They started in the Turonian at about 93.5 million years BP and lasted right to the beginning of the Santonian at 85.4 million years BP.
The first fossils of Futabasaurus were found in the Irimazawa Member of the Tamayama Formation, Futaba Group, Fukushima Prefecture, Japan, dating to the Inoceramus amakusensis zone of the early Santonian.
A remarkably well preserved and nearly complete Selmasaurus skull and partial postcranial skeleton was discovered by Steve Johnson and family in 1996, from the Santonian or Campanian marine horizon in the Niobrara Formation of Niobrara Chalk, western Kansas.
The cliffs are mainly of geological interest, containing many Santonian and Campanian fossils.
Located in strata of the Santonian Bajo de la Carpa Formation, the original remains were discovered by Oscar de Ferrariis, Director of the Natural History Museum of the Comahue National University in Neuquén around 1984-5.
These rocks together with overlying Santonian rocks form the limestone plateau of Légal, the divide between the drainage basins of the rivers Lisle and Vézère.