Internal structure and origin of the concentric lamination
Though not generally accepted, two basic models are used to explain the origin of the obligatory, active filling in concentrically laminated burrows. In one model, the basic features are interpreted as the result of radial backfilling associated with feeding activities (figure 8.A; Seilacher, 1986). In Asterosoma and Rosselia this model was related to the feeding activity of a worm-like organism that systematically reworked the sediment with spiral movements of its distal end, while periodically moving along an inner tube (figure 8.B; Chamberlain, 1971). In the other model, a tube-dwelling organism produces the concentrically laminated structure by adding successive wall layers to the tube and pushing and splitting previous layers outwards, and the resulting concentric laminae are more a thick wall-lining than a backfilled structure (figures 8.C-D; Goldring, 1996; Bromley, 1996 and the bibliography therein). Nara (1995) applied this model to explain the origin of the concentric laminae in Rosselia socialis Dahner, suggesting terebellid polychaetes as the trace markers. The superposition of successive spindle-shaped parts (figure 8.D) was interpreted as an equilibrium structure (Nara, 1995).
In Asterosoma radiciforme and the three new ichnospecies of Patagonichnus from Las Grutas, the bioturbated material that composes the concentrically laminated structures is readily differentiated from the host sediments by contrast in color and texture. The reconstruction of the burrow system (figure 3) and the geometry of the internal structure (figure 5) clearly indicate that the concentric lamination is best explained as the radial backfill produced by elongate, worm-like organisms spiraling or circling within the sediment while feeding (figure 9). The basic backfilling mechanism is similar to that proposed by Chamberlain (1971) but significant improvement of this model results from the analysis of the inner structure of the material from Las Grutas.
The analysis of transverse cross-sections shows three recurrent patterns for the arrangement of the internal laminae: 1, discordant groups of laminae defining two half spirals, which are arranged into an alternated pattern (figures 5.A-B); 2, laminae arranged into more continuous spirals (figure 5.C); and 3, circular laminae (figures 5.F, J). Transitions among these patterns are frequent. Patterns 1 and 2 are common in all studied Asterosoma and Patagonichnus from Las Grutas (figures 5.A-E, I, M). Pattern 3 is more frequent in the straight stem of A . radiciforme (figure 5.F); in all compound elements of P. stratiformis (figures 5.G-J); and in the backfilled branches of P . thalassiformis (figure 5.N). In addition, in P . stratiformis these three patterns are transitional with helicoidal bulbs (figures 5.K, 6.F).
By using the case of pattern 1 ?the alternated half-spiral geometry? as an example, the three-dimensional interpretation for the development of the whole structure is shown in figure 9. The basic plan of the structure results from the spiral movement of a worm-like organism with one end in the terminal position of the inner lined tube and the other end extended and spiraling within the sediment (figure 9.A). The interpretation of the sequential development of the structure is shown in figure 9.B. At time 0, during the initial production of the first half-spiral, the organism excavates a conical burrow and then moves the open burrow, excavating towards the center and backfilling the periphery. The process of movement and backfilling of the initial burrow is recorded by a set of slightly discordant, sub-concentric laminae, which in cross-section widens in the direction of progression of the structure and in longitudinal cross-section thins toward the inner lined tube. This process continues after completion of the first half-spiral (time 1) and is then repeated again, but now moving a little bit nearer the inner lined tube, forming the second half-spiral (times 1 and 2). At time 1, during the initiation of the construction of the second half-spiral its point of origin is displaced upwards respect to the similar point of the first halfspiral at time 0, and the resulting three-dimensional geometry is a helicoidal body (i.e. the spiral is stretched in the third dimension). At times 3 and 4, the open burrow now lies in a more central position in the plane containing the cross-section, the inner lined tube has grown in the direction of progression of the structure, and the widest parts of the sets of laminae corresponding to the third and fourth halfspirals were intercalated beside the thinnest parts of the previous sets of the first and second half-spirals. In this way, maximum exploitation of the sediment in the search for food is achieved. The model for the construction of the structure is very similar to the helicoidal phyllotaxis in plants, and in fact it was inspired by the helicoidal arrangement of alternated couples of leafstalks at the basal stem of the celery plant (Stevens, 1989, Fig. 127).
The contact between successive laminae is not even; it frequently is irregular and sinuous (figure 9.C), and this is best seen when the set of laminae consists of alternating thick clay- and thin sandylaminae. This sinuous contact is interpreted as having originated by the pressure exerted by the tracemaker, obviously with a cylindrical body, pushing against the tube-wall.
As shown in figure 10, the sequential development of the structure for the cases of patterns 2 and 3 is similar to that of the described model. In case 2, the continuous spirals partially wrap and the structure is developed much like a gastropod shell, and in case 3, the circular laminae result from the successive displacement of a moving cone. If the inner lined tube is kept straight, the combination of spiraling or circling movements results in helicoidal or conical backfilling, respectively (figure 10.A). If the inner tube is helicoidally twisted, the resulting backfilling is always helicoidal (figure 10.B).
Behavior of the trace-maker and function of the elementary components
All the Patagonichnus material from Las Grutas consists mainly of large, horizontally spread out structures connected by smaller vertical structures, both including an open, mud-lined tube, isolated or surrounded by various concentrically laminated structures (figure 3). The complex backfilling mechanisms clearly indicate mainly fodinichnia structures. However, all of these structures are large, compound burrow systems and the analysis of the elementary components suggests that some of them could be designed to also accomplish different, coordinated functions.
By definition, fodinichnia are characterized by the combined functions of dwelling and feeding. The open, inner mud-lined tube is interpreted to function mainly as a dwelling structure, providing also connection, irrigation and ease of movement of the organisms among different parts of the burrow system. The construction of an extensive mud-lined wall is generally associated with a long occupation of the burrow by the trace maker (Bromley, 1996), and this is consistent with the large area, in excess of 1 m 2 , covered by a single burrow system. The complex strategies used in the development of the backfilled bulbs and swellings indicate that they represent feeding activities; such interpretation is consistent with the restriction, and large horizontal extension, of these backfilled structures to specific horizons, as if the trace maker was differentially mining particular, food-rich layers. The contrast in color and texture of the bioturbated sediment respect to the host rock is also indicative of a feeding activity.
The vertical, mud-filled cone-in-cone structure of P. calyciformis is only present when a sand layer is interposed between mud layers, and it is always found in connection with distinct, horizontally extended backfilled structures, developed only in the lower and upper mud layers (figures 3.A, 4.I, 5.L). The mutual restriction to specific grain-sizes of these different, but connected, structures seems to indicate that the P. calyciformis producer was adapted to actively feed on mud but not on sand. The likely functional interpretation of the cone-in-cone structure is that of an elaborated wall made of mud cylinders. If, when exploring new mining layers, the organism was forced to cross a sandy layer, apparently it built a complex wall, designed mainly for protection of the connecting inner tube.
The vertical, conical laminated shaft of P. Stratiformis (figures 3.B, 5.J, 6.D) is another distinctive structure frequently found in thin alternating sand and mud beds. This vertical shaft is only connected to the horizontally extended laminated bulbs in sandy layers but such bulbs are not developed in the intervening muddy layers (figures 3.B3, 6.D). In this case it seems that the P. stratiformis producer was adapted to only feed in sandy layers. The best functional interpretation for the vertical, conical laminated cylinder is that of a probing structure; and probably it represents a vertical excursion of the organism to detect the preferred grain-size layer for mining.
The intricate tube network projecting outside the backfilled burrow in P. thalassiformis (figures 3.D, 5.N, 7.A-D) is the most intriguing structure. This tube network is frequently found in association with thin, distinctive tuff beds in unit III. Multiple anastomosing and division of the tubes with an irregular pattern is a common feature, but there is a definite tendency for the tubes to follow the surfaces of the backfilled branches at the boundaries of the tuff layers (figures 3.D, 7.B-C). We can only guess that the functional interpretation of this tube-network is that of a probing system searching for food. If the organisms actively mining a food-rich layer encounter a barren tuff bed, and some of them succeed in finding the way to a new food-rich layer, the others try to follow them looking for the successful passage.
The three new ichnospecies are elite burrow systems that distinctively dominate the bioturbated fabric in mud- or sand-rich layers. In P . thalassiformis , P . calyciformis and P . stratiformis the areal distribution of a single burrow system covers at least 1 m2. But as the burrow system consists of several, repeated horizontal layers connected by vertical structures, the total area covered by a single burrow system must be much larger. A single layer, exclusively and fully bioturbated to form P. stratiformis ( e.g . figure 6.E) was followed to at least 15 m2. It is unlikely that a single organism could make such a huge burrow system. On the contrary, it looks as though these large burrow systems, with many different elementary components, were constructed by gregarious organisms, living and working simultaneously. Measurements of the inner, mud-lined dwelling tube seem to confirm this interpretation. The diameter of the inner tube ?which represents the maximum transversal size of the dwelling organism? in any single burrow system of the three new ichnospecies, records a continuous range of variation in size. Continuous range is from 0.25 to 4 mm (average 1.66 mm) in P . calyciformis ; 0.5 to 3.8 mm (average 1.95 mm) in P. thalassiformis ; 1.15 to 7.44 mm (average 3.06 mm) in P . stratiformis with straight inner tube; and 1.47 to 9.2 mm (average 3.3 mm) in P . stratiformis with helicoidal inner tube (figure 11). The fact that this continuous range of sizes is not a representation of some kind of growth vector of a single organism is demonstrated by the frequent joining of inner tubes of different diameters into a single point (figures 7.C-D). This is strong evidence that many similar organisms of different sizes were living and working simultaneously within any single burrow system. Large, fully bioturbated areas exceeding 15 m2, can be explained by the close association of many single burrow systems.
Regarding the depth of penetration of the burrow system, both direct and indirect evidence indicate that the Patagonichnus producer penetrated several tens of centimeters into the sediment. The conical laminae of the vertical connecting-shaft of P. stratiformis recorded in thin, interbedded sand and mud layers, provide direct evidence that the burrow penetrated the sediment to a depth of at least 30 cm. As the sedimentation of the thin-bedded heterolithic beds was continuous, with no signs of internal erosion surfaces, the downward movement of the burrow is not an equilibrium structure but represents a true minimum depth reached by the organism. Crosscutting relationships with the palmate crustacean burrow and Ophiomorpha provide additional indirect evidence. Patagonichnus calyciformis and P. stratiformis always cut both the palmate branches and the main stem of the crustacean burrow. Even though Ophiomorpha more frequently cuts the Patagonichnus burrows, mutual crosscutting relationships were also observed. Crustacean burrows range from 20 to 50 cm in height, confirming that the Patagonichnus structures penetrated at least about 20-30 cm below the sediment-water interface.
Identification of the trace-makers
In general, biologic identification of trace makers is problematic ( cf. Frey, 1975; Seilacher, 1986). In the material from Las Grutas there is some direct evidence on size and form of the trace maker. Obviously, the cylindrical inner, lined tube indicates that the trace maker must have had an elongate, worm-like body, with only a few millimeters in cross-section (figures 4.E-F, 6.B, 7.B-D). Similar form and size of the producer are also indicated by the sinuous deformation of the laminae within the bulbs, probably produced by the repeated lateral pushing of the organism against the burrow wall (figure 9.C), and by the bioglyphs in the form of cylindrical furrows preserved on the external part of the bulbs in P. calyciformis (figure 4.F). Additional analogies with known burrow geometries, mode of life, and feeding strategies of extant worm-like organisms suggest polychaetes as the most probable architects for the Patagonichnus burrows from Las Grutas.
Irregular, intricate tube systems of burrowing polychaetes are relatively common in modern tidal flats and intertidal channels (Reineck et . al ., 1967; Schäfer, 1972; Barnes, 1974) and the analogies in size and form of some of them with the Patagonichnus burrows from Las Grutas are striking. In particular, Reineck et al . (1967), Schäfer (1972), Powell (1977), Ronan (1977), and Gingras et al . (1999, 2002), described polychaete burrows of similar form and size to those of the Patagonichnus inner tube from Las Grutas. Maldanid and capitellid polychaetes are the known trace-makers of Cylindrichnus -, Gyrolithes -, Trichichnus - or Chondrites -like burrows (Gingras et al . (1999, 2002). Similar forms are found at Las Grutas; in particular the helicoidal inner tube of P . Stratiformis seems to be a common feature developed by capitellid polychaetes (Reineck et al ., 1967; Schäfer, 1972; Powell, 1977; Gingras et al ., 1999, 2002). The capitellid Notomastus magnus Hartman makes large, relatively deep, irregular and meandering tube systems using its proboscis for feeding and locomotion; eversion of the proboscis is recorded in the burrow by a local expansion of the tube diameter (Ronan, 1977) and this structure seems to be present in the tube network of P. thalassiformis (figures 7.C-D). Some relatively deeply burrowing polychaetes commonly develop a complicated burrow system that tends to expand horizontally at certain levels ( e.g some species of Nereis and Heteromastus described by Schäfer, 1972) and this is a common feature of all Patagonichnus from Las Grutas. Different but related species of polychaetes are frequently adapted to burrow different substrates, preferring either fine sand or mud, where they are found in large numbers, usually several hundreds of specimens per square meter (Schäfer, 1972; Barnes, 1974), and the size of the inner tube is a diagnostic species-specific feature in some polychaetes (Gingras et al ., 1999). The Patagonichnus from Las Grutas share all of these polychaete features. Patagonichnus stratiformis is only developed in fine sand and P. calyciformis is mostly developed in mud, and both are characterized by a large number of crowded, associated inner tubes. To a certain extent the inner tube overlaps in size, but when large numbers of specimens are measured, two distinctive size-frequency distributions are clearly shown: the average diameter of the inner tube in P . Calyciformis and P. thalassiformis (mudstone beds) is less than 2 mm, and in P. stratiformis (sandstone beds) is more than 3 mm (figure 11).
Longitudinal cross-sections of P. stratiformis (figures 6.A-B) bear a general similarity to Nereites , which also is produced by a worm-like organism, probably an enteropneust, which sorted out the particles with its protosoma, stowed them in lateral backfilled lobes, and stuffed the ingested material into a median tunnel, behind the animal (Seilacher, 1986). The anterior part of the Patagonichnus animal probably processed the sediments with a similar mechanism but the lack of the stuffed material in its inner, median, tube indicates a different behavior, and probably a different organism. Worm-like organisms in general and deep burrowing polychaetes in particular, use several mechanisms for movement, including bolting, peristaltic and undulatory movements, multiple circular shoveling using specialized parapodia, or some combinations of these (Schäfer, 1972; Buatois et al ., 2002). Their feeding strategies are complex, and they could be selective or non-selective deposit feeders, carnivorous, or omnivorous (Ronan, 1977). Regardless of these different mechanisms and strategies, errant, burrowing polychaetes all have a hollow, and commonly lined, inner tube that connects the horizontal and vertical branches of the burrow system (Schäfer, 1972; Barnes, 1974). As explained before, the main function of the inner tube is that of a dwelling structure, providing both irrigation and ease of movement for the organism among the different parts of the burrow system. In analogy with extant polychaetes, the hollow, lined inner tube of Patagonichnus from Las Grutas probably was also functional to evacuate the fecal material to the surface; explaining why the residual of the ingested material is never found stuffed in the median, inner tunnel as in Nereites .
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