Probable palaeogeographic influences of the Lower Cretaceous Iberian rifting phase in the Eastern Cameros Basin (Spain) on dinosaur trackway orientations

摘要

The Cameros Basin sedimentary infill comprises a large, essentially continental, megasequence ranging from the Tithonian (Upper Jurassic) to the Albian (Lower Cretaceous). It occupies an area of some 5500km~2, and is home to around 300 dinosaur tracksites. Most of these tracksites are found in either the Huérteles Formation, which is part of the Oncala Group (Berriasian), or the Enciso Group (Lower Aptian), both of which represent early Cretaceous lacustrine episodes. Dinosaur trackways (n=1170) from both episodes were analysed in order to establish the preferred direction of dinosaur movement, and to determine whether these movements were influenced by the palaeogeographic and palaeoenvironmental conditions of the area.The Huérteles Formation is interpreted as a complex record of alluvial plain systems distally connected with a playa-lake. Its dinosaur tracksites are distributed throughout its alluvial plain facies and trackways show two preferential unidirectional orientations: 1) NW, more or less parallel to the distribution of the facies belt and 2) NNE. The Enciso Group is represented by a wide and shallow lacustrine system connected with marine environments towards the SE (Iberian Basin realm) and in close proximity to marine settings to the NW (Basque-Cantabrian Basin realm), rendering the Cameros Basin as the only continental connection between the Ebro and Iberian Massifs. The fluctuating (but always shallow) water level of the system with frequent desiccations probably allowed dinosaurs to pass through the lake basin. As a consequence of these paleogeographic restrictions, the dinosaur trackways generally show a bidirectional NE-SW orientation. Despite the temporal and geographical differences between the Huérteles Formation and Enciso Group, the ichnocenoses of both are dominated by theropod dinosaur trackways (85% as a mean value). This is probably explained by these dinosaurs being more active than others, a consequence of their searching/hunting behaviour.