Discover France and Explore the Fossils & Geodiversity of La Grive-Saint-Alban in Isère
- Wayne Munday
- 15 hours ago
- 4 min read
Sip back and discover France and explore the fossils and geodiversity of La Grive-Saint-Alban in Isère about 40 km southeast of Lyon and one of Europe’s foremost Middle Miocene fossil localities. Set within the Bajocian oolitic limestone hills of the Rhône basin its distinctive karstic fissure-fill deposits of marls, sand and red clays rich in iron oxides preserve a remarkable fossil assemblage dating from roughly 16 – 11 million years ago. Formed as rainwater widened Jurassic limestone fractures creating fissures that trapped sediment and fauna; the fossils occur only in the Miocene infill and not the bedrock. Iron oxides promoted mineral-associated organic matter, stabilising remains and slowing decay. The fossil record of La Grive records environmental shifts from the warm Miocene Climatic Optimum (MMCO) to the cooling Middle Miocene Climate Transition (MMCT), when Antarctic ice expansion drove temperate ecosystems and faunal turnover. Among the fossil highlights include cricetid rodents Hispanomys decedens, the extinct tree squirrel Lagrivea vireti and predatory carnivores like Albanosmilus jourdani and the owl Tyto sanctialbani.
La Grive-Saint-Alban is located in Isère and is one of Europe’s most significant Middle Miocene fossil sites set within the Bajocian Age oolitic limestone hills just about 40 Km southeast of Lyon. Its distinctive karstic fissure-fill deposits are composed of marls, sand and red siderolithic clays holding significant amounts of iron oxides that have preserved a remarkable assemblage of mammals, birds, and reptile fossil dating from approximately 16 to 11 million years ago. These fissures exhibit a complex, multi-phase depositional history, making the site a critical reference destination for Neogene biostratigraphy and palaeontology in Europe. Fossils from La Grive-Saint-Alban are curated and displayed at Lyon’s Musée des Confluences and tells a story of the Middle Miocene ecosystem of the Rhône basin. Beyond the scientific significance of La Grive-Saint-Alban the surrounding countryside enchants visitors with a patchwork of rolling limestone ridges, verdant meadows, and ancient woodlands, interspersed with traditional communes such as Saint-Alban-de-Roche.
La Grive-Saint-Alban is not a named rock formation but rather a classic karstic fissure-filled fossil network of at least eight fissures labelled L1–L8 and is best understood as a suite of named pits and fissure-fill localities rather than a single feature or formation. Rainwater and surface runoff chemically weathered and mechanically widened fractures in the exposed Jurassic limestone forming cavities and fissures typical of karst terrain. These voids acted as natural traps where episodic floods, slope wash and soil erosion transported sediments into the fissures. Over time these sediments gradually accumulated and buried the organic remains of teeth, jaws and limb fragments of Neogene fauna. The fossils are only contained in the Miocene infill and not the older limestone bedrock.
Interestingly, the abundance of iron oxide in the matrix has helped to stabilise and slow organic decay and preserve the La Grive fossil assemblage. Iron oxide slowed organic decay by forming strong physical and chemical bond with the organic matter making mineral-associated organic matter (MAOM) where organic compounds become less soluble and far less bioavailable so effectively forming a shield from microbial decomposition.
The fossil assemblages at La Grive shows that different fissures preserve subtly different communities from across the late Early to Middle Miocene and correlated to European Mammal Neogene (MN) Zones. One fissure may be dominated by arboreal rodents whilst another another has a higher proportion of carnivores or amphibians. The fossils at La Grive fit into the transitional interval of the Middle Miocene when Europe experienced cooling, habitat shifts and significant faunal turnover as temperate ecosystems spread.
The Middle Miocene was a pivotal time in Europe beginning with the warm, humid Miocene Climatic Optimum (MMCO) and culminating in the cooling Middle Miocene Climate Transition (MMCT). During the MMCO, temperatures were substantially higher and subtropical and warm-temperate forests spread across Western and Central Europe that supported a flourishing biodiversity of mammal including dinotheres, rhinocerotids, early deer, mustelids and primates.
Around 14 million years ago the Middle Miocene Climate Transition (MMCT) triggered Antarctic ice expansion, sea-levels fell and a persistent trend towards cooler and drier conditions prevailed. Europe shifted from the warm, humid conditions of the Miocene Climatic Optimum to a cooler, more seasonal climate. This change reshaped ecosystems, vegetation and animal communities, driving one of the most significant faunal turnovers of the Neogene. As cooling intensified, these subtropical habitats contracted and were replaced by more open woodlands, shrublands and expanding grass dominated landscapes. This restructuring of vegetation triggered corresponding changes in mammal communities, where species declined or became extinct, favouring the emergence of a temperate ecosystem favouring herbivorous grazers and more resilient and adaptable mammal groups.
The fossil assemblages at La Grive is dominated by mammals with well over a hundred taxa being identified across the limestone fissure network. Among the fossils of jaws, isolated teeth and limb bones are small insectivores and shrews, a rich array of rodents, notably cricetids, tree squirrels and flying squirrels, lagomorphs, carnivores and ungulates.
A distinguishing characteristic of the fossil assemblage is the presence of the large extinct tree squirrel called Lagrivea vireti and Hispanomys decedens a type of terrestrial cricetid rodent. Other notable mammals include cat and dog-like carnivores including the large barbourofelid Albanosmilus jourdani (a sabre-toothed cat relative) and Jourdanictis grivensis or Plioviverrops gaudryi an extinct, small, civet-like hyaenid. Avian remains include an extinct species of owl called Tyto sanctialbani, plus reptiles, amphibians and even fish fragments.
La Grive-Saint-Alban reveals a dynamic Middle Miocene landscape shaped by environmental change as portrayed in the site’s fossil assemblage of rodents, squirrels, amphibians and reptiles. These fossil-bearing fissures capture environmental fluctuations occurring over thousands to hundreds of thousands of years, offering rare insight into how ancient communities responded to environmental change.
