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Dinosaur Footprints and Trackways of La Rioja

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During the Early Cretaceous, lakes, meandering streams, and flood plains covered the region where the current foothills of Rioja now exist. Today the area is known for its wine and for the dozens of sites where footprints and trackways of dinosaurs, amphibians, and even pterosaurs can be seen. The dinosaurs that lived here 120 million years ago left their footsteps imprinted in the mud and moist soil. Now fossilized in rock, they have turned Rioja into one of the most valuable dinosaur footprint sites in all of Europe. Félix Pérez-Lorente and his colleagues have published extensively on the region, mostly in Spanish-language journals. In this volume, Pérez-Lorente provides an up-to-date synthesis of that research in English. He offers detailed descriptions of the sites, footprints, and trackways, and explains what these prints and tracks can tell us about the animals who made them.

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1 La Rioja Footprints

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THE SPANISH PROVINCE OF LA RIOJA IS AN AREA OF THE WORLD where a huge number of dinosaur footprints have been found, with many more likely yet to be discovered. This hilly region has many rock slopes with layers so full of tracks that, if the vegetation, loose rock, and debris could be removed, would yield from 8000 to as many as 25,000 footprints. Using the best estimates from some slopes – that is, the maximum estimate from that partial data – there may be as many as 70,000 footprints.

Many of the footprints are so easy to see that the first people to discover them were likely shepherds or hunters who passed through the area. However, the identity of the first person to correctly interpret them is another question. The footprints are so evocative that the inhabitants of the region have long associated them with animals. In the villages of Enciso, El Villar, and Poyales, there were people who thought the footprints now understood to be those of theropod dinosaurs had been made by giant chickens. In the village of Navalsaz, it was said that the ornithopod footprints of the Cuesta de Andorra had been made by huge lions. It is difficult to know exactly how long such claims have been made, whether the local population even knew about wild animals such as lions, or whether this interpretation was offered by visitors to the region.

 

2 Ichnology

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ACCORDING TO THE MOST RECENT PUBLISHED COUNT (PÉREZ-Lorente, 2003b), the number of footprints in La Rioja is 7967. Subsequent studies of additional sites have provisionally increased that number to 9150. This number only includes footprints that have been studied and that have data available regarding their form and dimensions. The 9150 footprints are distributed among 866 theropod, 146 ornithopod, 22 sauropod, and 34 unidentified trackways, as well as a number of isolated prints. There are 5236 theropod footprints, 1059 ornithopod prints, 1198 sauropod prints, and 950 as yet unidentified prints. Many footprints at other sites remain to be studied. For example, the site of La Pellejera contains more than 700 footprints in about 70 trackways.

The distribution of the tracksites in the Cameros Basin is heterogeneous. They occur in the rocks of the Tera, Oncala, Urbión, and Enciso groups. The rocks with footprints are limestone, sandstone, and shale. The neighboring outcrops may be adjacent to each other or separated by hundreds of meters.

 

3 The Tracksites

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3.1. Location of La Virgen del Prado (VDP) tracksite (also see Fig. 1.4). Local road is labeled LR 284.

After the discovery of the Virgen del Prado site (Fig. 3.1) in 1991 by an anonymous researcher, a team from the Universidad Autónoma de Madrid and an electrical company (Iberdrola) cleaned and mapped it during subsequent campaigns, finally describing the results in an unpublished thesis by Moratalla in 1993. The thesis stated there were only 36 footprints, of which four were part of a trackway. Years later, Moratalla and Hernán (2005) produced a new map of the site showing 200 footprints and six trackways (see Fig. 3.2A). This new study recognized the existence of a theropod with slender toes and feet, which was apparently abundant in the Oncala Group.

In 2004, Pérez-Lorente carried out a separate unpublished study of the site to better understand the contents of La Virgen del Prado (Fig. 3.2B). This was because the traces in La Virgen del Prado show noteworthy collapse structures, there were types of footprints previously not described, and there were a large number of unrecognized trackways. The map in Fig. 3.2B was not created by surveying the area but rather from partial surveys that were superimposed later. The procedure was as follows. First, each of the trackways and adjacent footprints were mapped. Footprints were drawn on the plan to obtain a picture of each trackway and the adjacent footprints. Second, all trackways were placed in their relative position on a single drawing (Fig. 3.2B). Relative position was established by superimposing the common footprints of the trackways. The footprints on the right-hand side of the site were situated according to the 2005 data of Moratalla and Hernán. Both the 2005 and 2004 maps are presented in Fig. 3.2. The Moratalla and Hernán (2005) map (Fig. 3.2A) is the more recently published version, but my version (Fig. 3.2B) provides additional detail. The count of footprints and trackways is taken from the latter (Table 3.1).

 

4 Conservation of the Tracksites

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THE SITES WITH DINOSAUR FOOTPRINTS IN LA RIOJA HAVE BEEN OF interest for many years. Concern for their conservation has spread throughout the population of this region. They are an asset that must remain in the countryside, and the inhabitants of the villages with the tracks view them as part of their heritage. If it were not so, dealers, collectors, and even some of the local inhabitants would have removed the best of them. Sites of interest must have some value that ideally can be quantified. Objects that have value are treated as assets, prompting states and other governmental entities to declare them part of the heritage of the land via legislation or other official regulations.

From the beginning, the teams studying the footprints have been interested in their conservation and protection as well as their publication (Pérez-Lorente, 2001b). Relevant to this is political action, which began in 1980 with the first proposals to include outcrops as places covered by laws protecting natural areas. Thanks to the government of La Rioja and its institutions, there have been grants for almost all fieldwork or research projects. The Instituto de Estudios Riojanos has funded research projects on dinosaur tracks in La Rioja and has published about a hundred articles in its journal, Zubía, relating to footprints, their conservation, and their relationship with the geological heritage. Colleges and universities have lent their staff and their experience. For example, the University of La Rioja currently offers summer courses on dinosaur footprints and conservation of sites. Support for these institutions has led to other such initiatives supporting their publicity and preservation, which can be classified as legal protection, physical protection, and maintenance protection.

 

5 Summary

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5.1. Las Mortajeras (LM) site. (A) Chaotic distribution of footprints. (B) Arrangement of paces. (C) More orderly arrangement of trackways (midlines).

AS IN OTHER PARTS OF THE WORLD, FOOTPRINTS OF THE SAME TYPE (theropod, sauropod, or ornithopod) dominate in small outcrops. They were probably made because in some cases, dinosaurs of similar taxa are dominant or exclusive in areas of definite habitats. Formerly it was said that the different dinosaurs were separated in space and time, I suppose because large outcrops were unknown – so much so that sometimes the same ichnotaxon was applied to all footprints found in the same place. It was assumed, therefore, that the habitat, the age of rocks, and the habits of dinosaurs were sufficient conditions for the dinosaurs to be in widely dispersed groups. In La Rioja we can see that as the size of the site increases, the ichnogroups present also increase (Casanovas et al., 1999).

Because theropod footprints are the most abundant, so are outcrops with theropod footprints. If all theropod footprints are of carnivorous dinosaurs, in La Rioja there is a problem of interpretation. Were there more carnivorous than plant-eating dinosaurs? Several hypotheses have been put forward on this topic. In the Igea Paleontology Center, the deposited dinosaur bones are identified as Baryonyx (Viera and Torres, 1995a). In that same center, the authors offer the theory that many of the theropod dinosaurs in La Rioja may have been piscivores (Baryonyx). This would justify, they say, the presence of large numbers of theropod footprints and the scarcity of herbivorous dinosaurs. However, the overabundance of theropod footprints can also be produced by the greater activity levels on the part of carnivores than herbivores (Coombs, 1980; Leonardi, 1984; Farlow, 1987).

 

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