Bow-shaped, concentrically laminated polychaete burrows: A Cylindrichnus concentricus ichnofabric from the Miocene of Tarragona, NE Spain

Palaeogeography, Palaeoclimatology, Palaeoecology 381–382 (2013) 119–127

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Bow-shaped, concentrically laminated polychaete burrows: A Cylindrichnus concentricus ichnofabric from the Miocene of Tarragona, NE Spain Zain Belaústegui ⁎, Jordi M. de Gibert 1 Departament d'Estratigrafía, Paleontologia i Geociències Marines, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain

a r t i c l e

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Article history: Received 13 September 2012 Received in revised form 11 April 2013 Accepted 18 April 2013 Available online 28 April 2013 Keywords: Cylindrichnus concentricus Terebellid polychaete Ichnology Bioturbation Miocene

a b s t r a c t Bow-shaped, concentrically laminated burrows from the Miocene of the El Camp de Tarragona Basin in northeastern Spain are described. Although the ichnotaxonomic assignment of these traces is controversial, they are here designated as Cylindrichnus concentricus. A detailed analysis of the overall architecture of the burrows and the constructional features of their characteristic linings allows direct comparison with structures produced today by terebellid polychaetes. Similar trace fossils have been described in the literature from shallow marine environments as old as the Jurassic, which highlights the recurrence of this ichnotaxon and its importance in these settings. C. concentricus occurs in Tarragona as an elite trace fossil in intensely bioturbated ichnofabrics formed in offshore settings with low sedimentation rates, a comparable depositional setting to that of similar Neogene ichnofabrics. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Concentrically laminated burrows are common in shallow and marginal marine depositional paleoenvironments throughout the Phanerozoic. They are mainly represented by the ichnogenera Asterosoma, Cylindrichnus and Rosselia, which have been collectively named as asterosomids by Seilacher (2007). Papers by Goldring (Goldring, 1996; Goldring et al., 2002, 2005) and others (e.g. Aguirre et al., 2010; Gibert et al., 2012) highlighted the importance of bow-shaped asterosomids as ichnofabric producers in Jurassic to Pliocene sedimentary rocks. These burrows were only tentatively designated as Cylindrichnus concentricus by these authors because of their obscure original description in an unpublished Master's thesis (Toots, 1962), which was later reproduced and thereby made available as an ichnotaxon by Howard (1966). Preliminary work by Ekdale and Harding (2011) at the type locality in the Cretaceous of Wyoming confirms that the type material has the bow-shaped geometry seen in other localities. In the present paper, we describe a new occurrence of C. concentricus that was previously reported by Belaústegui and Gibert (2009) and Belaústegui et al. (2011) from the middle Miocene of the El Camp de Tarragona Basin in NE Spain. The new observations allow greater precision of the morphological description of this bow-shaped trace fossil, and contribute to a better understanding of its paleobiology (tracemaker, construction, function). C. concentricus in Tarragona occurs as the dominant form of an intensely burrowed ichnofabric,

⁎ Corresponding author. Tel.: +34 934020177; fax: +34 934021340. E-mail address: [email protected] (Z. Belaústegui). 1 Deceased. 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.04.021

which is interpreted in consideration of ichnological and sedimentological data, and compared to the handful of occurrences of this ichnofabric previously described in the literature. 2. Geologic and stratigraphic setting The studied trace fossils are found in two neighboring sites (Waikiki and Altafulla sections) situated on the coastline of Tarragona in the northeast of the Iberian Peninsula. The Waikiki section is west of the locality of La Móra within the ‘Area of Natural Interest’ of Punta de la Móra, while the Altafulla section is located at the northeastern end of the Altafulla beach (Figs. 1 and 2). Sedimentary rocks at both localities are part of the Miocene fill of the El Camp de Tarragona Basin (Fig. 1A), which is also known as the Valls-Reus depression (Barnoles et al., 1983). This basin constitutes an extensional half-graben that involves Paleozoic to Paleogene rocks of the Catalan Coastal Ranges (Anadón et al., 1979; Cabrera et al., 2004). It is part of the onshore sector of the Valencia Trough, an extensional system developed during the latest Oligocene and Miocene between the eastern part of the Iberian Peninsula and the Balearic promontory to the east (Fontboté et al., 1990; Roca et al., 1999). The Miocene sedimentary record of the extensional basins located in the Catalan Coastal Ranges (mainly the El Camp and the Vallès-Penedès Basins) was characterized by constitution of two main depositional sequences (Cabrera et al., 1991): the Garraf and Tarragona Sequences with Langhian and Serravallian ages (middle Miocene), respectively. In the El Camp Basin, marine units of the Tarragona Sequence are the best exposed. These Serravallian units, consisting of shallow marine sediments deposited in a mixed siliciclastic–carbonate platform, correspond to the post-rift stage (Cabrera et al., 2004). Barnoles et al. (1983)

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Fig. 1. Geographic and geologic setting. A. Synthetic geologic map of the El Camp de Tarragona Basin, and its location in the Iberian Peninsula. B. Simplified geologic map of the studied area with location of the Waikiki and Altafulla outcrops (black stars). C. Stratigraphic sections of Waikiki and Altafulla (black arrows indicate the horizons of the studied ichnofabrics).

named these materials, which include calcisiltites, biocalcarenites, coquinas and sandstones, as the Ardenya Unit. Four main groups of facies have been recently described within the Ardenya Unit (Belaústegui and Gibert, 2011; Belaústegui et al., 2012b): 1) Unsorted conglomerates

related to paleocliffs and overlying the pre-Miocene substrate, through a bioerosion surface (Domènech et al., 2001); 2) inner-platform facies including a variety of bioclastic carbonate deposits dominated by bivalves, echinoids, bryozoans, and coralline algae; 3) outer-platform

Fig. 2. General view of the coastal outcrops of Waikiki and Altafulla. A. Waikiki section. B. Altafulla section. TS, transgressive surface; GCs, glauconitic calcisiltite; Cs, non-glauconitic calcisiltite; BCa, biocalcarenite; RCq, rhodolite coquina.

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Fig. 3. Cylindrichnus concentricus, general architectural features. A, B. Longitudinal sections displaying the overall bow-shaped architecture. C. A possible aperture with funnel-shaped morphology of the lining. D, E. Two transversal sections showing the thick, concentrically laminated, calcisiltitic lining enveloping the dwelling tunnel, which is infilled with the same glauconitic sediment that constitutes the host substrate.

facies, consisting of calcisiltites with or without glaucony; and 4) siliciclastic facies made up of bioturbated, cross-bedded nearshore sandstones (Gibert et al., 1995), or marly units related to open marine conditions. The C. concentricus ichnofabrics described herein from Waikiki and Altafulla occur in glauconitic calcisiltites located at the base of two different shallowing-upward sequences (Belaústegui and Gibert, 2009) (Figs. 1C and 2). These glauconitic units represent the deeper facies of their respective sequences, and in both localities they overlie transgressive surfaces developed on top of fine- to medium-grained biocalcarenites. C. concentricus burrows are intensely concentrated in these glauconitic levels, which contain disperse skeletal fossils, mainly complete valves of the pectinid Amusium cristatum, delicate bryozoan colonies (Metrarabdotos tarraconensis) and isolated spatangoid echinoids. Cetacean bones have also been reported from this unit at Waikiki (Belaústegui et al., 2011, 2012a). In the Waikiki section, these traces as

well as the glaucony gradually disappear upwards (Belaústegui and Gibert, 2009; Belaústegui et al., 2011, 2012a). On the contrary, in the Altafulla section (as originally described by de Porta, 1971) this intensely bioturbated unit is abruptly interrupted at its top by an erosive rhodolite coquina. It is unclear whether both localities represent the same glauconitic unit or similar depositional conditions at two different stratigraphic levels, as the area is strongly tectonized by extensional faulting making correlation difficult (Gaspar-Escribano et al., 2003). 3. Descriptive ichnology 3.1. C. concentricus Bioturbation structures from Waikiki and Altafulla are simple burrows preserved as full reliefs. Their overall architecture consists of a broad U- or bow-shaped geometry contained on a section perpendicular to bedding

Fig. 4. Cylindrichnus concentricus, structure of the lining. A. Transverse–oblique section showing the fusiform parcels of sediment that constitute the lining (interpretative sketch in the lower right corner). B. Thin section displaying the discrete laminae of glaucony grains within the silty lining.

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plane (Fig. 3A–C). The burrows have a central tunnel surrounded by a thick, concentrically laminated lining (Figs. 3D–E and 4). The infill of the tunnel is composed of glauconitic calcisitite same as the surrounding sediment, while the lining is very distinct because of the almost complete lack of glauconitic grains. Thus, the lining is mainly composed of clean, yellow calcisiltite, although some discrete, discontinuous, and very thin deposits of glaucony, and occasionally also of polychaete tube fragments (Ditrupa sp.), are observed between the different silt laminae. In some cases, owing to the color contrast provided by the dark green glaucony, it is possible to discern that the lining is composed of discrete fusiform parcels of non-glauconitic silty sediment (Fig. 4A). The traces were studied in natural exposures, which in both outcrops are mostly perpendicular to bedding. Occasionally loose specimens have been recovered. In order to carry out a detailed analysis of burrow geometry and morphology, exposed surfaces were ground with sandpaper and samples were taken for thin-sectioning. Outcrops provided a large number of oblique, longitudinal and transversal sections of the burrows, including internal morphological details of the linings. Despite this, no complete burrow was observed. Elongate (near-longitudinal) sections are mostly horizontal or gently inclined although dips of up to 45° may be reached. The burrows present subcircular cross-sections. In most cases the main tunnel occupies a central position, although eccentric positions have also been observed. The concentric layers of the lining are not totally continuous; instead some of them are truncated or broken. Average diameter of the main tunnel (internal diameter) ranges from 3.0 to 7.5 mm, while those of the complete burrow (external diameter) range from 14.5 to 16.0 mm. The thickness of the lining ranges from 2.5 to 8.5 mm, and the internal layers vary from 0.8 to 2.0 mm. The inner boundary of the lining is sharper than the outer one. The different sections visible in both outcrops allow the interpretation of the overall morphology of these traces as simple U- or bowshaped burrows, much wider than deep. Maximum estimated length may range from 600 to 700 mm, with depth from 200 to 300 mm. Only occasionally has an end interpreted as one of the two possible entrances been observed, and this presents an inclined disposition and an enlarged, funnel-shaped morphology (Fig. 3C), very similar to the funnel-shaped morphologies described for the ichnospecies Rosselia socialis by Nara (1995, 1997). This is in contrast with the constricted apertures described for C. concentricus in the Lower Cretaceous of Oxfordshire by Goldring (1996). 3.2. Ichnofabric C. concentricus occurs in glauconitic calcisiltites, where it is the dominant trace fossil. The ichnofabric is characterized by intense bioturbation (ichnofabric index, ii, 4/5 sensu Droser and Bottjer, 1986) with no preservation of primary sedimentary structures. C. concentricus can be considered as the elite trace fossil (sensu Bromley, 1990) of this ichnofabric, not only because of its abundance but also because it is the most prominent ichnofossil thanks to its characteristically nonglauconitic thick lining, which enhances its visibility (Fig. 5A–B and J). Thus, the ichnofabric is dominated by cross-sections of these concentrically laminated burrows, transverse cross-sections being dominant in vertical exposures as a consequence of its wide, bow-shaped form with a long horizontal or subhorizontal segment. Other exposures parallel to bedding reveal more abundant longitudinal sections. In both studied localities, especially at Waikiki, the amount of glaucony as well as the abundance of C. concentricus decrease upwards. The only other ichnotaxon that is easily recognizable is Teichichnus rectus (Fig. 5C–D). This corresponds to horizontal burrows (Ø = 17.5 mm) bearing retrusive spreiten, which may reach heights of up to 60 mm and do not show any segregation of glaucony grains. T. rectus is less abundant than C. concentricus. The rest of the bioturbation is poorly defined and is recorded as a fully mottled background with only occasional mud-lined burrows (cf. Ophiomorpha, Ø = 10.5 mm; Fig. 5H) and other large cylindrical burrows (cf. Thalassinoides, Ø = 25 mm). The

latter occur in the Waikiki section constituting a poorly defined horizon (Fig. 5E). Idiomorphic Thalassinoides are found at Altafulla just below the rhodolith bed that truncates the C. concentricus ichnofabric, and they are filled by the coarser-grained carbonate sandstone of the overlying bed (Fig. 5I). The linings of C. concentricus are commonly crosscut by cylindrical Planolites-like burrows (Ø = 6.5 mm; Fig. 5F–G and J). They are indistinguishable in the host sediment but display sharp boundaries where penetrating the linings, which emphasizes its cohesiveness. 4. Ichnotaxonomic discussion C. concentricus was proposed by Toots (1962) as a new ichnogenus and ichnospecies in an unpublished Master's thesis at the University of Wyoming based on the material from the Upper Cretaceous Mesaverde Formation of southern Wyoming. Subsequently, Howard (1966) used this ichnotaxon for the first time in a formal publication, thus making the ichnogenus taxonomically available and recorded its presence in similar Upper Cretaceous units in Utah. Howard (1966) did not emend Toots's diagnosis, according to which C. concentricus consisted of nearly horizontal to vertical concentrically laminated burrows with a central core and a circular to oval cross section. In later papers concerning the same units studied by Howard (1966), Howard and Frey (1984) and Frey and Howard (1985) provided the same diagnosis but figured C. concentricus as a vertical burrow and suggested that some specimens might represent the deeper parts of Rosselia. These later papers were seminal contributions in a time of rapid growth of ichnology and this imprecise description of C. concentricus became widespread despite the lack of a definitively described architecture for this ichnospecies. Thus, later authors used ichnogenus Cylindrichnus to designate unbranched, concentrically laminated burrows lacking conical to bulbous morphologies such as those of Rosselia. That led to the definition of various ichnospecies mostly based on the overall morphology of the burrow (e.g. C. elongatus Noda, 1984; C. pustulosus Frey and Bromley, 1985; C. errans D'Alessandro and Bromley, 1986; C. operosus Orlowsky, 1989; C. candelabrus Głuszek, 1998; and C. helix Gibert et al., 2006), but the true architecture of the type ichnospecies remained unknown. Because of that, Goldring (1996) considered C. concentricus to be a nomen dubium. More recently, several authors have addressed this problem although results are still preliminary. Nara and Ekdale (2006) revisited the localities in the Upper Cretaceous of Utah and described C. concentricus as showing a bow-like bend in a section perpendicular to bedding plane. After examination of type material and new specimens at the type locality in southern Wyoming, Ekdale and Harding (2011) noted that these trace fossils had a broad bow- or U-shaped morphology. This bow-shaped architecture had already been noticed by various researchers in concentrically laminated burrows from the Upper Jurassic of Saudi Arabia (Goldring et al., 2005), the Lower Cretaceous of England (Goldring, 1996; Fig. 6B), the middle Cretaceous of Iran (Sharafi et al., 2012), the Miocene of Spain (Gibert et al., 2012; Fig. 6D) and Malta (Goldring et al., 2002), and the Pliocene of Spain (Aguirre et al., 2010; Fig. 6E). Our observations have added concentrically lined burrows with the same overall geometry in the Middle Jurassic Carmel Formation of Utah (referred to as R. socialis by Gibert and Ekdale, 1999; Fig. 6A) and the Upper Cretaceous Horseshoe Canyon Formation of Alberta (Fig. 6C). Older occurrences of bow-shaped concentrically laminated burrows are limited to some Polish Upper Carboniferous material referred to as C. candelabrus by Głuszek (1998), but this shows upward branching indicating that the material is not Cylindrichnus. Liñán et al. (1995) described C. concentricus from the Middle Cambrian of Spain, but since the overall geometry is not described, its assignment to this ichnospecies is doubtful. Thus, bow-shaped, concentrically laminated burrows seem to be common in shallow marine trace fossil assemblages since the Jurassic. Based on the preliminary data provided by Ekdale and

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Fig. 5. Cylindrichnus concentricus ichnofabric. A, B. General view of a vertical section of the ichnofabric at the Waikiki and Altafulla outcrops, respectively. C, D. Teichichnus rectus. E. Thalassinoides-like structures. F, G. Several sections of C. concentricus burrows crosscut by Planolites-like burrows. H. Ophiomorpha-like structures. I. Thalassinoides burrows in the Altafulla section, filled with coarse-grained sediment derived from the upper rhodolite biocalcarenite bed. J. Vertical section of the ichnofabric showing multiple longitudinal sections of C. concentricus crosscut by Planolites-like structures.

Harding (2011), these trace fossils seem to conform with the type material of C. concentricus described by Toots (1962). Seilacher (2007) used the term asterosomids to gather ichnotaxa having concentric lamination, into a group whose main representatives are Asterosoma, Rosselia and Cylindrichnus. The three share similar constructional features of the lining, which may lead to confusion in poorly or partly preserved material. Nevertheless, the three have different

overall configurations. Thus, Asterosoma is characterized by being formed of generally horizontal multiple bulbous structures, while Rosselia is typically bulbous, unbranched and vertical. A more precise characterization of C. concentricus should serve to clarify the ichnotaxonomic significance of the ichnogenus within the group and to reevaluate other ichnospecies of Cylindrichnus, some of which may be reassigned to other ichnogenera such as Rosselia.

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Fig. 6. Bow-shaped, concentrically laminated burrows (Cylindrichnus concentricus) from various ages and localities. A. Middle Jurassic Carmel Formation of Utah (called Rosselia socialis by Gibert and Ekdale, 1999). B. Lower Cretaceous Lower Greensand of England (Goldring, 1996). C. Upper Cretaceous Horseshoe Canyon Formation of Alberta. D. Miocene Heterostegina Calcisiltite of Mallorca, E Spain (Gibert et al., 2012). E. Pliocene of Guadalquivir Basin, S Spain (Aguirre et al., 2010).

5. Tracemaker, construction, and function Tracemaker attribution is generally a difficult task. It is necessary to look for morphological trace-fossil characters that bear a high anatomical or constructional significance, the bioprint of Rindsberg and Kopaska-Merkel (2005) or the fingerprints of Seilacher (2007). In the case of C. concentricus, the overall architecture of the burrow is not diagnostic of a particular tracemaker, as U-shaped geometries are produced by a wide variety of invertebrates (Bromley, 1990). In contrast, the lining has a distinct and complex structure that is the result of a highly specialized constructional behavior, which may potentially be diagnostic of a tracemaker. Despite this, Goldring (1996) proposed several hypotheses to explain the formation of concentrically laminated linings in asterosomids, and one of them was that they were not actual linings but a particular form of passive fill similar to the draft fill described by Seilacher (1968) within ammonite shells and fossil crustacean burrows. In this scenario, the laminae would have been passively formed by slow and continuous sediment input favored by a current generated through the constricted apertures. The final fill would have given rise to a lumen located in the upper part of the burrow, which may consist of different sediment. In the material studied in Tarragona, as well as in other C. concentricus referred to in the literature, the position of the sand-filled tunnel is centered or eccentric but not necessarily located in the upper part. This does not support Goldring's passive fill hypothesis but rather active lining construction by the inhabitant of the burrow.

The lining of C. concentricus is multilayered and constituted almost exclusively of calcisiltite. Glaucony grains are limited to some discontinuous laminae and were probably passively introduced into the burrow by bottom currents. In contrast, the silty fraction of the lining is interpreted to have been introduced by the tracemaker from the surface. Infaunal segregation of silt and glaucony would not explain the composition of the lining, because segregated glaucony grains should then be concentrated elsewhere, as in Macaronichnus (e.g. Nara and Seike, 2004; Seike, 2008). The successive layers point to long-term habitation of the burrow, and the recognition of discrete sediment parcels suggests that they were constructed by successive addition of mucous-impregnated pellets or sediment parcels. These features were also observed in Jurassic C. concentricus (Fürsich, 1974). The constructive behavior interpreted from ichnological data bears striking similarities with that described for modern terebellid polychaetes. Thus, Aller and Yingst (1978) describe how the terebellid polychaete Amphitrite ornata, an inhabitant of U-shaped burrows in Cape Cod Bay (Massachusetts, USA) (Fig. 7B), uses its tentacles to collect fine-grained particles from the seafloor to produce mucusimpregnated fusiform aggregates, which are incorporated to the burrow wall in successive layers to produce a concentrically laminated lining (Fig. 7A). The same mechanism for construction of the lining had been observed in another terebellid, Neoamphitrite figulus, by Schäfer (1956, 1972) (Fig. 7A). Nara (1995), taking into consideration these neoichnological data, inferred that terebellids were the most likely tracemakers for the bulb-shaped asterosomid R. socialis. In their study of modern and Pleistocene deposits from Willapa Bay

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Fig. 7. Architectural features of modern terebellid burrows and fossil bow-shaped, concentrically laminated burrows. A. Constructional features of the lining of three modern terebellid burrows redrawn from different authors. B. Reconstruction of the burrow of Amphitrite ornata. Drawing modified from Bromley (1990), who in turn based his drawing on Rhoads (1967) and Aller and Yingst (1978). C. Two different apertural architectures: funnel-shaped (C1) as seen in Tarragona and constricted (C2) as depicted by Goldring (1996).

(Washington, USA), Gingras et al. (1999) proposed maldanid polychaetes as constructors of Cylindrichnus, and terebellids of Rosselia. Nevertheless, these authors described the lining of maldanid burrows as very thin and non-laminated and thus different to Cylindrichnus, while the Rosselia-like structures produced by terebellids were shown as occurring in the apertures of U-shaped burrows. Hence, the Willapa Bay material would share greater similarities with the burrow entrances occasionally seen in the Miocene of Tarragona (Fig. 7C1). Such a morphology is very different from that of the constricted apertures described by Goldring (1996) for C. concentricus in the Lower Cretaceous of Oxfordshire (Fig. 7C2). Recently, Zorn et al. (2010) described the burrow-wall micromorphology of nine modern intertidal invertebrates from various localities along the western coast of Washington and California and noted that it was strongly linked to the behavior of the burrower. Among all of them, those of the terebellid Cirriforma luxuriosa were unique in being concentrically laminated (Fig. 7A). Interestingly, the burrow of this polychaete is presented by these authors as a U-shaped tunnel with two concentrically laminated, funnel-shaped apertural enlargements similar to those seen in Tarragona. Zorn et al. (2010) described its lining as formed by laminae compacted by body contractions of the host organism when space was restricted due to active or passive sediment input. The continuous addition of inner layers and compaction of new material against the wall should produce deformation in both the lining and the host sediment. This centrifugal compressive deformation would result in perimetral extension of the lining layers. Nara (1995) described lengthwise microfaults in Rosselia linings interpreted as formed in this way. Also, the longitudinal striation seen in the outer surface of some asterosomids, including C. concentricus (Nara and Ekdale, 2006), is interpreted likewise (Seilacher, 2007). In the Miocene material from Tarragona, none of these features has been clearly observed. Nevertheless, the irregular outer boundary of the lining and the discontinuous layers that compound the laminated lining may be indicators of deformation and microfaulting.

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Thus, attribution of C. concentricus to the activity of terebellid worms seems to be well supported. The body fossil record of this group of worms, as with most other polychaetes, is very limited. Garraffoni et al. (2006) emphasized that the oldest terebellids were known from the Upper Silurian of Wales, thanks to the material described by Thomas and Smith (1998) as the trace fossil genus Oikobesalon, which designates burrows with thin, fusiform-banded linings. Although Oikobesalon lacks the characteristic thick, concentrically laminated lining typical of asterosomids, the fusiform fabric of its lining was interpreted by the authors as similar to that of some modern terebellids, and thus, also to the composite, brick-layered laminae of the C. concentricus seen in the Jurassic of Portugal (Fürsich, 1974) and the Miocene of Tarragona, as well as in some Pleistocene Rosselia from Japan (Kikuchi, 1966). If both Cylindrichnus and Rosselia were considered as terebellid burrows, the fossil record of this group of polychaetes would extend all the way back to the Cambrian. In terms of functional significance, the U-shaped architecture of C. concentricus is most commonly found among suspension or surfacedetritus feeders (Bromley, 1990). Fauchald and Jumars (1979) classified terebellid polychaetes as tentaculate, discretely motile to sessile, surface detritus feeders. Aller and Yingst (1978) described in detail the feeding activity of A. ornata, which inhabits broad U-shaped burrows very similar to C. concentricus. The function of the thick lining is less straightforward. It has been suggested that it may serve as a possible protection against disturbance by other burrowers or as a mechanism to promote water retention during low-tide or drier episodes (Aller and Yingst, 1978). The common occurrence of C. concentricus in sublittoral deposits allows the second hypothesis to be ruled out. Zottoli and Carriker (1974) observed that mucus secretion with a specific chemical composition that prevents foreign colonization is quite common in tubiculous polychaetes. Bromley (1990) also suggested that A. ornata may supplement its diet by microbial farming on the burrow walls, which would add an additional function to the lining. Such behavior was also proposed for C. helix by Gibert et al. (2006). 6. Paleoenvironmental significance The C. concentricus ichnofabrics found in the glauconitic calcisiltites at the Altafulla and Waikiki sections exhibit very intense bioturbation, with this ichnospecies being the most prominent and abundant trace fossil. C. concentricus crosscuts the mottled background with abundant but poorly defined bioturbation, and is only affected by some Planoliteslike burrows that distinctly nibble the concentric linings. These observations suggest a relatively simple tiering profile with shallow-burrowers producing indistinct bioturbation in a soupy or soft background and surface-detritus-feeder terebellid worms constructing thickly lined U-burrows reaching a deeper tier. Considering the overall bow-shaped geometry of C. concentricus, it may be estimated that most bioturbation was probably restricted to the uppermost 30 cm of sediment (=depth of C. concentricus). Only some vagile, possibly deposit-feeding worms (Planolites producers) would have burrowed deeper than that. This benthic community inhabited a low-energy, offshore setting below storm wave base as evidenced by the fine-grained calcisiltitic sediment and the absence of coarser event beds. The abundance of shells of A. cristatum is consistent with this interpretation, based not only on the environmental requirements of this pectinid species (Aguirre et al., 1996) but also on its taphonomic attributes (with complete and often articulated, concave-up valves). Bioturbation could keep pace with the slow sedimentation rate. This may be inferred from the presence of authigenic glaucony, which is today found in open marine sea floors with slow sediment accumulation at water depths of 50 m and deeper (Odin and Matter, 1981; Chafetz and Reid, 2000). In the case of the Waikiki section, the glauconitic unit is located at the base of a shallowing-upward sequence (Fig. 1C), and overlies a bored (Gastrochaenolites) transgressive surface (Belaústegui et al., 2011). This shallowing process is recorded in the lower part of the

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sequence by the replacement of the C. concentricus ichnofabric by an ichnofabric dominated by the crustacean burrows Thalassinoides suevicus in the non-glauconitic calcisiltites. A similar stratigraphic and ichnofabric succession is seen at Altafulla (Fig. 1C). Nevertheless, at this locality glauconitic and non-glauconitic calcisiltites are separated by a fining-upward rhodolite biocalcarenite, which is interpreted as produced by a high-energy event (large storm or tsunami). Below this bed, Thalassinoides burrows are exceptionally preserved filled with coarse-grained sediment derived from it. That provides a record of the presence of burrowing crustaceans as part of the benthic community. Although C. concentricus is a common component of high-diversity ichnoassemblages in shallow marine deposits (e.g. Fürsich, 1975; Howard and Frey, 1984; Sharafi et al., 2012), ichnofabrics in which this ichnospecies is abundant and occurs as an elite trace fossil are less common. They have been described in Miocene shallow and open-shelf pelagic carbonates on Malta (Goldring et al., 2002), in Miocene outer shelf calcisiltites on Mallorca (Gibert et al., 2012), and in Pliocene middle shelf deposits close to the storm wave-base in southern Spain (Aguirre et al., 2010). The Cylindrichnus ichnofabrics described in these Neogene occurrences bear general depositional and paleoenvironmental similarities to that of Tarragona. In particular, the occurrence in the Miocene of Mallorca (eastern Spain) shows striking resemblances to the succession from a Cylindrichnus to an Ophiomorpha ichnofabric recording shallowing-upward conditions (the ‘worm-to-shrimp’ sequence of Gibert et al., 2012). 7. Conclusions Bow-shaped, concentrically laminated burrows, such as those described herein from the Miocene of Tarragona, have been abundant in shallow-marine paleoenvironments since the Jurassic. This fact may have been disregarded by ichnologists because of the controversial status of the ichnospecies C. concentricus, which seems to be the most adequate available ichnotaxonomic name for these burrow architectures. The characteristic thick, concentrically laminated lining of C. concentricus, and in particular its brick-layered pattern, may be considered as a diagnostic feature to assess the identity of the tracemaker. Its constructive features display remarkable similarities to the linings found today in the burrows of some terebellid polychaetes. Previous work by Nara (1995) had assigned Rosselia to this group of worms based on the same construction criteria. C. concentricus is a common constituent of diverse offshore-toshoreface ichnofabrics in the Mesozoic and Cenozoic. Nevertheless, these trace fossils may be also found as elite trace fossils in true Cylindrichnus ichnofabrics developed in outer-to-medium-platform settings during the Neogene.

Acknowledgments During the revision of this article, the second author Jordi Maria de Gibert suddenly passed away. This was the last article we did together, as my PhD supervisor and my friend, this manuscript is dedicated to his memory. This paper was greatly inspired by the influence of the late Roland Goldring, who first noticed the commonness of bow-shaped, concentrically laminated burrows and raised some of the questions concerning C. concentricus discussed herein. Discussions with Masa Nara and Tony Ekdale greatly improved the manuscript. Comments of the editor (Prof. Finn Surlyk) and of two anonymous reviewers have been very useful and constructive. The authors acknowledge the help of Isabel González during the fieldwork and the tenacious job of Alejandro Gallardo in the preparation of thin sections. This study is part of the activities of the research project CGL 2010-15047 of the Spanish Science and Innovation Ministry.

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