Introduction
In the geologic record, omission surfaces are discontinuities within sedimentary sequences that represent major changes in paleoenvironmental conditions at the seafloor due to a pause in sedimentation that may or may not reflect a fluctuation in sea level and/or an erosional episode. Paleoecologic studies of omission surfaces are interesting and important because they highlight the ecologic and ethologic responses of members of the benthic community to dynamic seafloor conditions. Especially important in this regard is a detailed analysis of the ichnocoenoses (trace fossil communities) that inhabited the substrate before, during, and after the hiatus that is marked by the omission surface (Goldring and Kaźmierczak, Reference Goldring and Kaźmierczak1974; Bromley, Reference Bromley1975; Pemberton and Frey, Reference Pemberton and Frey1985; Lewis and Ekdale, Reference Lewis and Ekdale1992; Wilson and Taylor, Reference Wilson and Taylor2001; Taylor and Wilson, Reference Taylor and Wilson2003). This paper reports ichnologic and paleobiologic aspects of a well-preserved omission surface in the Miocene of southeastern Spain.
The first report of the unusual trace fossil locality near Alcoi in southeastern Spain was written by Saporta (Reference Saporta1887), who studied fossil material that had been sent to his institution, the Muséum National d'Histoire Naturelle in Paris, by Juan Vilanova y Piera, one of the most remarkable nineteenth-century Spanish naturalists. Saporta described two different types of fossils from the Alcoi site, which he interpreted as sponges. One of them, Taonurus ultimus, was already known from other localities, while the second, Spongeliomorpha iberica, was first described as a new genus and species. In the following years, little attention was paid to this fossil locality, except for Boscà (Reference Boscà1917), who provided a detailed description of the site and its location west of Alcoi near a farmhouse known as Can Pardinetes. Later, the actual nature of Spongeliomorpha iberica was made clear by several authors (Kennedy, Reference Kennedy1967; Bromley and Frey, Reference Bromley and Frey1974; Häntzschel, Reference Häntzschel1975), who interpreted it as a trace fossil created by crustaceans. Bromley and Frey (Reference Bromley and Frey1974) considered Spongeliomorpha iberica as a nomen dubium because of the inadequate original description and the lack of type specimens. Fortunately, Vilanova y Piera also had sent some material to the Museu Geològic del Seminari Conciliar in Barcelona, where it was examined by Calzada (Reference Calzada1981), who designated a new neotype specimen and provided a new description in accordance with the trace fossil nature of the taxon. Calzada (Reference Calzada1981) could not find the type locality, but he described a new Miocene site in Murcia, also in SE Spain (see also Gibert and Ekdale, Reference Gibert and Ekdale2010). Thanks to the help of a local amateur group, the Asociación Paleontológica Alcoyana ISURUS, the authors of the current paper were able to find the Boscà (Reference Boscà1917) site near Can Pardinetes.
The following trace fossils are abundant and well preserved at the Miocene locality at Can Pardinetes: Spongeliomorpha iberica, Glossifungites saxicava (Taonurus ultimus of Saporta, Reference Saporta1887 and Rhizocorallium jenense of Gibert, Reference Gibert2011), and Gastrochaenolites ornatus Kelly and Bromley, Reference Kelly and Bromley1984. These trace fossils are restricted to a single stratigraphic horizon and constitute a typical firmground ichnoassemblage of the Glossifungites ichnofacies as described by Seilacher (Reference Seilacher1967; see also Pemberton and Frey, Reference Pemberton and Frey1985). The stratigraphic significances of Glossifungites surfaces have been widely explored (and exploited) in a plethora of papers (e.g., Pemberton et al., Reference Pemberton, MacEachern and Saunders2004; Buatois and Mángano, Reference Buatois and Mángano2011), but other aspects concerning the paleobiology of firmground burrowers have not been addressed widely. Firmground trace fossil ichnocoenoses usually are relatively short-lived in comparison with the more complex softground ichnocoenoses. Firmground trace fossils often preserve bioglyphs that offer valuable potential for interpreting tracemaker identity and behavior. In addition, burrowing in firmgrounds can modify the seafloor, creating additional niche space by essentially turning a two-dimensional surface into a three-dimensional gallery, such as is the case here, where open burrows were occupied by Actinobalanus dolosus (Darwin, Reference Darwin1854), a balanomorph barnacle (Crustacea: Cirripedia). The objectives of this contribution are (1) to provide a description of the study site and the trace fossils, (2) to offer new ichnotaxonomic perspectives of some of the trace fossils, (3) to analyze the paleobiological significance of the traces and associated body fossils, and (4) to provide a genetic interpretation of the firmground paleocommunity based on ichnologic, paleobiologic, and sedimentologic data.
Repositories and institutional abbreviations
The material described here is stored in four institutions: the Museo Nacional de Ciencias Naturales (MNCN) in Madrid (Spain), the Museu Geològic del Seminari Conciliar de Barcelona (MGSCB) in Spain, the Ichnological Collection of the Faculty of Geology of the University of Barcelona (UB-IC) in Spain, and the Museo Paleontológico y de las Ciencias ISURUS (MPCI) in Alcoi (Spain). All the specimens were recollected at the type locality of S. iberica, 9 km west of Alcoi.
Geological and geographical setting
The study locality is at Can Pardinetes within the Parc Natural del Carrascal de la Font Roja, 9 km west of Alcoi (Alicante province, SE Spain). The Miocene units bearing the trace fossils were deposited in one of the so-called Eastern Prebetic Basins (Sanz de Galdeano and Vera, Reference Sanz de Galdeano and Vera1992) in the External Zones of the Betic Cordillera (Fig. 1), which, together with the Moroccan Rif, constitutes the westernmost part of the Alpine Mediterranean Chains. Folding and diapirism controlled these small Neogene basins (Cater, Reference Cater1987; Sanz de Galdeano and Vera, Reference Sanz de Galdeano and Vera1992; Ruig, Reference Ruig1992; Martínez del Olmo, Reference Martínez del Olmo1999), but their precise geometry is not well known due to the structural complexity of the region.
Figure 1 Geographic and geologic setting. (1) Geologic map of eastern Spain, showing the location of Alcoi. (2) Simplified geographical map of the study area (white star) and the location of the studied outcrop (Spongeliomorpha type locality) in the Iberian Peninsula.
Cater (Reference Cater1987) recognized three post-Burdigalian depocenters in the NE sector of the Prebetic Zone. One of them corresponds to the Alcoy-Concentaina area and constitutes a NE-SW elongated depression filled by Neogene sediments and bounded by two anticlines, which affect the Mesozoic and Paleogene units that form the Serra de Mariola and Serra de Menejador. The Miocene succession in this sector was described by Cater (Reference Cater1987). Most of it consists of about 1,000 m of pelagic mudstones known as the Masia del Garrofero Formation (Cater, Reference Cater1987), which correspond with the informally named ‘Tap’ marls that occur extensively in the Eastern Betics. The age of this unit is late Langhian to early Tortonian (Cater, Reference Cater1987; Ott d’Estevou et al., Reference Ott d’Estevou, Montenat, Ladure and Pierson D'autrey1988; Geel et al., Reference Geel, Roep, Kate and Smit1992).
The studied outcrop is poorly exposed and covered by vegetation. Trace fossils occur in the Tap marls and are found near the base of an approximately 3 m-thick carbonate unit, which is intercalated within the Tap. This carbonate unit is characterized by calcirudites with rounded lithoclasts derived from nearby Paleogene limestones and horizontally laminated calcarenites in the uppermost section. Trace fossils occur in situ as full reliefs filled by the same fine-grained quarzitic calcarenite that is present in the matrix of the overlying carbonate unit, as well as hyporeliefs at the base of this unit. With respect to body fossils, the Tap marls are rich in foraminifera, including Martinottiella communis (d'Orbigny, Reference d'Orbigny1846), Textularia sp., Lenticulina calcar (Linnaeus, Reference Linnaeus1767), Plectofrondicularia sp., Globulina sp., Bulimina sp., Florilus boueanum (d'Orbigny, Reference d'Orbigny1846), Ammonia beccarii (Linnaeus, Reference Linnaeus1758), Globigerina bulloides d'Orbigny, Reference d'Orbigny1826, and Orbulina sp. Macrofossils mainly consist of balanomorph barnacles (Actinobalanus dolosus [Darwin, Reference Darwin1854]) and pholadid bivalves (presumably Barnea sp.) preserved in life position within the burrows.
Systematic paleontology
Glossifungites saxicava (Fig. 2), Spongeliomorpha iberica (Fig. 3), and Gastrochaenolites ornatus (Fig. 4) are the three ichnotaxa identified in the Miocene of Alcoi (SE Spain). This is not an exhaustive systematic revision; however, remarks are provided in order to clarify some ichnotaxonomic problems.
Figure 2 Glossifungites saxicava Łomnicki, Reference Łomnicki1886, Miocene, Alcoi (SE Spain). (1) Very complete specimen (UB-IC 570); a Spongeliomorpha branch is observable in the lower part. (2, 3, 6–9, 12, 13) Different specimens with different sizes showing bioglyphs, i.e., longitudinal ridges along the outside edge and shorter crisscrossed ridges in the depressed central area (UB-IC 571, 572, 575 to 578, 580, MPCI CIAI-01332, and UB-IC 581, respectively). (4) Specimen colonized secondarily by the Spongeliomorpha tracemaker (UB-IC 573). (5) Specimen exhibiting two blind and twin tunnels in the lower part (UB-IC 574). (10, 11) Two sides of the same longitudinally cut specimen; the polished inner side (10) shows no evidence of any kind of spreite or filling structure (UB-IC 579). (14) Transverse polished section, no spreite or filling structure is observed in the central area (UB-IC 582). Scale bar=1 cm.
Figure 3 Spongeliomorpha iberica Saporta, Reference Saporta1887, Miocene, Alcoi (SE Spain). (1, 5) Three specimens showing the diagnostic Y-shaped bioglyphs (UB-IC 638 and 641, respectively). (2) Blind tunnel with a tapering termination (UB-IC 639). (3, 4) Y-shaped branching points (MPCI CIAI-01337 and UB-IC 640, respectively). (6) Spongeliomorpha blind tunnel associated with a Glossifungites burrow (UB-IC 642). Scale bars=1 cm.
Figure 4 Gastrochaenolites ornatus Kelly and Bromley, Reference Kelly and Bromley1984, Miocene, Alcoi (SE Spain). (1) Specimen showing two areas with bioglyphs and a Spongeliomorpha branch (UB-IC 671). (2) Basal view of (1). (3) Specimen exhibiting a clearly spiral ornamentation (MPCI CIAI-01342). (4) Fragmented specimen partially showing the shell of the bivalve tracemaker (UB-IC 672). (5) Specimen with a blind nipple-like protuberance in the apex of the base (UB-IC 673). (6, 7) Two sides of the same longitudinally cut specimen; the polished inner side (7) shows a white and rounded marl clast (UB-IC 674). Scale bars=1 cm.
Ichnogenus Glossifungites Łomnicki, Reference Łomnicki1886
Type species
Glossifungites saxicava Łomnicki, Reference Łomnicki1886 from the Miocene deposits of the Lviv region, Ukraine, by original designation.
Glossifungites saxicava Łomnicki, Reference Łomnicki1886
Holotype
No holotype specimen was designated by Łomnicki.
Emended Diagnosis
Horizontal to oblique, tongue-shaped burrows with a central area more depressed or narrower than the outside edge. The distal part is usually wider than the aperture, which approaches a figure-eight shape. The outer surface is covered by bioglyphs: longitudinal ridges disposed along the outside edge and shorter crisscrossed ridges located in the depressed central area. In both cases, bioglyphs are oriented roughly parallel to the overall tongue-shaped morphology. Burrows are passively filled, i.e., lacking any kind of spreiten (adapted from Łomnicki, Reference Łomnicki1886).
Description
It is possible to observe a gradation of burrow sizes, with lengths ranging from 45 to 162 mm and a maximum width varying between 28 and 89 mm, narrowing toward the aperture (from 51 to 23 mm wide). Aperture height ranges from 8–25 mm at the outside edge to 3–11 mm in the central area. With respect to the bioglyphs, scratches are 1 mm wide and vary from 4 to 31 mm long. No spreiten structures are observable in the infill of these burrows, which is totally homogeneous (Fig. 2.10, 2.14). Occasional ramifications may occur; exceptionally, a small specimen (35 mm maximum width; Fig. 2.5) shows two short, twin, and blind tunnels in its most distal part.
Materials
UB-IC 570 to 637 and MPCI (CIAI-00114, 00891, 01028, and 01332 to 01336). There are also specimens deposited by Prof. J. Vilanova y Piera in the MNCN (MNCNI-18238, 18240, 18241, 18612, and 38909).
Remarks
The original diagnosis of Łomnicki (Reference Łomnicki1886), as translated into English by Uchman et al. (Reference Uchman, Bubniak and Bubniak2000, p. 185, 187–188), described Glossifungites saxicava from the type section in the Ukraine as “…tongue-like or hoof-like borings, which reach down into the rock, and which are filled with coarse sand that is composed of smoothly abraded grains. Better preserved specimens show one of the surfaces (the lower, if horizontal) to be smoother, with cylindrically thickened margins, which are longitudinally striated, parallel to the margin. The second surface (upper is flat, with slightly thickened margins, and with much coarser sandstone grains ….” There is no mention that the sediment filling the trace fossil corresponds to material that was reworked by the burrowing animal, as in a spreite. Łomnicki ascribed the origin of these fossils to ‘lithophagous sponges,’ presumably somewhat akin to modern rock-boring clionaid sponges.
The Alcoi specimens of Glossifungites appear to be essentially identical to those described by Łomnicki from the Ukraine, and the original drawing of the trace fossil by Łomnicki (Reference Łomnicki1886, pl. 3, fig. 64a, b; reproduced in Uchman et al., Reference Uchman, Bubniak and Bubniak2000, fig. 4) looks exactly like Glossifungites specimens from Alcoi. The reported dimensions of the traces from the two sites also are similar. Thus, it is quite reasonable to conclude that the Alcoi specimens fit with the ichnogeneric and ichnospecific diagnoses of Glossifungites saxicava Łomnicki, Reference Łomnicki1886.
Some subsequent workers have regarded Glossifungites saxicava to be a junior synonym of Rhizocorallium jenense Zenker, Reference Zenker1836 (see discussions in Uchman et al., Reference Uchman, Bubniak and Bubniak2000; Knaust, Reference Knaust2013). Rhizocorallium generally is described as a U-shaped spreiten burrow, typically subhorizontal to horizontal, and usually entirely protrusive. In Rhizocorallium, the sediment occupying the interlimb area between the two tubes of the ‘U’ is clearly sediment that was reworked by the burrower as a spreite consisting of the ghosts of successive tunnels that have been shifted distally as the burrower extended the U tube. This sort of spreiten structure is absent from Glossifungites saxicava, so it is inappropriate to synonymize this ichnospecies with R. jenense.
In the Miocene of Alcoi, specimens of G. saxicava are preserved often as full reliefs and hyporeliefs. In the latter case, only the aperture area is preserved (Fig. 5). Burrow casts are filled by fine-grained carbonates from the overlying unit. In addition, calcareous basal plates of Actinobalanus dolosus (Cirripedia) encrusting the burrow walls are observable in the outer part (Fig. 6.1–6.2). Thin sections show that these barnacles are located in life position within the burrows and frequently occur only on the upper side (i.e., the roof) of them (Fig. 7). Fifty-five specimens have been analyzed, documenting that 22% exhibit basal plates of A. dolosus, which represent clusters of up to 41 individuals.
Figure 5 Alcoi outcrop. (1) Field view of the studied unit from Alcoi, trace fossils are in situ, included in the white marls (Tap), and immediately below the quarzitic calcarenite. (2) Diagram of the interpreted depositional sequence for the studied trace fossil unit. (3) Hyporeliefs on the base of the upper carbonate unit. (4) Schematic representation of (3). t1=time 1; t2=time 2; Qc=quarzitic calcarenite; OS=omission sirface; TAP=Tap white marls; Gs=Gastrochaenolites; Glo=Glossifungites; Spo=Spongeliomorpha. Scale bars=1 cm.
Figure 6 Cryptic barnacles. (1) Glossifunfites specimen exhibiting several clusters of calcareous basal plates with characteristic radial canals of Actinobalanus dolosus (UB-IC 582). (2) Detail of A. dolosus basal plates in (1). (3) The lower side of (1), without barnacle basal plates. (4) Spongeliomorpha specimen exhibiting a cluster of 15 basal plates of A. dolosus (UB-IC 583). (5) Detail of A. dolosus basal plates in (4). (6) White arrow indicates an A. dolosus basal plate in a Spongeliomorpha specimen (UB-IC 584). (1, 3–6) Scale bars=1 cm.
Figure 7 Test structures of Actinobalanus dolosus in thin section. (1) Section of the calcareous basal plate of the test showing each axial channel of epithelial blades without lateral extensions. In the upper part of the picture a fragment of an opercular plate can be seen. (2–4) Different sections of the lateral compartmental plates. Scale bars=500 µm.
Ichnogenus Spongeliomorpha Saporta, Reference Saporta1887
Type species
Spongeliomorpha iberica Saporta, Reference Saporta1887 from the Miocene locality of Can Pardinetes, 9 km west of Alcoi, southeastern Spain, by original designation.
Spongeliomorpha iberica Saporta, Reference Saporta1887
Neotype
MGSCB specimen number 33010, selected by Calzada (Reference Calzada1981, p. 192), Barcelona, Spain. Figured by the cited author (pl. 2, fig. 2).
Emended Diagnosis
Cylindrical to elliptical, simple to complex, and mainly horizontal burrow systems, showing Y- and/or T-shaped branching, characterized by an ornamented and unlined wall (i.e., with bioglyphs). Bioglyphs consist of sharp Y-shaped ridges, longitudinally disposed with respect to the main axis of the burrow, and sets of shorter, parallel, rectilinear ridges oriented perpendicularly to the axis of the burrow. Short, blind tunnels with a very characteristic tapering termination are very common (after Calzada, Reference Calzada1981; Gibert and Ekdale, Reference Gibert and Ekdale2010; Gibert, Reference Gibert2011).
Description
Cross sections of the burrows are subcircular to ovate (14 to 33 mm diameter). Tunnels are commonly rectilinear (Fig. 3.1, 3.5), and Y-shaped branching points are common (Fig. 3.3, 3.4). Blind tunnels with sharply tapering terminations are very abundant (Fig. 3.2). Bioglyphs consist of a rhomboidal pattern composed of Y-shaped scratches (1 mm thick, 2–4 mm wide, and 6–11 mm long). The sets of shorter, parallel, rectilinear ridges described by Gibert and Ekdale (Reference Gibert and Ekdale2010) in the S. iberica specimens from the Miocene of the Fortune Basin (Murcia, SE Spain) have not been observed in the specimens from Alcoi, probably because these latter bioglyphs have a slightly worse preservation potential.
Materials
UB-IC 638 to 670, and MPCI (CIAI-00115, 00116, 01029, 01030, and 01337 to 01341). There are also specimens (syntypes) deposited by Prof. Vilanova y Piera in the MNCN (MNCNI-06706 and 18239).
Remarks
Ichnogenera Thalassinoides Ehrenberg, Reference Ehrenberg1944 and Ophiomorpha Lundgren, Reference Lundgren1891 have been proposed as junior synonyms of Spongeliomorpha Saporta, Reference Saporta1887 (Fürsich, Reference Fürsich1973, Reference Fürsich1974a; Schlirf, Reference Schlirf2000) since all three ichnogenera are considered as branching burrow systems that are differentiated only by wall features, which are in turn related to different consistencies of the substrate during burrowing. Following Gibert and Ekdale (Reference Gibert and Ekdale2010), we consider these three ichnotaxa as separate and valid ichnogenera. While Ophiomorpha burrows exhibit a characteristic pelleted lining, a specialized burrowing behavior observed in some modern callianassid shrimps (e.g., Weimer and Hoyt, Reference Weimer and Hoyt1964; Gibert et al., Reference Gibert, de, Netto, Tognoli and Grangeiro2006), Spongeliomorpha and Thalassinoides never possess pellets in their walls. In turn, these two ichnogenera, respectively, are differentiated by exhibiting walls that may or may not be covered by bioglyphs. Besides bioglyphs, Gibert and Ekdale (Reference Gibert and Ekdale2010) also proposed two additional ichnotaxobases to differentiate between these two ichnogenera: (1) the abundance of blind tunnels with very sharply tapering terminations observed in Spongeliomorpha, and (2) the absence of an anastomosing-tunnel geometry as seen in Thalassinoides.
In summary and continuing the studies of Calzada (Reference Calzada1981), Gibert (Reference Gibert2011), and Belaústegui et al. (Reference Belaústegui, Ekdale, Domènech and Martinell2014), we emphasize the importance of S. iberica as a valid ichnogenus and ichnospecies and highlight the Miocene of Alcoi (SE Spain) as its type locality.
In the Miocene of Alcoi, S. iberica is preserved as full reliefs and hyporeliefs (Fig. 5). As with G. saxicava, the burrow infill has the same composition as that of the overlying carbonate unit, and barnacles (A. dolosus) encrusting the walls inside some burrows (Fig. 6.4–6.6) are observable. In this case, barnacles appear in 33% of the 33 analyzed specimens, and they are found in clusters of up to 12 individuals.
Ichnogenus Gastrochaenolites Leymerie, Reference Leymerie1842
Type species
Gastrochaenolites lapidicus Kelly and Bromley, Reference Kelly and Bromley1984 from the Basal Spilsby Nodule Bed, Spilsby Sandstone, middle Volgian, Nettleton, Lincolnshire, England, by original designation.
Gastrochaenolites ornatus Kelly and Bromley, Reference Kelly and Bromley1984
Holotype
NHMUK 32602, S. Woodward Collection, Natural History Museum of UK, London, UK. Figured by Kelly and Bromley (Reference Kelly and Bromley1984, fig. 7).
Diagnosis
See Kelly and Bromley (Reference Kelly and Bromley1984).
Description
Studied traces have the typical flask-shaped, or clavate, morphology of the ichnogenus Gastrochaenolites. They are circular to oval in cross section (23 to 29 mm maximum diameter); their morphology is commonly rectilinear although some specimens may be slightly curved (80 to 91 mm long); and they possess circular to oval apertures (16 to 23 mm diameter). The deepest parts (bases) of the boring exhibit circular or spiral bioglyphs, commonly continuous and concentrically serrated or arranged in zig-zag ridges (1 to 2 mm thick). Some specimens exhibit a small nipple-like protuberance in the apex of the base (up to 4 mm long; Fig. 4.5).
Materials
UB-IC 671 to 694, and MPCI (CIAI-00890, and 01342 to 01346). There are also specimens deposited by Prof. Vilanova y Piera in the MNCN (MNCNI-13543, 18242 and 18243).
Remarks
Ichnogenus Gastrochaenolites is defined primarily as clavate (club-shaped) borings made in hard substrates, which are commonly attributed to the boring activity of bivalves (Bromley, Reference Bromley2004). However, it is known that some boring bivalves also are able to colonize both soft (unlithified) and hard (cemented) substrates (Savazzi, Reference Savazzi1999; Belaústegui et al., Reference Belaústegui, Gibert, de, Nebelsick, Domènech and Martinell2013). In the Miocene of Alcoi, pholadid bivalves were the producers of these structures. Among the Pholadidae, there are species that may be both rock borers and firm (compacted but unlithified) mud burrowers, and they use the same mechanical technique to bore as to burrow into the substrate, producing almost the same traces in both cases (Savazzi, Reference Savazzi1999). Although substrate character may be considered as a high-ranking ichnotaxobase, Carmona et al. (Reference Carmona, Mángano, Buatois and Ponce2007) point out that in these cases (i.e., organisms able to produce identical structures by both boring and burrowing), the erection of new ichnotaxa based solely on substrate character could be misleading, so they regarded the ichnogenus Gastrochaenolites as available for bivalve burrows in firm but unlithified substrates as well as for borings in lithified substrates. Their approach is followed in this paper.
Specimens showing the nipple-like protuberance in the deepest part of the boring are similar to the amphora-shaped ichnospecies Amphorichnus papillatus Männil, Reference Männil1966. Regardless of its unclear taxonomic validity (Frey and Howard, Reference Frey and Howard1981), A. papillatus has been clearly identified as a burrow, not a boring. In any case, due to the great morphological affinities with the ichnogenus Gastrochaenolites and the lack of bioglyphs in Amphorichnus, we prefer the ichnospecies G. ornatus as the name of these bioturbation structures. These terminological considerations do not affect the paleoecologic and paleoethologic interpretations that follow.
G. ornatus is the most abundant of the three ichnospecies identified in the Miocene of Alcoi. This ichnotaxon occurs mainly as full reliefs; hyporeliefs also may be present, but commonly only the base (distal part) is preserved (Fig. 5). In one case, the shell of the bivalve tracemaker is partially exposed in the base of the boring (Fig. 4.4).
Discussion
Spreite and nonspreite burrows
An ethologically and ichnotaxonomically important feature of some trace fossils is a spreite, which is widely regarded by ichnologists as a site of reworked sediment consisting of closely spaced tunnel walls that were packed together as the burrow tunnel was repeatedly shifted laterally (broadside) through the sediment (Seilacher, Reference Seilacher1964, Reference Seilacher2007; Frey, Reference Frey1973; Häntzschel, Reference Häntzschel1975; Ekdale et al., Reference Ekdale, Bromley and Pemberton1984; Bromley, Reference Bromley1996; Buatois and Mángano, Reference Buatois and Mángano2011; Rindsberg, Reference Rindsberg2012; Uchman and Wetzel, Reference Uchman and Wetzel2012). The behavioral significance of a spreite is that it represents the continuous activity of a burrower that processes sediment by successively moving its burrow short distances sideways, creating a series of previously occupied, sediment-filled, ghost tunnels compacted side by side in either a protrusive or retrusive direction (i.e., away from or toward the apertures, respectively). The spreite thus represents sediment that was actively reworked by the animal and is not just a passive fill of an open burrow. Rhizocorallium, a well-known ichnogenus in the geologic record, is a predominantly horizontal U-shaped burrow with a distinct spreite located between the two parallel limbs of the U-shaped tunnel (Häntzschel, Reference Häntzschel1975; Schlirf, Reference Schlirf2011; Knaust, Reference Knaust2013). Although the U-shaped burrows at Alcoi were assigned to Rhizocorallium Zenker, Reference Zenker1836 by Gibert (Reference Gibert2011), it is clear that they do not contain a spreite. Instead, the Alcoi burrows are tongue-shaped slots that were passively filled by sediment that exhibits no evidence of biogenic reworking. The originally open nature of these tongue-shaped burrows is clearly evidenced by three crucial observations: (1) the clear-cut bioglyphs on the interior burrow margins, (2) the passively compacted sediment in the central part of the ‘tongue,’ and (3) the occurrence of barnacles attached to the interior of margins of the burrow. Łomnicki (Reference Łomnicki1886) appropriately assigned these burrows to Glossifungites saxicava, which is not a spreiten burrow. The ichnogenus Glossifungites therefore is not the same as Rhizocorallium. Despite some published opinions to the contrary (Fürsich, Reference Fürsich1974b; Knaust, Reference Knaust2013), Glossifungites is not a synonym of Rhizocorallium; it stands apart as a valid ichnogenus, to which the tongue-shaped burrows at Alcoi are appropriately assigned.
Tracemakers
Spongeliomorpha burrow systems, characterized by the presence of bioglyphs, commonly are attributed to the burrowing activity of decapod crustaceans (e.g., Fürsich, Reference Fürsich1973; Seilacher, Reference Seilacher2007). Burrowing activity of modern crustaceans is well known, especially by decapods such as thalassinidean and alpheid shrimps, astacideans (lobsters and crayfish) and brachyuran crabs, or by other kinds of crustaceans such as stomatopods (mantis shrimps) (Atkinson and Taylor, Reference Atkinson and Taylor1988; Atkinson et al., Reference Atkinson, Froglia, Arneri and Antolini1997). Gibert and Ekdale (Reference Gibert and Ekdale2010), based on a detailed study of the Miocene S. iberica from Muela de Maraón (Murcia, SE Spain), proposed thalassinidean and alpheid shrimps as the possible tracemakers of S. iberica since these kinds of crustaceans generate today geometrically complex burrow systems with turnarounds and chambers very similar to those of this ichnotaxon.
Seilacher (Reference Seilacher2007), Ekdale and Gibert (Reference Ekdale and Gibert2010), and Gibert and Ekdale (Reference Gibert and Ekdale2010) interpreted three burrowing behaviors from the three different bioglyphs present in Spongeliomorpha: (1) Y-shaped ridges were interpreted as the plucking action of the chelipeds and mainly related to digging ahead, (2) long longitudinal ridges were attributed to pereiopods widening the tunnel, and (3) shorter, transverse ridges were related to the ventilation tasks carried out by uropods or to the gnawing of maxillipeds or other mouth parts to graze on bacterial film located in the tunnel wall. In the S. iberica of Alcoi, only those interpreted as produced by chelipeds and very rarely by pereiopods have been identified.
Glossifungites was first interpreted by Łomnicki (Reference Łomnicki1886) as borings produced by lithophagous sponges and subsequently filled by coarse sands. However, despite the fact that G. saxicava sometimes has been referred to as R. jenense, this ichnotaxon commonly has been attributed to the burrowing activity of crustaceans (see Knaust, Reference Knaust2013 and references therein). In addition, Knaust (Reference Knaust2013) proposed chewing annelids as possible producers. However, as with Spongeliomorpha, the presence of different bioglyphs would support the proposal according to which an animal (very likely a crustacean) with rigid appendages capable of scratching the firm mud in different ways was the tracemaker. By contrast, it would be expected that the bioglyphs produced by annelid jaws (see Knaust, Reference Knaust2013) would be more repetitive (with a unique morphology) and smaller than those observed in the G. saxicava of Alcoi. The modern amphipod Corophium excavates U-shaped burrows and frequently has been suggested as a possible analogue of the Rhizocorallium tracemaker (Gibert, Reference Gibert2011). However, the tongue-shaped morphology of Glossifungites has not been observed in modern amphipod burrows or in other kinds of crustacean burrows. Nevertheless, this tongue-shaped architecture also might correspond to the slightly different burrowing behavior of amphipods adapted to colonizing firm substrates.
In the Miocene outcrop at Can Pardinetes, it is possible to observe a continuous gradation of sizes in Glossifungites specimens (Fig. 2), all of them sharing identical morphological features. This would suggest the presence of different ontogenetic stages and hence the coexistence of juvenile and adult individuals of the same species in the same sedimentary horizon.
As stated, Gastrochaenolites generally is defined as clavate (club-shaped) borings produced by bivalves in hard substrates (e.g., Bromley, Reference Bromley2004). However, it is known that traces with identical morphology can be produced in softer materials and preserved in the lithified record (Savazzi, Reference Savazzi1999; Belaústegui et al., Reference Belaústegui, Gibert, de, Nebelsick, Domènech and Martinell2013). An identical morphology enables the use of the same ichnotaxonomic nomenclature, despite the different original substrate. In the Miocene outcrop of Can Pardinetes, the origin of these structures is attributed to the activity of a bivalve, very likely belonging to the pholadid genus Barnea. In one Gastrochaenolites specimen, the shell of the tracemaker is partially exposed in the base of the boring (Fig. 4.4). A finely cancellate sculpture is visible, but there are no more traits to permit a more accurate identification. Some bivalve internal molds also have been collected, corresponding to sediment infills of closed shells. Presumably they also represent shells of the borers, but the absence of identifying features of a pholadid has impeded a more accurate identification.
Cryptobionts
Morton and Challis (Reference Morton and Challis1969) utilized the term ‘cryptobion’ to designate communities enclosed within living or dead coral and revealed only by cracking the substratum, thus distinguishing between pioneering species that bore or excavate galleries and secondary species that nestle or take refuge in those galleries abandoned by the pioneers. In the same sense as these authors, Shirayama and Horikoshi (Reference Shirayama and Horikoshi1982) used the term ‘cryptobionts’ to divide these secondary species in two groups: mobile species that utilize natural interstices and or biogenic crevices or holes in the coral skeleton, and immobile species that are embedded passively in the coral skeleton by coral growth.
In studying marine boring microorganisms, Golubic et al. (Reference Golubic, Perkins and Lukas1975) differentiated among ‘epiliths’ (living on the surface of the substrate), ‘chasmoliths’ (adhering to the surfaces of fissures and cavities within the substrate), and ‘endoliths’ (penetrating into the substrate). Subsequently, Golubic et al. (Reference Golubic, Friedmann and Schneider1981) redescribed ‘endoliths’ as colonizers of the interior of rocks, and he divided them into ‘chasmoendoliths’ (colonizers of clefts in the rock), ‘cryptoendoliths’ (hidden colonizers of structural cavities within porous rocks, including spaces produced and vacated by euendoliths), and ‘euendoliths’ (rock-boring organisms).
Finally, Kobluk (Reference Kobluk1988b) proposed a uniform terminology to designate cryptic marine organisms inhabiting reefs and other settings. Among other terms, Kobluk (Reference Kobluk1988b) defined ‘cryptobiont’ as “an individual organism or species within the cryptos, or living in a crypt”; ‘crypt’ as “general term to refer to the habitats within all kinds of cavities or completely or partially enclosed void spaces”; ‘cryptos’ as “organisms as a group with a hidden mode of life living protected from full or direct exposure to major physical environmental factors”; and ‘cryptic’ as “adjective referring to organisms belonging to the cryptos” (p. 381, table 1). Kobluk’s terminology is followed in this manuscript.
Cryptobionts are common in the fossil record. In the review of pre-Cenozoic cryptobionts associated with reefs and mounds that was offered by Kobluk (Reference Kobluk1988a), the oldest occurrences date back to the Cambrian and Ordovician (see also Hong et al., Reference Hong, Choh and Lee2014). To a lesser extent, even the activity of infaunal soft- and mainly hard-substrate cryptobionts is known in marine paleoenvironments (see Uchman et al., Reference Uchman, Mikuláš and Houša2003 and references therein). Today, the most common organisms with cryptic habits are algae, foraminifera, sponges, corals, polychaetes, mollusks, bryozoans, and brachiopods (Kobluk and Lysenko, Reference Kobluk and Lysenko1993; Lukeneder and Harzhauser, Reference Lukeneder and Harzhauser2003; Taylor and Wilson, Reference Taylor and Wilson2003; Zuschin and Mayrhofer, Reference Zuschin and Mayrhofer2009; Schlagintweit and Bover-Arnal, Reference Schlagintweit and Bover-Arnal2012).
In the Miocene outcrop of Can Pardinetes, it is possible to observe calcareous basal plates of balanomorph barnacles (Crustacea: Cirripedia: Thoracica) in the outer surface of several Glossifungites and Spongeliomorpha specimens (Fig. 6), but never on Gastrochaenolites traces. Thin sections clearly show that the basal plates of these barnacles are attached directly to the walls of the burrows since they are associated with the wall plates, which are preserved in their original life position inside the burrow infills (Fig. 7). Occasionally, opercular plates also have been observed inside the barnacle shells (Fig. 7.1). There is no doubt that these barnacles colonized the interiors of the burrows by attaching to the burrow walls, and therefore they can be regarded as cryptobionts. The total number of clustered barnacles in Glossifungites specimens is variable (from 3 to 41), and the diameter of basal plates varies from 3.7 to 10.3 mm. In Spongeliomorpha, basal plates range from 2.4 to 8.2 mm, with clusters of up to 14 barnacles. Maximum observed height is 2 mm. A. dolosus is a thoracican cirripede species that is present in the European Neogene, including the middle Miocene of France (Davadie, Reference Davadie1963) and Pliocene of the United Kingdom (Darwin, Reference Darwin1854; Menesini, Reference Menesini1964; Newman et al., Reference Newman, Zullo and Withers1969). This paper reports its first record of occurrence in cryptic environments.
Although the presence of cryptobionts inhabiting the inside of hardgound burrow systems (mainly Thalassinoides-like) is well documented (e.g., Fürsich and Palmer, Reference Fürsich and Palmer1975; Voigt, Reference Voigt1987, Reference Voigt1988; Wilson and Taylor, Reference Wilson and Taylor2001), only once before now has the presence of cryptobionts attached to the inner walls of noncemented burrows been described in the fossil record, in particular foraminifera (but not barnacles) within Thalassinoides burrows in the Upper Cretaceous of the Netherlands (Hofker, Reference Hofker1965). Furthermore, the occurrence of cirripeds in other fossil cryptic environments (e.g., caverns, grooves, crevices, or hidden spaces between cobbles or boulders) is not widely documented. Aguirre et al. (Reference Aguirre, Belaústegui, Domènech, Gibert and Martinell2014) described the presence of balanoid (Balanus trigonus Darwin, Reference Darwin1854) and pyrgomatid (Pyrgoma sp.) barnacles inhabiting grooves and crevices of a Pliocene Dendropoma reef from the Baix Ebre Basin (NE Spain). Rosso et al. (Reference Rosso, Sanfilippo, Ruggieri, Maniscalco and Vertino2015) recorded the occurrence of the barnacles Balanus perforatus Bruguière, Reference Bruguière1789 and Verruca spengleri Darwin, Reference Darwin1854 in a Pleistocene submarine cave in Sicily (Italy). Scarce examples of bioerosion structures attributed to the boring activity of acrothoracican barnacles (ichnogenus Rogerella) also have been recorded in fossil cryptic settings (Palmer and Fürsich, Reference Palmer and Fürsich1974). Today, such genera as Amphibalanus, Chthamalus, and Euraphia (Balanomorpha) are common inhabitants in grooves and crevices in different locales around the Mediterranean area (Linkin and Safriel, Reference Linkin and Safriel1971; Crisp et al., Reference Crisp, Southward and Southward1981; Guy-Haim et al., Reference Guy-Haim, Rilov and Achituv2015). The species Balanus glandula Darwin, Reference Darwin1854 has been observed colonizing firmgrounds at Willapa Bay, Washington (Gingras et al., Reference Gingras, Pemberton and Saunders2001).
When studying the effect of light on the growth rate of two Balaninae barnacles, Barnes (Reference Barnes1952 and references therein) concluded that wave action and water flow is much more important on the barnacle growth than the effect of light, which is almost insignificant. This fact could indicate that the barnacles colonized the interior of the burrows while these were still occupied by their producers, likely alpheid or thalassinidean shrimps (at least in the case of Spongeliomorpha burrow systems), since it is known that these kinds of decapod crustaceans, by pleopod beating, pump oxygen-rich surface water through their burrows promoting good irrigation and ventilation (Dworschak, Reference Dworschak1981; Forster and Graf, Reference Forster and Graf1995; Astall et al., Reference Astall, Taylor and Atkinson1997; Stamhuis and Videler, Reference Stamhuis and Videler1998a, Reference Stamhuis and Videlerb, Reference Stamhuis and Videlerc; Atkinson and Taylor, Reference Atkinson and Taylor2005). In addition to this, some thalassinidean shrimps collect and accumulate organic debris material as a food source in the walls or in chambers of their burrows, and these actions may increase the amount of organic content and bacteria in deeper layers of the sediment (Branch and Pringle, Reference Branch and Pringle1987; Dworschak, Reference Dworschak2001; Dworschak et al., Reference Dworschak, Koller and Abed-Navandi2006). Therefore, since the microhabitat generated within these burrows provides: (1) ventilated and oxygenated waters, (2) high bacterial and organic content, and (3) protection against predators, its colonization by cryptic barnacles seems reasonable. In fact, within the burrows of thalassinidean shrimps, the cohabitation (mainly commensalism) of the producers with different organisms such as other decapods (e.g., alpheid shrimps), amphipods, bivalves, polychaetes, or even gobiid fish have been described both in the fossil record and today (Dworschak et al., Reference Dworschak, Anker and Abed-Navandi2000; Anker et al., Reference Anker, Jeng and Chan2001; Kneer et al., Reference Kneer, Asmus and Vonk2008; Liu et al., Reference Liu, Kneer, Asmus and Ahnelt2008; Nara et al., Reference Nara, Akiyama and Itani2008).
Depositional sequence
The Can Pardinetes outcrop exemplifies several successive colonization stages of an exposed sea floor during the middle to upper Miocene (Langhian-Tortonian), which are represented by a succession within the same ichnocoenose (Fig. 5). The sequence stratigraphic implications suggest a regressive trend with submarine exposure during a sea level lowstand.
First, marly lime sediment was deposited under shallow subtidal marine conditions, and the seafloor was occupied by a benthic fauna, which presumably contributed a preomission suite of trace fossils, which could not be observed in the field during this study due to poor preservation. Then the water shallowed, and sedimentation apparently ceased (with or without erosional events). The seafloor became compacted and firm, creating a firmground omission surface that was occupied by burrowers capable of penetrating the stiff (but uncemented) sediment.
The omission suite of trace fossils includes tongue-shaped burrows (Glossifungites saxicava), branching burrow systems (Spongeliomorpha iberica), and club-shaped dwellings (Gastrochaenolites ornatus) that were produced contemporaneously by probably two taxa of crustaceans and one of pholadid bivalves. The three types of trace fossils clearly are connected in some specimens, testifying to their contemporaneity and the high bioturbation intensity at that time. In some cases it is possible to observe Glossifungites specimens that were colonized by the tracemaker of Spongeliomorpha, which partially adapted its burrow to the previous one (Fig. 2.4) or just cut across it (Figs. 2.1, 3.6). The latter case also occurred between Gastrochaenolites and Spongeliomoprha in some specimens (Fig. 4.1). This ichnocoenose was repeated at least two times due to successive erosive events (Fig. 5.2).
These trace fossils constitute a typical firmground ichnoassemblage of the Glossifungites ichnofacies as described by Seilacher (Reference Seilacher1967; Pemberton and Frey, Reference Pemberton and Frey1985). Glossifungites ichnofacies dominated by the ichnospecies G. ornatus, as occur in the Miocene of Alcoi, also have been described in the Miocene of Patagonia, Argentina (Carmona et al., Reference Carmona, Mángano, Buatois and Ponce2007). In firmgrounds at Willapa Bay (Washington, USA), Gingras et al. (Reference Gingras, Pemberton and Saunders2001) described modern Glossigungites ichnofacies trace assemblages that are dominated by the burrows of bivalves (genus Petricola) and decapod crustaceans (genus Upogebia), which generate Gastrochaenolites- and Thalashinoides-like structures, which share a lot of similarities with the Miocene case studied here.
Cryptic barnacles could have colonized the inner walls of Spongeliomorpha and Glossifungites burrows while these were still occupied by their producers, thus benefiting from the benign and protected microhabitats generated by some kinds of crustaceans within their burrows (see section ‘Cryptobionts’). Conversely, these burrows could have been colonized after being abandoned and before being filled by sediment. In either case, barnacles are mostly attached to the roofs of the more horizontally disposed burrows. This preference for the roof of the burrow over the bottom could be explained by: (1) the presence of loose sediment deposited passively in the bottom, which could prevent their attachment and their breathing; (2) avoiding trampling by the host; or (3) a greater degree of compaction of the roof than the bottom. In fact, the preservation of bioglyphs also is generally better on the roof than on the bottom.
Some specimens, mainly those belonging to Glossifungites and Spongeliomorpha, exhibit irregular cavities (Figs. 2.2, 3.2). Thin section evidence rules them out as bioerosion structures, since any clasts in their outer perimeter show no evidence of having been cut or dissolved. Irregular rounded marl clasts have been observed within many specimens (e.g., Fig. 4.7). Since they have the same lithology as those of the surrounding white marls (Tap), they can be considered as mud clasts. The origin of these mud clasts is due to erosional and transport processes and may be related to shrinkage and cracking of mud layers as well as to erosion and reworking by high-energy events (Knight, Reference Knight2005; Ghandour et al., Reference Ghandour, Al-Washmi and Haredy2013 and references therein). The marly firm substrate that constituted the seafloor in Alcoi was exposed by at least one erosional event, as evidenced by the record of two omission suites of firmground burrows. During the erosive process, the marly firm substrate may have become fragmented and some of these fragments were redeposited as marl clasts. Some marl clasts fell into the open trace fossils, were deposited by gravity on their bottoms, and finally were buried by the passive infilling of these trace fossils by a fine-grained quarzitic calcareous sand. After diagenesis, burrow casts were totally hardened and cemented as calcarenite. Once this bioturbated unit was exposed to weathering, the differential erosion of looser marly sediments, including the marl clasts located in the outer part of the trace fossils, promoted the formation of these irregular cavities in the burrow casts.
Conclusions
The type locality of Spongeliomorpha iberica, located in the Miocene of Can Pardinetes (Alcoi, SE Spain), was revisited and studied in detail. Besides S. iberica, the ichnospecies Glossifungites saxicava and Gastrochaenolites ornatus were identified.
Detailed ichnologic study demonstrates that the ichnospecies G. saxicava Łomnicki, Reference Łomnicki1886 is a valid ichnotaxon, and some of the trace fossils found in the Miocene site at Can Pardinetes are attributed to this ichnospecies. The tongue-shaped, nonspreite burrows belonging to the ichnogenus Glossifungites are fundamentally different from the U-shaped, spreite burrows of the ichnogenus Rhizocorallium, so it is clear that the two ichnogenera are not synonymous.
This paper contributes the first report of cryptic barnacles (thoracican cirripedes) colonizing the interior of open burrows that constitute a typical firmground ichnocoenose in the fossil record.
Acknowledgments
This manuscript is largely the result of the research begun in 2008, but unfortunately unfinished, by our respected colleague and dear friend Jordi Maria de Gibert. The authors dedicate this contribution to his memory.
The authors acknowledge the help of the members of the Asociación Paleontológica Alcoyana ISURUS during fieldwork, W.A. Newman (UC San Diego, USA) for his useful comments on fossil barnacles, C.M. Santos (MNCN, Madrid, Spain) for providing all the information related to the specimens housed in the MNCN’s collection, and A. Gallardo (University of Barcelona, Spain) for the preparation of specimens and thin sections.
We appreciate the comments provided by the editor B. Hunda (Cincinnati Museum Center, USA) and the reviews of M. A. Wilson (The College of Wooster, USA) and an anonymous reviewer; their suggestions have improved this paper.
This study is part of the activities of the research project CGL 2010-15047 of the Spanish Science and Innovation Ministry. Z. Belaústegui is supported by a BDR postdoctoral fellowship of the University of Barcelona. A. A. Ekdale’s involvement in the project was supported in part by a grant from the US National Science Foundation.
