The transition from the Precambrian biosphere, dominated by microorganisms, to the Phanerozoic world, where animals are a keystone group from a biogeochemical and evolutionary perspective, is arguably the deepest ecological change in the history of life on Earth (Crimes, 1992; Fedonkin et al., 2007; Seilacher, 2007). This revolution was mainly concentrated in the Ediacaran—Cambrian transition, whose fossil record in central Spain is not very abundant but quite diverse, containing stromatolites, trace fossils, soft-bodied organisms (vendotaenids, sabelliditids, acritarchs, possible Beltanelliformis), and early skeletal fossils (Cloudina, Sinotubulites, Cambrian “small shelly fauna”, archaeocyathans, early trilobites…) (Liñán & Palacios, 1987; Vidal et al., 1994; Liñán et al., 2004; Fernández-Remolar & García-Hidalgo, 2005; Jensen et al., 2007; Zhuravlev et al., 2012; Jensen & Palacios, 2016). In the Central Iberian Zone (CIZ), the Ediacaran—Cambrian transition is recorded in several rock units which have received different names depending on the region and author (Nozal Martín et al., 1988; Vidal et al., 1994; Pieren Pidal, 2000). The present work deals with the Alcudian unit (Alcudiense, as described by Crespo & Tamain, 1971) in its typical region, the Alcudia Anticline of the valle de Alcudia (Ciudad Real, Sierra Morena), at the southern margin of the CIZ (Fig. 1; after Pieren Pidal, 2009).

The axis of the valle de Alcudia is the Lower Alcudian subunit, a thick (4,000–6,000 m), generally subvertical, usually flysch-like alternation of greywackes and siltstones representing turbidites and gravitational breccias of probable late Ediacaran age, being younger than about 580–560 million years (Ma) according to radiometric geochronology using detrital zircons (Talavera et al., 2015). This subunit is considered to have been deposited on a continental slope (Pieren Pidal, 2000; 2009). Most of the Lower Alcudian is so fractured and folded that conventional stratigraphic study seems impossible. Its rocks are weakly metamorphosed, corresponding to the low and occasionally high anchizone (Pieren Pidal, 2000; 2009). This subunit is overlain by the Upper Alcudian, in angular unconformity (< 550–540 Ma according to Talavera et al., 2015).

Upper Alcudian beds tend to be much less distorted, suffered less metamorphism, and contain a wide range of lithologies, with frequent conglomerates. In the Alcudia Anticline, this subunit was divided by Pieren Pidal & García-Hidalgo (1999) into five formations which are interpreted to record sedimentation in a shallow siliciclastic platform. From bottom to top, these formations (Fm.) are the Tamujar, Hinojosas, Cabezarrubias (Pizarras de Cabezarrubias), San Lorenzo (Conglomerados de San Lorenzo), and Upper Formation (Pizarras Superiores) (Fig. 2). The nature of their contacts is still quite open to debate. Carbonate lenses are present in the Hinojosas Fm. (Pieren Pidal, 2000, 2009).

Up to date, the fossil record of the Lower Alcudian is restricted to a single report of sphaeromorph acritarchs in an unpublished work by Mitrofanov & Timofeev (1979, cited from Pieren Pidal, 2009). The Upper Alcudian starts with hints of bioturbation in the Tamujar Fm. (Pieren Pidal, 2000). The Hinojosas Fm. has yielded the richest fossil assemblage of the Alcudia Anticline (García-Hidalgo, 1993; Pieren Pidal, 2000), consisting on varied trace fossils. In the northern flank (Cabezarrubias-Hinojosas zone, Fig. 1), the Hinojosas Fm. have simple unbranched burrows (Planolites), arthropod scratches (Monomorphichnus lineatus), and possible treptichnids assigned to Hormosiroidea canadensis. The presence of Monomorphichnus indicates a Cambrian age (Liñán et al., 1984; Gámez-Vintaned & Liñán, 1996, 2007). In the southern flank of the anticline (Macizo de Valdoro area), this formation contains simple traces with loops (Gordia molassica), and meandering bilobed traces initially identified as Taphrhelminthopsis but reinterpreted by Jensen & Palacios (2016) as possible Psammichnites circularis. However, according to the diagnosis of Taphrhelminthopsis, this ichnogenus refers to traces that are usually big (1–3 cm wide) and very long, whereas the bilobed trails of the Hinojosas Fm. are small (~4–5 mm wide) and not very long (see photograph 4 in Pieren Pidal, 2009). Thus the ichnogenus Archaeonassa (1–7 mm wide) seems more adequate for these trails (see the diagnoses of both ichnogenera in Häntzschel, 1975). An example of Archaeonassa from the upper Ediacaran of the White Sea region (figure 3 in Jensen, 2003) in fact looks very similar to these bilobed traces in size and path.

Above the Hinojosas Fm., possible cnidarian burrows (reported as Bergaueria aff. langi) and Planolites have been found in the Cabezarrubias Fm. The San Lorenzo Fm. contains the early mollusks Anabarella plana and undescribed members of the family Coreospiridae (previously Latouchellidae). Phosphatized remains of the Tommotian species Anabarella plana are extremely abundant in some massive mudstone beds of the Upper Fm. (Vidal et al., 1999; Pieren Pidal, 2009).

The specimens have been studied in outcrop when they appear in hard rocks and extensive beds that make them uncollectable without high risk of damage. They were photographed using a Canon EOS 550D and Canon EOS 7D DSLR cameras with a zoom lens Canon 18–55mm f3.5–5.6 ISII, and macro lenses Canon 60mm f2.8 and Canon 100mm f2.8. Collected specimens are deposited in the Museo de Paleontología de Castilla-La Mancha (Cuenca, Spain) under accession numbers MPCM-VA-0001 to MPCM-VA-0005.

The sedimentary environment inferred for the Cambrian trace fossils here described is the shallow platform where the Hinojosas Fm. was deposited (Pieren Pidal, 2000, 2009). The carbonates and abundant bioturbation point to low sedimentation rates. Age is constrained by the presence of Anabarella plana in overlying beds (Fig. 2). The age of this mollusk species ranges from about 535 to 531 Ma according to Gubanov & Peel (2003), which implies a Fortunian stage (Cambrian stage 1) for the ichnofossil locality, if the base of the stage 2 is at about 529 Ma as indicates the GSSP Table of the International Commision on Stratigraphy (consulted in June 5^th 2017). In the regional chronology, this corresponds to the Lower Corduban substage (see its definition in Liñán et al., 2006).

The possible taxonomic affinities of the organisms which created the traces of El Hontanar and Arroyo del Barranco are varied. Monomorphichnus lineatus corresponds to arthropods or arthropod-like organisms (Crimes et al., 1977). As a possible teichichnid, the spreiten-burrow could have been excavated by annelid-grade vermiform creatures, or arthropods (Seilacher, 2007). In the original description of Archaeonassa, Fenton & Fenton (1937) showed that it closely resembles modern trails of the gastropod Littorina, and thus early mollusks (may be Anabarella plana or a similar species), or the mollusk-like halkieriids, could have produced the small bilobed trails of Fig. 3D. Seilacher & Gámez-Vintaned (1996) argued that the Psammichnites organism was an infaunal, soft-bodied, vermiform animal which bulldozed the sediment and had a “snorkel” appendage that protruded and collected food on the seafloor (Seilacher, 2007). The taxonomic affinity of such an organism is open to debate and may be close to mollusks or proboscis-bearing “worms”.

The Cambrian traces here described could belong to the lowermost Cambrian ichnozone (that is, the Treptichnus pedum + Monomorphichnus lineatus Zone in Spain, and the global and probably equivalent Treptichnus pedum Zone, see Narbonne et al., 1987; Gámez-Vintaned & Liñán, 2007). However, the time spans of these traces extend into younger zones, and so they could as well represent the second Cambrian ichnozone (the global Rusophycus avalonensis Zone and the very similar Spanish Rusophycus avalonensis + Rusophycus bonnarensis Zone; Narbonne et al., 1987; Gámez-Vintaned and Liñán, 2007). This is the ichnozone information provided by the Cambrian traces:

Regarding the Ediacaran structures here described, possible fossils comparable to the disc and the “stalked diamond” are not known for the late Ediacaran of Spain (Pieren Pidal, 2000; Jensen et al., 2007; however, see a possible occurrence of Nimbia in Liñán et al., 2009). These two structures remain as isolated findings in spite of extensive search, but rarity is not necessarily an argument against biogeneicity. The reason is that well exposed bedding planes are extremely rare in the Lower Alcudian, and deep marine complex organisms probably were scarce in the latest Ediacaran after the extinction of most rangeomorphs (Xiao & Laflamme, 2009), and so very few fossil findings would not be surprising in this unit. Biogeneicity seems more likely for the “stalked diamond” due to the very unusual arrangement of a ringed tubular structure with a quite symmetric sheet that displays ribs and hints of taphonomic folding and internal structure. This arrangement of different objects is very difficult to explain from an abiotic point of view. It may be a (degraded?) organism of uncertain taxonomic affinity, or an abiotic structure of unknown origin.

The occurrence of Torrowangea aff. rosei may represent the first finding of a trace fossil in the Lower Alcudian of the valle de Alcudia, if this kind of filamentous reliefs represented traces, as the possible meandering suggests. However, the options of microbial strings and tubular organisms cannot be definitely ruled out with this material. If they were trace fossils of bilateral organisms, they would imply that at least some parts of the Lower Alcudian are younger than about 555 Ma, since this is the age of the oldest unambiguous bilateral traces (Jensen, 2003; Fedonkin et al., 2007). This age would be in agreement with the radiometric dating of this subunit (< 580–560 Ma according to Talavera et al., 2015).

A trace fossil origin for these filaments is compatible with their occurrence as positive epireliefs, since Torrowangea could represent a string of faecal pellets (Liñán & Palacios, 1987). The deep marine environment is also coherent with the trace fossil hypothesis, because similarly simple traces have been described from turbidite outcrops that document the Ediacaran—Cambrian transition, for example in the Puncoviscana Formation (Buatois et al., 2014). More material is required to reach any solid conclusion about the origin of all these structures.