also the first Jurassic mammal-bearing site in France. Together, these assemblages provide a unique window into Late Jurassic coastal ecosystems along the western Tethyan margin of Europe.
Oléron Island or Île d’Oléron, sits just off France’s Atlantic coast in Charente-Maritime, linked to the mainland by a free 3 Km Viaduc d'Oléron near Marennes. Positioned between La Rochelle to the north and the Gironde estuary to the south, the island faces the open Bay of Biscay. To the west stretches wide Atlantic beaches, rolling dunes and major migratory bird routes and to the east, sheltered oyster farms, salt marshes, and extensive wetlands reveal a calmer environment rich in biodiversity. At the island’s northern tip is the Chassiron Headland which is not only the site of fossiliferous limestones and marls of the Purbeck beds but is also distinguishably overlooked by the striking white-and-black stripes of the 46-meter-tall Phare de Chassiron a lighthouse built to protect sailors who have long feared its treacherous shallow reefs often called the “end-of-the-world”.
The Late Jurassic deposits at Chassiron, situated on the northern tip of Oléron Island along France’s Atlantic coast, preserve one of the most diverse and scientifically significant vertebrate fossil assemblages known from the French Jurassic. The Chassiron Fossil Bonebed is not open to the public and is located on private property. Encased within Early Tithonian Purbeck Beds, these rich bonebeds capture the complexity of an entire paralic ecosystem of fish, amphibians, reptiles, mammals, plants and abundant invertebrates preserved in exceptional detail. Chassiron stands out as a rare Konzentrat-Lagerstätte, for its extraordinary fossil concentration rather than for any notable soft tissue preservation.
A paralic ecosystem is a dynamic coastal environment that forms where the land and sea meet creating an environment of mixed salinity conditions influenced by tides, sediment supply and shifting sea levels. Found in settings such as lagoons, estuaries, deltas, tidal flats and salt marshes these transitional habitats range from fully marine to nearly freshwater and support a diverse array of organisms. Paralic ecosystems are common in ancient sedimentary sequences like the Purbeck Beds of southern England where changing coastlines produced a complex mosaic of coastal wetlands and shallow-water environments.
The Early Tithonian Purbeck Beds record a dramatic environmental shift at the time of the transition from the Jurassic to the Cretaceous Period capturing the moment when a major marine regression transformed southern England and parts of Europe including Chassiron into a landscape of lagoons, lakes and coastal plains. Deposited over roughly six million years between 146–140 million years ago they mark a transition from marine to a marginal-littoral environment, characterised by fluctuating salinity and a mix of brackish and freshwater conditions.
The “Phare de Chassiron” section of the Purbeck Beds preserves a succession of predominantly limestones, shales, mudstones and evaporites formed in warm, shallow lagoons and coastal swamps with high evaporation rates, similar to modern environments like the Persian Gulf.
The diversity and complexity of the plant fossils from the Chassiron Bonebed provide a detailed insight into the vegetation that bordered this dynamic coastal environment. Charophytes a group of multicellular green algae, particularly Latochara latitruncata and Mesochara voluta are abundant, live in freshwater habitats although some can be found in brackish water. These algae mark part of a much large complex transitional zone represented across much of southern Europe, encompassing modern Southern France, the Iberian Peninsula, and the Carpathian Basin.
Mesofossil plants from the Chassiron deposits that are visible but typically need to be studied under a microscope are dominated by conifer fragments preserved as compressions, charcoals and cuticles, featuring spirally arranged scale-like leaves, differentiated cones and numerous seeds. Fern fronds, though less frequent, are occasionally preserved with intact sori, the spore-producing structures essential for reproduction, while a diversity of megaspores points to the presence of lycopsids in the surrounding landscape. Large accumulations of driftwood, commonly identified as Agathoxylon or Brachyoxylon indicate that extensive conifer forests once bordered the coast supplying organic debris into the paralic environment.
The invertebrate fossils from this horizon highlight the strongly transitional nature of the Late Jurassic coastal environments at Chassiron revealing a dynamic mix of marine, brackish, and freshwater influences. Abundant molluscs dominate the assemblage, particularly neomiodontid bivalves such as Neomiodon and Psammobia tellinoides alongside freshwater and oligohaline gastropods including Gyraulus, Provalvata, and Viviparus. This distinctive fauna closely parallels the classic Purbeck Group of southern England and evidence of long-range ecological and sedimentary connections across the European epicontinental seas during the Late Jurassic. The rich ostracod assemblage further supports this interpretation, featuring both the euryhaline Fabanella boloniensis ornata and the freshwater Mantelliana perlata, whose contrasting salinity tolerances document repeated shifts between marine incursions and terrestrial freshwater input.
The vertebrate fossil assemblage at Chassiron is dominated by two euryhaline hybodontiform sharks were capable of inhabiting freshwater, brackish, and marine environments, and both are known from fossil assemblages across Europe, South America, and Asia. These chondrichthyans include Parvodus and Planohybodus who form an important part of the Late Jurassic vertebrate record. Parvodus is especially abundant, represented by numerous teeth, dorsal fin spines, and distinctive cephalic spines that reflect strong monognathic heterodonty or tooth variation within a single jaw where teeth at different positions (e.g., anterior, lateral, and posterior) have different shapes and functions adapted to a wide range of feeding strategies. Planohybodus occurs less frequently but is easily recognised by its larger, densely ornamented teeth, adapted for tearing large soft-bodied prey such as fish, squid and and occasionally tackling hard shelled prey like ammonites.
The bony fish assemblage from Chassiron preserves abundant remains of the ginglymodian Scheenstia mantelli, including distinctive scales and hemispherical teeth, alongside diverse Ionoscopiform fishes such as Ionoscopus and Ophiopsis. Additional fish include caturids, ichthyodectids like Thrissops and rare pycnodontids. All of these groups were euryhaline and well adapted to environments where salinity, water depth and energy conditions fluctuated.
Chassiron has an unexpectedly diverse amphibian community. Among the fossil assemblage are Turtles both pleurosternids and shallow-marine plesiochelyids, Albanerpetontids, Salamanders and early frogs. Their presence indicates stable freshwater or consistently humid habitat.
The crocodilian assemblage from Chassiron reveals a strikingly diverse and well-partitioned predator community. Teleosaurids such as Steneosaurus are identified by their slender, ridged teeth, typical of longirostrine piscivores adapted to hunting in shallow marine and brackish waters. In contrast, goniopholidids, particularly the widespread Goniopholis, are represented by robust teeth, heavy osteoderms, vertebrae, and jaw fragments, reflecting powerful semi-aquatic predators that patrolled, hunted and ambushed their prey in the lagoons and protected bays. Complementing these larger forms are much smaller neosuchians, likely Atoposaurids and Bernissartia, recognised from their diminutive, finely ornamented teeth and delicate osteoderms. Together, these crocodiliforms illustrate a structured ecosystem in which multiple crocodylomorphs coexisted by occupying distinct ecological niches within the coastal landscape of Chassiron.
The Chassiron–La Morelière site on Oléron Island also has dinosaur footprints preserved in the evaporitic intervals of the Purbeck Beds that include tridactyl theropod tracks, as well as sauropod and thyreophoran impressions, all formed on tidal flats. These track-bearing limestones exhibit mudcracks, ripple marks and laminites from calcite recrystallised after gypsum dissolution, confirming a shallow marginal-marine environment during the Tithonian. The trackways were likely made by a medium-sized non-coelurosaur tetanuran, such as a megalosauroid or allosauroid.
Remarkably, Chassiron is also the first Jurassic mammal-bearing locality in France, preserving microvertebrate remains including isolated teeth, jaw fragments, and small postcranial elements, representing early therian and possibly eutriconodont mammals.
Chassiron reveals a dynamic Late Jurassic coastal landscape where terrestrial forests, freshwater wetlands, brackish lagoons, and shallow marine platforms existed close together. Recurrent shifts in sea level and salinity, driven by Tithonian transgressive cycles, created diverse habitats capable of supporting a rich biodiversity of plants and animals living in a Late Jurassic ecosystem to the west of the Tethyan Ocean.
