Les traces de pas de Dinosaures et autres Archosaures du Lias inférieur des grands Causses, Sud de la France

Georges Demathieu, Georges Gand, Jacques Sciau, Pierre Freytet and Jacques Garric

The Causses" is a near 3400 km2 large plateau located in the south of France. Here the first dinosaur footprints where found in 1935. After this, this area has yielded an ever-increasing number of ichnites now in excess of 500 specimens. These latter, 15 to 50 cm long, tridactyl or tetradactyl footprints of generally biped animals, were discovered at the surface of Hettangian to lower Sinemurian dolomite layers within 4 distinct stratigraphic units. The 35 sites bearing ichnites are located on the plateau margin. For the first time, morphologic characters studied through descriptive statistic methods with the usual parameters and classical Student and Snédecor tests, allowed us, to divide the whole set of biped traces into 6 ichnospecies. Their definitions are further constrained by multivariate statistical results using Principal Component Analysis (PCA), Factor Analysis of correspondances (FAC) and Discriminant Analysis (DA). All have confirmed the morphologic observations. So that now, the following taxa are identified : Grallator variabilis, G. lescurei, G. sauclierensis, G. minusculus, Eubrontes giganteus, Dilophosauripus williamsi, cf. Moraesichnium, Orníthopus fabrei nov ichnosp. The more immediately visible differences relate to the interdigital II-IV divarication and the digit length ratio. To this panel, we must add Batrachopus deweyi and shapes suggesting Trisauropodichnus and/or Anomoepus. Among all ichnite associations described in the lower Liasic, the New England assemblage presents the most affinities with ours. It shows the ichnotaxa Grallator, Dilophosauripus, Eubrontes, Batrachopus without forgetting Ornithopus fabrei nov. ichnosp. which is close to Ornithopus gallinaceus from the Massachusetts and Connecticut basins. On comparing the present early Jurassic ichnofauna of the Causses with the ones of the Middle and Upper Triassic formations of the eastem border of the Massif Central (France), it appears that tridactyl footprints become more and more numerous and large from Triassic to Early Jurassic. In the Causses, these latest are prevalent but in Quercy (France), Poland, Italy, USA, they are also associated with Omithopoda, Thyreophora and Sauropoda ichnites. Footprint areas considered here were generaly under an arid climate. Animals that passed by were heavy and bulky possible Megalosaur trackmakers, and lighter and slender Coelophysids or Ceratosaurs. For all, these areas were pathways as the orientations of the trackways seem point out. The directions followed by these reptiles were without any important variation during the Hettango-Sinemurian stages. These areas were also used from time to time by Crocodilomorpha and may be tetradactyl (I-IV) bipedal avian Theropods. However, the number of such trackways in sites, sometimes substantial, should not lead us to overestimate the trackmakers populations. These last were probably relatively moderately abondant in this inter-supratidal swamp environment. In the Causses, ichnites are connected with former algo-laminated deposits (Algal mats) which were rapidly hardened by means of calcitisation of cyanobacteria. The result has been a moderate depth of footprints; autopodia disturbing only a few cm of the carbonate substrate. Other fossils have been discovered : invertebrates with thin bivalve and gastropod shells, crustaceans tests and plants. These latter suggest the existence of paleomangroves like environments but also continental vegetation periodically overruning the swamp environment during regression/transgression cycles. At these times, wooded parts of it, could become protecting, feeding, resting and nesting places.