New ornithopod tracks from the Lower Cretaceous El Castellar Formation (Spain): Implications for track preservation and evolution of ornithopod footprints

Dinosaur tracks have been identified in several Upper Jurassic-Lower Cretaceous sedimentary units across the Maestrazgo Basin in eastern Iberia, which are preserved in a variety of transitional and continental paleoenvironments. Here, we described the lower Barremian San Benón tracksite with the first occurrence of clear dinosaur tracks in the El Castellar Formation within the Galve subbasin. This palustrine-lacustrine formation has yielded a rich osteological record although dinosaur tracks are notably scarce. The new footprints represent an uncommon case of dinosaur track preservation since the site has yielded several non contemporary tracks (part of different ichnoassemblages) preserved as carbonate casts at the base of a limestone bed forming a composite ichnofabric. The tracksite shows a complex history of sedimentation, track production and preservation linked to the lake level variations. The ornithopod tracks are identified to belong to the ornithopod ichnogenus Caririchnium, concretely to the ichnospecies C. magnificum. The studied tracks represent the oldest occurrence of this ichnotaxa in the Maestrazgo Basin and are coherent with other coeval (or almost coeval) occurrences in the Iberian Peninsula. The presence of C. magnificum in the El Castellar Formation fills a gap between the oldest (Tithonian-early Valanginian?) and younger (Barremian) occurrences of ornithopod tracks within the Maestrazgo Basin, being one of the most complete successions of ornithopod tracks in Europe. Interestingly, the underlaying formations have ichnoassemblages with Dinehichnus-like and Iguanodontipus-like tracks, whereas Caririchnium is mainly found in El Castellar Formation and other Barremian units. These changes in the ichnoassemblages reflect the ornithopod faunal changes shown by osteological data in the Iberian Peninsula recording a Late Jurassic-earliest Cretaceous stage dominated by basal iguanodontians followed by an Early Cretaceous stage with abundance of more derived and large-sized iguanodontians.