Dinosaur Footprints & Trackways of La Rioja

Dinosaur Footprints &

Trackways of La Rioja

Life of the Past    James O. Farlow, editor

DINOSAUR

FOOTPRINTS

& TRACKWAYS

OF LA RIOJA

Félix Pérez-Lorente

This book is a publication of

Indiana University Press

Office of Scholarly Publishing

Herman B Wells Library 350

1320 East 10th Street

Bloomington, Indiana 47405 USA

iupress.indiana.edu

© 2015 by Félix Pérez-Lorente

All rights reserved

No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage and retrieval system, without permission in writing from the publisher. The Association of American University Presses’ Resolution on Permissions constitutes the only exception to this prohibition.

The paper used in this publication meets the minimum requirements of the American National Standard for Information Sciences – Permanence of Paper for Printed Library Materials, ANSI Z39.48-1992.

Manufactured in the United States of America

Library of Congress Cataloging-in-Publication Data

Pérez-Lorente, Félix.

Dinosaur footprints and trackways of La Rioja / Félix Pérez-Lorente.

pages cm. – (Life of the past)

Includes bibliographical references and index.

ISBN 978-0-253-01515-0 (cl : alk. paper) – ISBN 978-0-253-01541-9 (eb)

1. Footprints, Fossil – Spain – La Rioja.

2. Dinosaur tracks – Spain – La Rioja.

3. Dinosaurs – Spain – La Rioja.

4. Paleontology – Spain – La Rioja.

5. La Rioja (Spain) I. Title.

QE845.P465 2015

567.90946'354 – dc23

2014030006

1  2  3  4  5    20  19  18  17  16  15

1     La Rioja Footprints

2     Ichnology

3     The Tracksites

4     Conservation of the Tracksites

5     Summary

·      REFERENCES

·      INDEX

Dinosaur Footprints &

Trackways of La Rioja

1.1. Schematic geological and geographical representation of Iberian Peninsula showing location of dinosaur tracksites (modified from Pérez-Lorente, Romero, and Torcida, 2002). Locations of outcrops of Mesozoic rocks in Iberian Peninsula that potentially contain dinosaur fossils are shown in white; areas marked with lines show those that cannot contain fossils because they are the wrong age. Regions marked with vertical lines have rocks from before the time of dinosaurs (Precambrian and Paleozoic); regions marked with horizontal lines are post-Cretaceous (Cenozoic: after the dinosaurs).

Introduction

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.

The footprints have also been attributed to animals from medieval mythology, in some cases inspired by religion. For example, in Igea, it was said that the footprints had been left by the horse of the apostle James on his travels. Popular tradition has it that James helped the Christians in their wars against the Muslims. As with the legendary mule tracks of Setubal in Portugal, there is no end of imaginative interpretations. In some cliffs to the south of Lisbon (Portugal) there are some dinosaur footprints which the ancient Portuguese interpreted as being miraculous. Miguel Telles Antunes (1976) says that according to legend, the Virgin Mary Santa Maria da pedra da mua (or an image of her from the 18th century) had come out of the sea and ascended to the top of the cliff while sitting on a mule. During the ascent, the mule left the footprints on the wall. According to Antunes, this tradition may date back to the 13th century. In Igea, the horse’s footprints are visible near the Santa Ana chapel, at the place where the apostle’s horse was said to have jumped 3 km to land near the shrine of the Virgen del Villar, where it also left footprints. Interpretations such as these are to be expected, given the knowledge of the population. For example, the presence of marine fossils in many places had to be explained as a whim of nature. Even if no one could explain why, the sea must have been there. Nature is capable of wonderful things! Even today, if you ask an old farmer or shepherd how the sea could ever have been so far inland, although he may not be able to provide an explanation, no one will be able to convince him it was not so. For the people, the rocks, rivers, sea, and mountains have always been where they are at present. In their village, there was never a sea.

Nor would any of the local inhabitants have been able to predict that the sloping rock strata on which the tracks appear had once been horizontal mudflats. It is understandable that people did not consider that normal animals could leave their footprints in such hard rock, whereas the horse of Saint James might have had magical properties that allowed it to make an impression in solid rock. It may have been the wish of the saint – or even the horse – that the footprint left behind had a shape very different from that left by a typical horse.

It is likely that almost all settlements with dinosaur footprints had traditions and legends surrounding them that have since been lost or that the older generation do not want to tell to strangers.

History of Discoveries

The first publication about dinosaur footprints in the Iberian Peninsula that I have in my possession is from Jacinto Pedro Gomes (1915–1916) for Cabo Mondego (Portugal), and the first in Spain is from Albert F. de Lapparent (1965) for the east (province of Valencia). Although geological research began in La Rioja a long time ago, the footprints were not recognized until 1969, when the first publication about dinosaur footprints in La Rioja appeared in the newspaper El Correo Español – El Pueblo Vasco. The authors were Moisés Iglesias Ponce de León, a geologist, and Luis Vicente Elias, an ethnologist, who found the footprints while doing fieldwork on the customs of the people from the Cameros region. After this discovery, a number of favorable events occurred.

The first was that the news was not published just in the newspaper. Almost immediately, a learned researcher, Blas Ochoa, who was a schoolteacher from Enciso, began collaborating with a team of vertebrate paleontologists, Maria Lourdes Casanovas and José Vicente Santafé. The team described five sites in two publications in 1971 and 1974. The two publications and comments from townspeople who had known the paleontologists inspired two groups of amateurs to search for new sites in their spare time and publish their findings as a challenge to other researchers. By 1979, nine sites had been identified, all near the village of Enciso. (These groups are still working, partly because they have active members and partly because we have followed in their footsteps in looking for new track sites.) Later, a schoolteacher from Igea, Angel Gracia, taught his pupils the importance of fossils and showed them how to search for, classify, and preserve them. His students found sites with footprints near the village, one of which remains to be studied.

More recently, traces of other vertebrates have been discovered: birds, turtles, and pterosaurs (Moratalla and Hernán, 2009; Moratalla and Sanz, 1992; Moratalla, Sanz, and Jiménez, 1992), crocodiles (Ezquerra and Pérez-Lorente, 2002, 2003), and fishes (Costeur and Ezquerra, 2008; Ezquerra and Costeur, 2009; Ezquerra and Pérez-Lorente, 2002, 2003). Today, the followers of Blas and Angel have their own specialist centers. In Enciso, there is a paleontological museum, and the first phase of a learning center on the lost ravine has been built, as well as another center in Igea that houses some interesting paleontological material collected mostly by those pupils, who in 2010 are about 40 years old. Both centers are strongly committed to scientific activities and educating tourists.

Just as dinosaur track sites are still being found in La Rioja, so are they also being found in other locations of the Iberian Peninsula, although in rocks of different ages. There are examples of dinosaur footprints from the Late Triassic, Middle and Upper Jurassic, and Early and Late Cretaceous periods.

Cameros Basin

The geological diagram in Fig. 1.1 shows the locations and ages of outcrops of Mesozoic rocks in the Iberian Peninsula that potentially contain dinosaur fossils. The Iberian sites with dinosaur tracks occur in the regions with Mesozoic outcrops. Thus, this figure indicates the geographic limits for the areas of possible dinosaur remains in Iberia.

The Iberian Peninsula consists of four major geological areas, as follows:

1. A central region of ancient volcanic and sedimentary rocks, metamorphic rocks, and plutonic rocks (Paleozoic and Precambrian), considered a stable zone (not folded) during the Alpine Orogeny. The Alpine Orogeny is the mountain-building event that created the Alps and in Spain the Pyrenees, Iberian Range, and Betic Cordillera. The Alpine Orogeny affects (folds) the Mesozoic strata between or adjacent to the stable Paleozoic and Precambrian zones.

2. A border with outcrops of Mesozoic sedimentary rocks, sometimes mixed (at the eastern edge) with older ones.

3. Two Tertiary depressions filled with Tertiary and Quaternary rocks (depressions associated with the Ebro River to the northeast and the Guadalquivir River to the south).

4. Two external Alpine chains with deformed ancient and modern sedimentary rocks (the Pyrenees bordering France and the Betic Cordillera to the south).

The Cameros Basin is located on the northeastern edge of the stable zone, where the Paleozoic and Precambrian Mesozoic sedimentary rocks, folded by the Alpine Orogeny, are found. It is between the Ebro Basin and the stable zone. The Cameros Basin was named by Brenner and Wiedmann (1974), although Götz Tischer (1966) and Gerhard Richter (1930) have previously used the word basin to encompass the Wealdian sediments ranging from the province of Burgos to north of Ricla (Zaragoza Province).

1.2. Upper Jurassic (Kimmeridgian–Tithonian)–Lower Cretaceous (Berriasian–Albian) continental stratigraphic groups of continental Cameros Basin, indicating Cameros Basin groups and locations of dinosaur footprint sites (2003). Area covers parts of La Rioja, Burgos, and Soria provinces.

The first stratigraphic summary of this region was published by Palacios and Sánchez Lozano (1885), who divided the Wealdian formation into strata of lacustrine sediments divided into different superimposed lithological parts called sections (B and C), bank (A) and levels (a, b, c). The terms are literal translations from the Spanish (sección, banco, nivel) used in an ancient geological language. The possible equivalents are unit or member, bed, and level. Rafael Sánchez Lozano’s five superimposed lithological parts (Fig. 1.2) correspond in part to the Tera, Oncala, Urbión, Enciso, and Oliván groups of Tischer (1966). The names of the groups are still used, with virtually the same meaning and positions defined by Tischer.

Long before footprints were discovered, the Early Cretaceous (Wealdian) age of the rocks containing them was already known and had been more precisely classified into five sedimentary groups defined by Tischer and by other followers of Professor Hans Mensink (Bochum University). It was also known that the lowest group (Tera), with abundant conglomerates at its base, was underlain by marine rocks of Late Jurassic age.

The exact age of the base and the top of the sediments in the Cameros Basin is difficult to establish because they lack appropriate fossils. The transition from the Late Jurassic to the Early Cretaceous occurs in continental sediments. The uppermost Jurassic marine sediments are limestone reefs of Kimmeridgian age (Alonso, Meléndez, and Mas, 1986–1987; Mas et al., 2002). The age of the continental sediments in the Cameros Basin ranges from the mid-Kimmeridgian (sensu lato) to the mid–late Albian (see Doublet, 2004) (Fig. 1.3). This period extends from the base of the Tera Group to the top of the Oliván Group. So far, all dinosaur footprints in La Rioja are of Early Cretaceous age (Fig. 1. 2). Two sites, El Encinar and La Vuelta de los Manzanos (Moratalla, Sanz, and Jiménez, 1996, 2000b), situated at the boundary of the Tera and Oncala groups, have been assigned to the Late Jurassic (see Cámara and Durantez, 1982) (Fig. 1.4, Tables 1.1, 1.2).

1.3. Position of Cameros Basin in geological time table of dinosaur age.

The Cameros Basin is situated at the northeast end of the Iberian Range. Sites with dinosaur footprints or bones occur from this point to the Mediterranean coast. Correlation of the Cretaceous sediments in La Rioja is simple in all directions except toward the north. The Cameros Basin continues into the provinces of Burgos to the west, Soria to the south, and Zaragoza to the east (Fig. 1.2). To the north, the continuity of Mesozoic outcrops is lost because the Tertiary rocks of the Ebro Basin cover them.

Outcrops in the south and southwest of the Cameros Basin reveal rocks of detrital material, the source of which was further south and west. The five stratigraphic groups come together in the northern outcrops in a spectacular pattern of thinning and overlap, and both the limestones and shales and dark sandstones of the Urbión and Enciso groups grade into light (sand, silt, and clay) and red-colored siliciclastic rocks (Doublet, 2004).

The deepest part of the Cameros Basin was initially in the province of Soria (the Oncala and Urbión/Berriasian–Barremian) and later in La Rioja (first the Enciso Group to the southwest/Barremian–lower Albian, and finally in the Oliván Group situated more to the northeast/Albian). It seems, therefore, that the center of subsidence of the basin drifted from Soria to La Rioja during the Early Cretaceous, at least for the Oncala, Urbión, Enciso, and Oliván groups. All fossils found in this sequence are continental freshwater, or brackish at most. Only two stratigraphic levels, located in the Oncala Group, have been shown to contain marine microfossils (Alonso and Mas, 1993; Suárez-González et al., 2011).

In the Cameros Basin area of La Rioja, dinosaur bones have been found that are attributed (Fig. 1.5) to Baryonyx (Torres and Viera, 1997; Viera and Torres, 1995a) and Hypsilophodon (Torres and Viera, 1994); also found were eroded vertebra and bones from sauropods and ornithopods related to Iguanodon. In addition, theropod and ornithopod teeth have been discovered, and some theropod phalanges are being studied. In addition to dinosaur bones, fossils include osteodermal and other bony elements of turtles, crocodiles, pterosaurs, and various kinds of fish (Lepidotes, Hybodus); charophytes, ostracods, and shells from bivalves and turritellid gastropods. Many sedimentary layers are composed of algal laminations, and in some layers there are higher plants, such as transported conifers and ginkgoales, as well as abundant roots of variable thickness, but less than 10 cm in diameter.

1.4. Diagrammatic map of location of footprint sites in La Rioja. (Top) Location of sites 3.1 through 3.24 as numbered in this book. (Bottom) More detailed key to location of tracksites. Each published tracksite is labeled with letter abbreviation, as identified in tracksite description that follows (e.g., VDP indicates La Virgen del Prado) (Table 1.1). Black footprints without labels indicate unpublished tracksites.

Skeletal remains of dinosaurs are abundant in the Early Cretaceous of Spain; they perhaps constitute the richest fossil association in the sediments of continental Europe in the Hauterivian–Aptian interval. Proof of this diversity is that remains from 15 different dinosaur taxa have been found at a single site from the Early Barremian of Teruel (Canudo et al., 2009, 2010; Ruiz-Omeñaca, 2011). Another significant aspect of the Spanish dinosaurs of the Early Cretaceous is their great paleobiogeographical complexity, including taxa with Asiatic, Gondwanan, and North American affinities as well as European ones (Canudo, Royo-Torres, and Cuenca-Bescós, 2008; Canudo et al., 2009; Ortega, Escaso, and Sanz, 2010; Pereda-Suberbiola et al., 2007; Ruiz-Omeñaca et al., 2004). The best-represented dinosaurs are the sauropods. Representatives of neosauropods have been found, both diplodocimorphs such as Demandasaurus (Torcida et al., 2011) and macronarians such as Tastavinsaurus and Aragosaurus (Canudo, Royo-Torres, and Cuenca-Bescós, 2008; Sanz et al., 1987). Nonavian theropods are represented by basal tetanurans such as allosauroids and spinosaurids and by derived tetanurans such as ornithomimosaurs and dromaeosaurids. To date, two taxa have been described: the ornithomimosaur Pelecanimimus (Pérez-Moreno et al., 1994) and the carcharodontosaur Concavenator (Ortega, Escaso, and Sanz, 2010). Thyreophorans are scarce, though remains of polacanthid ankylosaurs and stegosaurs have been described (Pereda-Suberbiola and Galton, 2001; Pereda-Suberbiola et al., 2007). Ornithopods are represented by basal iguanodontoids, dryosaurids, and hypsilophodontid-like basal euornithopods. The iguanodontoid Delapparentia has been described (Ruiz-Omeñaca, 2011). The basal iguanodontoids are the most abundant dinosaurs in terms of the number of dinosaur specimens in the Early Cretaceous of Spain.

Table 1.1. La Rioja ichnological sites.

Table 1.2. Stratigraphic groups, lithology, age, and tracksite distribution.

Note: Site abbreviations as in Table 1.1.

Sedimentological studies indicate that a large portion of the sedimentary succession is lake or marsh deposits, with both siliceous and limestone strata, as well as other fluvial siliciclastic (silt, sand, and conglomerate) sediments. In some places there are sebka-type deposits containing gypsum and other salts.

The Enciso Group was the subject of a study that detected the presence of a large lake (about 500 km²) (Doublet, 2004), the existence of which is demonstrated by, among other things, the great lateral continuity (in composition and thickness) of certain distinctive sedimentary layers. After the sedimentary filling of the subsiding Cameros Basin in the earliest Cretaceous came the compressive and thermal stage. The Alpine Orogeny began in the Albian period with compression and metamorphism. Silt and loose sand were compacted and their minerals recrystallized. Where the temperature was greater, veins of quartz and phyllosilicates grew. Chloritoid was formed, which indicates that the maximum temperature reached nearly 400°C, with large pyrite crystals forming at the same locations. The high temperature had a positive impact on the preservation of the tracks because it hardened the rocks in which they are located without destroying structures directly or indirectly related to them.

1.5. Reproduction of Baryonyx walkeri. Paleontological Center of Igea (La Rioja).

Number, Distribution, and Area of Sites

Dinosaur footprints occur in all the stratigraphic groups of the Cameros Basin (Fig. 1.2) except the Oliván Group. In La Rioja, the distribution of the sites is approximately linear in association with outcrops of the Enciso Group (Fig. 1.4) (Blanco et al., 1999b; Casanovas and Santafé, 1995; Moratalla, 2002; Moratalla and Hernán, 2007; Pérez-Lorente, 2006). The number of outcrops containing footprints is difficult to quantify. In some cases, multiple sites were recognized along the same layer as they were being discovered (such as six in Era del Peladillo). In other cases (e.g., La Torre in the same area), multiple sites were grouped together because they all occur in a particular exposure (e.g., one side of a hill), even though there may be no continuity between them and they may occur in different sedimentary layers. The number is in the range of 110 to 156 sites or exposures with footprints (Table 1.1) (Caro, Pavía, and Pérez-Lorente, 1997; Pérez-Lorente, 2003a, 2003b).

Outcrops with easily recognizable footprints are more abundant in the Enciso Group than in the other groups. However, this is not necessarily an indication of the number of prints that actually exist because the footprint-bearing stratification surfaces may be obscured as a result of the nature of the rocks. The interaction between the composition and structure of the rock and weathering is the main factor in how well footprints are displayed. Many of the rocks in the Urbión Group, for example, contain many footprints that go unnoticed by observers. Isolated footprints in single rock surface fragments on the slopes are not considered to be sites.

The size of the sites depends on what is shown and the source of the information – area with footprints, area cleaned and prepared, area with a surrounding protection zone, lateral areas where more footprints are expected, or whether the area is considered a heritage site and is protected, and so forth. Most of the outcrops could be extended by additional digging, so the presently exposed outcrop surface provides little indication of the total surface area that may contain footprints. Indeed, the area of the site occupied by tracks has changed in many outcrops as a result of the incorporation of lateral areas with newly discovered footprints.

The number of footprints in the rocks of the Cameros Basin is difficult to calculate. The superimposition of layers with footprints is spectacular. For example, at El Villar-Poyales there are seven surfaces with tracks at the tops of the seven strata in the site. Similarly, in Los Cayos A–C, the tracks are visible not only on the two surfaces of the strata (the top and the bottom/hollows, casts, and undertracks) but also in the transversal sections that show the interior in the deformed sedimentary laminas of the strata. In Los Cayos A, B, and C there are strata formed by superimposed flat laminated structures.

Age of Sites

All authors who have written recently about the Cameros Basin (Doublet, 2004; Mas et al., 2002) agree on the age of the sediments that fill it. Kimmeridgian reef limestone deposits are the most recent marine sediments. Continental sedimentation began during the mid-Kimmeridgian–Tithonian (Tera Group) and did not end until the Cameros Basin was filled. During the mid-Berriasian, a regional discontinuity was created when deposition of the Oncala Group ended; there is, however, a large deposit of siliceous sediments of this group to the south, outside of La Rioja. Missing from much of the basin are rocks of mid-Berriasian through Late Hauterivian age. The Urbión Group is Late Hauterivian to Upper Barremian in age. The age of the Enciso Group ranges from Late Barremian through Early Albian. The Oncala Group is Early Albian in age. The correlation between the groups and the subdivision of the Enciso Group, with the sites, are detailed throughout the text (Table 1.2).

All the groups thin out toward the north; this is clear on the maps (Fig. 1.2). The outcrops of the groups are much larger to the south, which indicates that they are thicker there. In the northeast of the Cameros Basin, the geological map shows that the outcrop of the Tera and Oncala groups is a very narrow strip, and that of the Urbión and Enciso groups is almost unnoticeable or has disappeared. The Urbión and Enciso groups end toward the north in detrital sediments, which are sometimes quite thick and show evidence of total emergence above lake level, erosion, and processes of pedogenesis and colonization by roots. Exposures of these five groups south of the basin are more than 6000 m thick, while to the north they are reduced to 500 to 600 m. The bases of the younger strata moving northward rest on the lower layers, forming an onlap.

Erosion of layers beneath the Oliván Group has, in the most northeasterly part of the basin, established direct contact between the sandstones of this group and the underlying Oncala Group limestones (Fig. 1.2). The last of the outcrops in the Oncala, Urbión, and Enciso groups, or the fact that these are so thin, is the reason why there are not many dinosaur footprint sites in the northeastern part of the basin. The only rocks appropriate for preserving fossil footprints in this region are the limestone and some of the intervening sandstone levels in the Oncala Group.

Field Guides

Most of the tracksites in the area are accompanied with placards and information panels. Tourist offices, centers, and museums in La Rioja (Igea Paleontological Center, Museum of Enciso, Barranco Perdido Park) provide brochures and situation maps. Several sites are prepared for visitors and include fences, walkways, informational panels, and even dinosaur reproductions: Allosaurus, Tarbosaurus, Brachiosaurus, Iguanodon, and Stegosaurus (Barranco de Valdecevillo [Fig. 3.112], La Virgen del Campo [Fig. 3.126], Peñaportillo [Fig. 3.165], and La Pellejera [Fig. 3.193]).

Many popular-science articles have been published in addition to scholarly ones, so information on the track sites may be accessed via many levels. Field guides suggest excursions; books suggest educational or teaching activities; and texts provide general informative descriptions (Blanco et al., 1999b; Brancas, Martínez, and Blaschke, 1979; Casanovas et al., 1996; Casas et al., 1996; Jiménez, 1978; Moratalla and Hernán, 2005; Moratalla, Sanz, and Jiménez, 1990, 1997a, 1997b; Moratalla et al., 1988; Pérez-Lorente, 1992b, 2003a, 2007; Pérez-Lorente, Fernández, and Uruñuela, 1986; Torcida, 1996, 2003). Some works have been published with support from public institutions or private companies.

2.1. Different footprint features related to phases of foot interaction with substrate identified by Thulborn and Wade (1989). T phase features are made during initial touchdown of foot with substrate (a). W phase features are made as the animal’s weight is supported by the foot, causing the foot to exert maximum pressure against the ground (b). K phase features are made during final kickoff as the foot leaves the ground (not illustrated). Footprint features made during T phase include slashlike incisions (c), slide marks (e–h), and smooth axial downfolds (i). W phase features include extrusion rims (e, f, i–k), lateral folds (i–k), undertracks (d), downfolds (i–k), stamps (f), and dead zones (j). K phase includes collapse track walls (g, h) and claw marks (e, g–k). Subtracks (k) can be created during all three phases.

Number of Footprints and Their Distribution

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.

Lockley (1991) defines megatracksites as ichnologic strata covering a large geographic area of hundreds or thousands of square kilometers. Such megatracksites often consist of a series of small outcrops, which are part of extensive strata with footprints. There may be only sporadic outcrops of strata containing footprints. Lockley does not consider a megatracksite as having to be exclusively on the same bed. Footprints may therefore be located in strata of the same geological interval (formation, unit, or stratigraphic sequence) in terms of their stratigraphic situation, sedimentary environment, or lithological composition.

In La Rioja, for both sandstone and limestone (detrital and chemically precipitated rocks, respectively), it can be difficult to ascertain whether there are many sites on the same stratum for several reasons. First, the continuity has not been studied. For example, within each mappable member in the Enciso Group, the layers are often part of large sheets of cross-bedding or linsen (or lenticular) bedding. Second, in each mappable member, there tend to be multiple layers containing tracks, even at the same site. It cannot be determined which outcrops with footprints from the same member reveal the same stratum. Finally, the more widely separated the sites are, the more difficult it is to correlate the beds and even the stratigraphic units.

If it is assumed that the megatracksites are large accumulations of traces that occur in the same sedimentary event level (sedimentary sequence, stratigraphic unit) and in rocks of the same composition, one can speak of several types of megatracksites in the Cameros Basin area.

Enciso Group Carbonate Megatracksite

The Era del Peladillo site (PL) consists of several layers of dark limestone, which overlie a colmatation sequence in the Enciso Group. In 1PL–6PL, the top two of the strata containing tracks can be seen. The two strata are in the upper part of a sedimentation sequence (Meléndez and Pérez-Lorente, 1996). The 1PL–6PL sequence is the third of five such superimposed sequences that would comprise a stratigraphic unit. The five sedimentation sequences, which begin with a period of increased energy (evidenced by small peaks of sandstone dispersed in a shaly matrix) end in large lakes (limestone) that cover more than 500 km² (Doublet, 2004). I have followed this limestone member and the two lower sedimentation sequences (shaly sandstone and limestone members) uninterruptedly for 15 km from this point eastward. The strata continue in the two directions, and it is not known where they stop. The two upper sequences are probably less extensive because they are eroded by the Oliván Group. The entire unit (five sedimentation sequences) is between 80 and 90 m thick. Underneath the unit are at least three additional levels with footprints.

In a stratigraphic log, the vertical separation between the upper and lower track-bearing limestone horizons is in tens of meters, but the distribution of their ichnological outcrops may be separated by hundreds of meters. The separation between tracksites in the same unit verifies the same pattern. Great geographical distance does not imply that the track sites are stratrigraphically, paleogeographically, and temporally widely separated.

Enciso Group Sandstone Megatracksites

In the total Enciso Group there are different limestone- and sandstone-rich members. In addition to the sites in limestone there are many tracksites in sandstone-rich parts of the same Enciso Group (above all in the western part of the basin) in two different stratigraphical units.

The dinosaur tracksites in La Rioja could be considered as forming part of a megatracksite at several alternative scales. If the megatracksite is considered a facies, La Rioja and two neighboring provinces (Soria and Burgos) contain it. This Weald Facies ranges in age from the Late Jurassic (part of the Kimmeridgian) to the end of the Early Cretaceous (Albian), covering an area of 1100 km², with more than 500 outcrops. If it is considered a formation or group, the entire Enciso Group would be a megatracksite exposed over an area in excess of 100 km², with more than 200 outcrops. Finally, if it is a sedimentary unit or depositional sequence, then there are at least three potential candidates for megatracksites (>100 km²/35 tracksites): the Enciso Group carbonate sediments (Aptian), and the lower (Barremian–Aptian) and upper (Aptian–lower Albian) siliceous sediments (Table 1.2).

Most of the described and published footprints and tracksites are in the Enciso Group, which is second to the Oncala Group in the number of sites. There are some tracksites in the Urbión Group, and one site is referred to in the Oliván Group. The Tera Group has hardly any outcrops in La Rioja, and only one locality, a place with casts, is known from the wall of a ravine.

Pérez-Lorente (2002a) attributed the difference in the number of sites between the Enciso Group and the other groups to the difference in the outcrop exposure area and the resistance to weathering and erosion of the rocks (which is less in the remaining four groups). The Enciso Group contains strata surfaces that are wider, have fewer joints, and are less weathered. However, some rocks in all the groups are resistant to erosion. These include sandstones and conglomerates in the Tera Group, limestone in the Oncala Group, sandstone in the Urbión Group, limestone and sandstone in the Enciso Group, and sandstone in the Oliván Group.

Paleoenvironment

The dinosaur footprints in La Rioja occur in sandy mudstones or shales, fine-grained sandstones, and carbonates (Pérez-Lorente, 2002a). Doublet (2004) and Doublet and Garcia (2004) provided data on sedimentology, environment, and climate that enabled the paleoenvironment and footprints to be linked. Most of the sediments with footprints accumulated in lakes and marshy areas with a continuous sheet of shallow water and some more deeply flooded zones. The lakes may have been freshwater or brackish as a result of drying.

The oldest footprints in La Rioja occur in an as yet unstudied Tera Group site in the Leza River Canyon (probably fluvial sediments). The oldest published footprints in La Rioja, however, are in carbonate rocks of the Oncala Group, a dolomitic unit known as the Inestrillas Formation at the site of Valdeprado. This formation consists of fine-grained deposits of carbonate and shale, with gypsum and salts. These sediments were deposited in a lake where the rate of evaporation exceeded the rate of water replacement. Other lake deposits with little salt in the same group (Leza Formation) contain many sites with footprints (Camino a Treguajantes; Soto 1, 2, and 3; Trevijano; San Martín 1, 2, and 3). Additional localities in this group with footprints that are not described in this book are Fuenteamarga, Sol de la Pita, Cuesta del Peso, Valdemurillo, San Vicente de Robres, and Barranco de Antoñanzas (Casanovas et al., 1990b; Moratalla, 1993; Moratalla, Garcia-Mondejar et al., 1994a; Moratalla, Sanz, and Jiménez, 1999, 2000a; Pérez-Lorente, 1993b, 1993c).

In the Urbión Group, the rocks include uncommon carbonates and more abundant sandstone, mudstone, and shale. The sedimentary environment was fluvial (deltas, channels, and floodplains) as well as open, slightly haline lakes. Floodplain deposits show soil horizons with root traces. Sites in this group discussed in the book are Mina Victoria, Valdeperillo, Cabezón de Cameros, and Los Chopos. Additional sites not described in the text are Cabezuelos, Barranco de Acrijos, La Moga, San Prudencio, Rio Maguillo, and Barranco de la Muga (Caro et al., 1995; Caro, Pérez-Lorente, and Requeta, 2002; Casanovas et al., 1995e; Moratalla, 1993; Pérez-Lorente, 2001c).

In the Enciso Group, the sedimentary rocks are sandstone, mudstone, shale, and abundant limestone. These were deposited primarily in freshwater rivers and lakes. The river environments are represented by channels, floodplains, and marshy areas with abundant vegetation, deltas, and emergent coastal areas (beaches). The lakes were shallow and extensive in area (several hundred square kilometers), with large sublittoral areas. Fossil footprints are found in almost all environments except in the open lake carbonate zones (which were deeper) and the swampy areas.

The Oliván Group is entirely fluvial. There are very few sites with footprints; they are located near the base of the unit. There is oral reference to a site with four footprints near the village of Oliván, but these footprints were stolen.

The climate was warm, with periods of both semiarid and humid tropical conditions during the depositions of the Oncala and Enciso groups, and a humid tropical climate during the deposition of the Urbión Group (Doublet, 2004).

Description of the Tracks

Nomenclature

For identification and location of tracksites, trackways, and footprints, a combination of letters and numbers is used in the following order: first, identification of the tracksite (or outcrop) through a combination of letters that may be preceded by a number; second, identification of the trackway with a number; and third, identification of the footprint with an ordinal number. For example, 7PL1.4 means the following: 7PL is outcrop 7 of the Era del Peladillo tracksite (PL); 7PL1 indicates that it is trackway 1; and 7PL1.4 is the symbol of footprint number 4 of trackway 1 from outcrop 7 of the Era del Peladillo tracksite.

In quadrupedal trackways, the pes footprint is distinguished from the manus mark with the letters p (pes) and m (manus). Thus, 7PL1.4p refers to the print of the fourth pes in the 7PL1 sauropod trackway.

In studies done by our team, values are taken from footprints and trackways, then analyzed to infer characteristics of gait and trackmaker. Data, ratios, and abbreviations used in the book are provided in Table 2.1.

Types of Features: Tracksite Surfaces

The sites and the tracks they contain are complex sets of structures that record sequences of events in time and need to be examined carefully. We will first consider the outcrops and tracks, followed by the relationship between