Introduction

The law 16/1985 of the 25th June regarding Spanish Historical Heritage states that the highest level of protection that can be afforded to palaeontological sites is that of Bien de Interés Cultural or Good of Cultural Interest (GCI). In agreement with Law 3/1999 of the 10th March regarding the Cultural Heritage of Aragón, the government of the eponymous Autonomous Region, having the necessary jurisdiction in such matters, has determined that its territory’s most important cultural heritage goods (both material and immaterial) be formally declared GCIs, be recorded as such in the inventory of the Region, and be adequately managed by the department responsible for cultural heritage. The category of Place of Cultural Interest-Palaeontological Zone (Conjunto de Interés Cultural-Zona Paleontológica) has been established for immovable goods such as dinosaur tracksites (which, according to the above law, is a place where there are remains, fossilised or otherwise, that together make up a coherent unit of clear identity). The Province of Teruel holds a whole of 16 sites that have been declared GCIs within the latter category (Table 1). Thirteen of these are dinosaur ichnite sites (declared GCIs by Aragón Governmental Decree 20/2003 of the 28th January and 18/2004 of the 27th January) that together cover the geological period from the Upper Jurassic to the Upper Cretaceous.

Table 1 Palaeontological sites in the Province of Teruel (Aragón, Spain) declared GCIs (within the category Place of Cultural Interest-Palaeontological Zone) and thus protected at the highest level
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The site of El Hoyo (CT-4), in the municipal area of El Castellar, is one of two GCIs representing the Lower Cretaceous in Aragón (Fig. 1). It is only GCI in the Camarillas Sandstone Formation. Since the presence of theropod tracks was reported by Alcalá et al. in 2003, other sites and ichnites have been discovered in this same lithostratigraphic unit; most are tracks make by large ornithopods, but those of sauropods and theropods are also present (Cobos and Gascó 2012; Herrero-Gascón and Pérez-Lorente 2013; Royo-Torres et al. 2013; Navarrete et al. 2014). The site of El Hoyo lies within the area designated as the Maestrazgo y Sierra de Gúdar Place of Community Interest (Lugar de Importancia Comunitaria), which occupies some 81,000 ha in the centre-west of the Province of Teruel on land that forms part of the Natura 2000 Network. The site was declared a GCI in 2004 as a consequence of the dinosaur ichnites preserved in it. This declaration was made when several Spanish autonomous regions were promoting the inclusion of all the dinosaur tracksites of the Iberian Peninsula within the UNESCO list of World Heritage items (Alcalá 2012). The background to this candidature goes back to 2002 when the Spanish Government included the item “Dinosaur Ichnite Sites of the Iberian Peninsula” (referring to sites in six Spanish autonomous regions—Aragón, Castilla y León, Catalonia, the Community of Valencia, La Rioja, and the Principality of Asturias) in its tentative list for recognition of World Heritage status (http://whc.unesco.org/ en/tentative lists). Spain presented this list to UNESCO in 2006, but in 2008 Portugal joined the initiative, and in 2009 the two countries made a joint presentation. The proposed item Dinosaur Ichnites of the Iberian Peninsula represented a trans-frontier heritage values. On both occasions (2006 and 2009), i.e., the Spanish and Spanish-Portuguese attempts, the UNESCO reviewers of natural heritage returned a verdict of ‘deferred’, communicating the need to justify the outstanding universal value of these ichnites as heritage values complementary to others already recognised as part of World Heritage related to dinosaurs (Table 2). The conferral of GIC status before 2006 to all dinosaur tracksites in Spain (including the site of El Hoyo) within the autonomous regions cited above was thus motivated by UNESCO requirements (see UNESCO World Heritage Centre 2005). All relevant sites that afforded the World Heritage status must first be protected by legal, institutional and/or traditional mechanisms that guarantee their long-term preservation. States proposing candidate items must provide documentation that these have been afforded adequate local, regional, national and/or traditional protection. In addition, all candidate fossil sites must be presented with an appropriate management plan or a system that specifies how they will be preserved. Thus, the site of El Hoyo site, and the 215 other dinosaur tracksites documented before the first Spanish proposal in 2006, was declared an GCI within the category ‘Place of Cultural Interest-Palaeontological Zone’ as part of the Spanish land (including the six of its autonomous regions involved) strategy for seeking their recognition as World Heritage. These sites were not, therefore, declared GCIs because of any known scientific importance or uniqueness. The later joint proposal made by Spain and Portugal did, however, select 11 sites regarded as being the best candidates for being considered of outstanding value. After the proposal presented in 2009, other dinosaur tracksites discovered in the same autonomous regions have not been awarded a GCI status, despite some almost certainly being of great scientific relevance.

Fig. 1
figure 1

a Geographical location of El Castellar and the El Hoyo site (Teruel, Spain), in the Maestrazgo y Sierra de Gúdar Place of Community Interest. b Lutite outcrops of the Areniscas de Camarillas Formation close to the site of El Hoyo

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Table 2 Dinosaur fossil sites in the UNESCO World Heritage list
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In addition to being legally protected by the Law of Aragonese Cultural Heritage, the El Hoyo site has been designated a Place of Geological Interest by the Aragón Regional Government within the category ‘Palaeontological Site’ (there are 24 such sites in this category in Aragón) via Decree 274/2015 of the 29th September. This also established the Catalogue of Places of Geological Interest of the region. The instrument for the protection of these places is the National Law 42/2007 of the 13th December regarding Natural Heritage and Biodiversity (Patrimonio Natural y Biodiversidad). The aforementioned decree establishes that Places of Geological Interest within the boundaries of a Protected Natural Area (Espacio Natural Protegido) or a GCI, are protected by the legislation most specific in this regard. Thus, the site of El Hoyo is protected as a GCI in the category of Place of Cultural Interest-Palaeontological Zone. The site is therefore well protected from a legal standpoint.

The dinosaur tracks at the site of El Hoyo not only have scientific peculiarities. The site also holds many fossil traces of invertebrates and other fossil groups made by invertebrates, which has increased its multidisciplinary interest. The following highlights some of the characteristic of the site and describes the methods used to study and preserve its ichnites, and to make them more widely known.

Geographical and Geological Setting

The site of El Hoyo (CT-4) is placed within the municipal area of El Castellar, a village some 40 km from the city of Teruel [Aragón, Spain (0° 48′ 10.78″ W, 40° 21′ 4.51″ N)]. Geologically, it lies in the Maestrazgo Basin, within the Peñagolosa sub-basin, in facies corresponding to the Camarillas Sandstone Formation (Fig. 1). This lithostratigraphic unit, which overlies the El Castellar Formation in this sector of the Iberian Range, was originally defined by Canerot et al. (1982) and then redefined by Salas (1987). The unit is formed by a stratigraphic alternancy of lutites and sandy conglomerate bodies. Its genesis has been interpreted as the consequence of the action of fluvial systems that fine upwards into delta systems (Canerot et al. 1982; Salas 1987). The study of their charophyte content has allowed dating them as Lower Barremian (Martín-Closas and Salas 1994). The unit passes gradually upward to the Artoles Formation, a carbonatic-siliciclastic unit deposited on a shallow platform (Mas et al. 2004). The Camarillas Sandstone Formation is known for its record of dinosaur bones, mainly those of ornithopods (Ruiz-Omeñaca 2011; Verdú et al. 2015).

The site, an outcrop with an area of some 20 m2, lies on a small slope out of which crops a siliceous sandstone layer. The grains of the latter are red-brown, of medium size, and very cemented. Along with several dinosaur ichnites, the site has many microinvertebrate structures and shows signs of abundant bioturbation (Alcalá et al. 2003; Cobos and Alcalá 2007; Rodríguez-Tovar et al. 2016) (Fig. 2a).

Fig. 2
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The site of El Hoyo (CT-4). a General view of the outcrop. b Trackway 1CT-4 with its four theropod tracks. c 2CT-4 with its three theropod tracks. d 3CT-4 with two theropod tracks. e Close-up of an individual track in the 1CT-4 trackway

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Ichnological Record

Dinosaur Tracks

The nine dinosaur tracks at the site of El Hoyo are arranged in three trackways composed of four (1CT-4) (Fig. 2b), three (2CT-4) (Fig. 2c), and two (3CT-4) (Fig. 2d) tracks, respectively. All the tracks are tridactyl and mesaxonic, and all are shallow due largely to the erosion suffered by the slope. This erosion recently allowed the visualisation of the 3CT-4 trackway, which was not reported so far. In general terms, the ichnites show the same morphological features and relationship with the substrate. They are between 19 and 28 cm long and 19–26 cm wide. The marks left by the digits, though deformed, show them to have been of varied length and relatively narrow. The longest impression is that of digit III, and the shortest that of digit II. All three digits had long, sharp nails (Fig. 2e). These findings show the makers of these tracks to have been small–medium-sized theropods (Thulborn 1990). Trackways 1CT-4 and 2CT-4 both travel towards N190° E. Although they are 4.5 m apart, the fact that the tracks belong to the same ichnotype are orientated in the same direction and show the same relationship with the substrate, suggests that two theropod dinosaurs were travelling together. However, the 3CT-4 trackway, which lies some 3 m from 1CT-4, was made by an animal moving in a very different direction; the above suggestion should therefore be viewed with caution.

Bioturbation Caused by Invertebrates

The same sandstone layer that contains the dinosaur ichnites shows an ichnofabric of invertebrates, mostly reflecting the presence of Taenidium (Fig. 3). The bioturbation present is characterised by structures with a meniscate infill, preserved as non-ramified cylindrical, straight, and wavy epireliefs, showing the absence of lining, mantle, and longitudinal striations. These ichnological characteristics of the bioturbations, including the length and width of these structures, their infill, and general shape, allow differentiation of the ichnospecies Taenidium barretti and T. serpentinum (Rodríguez-Tovar et al. 2016).

Fig. 3
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Signs of Taenidium at the site of El Hoyo. a General view of the surface showing abundant specimens of Taenidium. b–d Close-up views of Taenidium showing meniscate infill

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Dominant, nearly exclusive Taenidium in the invertebrate trace fossil assemblage allows for assignation to the Scoyenia ichnofacies. This is coherent with the presence of vertebrate tracks, the low ichnodiversity in invertebrate structures, and the environment in which the facies containing the ichnites originated (a transitional zone between terrestrial and non-marine aquatic environments, for instance floodplain areas adjacent to rivers and affected by low energy conditions). The Taenidium-dominated ichnofabric reveals the opportunistic behaviour of its tracemakers. These animals bioturbated the layer soon after its deposition on a damp, soft substrate during favourable but ephemeral conditions. This behaviour allows a rapid bioturbation of moist to wet substrates or softgrounds during favourable, episodic conditions, such as a rapid accumulation of nutrients buried in the sediment during alluvial discharges (Rodríguez-Tovar et al. 2016).

Research, Preservation and Knowledge Dissemination Methods

Dinosaur tracksites are commonly vulnerable since they are exposed to the weather changes and other atmospheric weather and other destructive atmospheric and biological agents. They generally require in situ preservation work to reduce these problems. The slope on which the site of El Hoyo is situated is generally dry for most of the year. However, water can stream across it during storms and times of heavy rain, which could damage the ichnological record (especially trackway 2CT-4, which can become covered in the red clay carried by such water). Despite these dangers, nothing has been done to divert the path of the water, although some actions have been taken to preserve the ichnological information of the scientifically most interesting parts of the site. In 2007, the Government of Aragón’s II Palaeontological Restoration Workshop made a 2.7 × 2.7-m mould of the 1CT-4 trackway (as recorded in its unpublished report entitled ‘Moulds and Replicas of Dinosaur Tracks at the sites of El Pozo and El Hoyo (El Castellar, Teruel)’ (Spanish title: Moldes y réplicas de huellas de dinosaurio de los yacimientos “El Pozo” y “El Hoyo” [El Castellar, Teruel]) (Fig. 4). The work involved sealing numerous surface cracks and fissures with clay (Fig. 4a), then applying a demoulding agent (polyvinyl alcohol), three layers of silicon elastomer (the first of those being a thin layer of silicon to ensure that fine details of the fossils would be picked up) and a sheet of aluminium foil. Finally, a rigid casing of glass fibre-reinforced polyester was made (Fig. 4b–d). The resulting mould was deposited in the Aragón Museum of Palaeontology (Fundación Conjunto Paleontológico de Teruel-Dinópolis). Casts were also made. Casts faithfully reproduce the original and important tools, removing the need to travel to the field. These were obtained by first applying a demoulding agent (again polyvinyl alcohol) to the surface of the silicon, then applying a first coat of white Gel Coat resin, which picks up all the details. Once this resin is catalysed, new layers of glass fibre-reinforced resin are applied. The replicas obtained can be painted with natural pigments and protected with matt varnish (Fig. 4e). One of these replicas is exhibited at one of the stops along the Dinosaur Palaeontological Route (DINOpaseo por El Castellar) around the streets of El Castellar (Cobos and Alcalá 2017). The replica was sited close to the castle of the village so that visitors could see some ichnites without actually visiting the site of the El Hoyo site (which is some way away in a wood) (Fig. 4f).

Fig. 4
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Mould and cast making of the 1CT-4 trackway at the site of El Hoyo by the Government of Aragón's II Palaeontological Restoration Workshop (Escuela Taller de Restauración Paleontológica II del Gobierno de Aragón). a Sealing fissures and cracks with clay. b–d Steps in mould-making. e Painting a replica with natural pigments. f One of the stops on Dinosaur Route in El Castellar (‘DINOpaseo por El Castellar’) showing the replica of the 1CT-4 trackway

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A second conservation action was undertaken in 2016, involving the taking of photogrammetric data in order to gather as much ichnological information as possible. The idea of this work was to provide a snapshot of the state of the site, and to provide a database of 3D information to assist in ichnological analyses. Moreover, all the information obtained could also be used for the diffusion of the site through digital platforms in exhibitions, theme networks, etc.

The term ‘photogrammetry’, referring to taking measurements from photographs, was first coined in 1867 (Matthews et al. 2016). The technique involves the correlation of images to reconstruct the geometry of an object and the eventual construction of 3D models. Similar to human vision, if an object is seen in at least two images from different viewpoints, the relative positions of the object in the images (parallax) allow for a stereoscopic view to be built and 3D information derived. By intersecting the rays coming from homologous 2D points visible in different images that pass through to the centre’s projection of the camera, 3D coordinates can be derived (Petti et al. 2008).

Over the past decade, 3D surface digitisation has become of growing interest to palaeontologists (Falkingham 2012). Some of the most valuable clues to the Earth’s history may be found in trace fossils, and these paleontological resources should be managed following scientific principles and using scientific expertise to safeguard their value (Matthews et al. 2016). The use of digital imaging has made possible the study of inaccessible natural sites and the making of non-destructive observations. The images and numerical models obtained allow quantitative investigations to be used in research and protection, and may be employed, e.g., through the use of virtual models, in the diffusion of knowledge regarding our geoheritage (Cayla 2014). During the collecting of data at the site of El Hoyo, special attention was paid to ensuring homogeneous illumination, and the best time of day to undertake readings was determined. After a complete visual inspection of the site, a diagram was drawn showing the orientation of the layer containing the ichnites. Overlapping high-resolution photographs were then taken using a Canon Powershot G12 compact camera. Photogrammetric data were then recorded, first in a panoramic sweep of the site (Fig. 5) (anticlockwise to magnetic north), and then in sessions focused on the three trackways (Fig. 5a–c) and the details of the bioturbation caused by Taenidium (Fig. 5e). High accuracy point clouds were generated from all sets of photos using Agisoft PhotoScan software. The 3D models produced were then converted to colour maps using CloudCompare software, importing the models orientated with respect to the Z-axis (Fig. 6). The models were scaled against three 1-m-long (red and white) and 10-cm-long sticks positioned on the surface of the site.

Fig. 5
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General photogrammetric model of the site of El Hoyo, and details of its three dinosaur trackways (a 1CT-4, b 2CT-4, and c 3CT-4) and the ichnofabric produced by Taenidium (d). Scale bar (red and white): 1 m

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Fig. 6
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a Photogrammetric model (phototextured) of a single theropod left pes track from the 1CT-4 trackway (scale bar = 10 cm). b Depth-colour image showing relative elevation (Z-axis)

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Although no museographic activities have been undertaken in situ, a number of reports have been prepared detailing the work needed to secure the preservation of the site and the information that should be transmitted in order for it to become part of El Castellar’s (and indeed the Province’s) working geotourism resources (Cobos and Alcalá 2017) (Fig. 7a). It should be highlighted that, of the 13 sites with dinosaur tracks in the Province that have been declared GCIs, only the site of El Castellar (Fig. 7b) (in the village of the same name) and those of Corrales del Pelejón and Las Cerradicas (Fig. 7c, d) in Galve have been completely conditioned for geotourism. Another two sites with dinosaur tracks—in Miravete de la Sierra and Ariño—have undergone minimal conditioning (fenced or supplied with an information sign) (Fig. 7e, f). It should be remembered that Galve and Miravete de la Sierra lie within the Maestrazgo Geopark (Alcalá et al. 2013, 2015) and that several institutions have proposed routes that promote geotourism to dinosaur sites, such as The Dinosaur Track Road between Galve and Teruel, passing through Aguilar del Alfambra, Ababuj, Cedrillas, El Castellar and Formiche Alto (Fig. 8) (Alcalá et al. 2016).

Fig. 7
figure 7

a Infographic simulation of some of the preservation and knowledge dissemination actions undertaken at the site of El Hoyo. Museographic conditioning of some dinosaur tracksites in the Province of Teruel: b El Castellar, c Corrales del Pelejón (Galve), d Las Cerradicas (Galve), e Miravete (Miravete de la Sierra) and f Puente del Río Escuriza (Ariño)

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Fig. 8
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The Dinosaur Track Road in Teruel (Spain). Figure modified by Alcalá et al. (2016)

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To establish how the research, preservation and knowledge diffusion actions mentioned above have affected the El Hoyo site and its surroundings, its heritage value has been examined using the method of Mampel et al. (2009). The results show the values of the site to have increased considerably from a rating of 0.23 sensu Alcalá et al. (2016) [with data from Cobos and Alcalá (2007)] to a current 0.33 (scale 0–1 where 0 = lowest possible, to 1 = highest possible). These findings conclusively show the importance of interdisciplinary research and the dissemination of scientific results to society.

In addition, while some of the proposals submitted to the institutions would be achieved in the future, the site is being used socioculturally through the current Dinosaur Route in the village, fieldtrips of several types of meetings and Summer Courses, to celebrate the International Museum Day, etc. (Cobos and Alcalá 2017).

Conclusions

The multidisciplinary study of the tracksite of El Hoyo, the only palaeontological GCI in the Camarillas Sandstone Formation (Barremian), is focused on its three trackways made by small–medium-sized theropods. These tracks are associated with abundant fossil invertebrate traces made (in large part) by Taenidium (suggesting it should be assigned to the Scoyenia ichnofacies). This combination of vertebrate and invertebrate fossils suggests environmental conditions related to a transition zone between a terrestrial and an aquatic (non-marine) environment. Given the risk of deterioration, moulds of the longest trackway have been made and replicas produced using standard techniques. In addition, the whole site has been subjected to photogrammetric analysis in order to obtain 3D data and thus preserve the most scientifically interesting ichnological information. The site is difficult to reach, but is made available for geotourism purposes via a replica in the village of El Castellar. The fact that the site has been declared a GCI with a mix of vertebrate and invertebrate-associated fossils, that it is also recognised as a Point of Geological Interest and that it lies on land given over to the Natura 2000 Network increases both its heritage value and its potential to form part of future tourism routes.