Uncommon preservation of dinosaur footprints in a tidal breccia: Eubrontes giganteus from the Early Jurassic Mongisty tracksite of Aveyron, southern France

Abstract

Abstract A recent excavation yielded 118 large tridactyl footprints in the Lower Jurassic Dolomitic Formation of the Causses Basin, at Mongisty in southern France. Most of the tracks are ascribed to Eubrontes giganteus Hitchcock, 1845. They are preserved on a surface of 53 m2 and form parallel rows with a preferential orientation towards the north. Such an abundance and density of E. giganteus is observed for the first time in the Early Jurassic from the Causses Basin. Sedimentological and ichnotaphonomical analyses show that the footprints were made at different time intervals, thus excluding the passage of a large group. In contrast to all other tracksites from the Dolomitic Formation, where tracks are preserved in fine-grained sediments corresponding to low-energy depositional palaeoenvironments, the tracks from Mongisty are preserved in coarse-grained sediment which is a matrix- to clast-supported breccia. Clasts consist of angular to sub-rounded, millimetric to centimetric-scale (up to 2 cm), poorly sorted, randomly oriented, homogeneous dolostone intraclasts floating in a dolomudstone matrix. Sedimentological analysis shows that the depositional environments of Mongisty varied from subtidal to intertidal/supratidal settings in a large and protected flat marsh. The lithology of the track-bearing surfaces indicates that the mudflat of the Causses Basin was sporadically affected by large mud flows that reworked and redeposited mudstone intraclasts coming from the erosion of upstream, dry and partially lithified mud beds. Throughout the world, this type of preservation of dinosaur tracks in tidal matrix- to clast-supported breccias remains rare.