We present a new locality with at least 880 vertebrate tracks found at the top of a limestone bed from the lower Miocene Tudela Formation (Spain). The trampled surface was formed by artiodactyls that crossed a muddy carbonate accumulated under the influence of water level variations in a palustrine environment. The tracks reflect different types of morphological preservation. The well-preserved tracks have tetradactyl digit impressions caused by both manus and pes, and are the type series of a new artiodactyl ichnotaxon, Fustinianapodus arriazui ichnogen. nov. and ichnosp. nov. The rest of the tracks, which are not as well preserved, are didactyl and were classified as undetermined artiodactyl tracks. According to their preservation, morphology, size, arrangement and orientation, we propose that this tracksite is the product of a social behaviour, particularly gregariousness, of a multi-age group of artiodactyls ~19 Ma ago. The morphologic and palaeoecologic data presented here suggest that the trackmakers were a group of anthracotheres with a livelihood similar to current hippos. They crossed, periodically, a fresh water palustrine area along some preferential pathways (trails). The Artiodactyla is a large order of placental mammals which includes pigs, peccaries, hippopotamus, camels, giraffes, deer, cows, antelope and sheep 1,2. The oldest fossil artiodactyls are from the lower Eocene in North America, Europe and Asia, and evolved to the large land mammals of today 3. They are "even toed" or "cloven hoofed" ungulates because the autopod axis of symmetry is between the third and fourth digits 2. They normally have autopods with two or four digits, although several basal members presented pentadactyl mani 4. Artiodactyls occupy a wide range of habitats, including tropical rain forest, temperate deciduous and evergreen forests, prairie, steppes, deserts and mountainous regions, and are present in all the continents except Antarctica 1,2. Many artiodactyl taxa live in small social groups or large herds that often have a hierarchy, with the social group size related to body size and feeding behaviour 1. The fossilized tracks and trackways of extinct vertebrates offer valuable information about locomotion, behaviour, palaeoecology, substrate conditions, and paleoenvironment 5,6. Indeed, fossil tracks can also complement information from the osteological record by providing additional data on geographical distributions 7 , first and last occurrences 7,8 and evolutionary radiations 9. For instance, the oldest artiodactyl tracks are known from upper Eocene strata of Europe and North America 10-12. Normally, artiodactyl tracks are didactyl 13,14 , but they may be tetradactyl too 11,15. They are paraxonic and the impression of digits III and IV appear to be equally important in both didactyl and tetratractyl tracks 16. Moreover, artiodactyl fossil tracks have been found in all continents except Australia and Antarctica 17 , and in different palaeoenvironments such as aeolian, fluvial, alluvial, palustrine and lacustrine ...

A new tracksite with bird footprints, found in the Bardenas Reales de Navarra Natural Park (Navarre, Spain), is presented in this study. The footprints are preserved in four sandstone blocks of the Lerín Formation from the northwest sector of the Ebro Basin. According to the magnetostratigraphic data, the age of these blocks is 20.4 Ma (Agenian, lower Miocene). The footprints are more than 100 mm in length, mesaxonic, and tridactyl, and have a prominent central pad impression with the digit impressions not jointed proximally. These features allow classifying them as Uvaichnites riojana. Some of the studied footprints are better preserved than the type series of Uvaichnites, which were found also in the northwest sector of the Ebro Basin. Therefore, the original diagnosis has been emended. Available chronostratigraphic data for these localities as well as for other footprints from China indicate a latest Oligocene-earliest Miocene age (from about 23 to 20 Ma) for Uvaichnites-like footprints. Sedimentological data also indicate similar continental environments, namely perilacustrine deltaic systems and distal alluvial systems. The information about early Miocene avian remains (bones, eggs and footprints) in the Iberian Peninsula is scarce. The skeletal and oological record of this age has been included within the families Phoenicopteridae, Phaisanidae and Cathartidae (or incertae sedis), while the ichnological record was related with trackmakers belonging to Charadriiformes, Ardeidae and Gruidae taxa. For this scenario, in which there are few avian remains, the ichnological diversity shown in this paper complements and improves the knowledge about the Iberian avian diversity in the early Miocene.