A Discovery of Toarcian Proportions
During the early 2000s, engineers and construction crews were focused on erecting the Millau Viaduct, a structure that would become the world's tallest bridge with a structural height of 343 meters. As they prepared the foundations for its seven massive piers, they cut deep into the Jurassic-era rock of the Tarn valley. This excavation unexpectedly exposed a record of a prehistoric world. The earthmoving equipment uncovered a large deposit of marine fossils dating back approximately 180 million years to the Toarcian Age of the Lower Jurassic.
The region was then submerged beneath a warm, shallow body of water called the Tethys Ocean. The sedimentary rocks that formed on this ancient seafloor, primarily fine-grained marls and limestones, created ideal conditions for fossilization. The discovery was not of isolated bones, but of remarkably well-preserved, and in some cases nearly complete, skeletons of the animals that swam in this sea long before the dinosaurs dominated the land. The construction of a 21st-century engineering marvel had revealed an ecosystem from the deep past.
The Creatures of the Tethyan Sea
The most famous fossil found here is a species of plesiosaur, a group of marine reptiles known for their long necks and paddle-like limbs. The most significant specimen is a nearly complete skeleton of Occitanosaurus tournemirensis, a four-meter-long plesiosaur. This specific animal is considered an elasmosaur, a family of plesiosaurs characterized by extremely long necks which, in some species, contained up to 80 vertebrae. The Occitanosaurus skeleton, now housed in the Musée de Millau, is the first complete elasmosaur discovered in France.
Alongside the plesiosaur, the fossil beds contained a variety of other marine life that showed details of the Toarcian environment. Remains of ichthyosaurs, another group of marine reptiles with a more dolphin-like body shape, were also found near Millau. The ecosystem had many cephalopods, including ammonites, nautiluses, and belemnites, which were likely prey for the larger reptiles. The excellent preservation suggests the animals were buried quickly in sediment on the seafloor, protecting them from scavengers and decay. In addition to marine reptiles, the broader Grands Causses area contains slabs with dinosaur footprints, including those of large carnivorous theropods up to nine meters in length.