Palaeoecological inferences about dinosaur gregarious behaviour based on the study of tracksites from La Rioja area in the Cameros Basin (Lower Cretaceous, Spain)

Abstract

In La Rioja the widespread ichnological record of dinosaurs ranges from the Berriasian to the Aptian age (Early Cretaceous). There, due to the high palaeodiversity, several evidences of gregarious behaviour can be observed in different types of footprints (sauropods, theropods and ornithopods). Moreover, due to the huge dimensions of some sites, different types of dinosaur tracks representing gregarious behaviour can be found very close in space and time. All these evidences have been compiled and analysed, obtaining an overview of this type of behaviour in relation to the type of trackmaker. Twenty-eight evidences of gregariousness have been found, of which nine are of sauropod footprints, twelve of theropod, seven of ornithopod, and one of unknown bipedal dinosaurs. These data have been further divided into accumulations and sets of parallel trackways. In sauropod footprints the main evidence is represented by the accumulations of tracks made by herds of individuals of the same size. In ornithopod footprints there are accumulations and parallel trackways of large size individuals and even family groups. In theropod footprints, sets of parallel trackways of few individuals of the same size (from small to medium) are the most common evidence and there are no data about gregariousness in large theropod footprints. Thus, data were obtained on different sizes of herds, size of individuals and structure of the groups depending on different ichnotypes, providing information about important palaeoecological aspects of their trackmakers, such as population structure.