In the time of Darwin, one of the biggest challenges for his theory of evolution was the apparently sudden occurrence of the first fossils in the stratigraphic column, at the point currently recognized as the Lower Cambrian (Chapter X in Darwin, 1859). Nowadays, this seeming jump in the fossil record has been tempered by a huge amount of data showing that the early animal evolution was a much more gradual process than previously thought, with simple multicellular organisms preceding the first skeletal animals, and these appearing before the Cambrian rise of shelly fossils (McMenamin, 1987; Knoll, 2003; Fedonkin et al., 2007). However, a drastic evolutionary discontinuity is still widely thought to have occurred exactly at the Ediacaran–Cambrian boundary, where many Ediacaran organisms seem to disappear from the fossil record (Amthor et al., 2003; Fedonkin et al., 2007; Buatois et al., 2014). In this usual view, that crisis preceded the rapid unfolding of the diversity of bilateral animals that characterizes the beginning of the Cambrian (the “Cambrian explosion”, Gould, 1989). According to this scheme, the Ediacaran to Cambrian transition signals a mass extinction followed by an evolutionary jump. This would explain why Ediacara-type fossils are absent from the Cambrian onwards, with a few exceptions (for example, Jensen et al., 1998; Hagadorn et al., 2000), although they could have been declining before the Ediacaran–Cambrian boundary (Darroch et al., 2015).

This perspective about the Ediacaran to Cambrian transition must be questioned on the basis of recent data about the early skeletal organism Cloudina. Its fossils are widespread in late Ediacaran carbonates worldwide (Cortijo et al., 2010), and its disappearance from the fossil record was taken as a major proof of an Ediacaran–Cambrian mass extinction (Amthor et al., 2003; Knoll, 2003). However, new findings from China and Siberia demonstrate that the cloudinids coexisted with shelly animals previously considered to be typical of the lowermost Cambrian, such as anabaritids and protoconodonts (Yang et al., 2016; Zhu et al., 2017). If these transitional faunas represented the general trend, and not isolated cases, then they would bring a second turn of the screw for gradualism in our view of the early animal evolution, implying the Ediacaran–Cambrian extinction event was not as dramatic as previously thought. This is suggested by the present work, where another one of these transitional faunas, consisting of Cloudina and protoconodonts, is reported for the valley of the Navezuelas stream in the Abenójar anticline (Iberian Massif, Spain), together with other fossils that depicts a possible Ediacaran–Cambrian transition and will help to disentangle the regional geology.

Fossil prospection was focused mainly on relatively fresh pieces found loose on exposures. The specimens were photographed using a Canon 7D MkII camera with the macro lenses Canon 100mm f2.8 and Canon 60mm f2.8. Microfossils were extracted from their carbonate matrix using 7-10% acetic acid buffered with calcium acetate during two to five days (Jeppson et al., 1999). They were observed with a stereomicroscope Seben Incognita III and photographed by mounting on it a Canon S95 camera with a digiscoping adapter. The resulting images were digitally stacked to increase depth of field and focus. The photographs for the stackings of Fig. 5G to J were taken using an optical microscope Optika B-120 and a digital camera Bresser MikrOkular Full HD. All the specimens here described and illustrated are deposited in the Museo de Paleontología de Castilla-La Mancha (Cuenca, Spain) under the accession code MPCM-AB.

Since the most detailed geological map available for the northern part of the Abenójar anticline is rather incomplete in the Navezuelas stream area (MAGNA 1:50,000, sheet 783, Abenójar), some basic geological mapping was carried on, in order to clarify or confirm the stratigraphic succession of the fossils. This was a traverse, exposure-based mapping and consisted on location of the individual exposures using GPS and aerial photography (Apple maps), measurement of strike and dip of representative beds with a Brunton-type compass-clinometer, and rock sampling (Barnes and Lisle, 2004; Coe, 2010). The results were mapped on a portion of the corresponding national topographic map (sheet 783-I, Hojalora, 1: 25,000) and complemented with information from the geological map 1: 50,000.

The fossil record of the area displayed in Fig. 2, along the sequence of Fig. 3, can be summarized as follows, from base to top. The points refer to those of Fig. 2.

(I) Lower siltstone: The siltstones and mudstones near the mouth of the Navezuelas stream contain abundant internal and external moulds of annulated tubular fossils (points 3 and 11, Fig. 4A-D). These moulds depict an organism with a tubular funnel-in-funnel structure that is clearer in the specimen of Fig. 4A. These remains are Cloudina cf. hartmannae preserved in siliciclastics. In this basal sequence there are also some fine-grained sandstone beds with sinuous oxidized ribbons (points 1 and 2). The specimen of Fig. 4E shows very fine longitudinal striation (Fig. 4F, point 2), supporting identification as Vendotaenia antiqua (Gnilovskaya, 1971; see Hofmann, 1985; Cohen et al., 2009).

(II) Middle carbonate: Acid digestion of the lower limestones of the Navezuelas stream valley (point 4) yielded phosphatized skeletons of Cloudina sp. (Fig. 5A-D) together with protoconodonts similar to Protohertzina unguliformis (Fig. 5E-J). Other microfossils from these lower limestones are steinkerns and hollow tubes of unidentified tubular organisms, may be Cloudina sp. (Fig. 5L), in phosphate and pyrite. There are also tiny specimens of Vendotaenia antiqua, preserved as flattened filaments of carbonaceous aspect, with fine longitudinal striation and hints of tubular structure (Fig. 5K). The upper carbonate level (point 6) provided fewer and more incomplete microfossils after acid maceration: fragments of hollow tubes, and vendotaenids. Both carbonate levels contain abundant organic debris, including soft sheets resembling algal remains.

In the sandy beds between the carbonates, some soles show small isolated semireliefs similar to Palaeophycus (Fig. 6B, point 5) that may be trace fossils or body fossils of tubular organisms. At about this level, a slab was found with trace fossils (Fig. 6F, point 10) interpreted as burrows of sediment feeders due to their form and compositional difference with the surrounding matrix of quartz-arenite. They are preserved as sinuous hyporeliefs, with ellipsoidal section and irregular constrictions (peristaltic movement?). The burrows are unlined, mainly horizontal, and sometimes show possible branching. All these features suggest the ichnogenus Planolites, but the irregular constrictions remind Torrowangea (Pemberton & Frey, 1982; Webby, 1970, Jensen & Palacios, 2016), so the ichnogeneric assignment of these trace fossils is unclear.

(III) Upper sandstone: At the lowermost part of this unit, immediately above the upper carbonate level, there are thick sandstone beds where a loose slab was found with the complex trace fossil Treptichnus pedum (point 7, Fig. 6A), developed as curved series of almond-shaped knobs. The Treptichnus pedum assemblage of trace fossils identifies the beginning of the Cambrian, but the range of this trace fossil extends about 4 m below the Ediacaran–Cambrian boundary in Newfoundland, and similar treptichnids are also known from the latest Ediacaran (Gehling et al., 2001; Seilacher, 2007).

Just below the guide level of quartz-arenites, there are hard sandstones (point 9) with abundant trace fossils that look very similar to the assemblage described by García-Hidalgo (1993) at the top of the Hondón stream beds. The commonest trace fossils are simple burrows that frequently cross themselves, sometimes to the extent that it is difficult to follow individual traces (Fig. 6C). On some bed soles there are isolated burrows, about 4 mm wide, with very fine transversal striation (Fig. 6D). This is typical of the ventral surface of the ichnogenus Psammichnites, but identification of these specimens as Psammichnites is uncertain, according to Mángano et al. (2002), since the emended diagnosis they proposed depends on the upper surface. Most of the burrows around these particular ones may be Palaeophycus if their lack of striation was an original feature, which is not clear since preservation is not optimal.

Other fossils in the same sandstone exposure are simple, unbranched and thin traces, approximately 1.5 mm wide, that describe very tight curves and loops in the bedding plane (Fig. 6E). They could be ascribed to Gordia, but the possibility exists that they are fossils of filamentous organisms preserved with the aspect of traces (Jensen et al., 2006; Cohen et al., 2009). Possible bilobed furrows occur isolated or in the mass occurrences of intercrossing burrows (point 8), but it can’t be ruled out whether they are just random juxtapositions of unilobed burrows.

Remarks: Cloudina was one of the earliest animals with a biomineralized skeleton, and it occurs almost worldwide in late Ediacaran carbonates (Grant, 1990; Zhuravlev et al., 2012, Cortijo, 2015; Cai et al., 2017). It has been found in several sites of the Central Iberian Zone (Vidal et al., 1994; Cortijo et al., 2015), including the Abenójar anticline (Zhuravlev et al., 2012), but few of these occurrences shows unambiguously the diagnostic characters of this genus (Vidal et al., 1994). The affinity of Cloudina is uncertain, with available evidence suggesting a polypoid cnidaria or annelid-grade bilaterian (Hua et al., 2005; Cortijo, 2015, Cai et al., 2017), but the structure and development of the skeleton support a cnidarian rather than annelid affinity (Vinn & Zaton, 2012). Cloudina was previously thought to have disappeared in a mass extinction at the end of the Proterozoic (Amthor et al., 2003), but this idea would require revision after the finding of Cloudina with typically Cambrian microfossils by Yang et al. (2016) and the present work.

Material: tens of specimens collected in siltstone or shale slabs, found in situ or loose near the outcrops; numerous smaller or less complete specimens were examined in the field. Sampling points 3 and 11 in Fig. 2. Accession codes: MPCM-AB-0009 to 0012, MPCM-AB-0021, MPCM-AB-0022 and MPCM-AB-0024.

Description: Cylindrical to gently conical tubes, preserved as external and/or internal moulds, with the surface divided into segments by grooves or rims. The best preserved specimen (Fig. 4A), found by splitting siltstones in the exposure of point 3, display an external mould with flanges (see the arrows) that suggest a funnel-in-funnel structure, with transversal irregular wrinkles on the outer surface of the funnels (Fig. 4A, below the scale bar; Fig. 4B). Some specimens start as quite horizontal tubes (Fig. 4D) that bend towards the bed top (MPCM-AB-0009). Tube width ranges from about 0.5 to 3 mm, with a length from approximately 3 to 17 mm. No biomineralization is apparent, being the fossils preserved in what appears to be limonite.

Remarks: The funnel-in-funnel structure is diagnostic of the family Cloudinidae. The transversal wrinkles of the funnels and the lack of conical shape in long specimens (Fig. 4B) support its assignation to Cloudina instead of Conotubus (Hahn & Pflug, 1985; Cai et al., 2011, 2017), and the lack of longitudinal crests rules out C. carinata (Cortijo et al., 2010). The specimens resemble C. hartmannae, but preservation is not good enough to identify the species unambiguously since it depends on fine details of the external surface of the funnels (Cai et al., 2017).

Comparable fossils of Cloudina in fine-grained siliciclastics have been reported in the Navalpino anticline (Cloudina carinata), well below a carbonate level with phosphatized Cloudina (Cortijo et al., 2010; 2014; 2015). Fossils resembling moulds of cloudinids in siliciclastics were also found in the area of Garganta Honda (Cáceres province, see fig. 27 in Cortijo, 2015). The unmistakable crests of C. carinata are very clear in the material from the Navalpino anticline, demonstrating that Cloudina fossilized not only in carbonates but also in siliciclastics, preserved as moulds lacking carbonate. This makes fuzzy the difference between Conotubus and Cloudina, because the fossils assigned to Conotubus differ from Cloudina mainly in their restriction to siliciclastics and lack of carbonate (Cai et al., 2011). Although some specimens of Conotubus are more conical, overall the genus seems morphologically almost identical to Cloudina (Wood et al., 2017).

In spite of the many tubular body fossils that have been described from the latest Ediacaran and earliest Cambrian, only Cloudina meets the characteristics of the present specimens. Anabaritids and Cambrotubulus (Matthews and Missarzhevsky, 1975; Yang et al., 2016; Zhu et al., 2017), Gaojiashania (Cai et al., 2013), Multiconotubus (Cai et al., 2017), Platysolenites (McIlroy et al., 2001), Rugatotheca (Yang et al., 2016), Shaanxilites (Meyer et al., 2012), Sinotubulites (Cai et al., 2015) and Wutubus (Chen et al., 2014) do not have a funnel-in-funnel structure. The cloudinid Rajatubus differs in its quite regular collars flaring almost perpendicularly to the tube (see Yang et al., 2016).

Material: several phosphatized specimens obtained from acid maceration of the limestones of point 4 (Fig. 2), including MPCM-AB-0013 (Fig. 5A-D), which comes from a hand sample that provided protoconodonts. Lower carbonates of the Navezuelas stream.

Description: The best preserved specimen consists on two nested phosphatic tubes, circular to elliptical in section, and quite separated. The external tube is a hollow and multilayered structure, 1.5 mm long, showing fine annulations in its inner surface and a smooth but slightly irregular outer surface (Fig. 5A-D). One extreme has a flaring border (Fig. 5A, D) with hints of division into segments (Fig. 5D). The other extreme displays two annulations defining large segments (Fig. 5C, left arrows) and a small flange (Fig. 5C, right arrow). Taken together, these characteristics suggest a tubular skeleton made by stacked funnel-like segments with flaring borders.

Remarks: The morphology of this skeletal fossil is only compatible with Cloudina among the late Ediacaran and early Cambrian fossil record (Yang et al., 2016; Cai et al., 2017). The funnel-in-funnel disposition diagnostic of this genus can originate a tube-in-tube arrangement when the funnel flanges bend forming an outer wall, as describe Hua et al. (2005). The resulting structure, observed in phosphatized remains of Cloudina from the Dengying Formation (China), is illustrated in the figure 1C of Hua et al. (2005), which resemble the present specimen. The lack of regularly spaced collars projecting quite perpendicularly from the tube rules out the early Cambrian cloudinid Rajatubulus (Yang et al., 2016). In the present material, deficient preservation impedes identification at the species level, but they could be C. hartmannae or a very similar species (sensu Cai et al., 2017). Cloudina was capable of asexual reproduction by budding, and in fact the relative width of the outer and inner tubes of the specimen here described reminds a budding Cloudina without one of the tubes (see for example fig. 3C in Cortijo et al., 2015).

Material: four fragments obtained from acid digestion, all from Cloudina-bearing samples from point 4 of Fig. 2. Lower limestone level of the Navezuelas stream valley. Accession codes MPCM-AB-0014 (specimens from the same hand sample that yielded the Cloudina fossil of Fig. 5A-D) and MPCM-AB-0026.

Description: Slender structures of ellipsoidal to flattened section. They are glossy, dark brown, with very chitinous aspect, composed mostly of a fibrous and ductile organic material that reminds chitin, with phosphate. The largest specimen (Fig. 5E) is a scimitar-like spine with pointed tip and flattened section. It is slightly curved and thins towards the inner side. The other specimens are more incomplete, and one of them shows a lateral ridge (Fig. 5F, arrow). All of them are about 0.2 mm wide and up to approximately 2 mm long. They have a narrow central cavity running longitudinally almost up to the tip, filled with pale crystalline matter (Fig. 5G-J). Some similar crystals are present out of the cavity within the main wall (Fig. 5H). The texture of this wall is fibrous in longitudinal direction (Fig. 5I). Well preserved specimens show an external surface with a pattern of fine transversal striae (Fig. 5G, right arrow; Fig. 5J, arrow). Hints of concentric layers are visible in section (Fig. 5G, left arrow). The outer layer is very thin and it lacks visible crystals like those of the wall and medullar cavity.

Remarks: The shape of the specimens, the presence and nature of the internal cavity, the organic-rich wall (Szaniawski, 1983), and the pattern of the external surface suggesting stacked lamellae (Fig. 5G and 5J) are diagnostic elements of the structure and composition of protoconodonts (see fig. 1 in Szaniawski, 1983), specifically type I protoconodonts such as Protohertzina (Müller & Hinz-Schallreuter, 1998). The material here described is similar to Protohertzina, the earliest form of protoconodonts (Kouchinsky et al., 2012). The elements here found lack the characteristic inner ridge that runs longitudinally in that genus, although the specimen in Fig. 5E has a sharp inner flank that may be related to this feature, like the keel of Fig. 5F. The very elongated shape suggests P. unguliformis (Missarzhevsky, 1973; Matthews & Missarzhevsky, 1975; Bengtson, 1982; Bengtson et al., 1990), rather than P. anabarica or other protoconodonts or conodont-like genera from the lowermost Cambrian (see Yi et al., 2004, Bengtson, 1982). Protohertzina occurs almost worldwide in lowermost Cambrian sequences, in the Siberian Platform, Kazakhstan, Mongolia, India, Iran, Australia, and Canada (Yi et al., 2004; Kouchinsky et al., 2012). In western Europe, this genus has been cited from English trilobitic beds (Cobbold, 1921; Hinz, 1987).

This is the first time that protoconodonts are found associated to an Ediacaran–Cambrian transition in Europe. Protoconodonts are typical members of the low diversity assemblage of small skeletal fossils of the earliest Cambrian (Anabarites trisulcatus-Protohertzina anabarica Zone, Nemakit-Daldynian, lower Fortunian, see Yang et al., 2016). The discovery of protoconodonts co-occurring with Cloudina and other cloudinids in the Yangtze Platform led Yang et al. (2016) to distinguish a new subzone of small shelly fossils at the very beginning of the Phanerozoic, the Ganloudina symmetrica-Rugatotheca typica Subzone.

Protoconodonts are the first fossils that can be attributed to predatory animals up to date, since there is compelling evidence that they are grasping spines of chaetognaths (arrow worms). Chaetognatha is an early branching phyla of bilateral animals with chitinous grasping spines, being important predators in marine plankton today (Szaniawski, 2002; Vannier et al., 2007). The commonness of protoconodonts during the beginning of the Phanerozoic suggests a substantial diversity and ecological role of chaetognaths at the beginning of the Cambrian radiation (Szaniawski, 2002), although it is unclear whether they were crown-group or stem-group forms. The large morphological variation recorded within the genus Protohertzina is probably because the spines of the grasping apparatus of an individual chaetognath can have quite different shapes, so P. unguliformis may be synonymous with P. anabarica (McIlroy & Szaniawski, 2000). Clusters of protoconodonts forming apparatuses are known from the lower Cambrian (McIlroy & Szaniawski, 2000) and have been found in the head of exceptionally preserved fossils of arrow worms (Vannier et al., 2007).

During the reviewing of this manuscript it was suggested that the specimens here described may be modern arthropod appendages due to contamination. This possibility is rejected due to the following reasons: First, the procedure of extraction of these microfossils ruled out external contamination: they were obtained after a sequential acid digestion of the corresponding sample, in closed vessel, involving respectively five and four rounds of extraction of the digested sediment, each one followed by washing the container and the etched rock fragments in tap water before adding new buffered acid. As a result, not a single particle attributable to a modern origin was detected in the sediment of the specimens, since they came from perfectly clean rock fragments and vessels. Second, the following characteristics of these microfossils support they are not modern arthropod appendages: (a) the very thick wall, compared to the very thin cuticle of an arthropod leg of this size (personal observations); (b) the crystalline filling of the central cavity and the presence of crystals within the wall; and (c) the transversal grooves suggesting a kind of growth different from that of an arthropod leg, whose cuticle is secreted by the epidermis as layers of chitin that are continuous and so lack the striated pattern observed in the specimens.

The fossil record of the Navezuelas sequence and the surrounding area suggests that it crosses the Ediacaran–Cambrian boundary. The co-occurrence of Cloudina and protoconodonts in the lower carbonate level of the sequence indicates a latest Ediacaran or lowermost Cambrian age. Thus, the lower siltstone (Fig. 3), extending more than 200 m below, could be terminal Ediacaran. At the other extreme of the succession, the upper sandstone contains examples of intense bioturbation (Fig. 6C) and trace fossils supporting a Cambrian age. Treptichnus pedum (Fig. 6A) may be terminal Ediacaran or Cambrian (Seilacher, 2007), but ornamented burrows like the possible Psammichnites of Fig. 6D are unknown from the Ediacaran (Mángano & Buatois, 2016). So this trace fossil assemblage probably indicates a Lower Cambrian age.

In the middle part of the sequence, the presence of Cloudina and protoconodonts implies either the youngest Ediacaran or the oldest Cambrian age, and there are no compelling reasons for choosing one or the other with our current knowledge about the biostratigraphy of this interval worldwide. This ambiguity should be accepted in the present case because of the following considerations. First, cloudinids were previously considered as index fossils of the latest Ediacaran, but this was quickly challenged since the stratigraphic range of Cloudina was found to overlap with that of Anabarites trisulcatus, which appeared at the very beginning of the Cambrian (Vidal et al., 1994). On the other hand, the first protoconodonts belong to the Protohertzina anabarica - P. unguliformis group and appear in the oldest zone of small shelly fossils of the Lower Cambrian (Kouchinsky et al., 2012). Yang et al. (2016) assigned to the lowermost Cambrian the level with Cloudina and Protohertzina unguliformis of the Yangtze Platform, due to the presence of typical Cambrian small shelly fossils. The same reason would imply a lowermost Cambrian age for the level where Protohertzina unguliformis and cloudinids (probably Cloudina) co-occur in the Siberian Platform, but Zhu et al. (2017) considered that its age is not Cambrian on the basis of chemostratigraphy, because these beds are only at the very beginning of the isotopical change that will lead to the basal Cambrian δ¹³C excursion, just above the late Ediacaran δ¹³C positive plateau. Thus these authors refused to assign an Ediacaran or Cambrian age for this level, and concluded that the biological Ediacaran–Cambrian transition was much more gradual than previously thought, lacking any major extinction event. This conclusion is strengthened by the coexistence of Cloudina and protoconodonts here described, since this is the third time that both are found together worldwide.

The Navezuelas sequence is the second possible Ediacaran–Cambrian transition found in the Iberian Massif, being the first one the Río Uso section in the Valdelacasa anticline (Fig. 1). This classical sequence of the Iberian geology is very different from the present one in lithostratigraphy and fossils (Brasier et al., 1979; Palacios, 1989; Vidal et al., 1994; Gámez-Vintaned, 1996; Gámez-Vintaned & Liñán, 2007). However, there are some problems with the widespread idea that it records the Ediacaran to Cambrian transition (see Jensen & Palacios, 2016). The Ediacaran–Cambrian boundary was proposed to be in the lower part of the Pusa Formation, approximately 250 m above the base of this fine-grained siliciclastic unit. The biggest problem is the lack of unambiguous Ediacaran fossils below the supposed boundary down to the base of the underlying unit, the Fuentes Olistostrome (“Nivel de Fuentes”), a megabreccia which lies unconformably above the Domo Extremeño Group. Between this unconformity and the proposed boundary, the fossils found could be either terminal Ediacaran or lowermost Cambrian, but no exclusively Ediacaran forms have been reported. The lower part of the Pusa Formation, between La Nava-Fuentes station and tunnel 9, has Chuaria-like carbonaceous discs, found also around the station among beds with Cambrian trace fossils such as scratch marks (Monomorphichnus, Vidal et al., 1994). The Fuentes Olistostrome contains carbonate pebbles and boulders with Cloudina in the neighboring area of the Arroyo del Cubilar (Cortijo, 2015), but these carbonates may have fallen to the sequence at Cambrian times, and Cloudina itself may be of lowermost Cambrian age, as discussed above. Thus at present there is no positive evidence that the lower Pusa Formation records the Ediacaran to Cambrian boundary. In the Navezuelas section, the Ediacaran age of the lower siltstone is also uncertain but seems more likely, due to the presence of Cloudina in beds that do not represent an olistostromic event of uncertain timing and were deposited hundreds of meters below the level where Cloudina co-occur with protoconodonts, since they indicate an age very close to the Ediacaran to Cambrian boundary. Further work would be desirable to characterize the fossil record and time relationships in this area as well as in the Río Uso section.

A transitional Ediacaran–Cambrian biota is present in the Navezuelas stream area, with the early skeletal organism Cloudina co-occurring with protoconodonts similar to Protohertzina unguliformis. The coexistence of cloudinids and protoconodonts is here documented for the third time worldwide and implies an age very close to the Ediacaran to Cambrian boundary.

This transitional fauna suggests the biological change from the Ediacaran to the Cambrian was more gradual than previously thought, blurring the notion of a dramatic faunal boundary between Ediacaran and Cambrian times.

The Navezuelas sequence possibly records an Ediacaran–Cambrian transition, since its base contains Cloudina hundreds of meters below the level with protoconodonts, thus suggesting an Ediacaran age, whereas its top has Cambrian-type trace fossils, well above the protoconodonts.