Document Type : Original Article

Authors

1 Naturtejo UNESCO Global Geopark. Geology Office of the Municipality of Idanha-a-Nova, Centro Cultural Raiano. Avenida Joaquim Morão, 6060-101, Idanha-a-Nova, Portugal

2 Instituto Dom Luiz, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal

3 DISTAV, University of Genova, Italy

4 Villuercas-Ibores-Jara UNESCO Global Geopark. Diputación de Cáceres. C/ Pintores nº 10, 10003 Cáceres, Spain

5 Área de Paleontología, Facultad de Ciencias, Universidad de Extremadura, 06006 Badajoz, Spain.

Abstract

The Early Paleozoic oceans were generally characterized by short trophic chains and simple ecological tiering dominated by suspension-feeding organisms. However, the Great Ordovician Biodiversification Event (GOBE) was responsible for the complexification of food webs, increasing depth and diversity of substrate ecospace utilization and increasing benthic competition for resources near the water-substrate interface. Daedalus is an enigmatic trace fossil that precedes this period of nearshore innovation and its disappearance is probably related to the escalation in tiering complexity, especially among benthic surficial feeders, that may have cut access to their main food sources. Daedalus producers were responsible for the occupation of some of the earliest deep substrate tiers, probably feeding from particulate organics and meiofauna using complex architectures for resource exploitation. They were responsible for characteristic ichnofabrics that can be found, especially in Lower-to-Middle Ordovician sandstones, in the “Armorican Quartzite” and similar nearshore facies around Gondwana. Villuercas-Ibores-Jara and Naturtejo UNESCO Global Geopark show several geosites where the best-preserved forms of Daedalus can be found and where its crowded ichnofabrics can be followed in large-scale exposures. Indeed, these are mega-ichnosites of international paleontological relevance for understanding behavior and ecospace utilization of a characteristic trace fossil during the GOBE. This paleoecological significance meant that Daedalus became a symbol for a cross-border Interreg Project that connects the two UNESCO geoparks. The Armorican Quartzite Route is an 800 km-long road following the main geological structures and promoting sustainable tourism attractions along the way. The Bridge over the Armorican Quartzite has the goal of fostering sustainable development through common scientific and education tools, raising awareness of the GOBE as one of the most important events of biodiversification in the Earth history and a key element of the geological heritage of both UNESCO Geoparks.

Keywords

Daedalus: A Trace Fossil Expression of Complex Behavior

Daedalus is among the most complex and least understood Lower Paleozoic trace fossils. Despite that, its architecture is well known from the work of Sarle (1906) and Lessertisseur (1971). Daedalus is large, vertical spreiten (backfilled) structure, composed of J-shaped burrows whose displacement twists spirally inwards. Spreiten surfaces, resulting from the repeated translocation of the vertically oriented burrow hundreds of times, may cross themselves or other similar structures. They started formation at the water-substrate interface from an (almost) fixed central point and may pass down through one or more horizons and reaching as much as ~1 m deep, resulting in a succession of helicospiral, conical or spindle-shaped bodies.

Daedalus is a characteristic Lower Paleozoic trace fossil, particularly common in the Ordovician, high-paleolatitude shallow water sandstones of the “Armorican Quartzite” Formation and similar facies in France, the Iberian Peninsula, North Africa, the Middle East and Argentina (Rouault 1850; Delgado 1885; Lessertisseur 1971; Beuf et al. 1971; Seilacher 1964, 2000, 2007; Durand 1985; Poiré et al. 2003; Gibert et al. 2011; Heward et al. 2019). Daedalus not uncommonly occurs in high densities often completely dominating the physical structure of beds (Neto de Carvalho et al. 2016). The different ichnospecies of Daedalus, each developing characteristic, almost monoichnospecific ichnofabrics in the Lower-to-Middle Ordovician “Armorican Quartzite” of the Iberian Peninsula, are differentiated by the size and shape of the generating tube, and how these tubes are bundled into a single complex burrow: the type ichnospecies D. desglandi (Rouault) possesses complex helicoidal spindle patterns produced by descending translocations of a J-shaped tube, whose distal end was turning back towards the center (Lessertisseur 1971; Seilacher 2000). In D. desglandi the tube is relatively wide (10-20 mm) and forms bodies with an irregular, apparently chaotic distribution of burrow translations, with numerous intersections of previously exploited areas that usually expand in curvilinear bands defining the outer expression of the burrow complex until moving downwards and starting once again from the central axis. In Daedalus halli, the tube is narrow and straight (usually less than 5 mm wide, up to 10 mm), showing no evidence of active filling, and its displacement forms smooth carpet-like walls. A cone-like, helical 3D shape may be defined by a change of level in the sediment and the comma-to-spiral/circular shape of the spreiten on the bedding plane.

The producers of Daedalus are unknown and unlikely ever to be found, but were probably “worm-shaped” animals that opportunistically exploited the substrate for food. The distribution pattern and morphology of D. desglandi suggests they harvested sand-enveloping biofilms as a possible feeding strategy (as suggested for D. halli by Noffke, 2012). A long, worm-like animal burrowing downwards must have been capable of sediment displacement from one flank of the body to the other to produce a lateral backfill (Seilacher 2000). A possible process to explain the occurrence of Daedalus in clean quartz sands is the “subtidal pump” model of Riedl et al. (1972). In energetic shorelines or shallow marine bottoms frequently disturbed by storms, breaking waves act as a pump and bring down oxygen and nutrients in deeper levels of the substrate (Bock and Miller 1995). They also reduce the diversity of benthic life in shallower tiers in high-energy environments. The clean sands that constitute the quartzites may have acted like a sieve, and only fine organic particles would have reached the deeper tiers. This, together with the meiofauna that exploited the same food source among the sand grains, and the grain enveloping biofilms might explain why such complex burrows were built (Neto de Carvalho et al. 2016).

In the Iberian Peninsula, the Villuercas-Ibores-Jara and Naturtejo UNESCO Global Geoparks, located in the southern part of the Central Iberian Zone, in eastern Extremadura, Spain, and central Portugal, respectively, are territories with outstanding paleontological heritage. Among these, Daedalus mega-ichnosites are found in the “Armorican Quartzite”, corresponding to quartzite beds with dense D. desglandi ichnofabrics showing exceptional preservational and paleoecological features that can be followed for hundreds of meters or sometimes kilometers, with great lateral homogeneity. For these reasons, they were included as geosites in the geological heritage lists of both geoparks (Barrera & Gil Montes 2013; Cortijo et al. 2016; Neto de Carvalho et al. 2016; López Caballero et al. 2018; Fig. 1).

Figure 1. Location of D. desglandi geosites identified in Naturtejo and Villuercas-Ibores-Jara UNESCO Global Geoparks in the Lower-to-Middle Ordovician “Armorican Quartzite”. 1) Muradal-Fajão syncline. 2) Monforte da Beira. 3) Penha Garcia syncline. 4) Las Amoladeras. 5) Risco Carbonero. 6a and 6b) Camorros de Castañar de Ibor and Navalvillar de Ibor. Limits of both geoparks are placed on extract from the Geological Map of the Iberian Peninsula, Balearic and Canary Islands 1:1.000.000, 2015 edition of IGME/LNEG (all rights reserved). The symbols for the main stratigraphic units can be consulted at:   

http://info.igme.es/cartografiadigital/datos/geologicos1M/Geologico1000_(2015)/pdfs/EditadoG1000_(2015).pdf

 

Daedalus Ichnofabric: Evolutionary Paleoecological Implications and Paleogeographic Distribution

An ichnofabric includes all structure and textural changes of the sediment resulting from bioturbation (and bioerosion) at all scales (Reineck 1963). The abundance and distribution of burrows reflect the non-linear sorting effects of physical and biological parameters, resulting in a disturbance regime at different degrees of patchiness in the colonization of the substrate. This is particularly true for the colonization of sessile to semi-sessile filter-feeding benthic fauna, or by vagile organisms with complex feeding strategies and/or cosmopolitan habits. Dense Skolithos ichnofabrics, known as piperocks, are usually considered to record the advent of deep burrowing by coelomate metazoans (possibly polychaetes or phoronids) during the Cambrian radiation (Droser 1991; Mángano & Buatois 2011). They first appear in the Early Cambrian in high-energy, nearshore, and storm-related siliciclastic deposits. The decline of the Skolithos piperock corresponds with the Ordovician faunal diversification and tiering complexification (Thayer 1983; Desjardins et al. 2010; Mángano & Buatois 2011, 2015; Liang et al. 2012). Together with Skolithos, which is also a common ichnofabric in the “Armorican Quartzite”, Daedalus may be seen as a pioneering product of the Cambrian substrate revolution (Early Cambrian forms described by Desai et al. 2010), a complex burrow that deeply penetrates the substrate for food processing and directly contributes to changing physical and chemical gradients in the sediments, similar to Dictyodora Weiss and Syringomorpha Nathorst. However, the general feeding behavior represented by Daedalus may have reached its evolutionary climax during the Early Ordovician. Dense Daedalus ichnofabrics are highly pervasive in the “Armorican Quartzite” and similar peri-Gondwanan, high latitude siliciclastics from Portugal (Delgado 1885; Neto de Carvalho et al. 2016), Spain (Gutiérrez-Marco et al. 2017) and Oman (Heward et al. 2019). These ichnofabrics occur in facies corresponding to very shallow marine, drifting sand bodies to lower shoreface and high hydrodynamic conditions (Figs. 2A, 3A), like amalgamated storm beds with planar lamination or hummocky cross-stratification.

To analyze the patch dynamics of trace fossils as a result of environmental disturbance, multifractal spectrum analysis was used as a measure of spatial ichnofabric heterogeneities in dense D. halli ichnofabrics from the Armorican Quartzite (Neto de Carvalho 2003; Neto de Carvalho and & Baucon 2013). The magnitude of the Daedalus ichnofabric fluctuations for the two stratigraphic sequences (Martim Preto and Serapicos in the NE of Portugal), both in area and in recurrence along the succession, shows that sandflat substrate colonization by the D. halli producer after each storm event was opportunistic, mostly multigenerational, with an exclusive and significant occupation of emptied ecospace (Neto de Carvalho and Baucon 2013). The patchy distribution pattern suggests a similar model of ecospace colonization to explain also D. desglandi architecture based on the subtidal pump mechanism of interstitial water exchange by wave action.

At Naturtejo UNESCO Global Geopark, Neto de Carvalho et al. (2016) described mega-ichnosites composed of quartzite beds with dense D. desglandi ichnofabrics which, in certain circumstances, can be followed for kilometers (Fig. 2). The Penha Garcia Formation (seen at site 3 in Figure 1), a formal regional name for the “Armorican Quartzite” at the Penha Garcia Ichnological Park (see this volume), is classified into two main groups of facies associations, which are interpreted as deposited in a mixed asymmetrical delta with along-strike variations between wave-dominated strandplain (updrift) and river-dominated deltaic settings (downdrift) (Bayet-Goll & Neto de Carvalho 2020). In the wave-dominated facies, D. desglandi beds are occasionally found in tide-influenced distributary channel fills at the delta front (Bayet-Goll & Neto de Carvalho 2020). The preservation of only one behavioral strategy, the substrate depth affected by these structures, the high density of burrows, and passive patchiness rates are all characteristics of r-selected populations. Such large-scale and frequent disturbance events as flood discharges at the delta front or storms (with remobilization and redistribution of sediments, endobenthos, and particulate food) made these Daedalus ichnofabrics one of the earliest opportunistic behaviors in the fossil record. They are also found in similar facies at Villuercas-Ibores-Jara UNESCO Global Geopark, where Daedalus beds can be followed for hundreds of meters (Figs. 3A–C; see also Cortijo et al. 2016), showing one of the best preservations known for this ichnogenus, and even including behavioral deviations to the normal architecture (Figs. 3D-G).