Document Type : European UNESCO Geoparks:Original Article

Authors

1 Villuercas-Ibores-Jara UNESCO Global Geopark. Diputación de Cáceres, C/ Pintores 10, 10003 Cáceres, Spain

2 Área de Paleontología, Faculta de Ciencias, Universidad de Extremadura, Avenida de la Física s/n, 06006 Badajoz, Spain

3 C/ Méjico 2, 06220 Villafranca de los Barros, Spain

Abstract

Cruziana is a common and widespread trace fossil in Lower Palaeozoic strata that is generally attributed to the activity of trilobites. The Lower to Middle Ordovician Armorican Quartzite Formation of southern Europe contains well-preserved examples of the Cruziana rugosa Group. This paper outlines how Cruziana forms an important part of the geological heritage in the Villuercas-Ibores-Jara UNESCO Global Geopark, Cáceres Province (SW Spain).

Keywords

Introduction

Cruziana is a common and widespread trace fossil in Lower Paleozoic siliciclastic successions. It is typically found preserved on the soles of sandstone beds in the form of two parallel lobes, each decorated with ridges that represent the imprints of movement by the producers’ limbs. The arrangement and form of these ridges are important for the differentiation into ichnospecies. In Cruziana the length of the trace fossil exceeds its width whereas in the otherwise similar Rusophycus these are approximately equal. Cruziana was named by the French paleobotanist Alcide d’Orbigny in 1842 in honor of the general and politician Andrés de Santa-Cruz y Calahumana, based on material from what is now Bolivia (Egenhoff et al. 2007). As was the norm at the time, Cruziana was generally interpreted as the remnants of a plant, but already by the end of the 19th century, it was increasingly realized that it is a trace fossil. Most Lower Paleozoic Cruziana can with reasonable confidence be attributed to the activity of trilobites. This is based on the general coincidence in the diversity trends of Cruziana,and Rusophycus with that of trilobites and, more directly, from imprints of hard parts compatible with trilobites in some specimens and, very rarely, trilobites found in direct association with a trace fossil (e.g., Seilacher 1970, 2007; Fortey & Seilacher 1997). However, Cruziana and Rusophycus are found also after the extinction of trilobites and there is always the possibility that the producer was an arthropod other than a trilobite. Regardless, Cruziana is unusual among trace fossils in that it can be reasonably narrowed down to a particular type of producer and studies on Cruziana have provided much material for discussion of the habits of trilobites (e.g., Seilacher 2007).

Cruziana isa highly characteristic trace fossil in the Villuercas-Ibores-Jara UNESCO Global Geopark, Spain, and the Naturtejo (e.g., Neto de Carvalho 2006; Neto de Carvalho & Baucon 2016; Neto de Carvalho et al. 2021) and Arouca (e.g., Sá et al. 2006, 2009) UNESCO Global Geoparks in Portugal. The purpose of this paper is a brief presentation of Cruziana in the Villuercas-Ibores-Jara Geopark (Fig. 1).

 

 

Figure 1. Map showing the location and geological context of the Villuercas-Ibores-Jara UNESCO Global Geopark and selected locations relevant to Cruziana. Inset shows location within Iberian Peninsula. Geology in main map is based on “Mapa Geológico de Extremadura a escala 1:250.000”. The Armorican Quartzite is represented by green color with white dots. Details on other lithostratigraphic units can be consulted at: http://sigeo.juntaex.es/portalsigeo/web/guest/geologia-de-extremadura. Symbols: 1, Geosite Estrecho de la Peña Amarilla; 2, outcrop at La Sierra de la Madrilla; 3, Geosite Canchos de la Sabana; 4, outcrop at Los Moros; 5, reception center in Cañamero village; 6, interpretation center in Navatrasierra village; 7, interpretation center in Logrosán village; 8, urban fossils in Cañamero village; 9, urban fossils in Berzocana village; 10, urban fossils in Navezuelas village; 11, urban fossils in Cabañas de Castillo village.

Cruziana in the Armorican Quartzite of the Villuercas-Ibores-Jara UNESCO Global Geopark

During the Early to Middle Ordovician (approximately 470 million years ago) sand-rich sediments were deposited in deltaic and shallow marine settings over vast areas marginal to northern Gondwana, including territories in present-day Spain, Portugal, France and northern Africa. Distributed by rivers, these sands were reworked by tidal currents and storms into beds separated by muddy sediment deposited during calmer periods. In France and the Iberian Peninsula these rocks are known as the Armorican Quartzite (e.g., Sá et al. 2014; Bayet-Goll & Neto de Carvalho 2020). Composed largely of sand, in which skeletal fossils typically degrade and dissolve, body fossils are rare but trace fossils are diverse and well preserved. The lower, sandstone-dominated, part of the Armorican Quartzite typically contains dense concentrations of Skolithos, simple vertically oriented tubes constructed by unknown animals. Higher in the unit is found the more complex Daedalus and, along sandstone-mudstone alternations, the bilobed Cruziana.

 

Figure 2. Cruziana as an urban fossil in building decoration: A, B) Well-preserved Cruziana from Navezuelas village. The larger forms are Cruziana furcifera. Most of the narrower forms are Cruziana goldfussi. B provides a closer view of the right portion of A to better show scratch marks and the marginal ridges flanking the bilobed surface. Coin for scale in A is 25 mm. C) Large and small Cruziana from Cabañas de Castillo village.

 

The most common ichnospecies of Cruziana in the Armorican Quartzite belong to the Cruziana rugosa Group, characterized by multiple ridges in groups up to 12. These include Cruziana furcifera, first described by d’Orbigny in 1842, and Cruziana goldfussi (Rouault) first described from the Armorican Quartzite in France in 1850 (Fig. 2). Together with Cruziana rugosa d’Orbigny, these are widely distributed Ordovician trace fossils that have found use as biostratigraphically significant fossils in otherwise unfossiliferous sandstone-dominated areas (e.g., Seilacher 1970; Meischner et al. 2020). The most likely producers of these trace fossils are asaphoid trilobites, and an exceptionally preserved trilobite from Morocco shows limb details that may explain the ridges observed in this type of Cruziana (Gutiérrez-Marco et al. 2017). These Cruziana were made within the sediment and probably represent combined feeding (of small food particles but possibly also scavenging) and locomotion (Figs. 3, 4). Bed soles may be densely covered with Cruziana allowing the movement of their producers to be traced out for great distances on large slabs (Fig. 4A).

 

Figure 3. Interpretative reconstruction of an asaphoid trilobite producing Cruziana. Artwork by Manuel García.

 

Cruziana as an Urban Fossil

Slabs of Armorican Quartzite covered with Cruziana have been traditionally used as ornamentation on buildings. This practice has been common in villages in the border regions of the provinces of Cáceres and Salamanca, a particularly striking example being the village of Monsagro (Simón-Porcar et al. 2020). In the Villuercas-Ibores-Jara UNESCO Global Geopark this practice can be found in several villages (see Fig. 1). These urban fossils often include particularly well-preserved material of Cruziana (Figs. 2, 5A).

 

Figure 4. A) Large surface covered with Cruziana goldfussi from Los Moros outcrop. The finer scale divisions are in centimeters. B) Armorican Quartzite with thick and thin sandstone beds (lighter) and mudstone (darker). Several Cruziana are seen in cross-section close to base of the thicker sandstone bed. Geosite Estrecho de la Peña Amarilla. Scale is 5 cm.

 

Cruziana in the Villuercas-Ibores-Jara UNESCO Global Geopark: Paths to Sustainable Development.

An important aim of UNESCO Global Geoparks is to protect the paleontological heritage and use it to tell the history of our planet and to improve the lives of its inhabitants through education and sustainable development. In the Villuercas-Ibores-Jara Geopark, information on Cruziana is provided by samples and interpretative material on display in visitor centers, especially in the Geopark Visitors’ Reception Center in Cañamero village, the Fossil Interpretation Center in Navatrasierra village and in the Vicente Sos Baynat (1895–1992: Spanish geologist who contributed significantly to mining exploration in Extremadura) geo-mining museum in Logrosán village (Figs. 1, 5B). Many of the geological itineraries and touristic routes run through sections of the Armorican Quartzite and include outcrops with well preserved Cruziana. At such geosites and viewpoints are generally found informative panels on Cruziana, for example in the Estrecho de la Peña Amarilla geosite (Fig. 5C).

Cruziana, and the manner in which it was created, is a popular educational tool. In addition to the visitor centers mentioned above, urban fossils provide a popular and effective resource (Fig. 5A). Replicas of trilobites and Cruziana form part of a “Geobox” that is distributed to local schools. The use of games has proven to be very helpful in generating interest and understanding of the Geopark. This includes the Trilobite Hunt game (Fig. 5D), developed from a game described in the “Libro de los juegos” ("Book of games"), commissioned by King Alfonso X, “The Wise man”, and completed in his scriptorium in Toledo in 1283. The lines of the board are Cruziana. This not only is an abstract board game for the development of logical reasoning but also touches upon Ordovician ecosystems (bricks depict trilobites and Anomalocaris, the latter a potential predator on trilobites). As an example of cultural heritage related to Cruziana can be noted stories that relate Cruziana to giant snakes or dragons, such as that of the La Chiquita cave in Cañamero village.

During the confinement in 2020 due to the Covid-19 crisis, diverse material was developed for the Geopark's social networks, including a series of cards called "Learning with Geopaca." In these, the Villuercas-Ibores-Jara Geopark mascot explained curiosities related to the geological, natural and cultural heritage, including Cruziana.

 

Figure 5. Paleontology of the Armorican Quartzite in the Villuercas-Ibores-Jara Geopark’s infrastructures and activities: A) Urban fossils in Navezuelas village being used as an educational tool. B) Cruziana samples in the reception center in Cañamero village. C) Panel installed in the Estrecho de la Peña Amarilla geosite with interpretation of Cruziana. Armorican Quartzite towering in the background. D) The Trilobite Hunt board game, an example of educational material (created by Jesús Vázquez). E) Discussion on the formation of Cruziana during a European Geoparks Week activity.

 

Cruziana invariably stimulates discussion and interest among specialists and non-specialist alike (Fig. 5E) and is particularly appreciated by the tourism companies in the Geopark. Many aspects of Cruziana remain incompletely understood, including details on its formation (cf. Goldring 1985), for which outcrops such as that in Fig. 4B provide fantastic material for discussion. Research on Cruziana and other trace fossils in the Armorican Quartzite is, therefore, an ongoing process.

Acknowledgments

We thank Carlos Neto de Carvalho and two anonymous reviewers for their constructive feedback, and Dan Grigorescu for the invitation to contribute and for his patience. In loving memory of Rosario Cordero (Charo), our "Geopresidenta" forever.

Conflict of Interest

The authors have no known conflict of interest.

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