Ichnogenus Asterosoma von Otto 1854
Type ichnospecies. Asterosoma radiciforme von Otto 1854. Original figures are reproduced in part by Häntzschel (1975, fig. 25 1a).
Remarks. The original diagnosis of Asterosoma von Otto (1854) was mostly based on the star-like arrangement of bulbs, typical for A. Radiciforme (Häntzschel, 1975). Subsequently, the ichnogeneric diagnosis was expanded to include the dichotomous to fan-like pattern of branching bulbs typical for A. ludwigae Schlirf (2000). Seilacher (writ. comm., 2003) considers that the radial pushing exerted internally by the tracemaker produces both the concentric lamination and the external longitudinal furrows and striae -interpreted as microfaults- commonly preserved in the German type material. In our material, there is limited evidence for this radial pushing mechanism (see Discussion).
Asterosoma radiciforme von Otto 1854
Figures 3.A4, 4.A-D, 5.A-F
A. radiciforme von Otto, 1854, in Häntzschel (1975, Fig. 25 1a)
A. radiciforme von Otto, Müller, 1971, Pls. 1-3
Material. Many field-specimens, and part of several bulbs, CADIC PI 21 to 26.
Description. Large, concentrically laminated, subhorizontal to inclined bulbs with a star-like arrangement radiating from a central point. The concentric structure of bulbs consists of thick mud-rich laminae alternating with thin sandy laminae, surrounding a central mud-lined tube. The internal structure of bulbs frequently consists of alternated half-spirals (figures 5.A, B, D, E). Bulbs egg-shaped, 10-15 cm long and 3-5 cm wide, connected by an inner lined tube to a central point that continues into a vertical, lined tube encircled by concentric lamination (figure 5.F). Inner tube ranges from 5 to 10 mm (average 7 mm) in width.
Preservation. Endichnial; mud-rich bulbs, commonly present in thin alternations of sandstone and mudstone beds.
Remarks. The construction of the burrow structure in A. radiciforme was interpreted to proceed in a downward direction (Uchman and Krenmayr, 1995). In addition, our material indicates that the tracemaker frequently followed an upward direction during the construction of the burrow as it is indicated by the upward displacement of mud laminae at the contact between mudstone-sandstone beds (figures 5.A-B) and by the conical-concentric backfilling in the central shaft (figure 5.F).
Localities. Relatively common in two or three levels near the top of unit I. Best examples are found between Bajada del ACA ( Bajada is a local name for the access to the beach) and just to the NE of Bajada Los Acantilados at Las Grutas.
Ichnogenus Patagonichnus n. igen.
Type ichnospecies. Patagonichnus stratiformis n. isp.
Derivatio nominis. Patagon -, from Patagonia; ichnus , trace.
Diagnosis. Large, compound, endichnial burrow system characterized by many distinctive concentrically laminated elementary components budding from an intricate system of horizontal to vertical connecting mud-lined tubes. Concentrically laminated elementary components may consist of vertical to horizontal spindle-shaped bulbs, bulb-like swellings, helicoidal bulbs, and backfilled tubular branches. The burrow system tends to cover large, irregular, horizontal layers. Vertical, concentrically laminated shafts or cone-in-cone structures surrounding a lined tube could connect different layers.
Remarks. Two basic taxonomic procedures have been applied to the nomenclature of compound trace fossils. In one case, the trace is named after the most common ichnotaxon or ichnotaxa ( e.g . Pickerill and Narbonne, 1995; Ekdale and Bromley, 2001). In the other case, a new name is given to the whole structure because it is thought that the distinct components, representing different but simultaneous behavioral activities of the same animal, contribute to an ichnological whole (Bromley et al ., 2003).
The complex and compound burrow system of Patagonichnus has many distinct, elementary components that in isolation could be assigned to several already established ichnogenera. In particular, the three newly proposed ichnospecies of Patagonichnus bear concentrically laminated bulbous structures that in isolation resemble the ichnogenus Asterosoma . Also, the constructional mechanism of the bulbs in Asterosoma radiciforme from Las Grutas is similar to that of some bulbs in the three new ichnospecies of Patagonichnus , suggesting that they represent the work of the same organism (see Discussion). However, the burrow system in Patagonichnus is much more complex than in A. radiciforme , or other Asterosoma ichnospecies characterized by discrete bunches of radiating bulbs ( cf . von Otto, 1854; Müller, 1971; Bromley and Uchman, 2003). The large Patagonichnus burrow system (figure 3) is characterized by complex, connected bulbous structures ?including series of linearly arranged, vertical calyx-like bulbs; sinuous, dense, horizontal branches with lateral bulb-like swellings; helicoidal bulbs; or aggregate, cylindrical structures with isolated or associated bulb-like swellings? systematically associated with a variety of structures, which are interpreted as representing different, simultaneous behavioral activities of the same animal. Consequently, we followed the taxonomic procedure of Bromley et al. (2003) and a new name is given to the whole structure.
Patagonichnus calyciformis n. isp.
Figures 3.A1-3, 4.F-I; 5.L-M
Type Material. Holotype: Partially preserved cup-like bulbs (CADIC PI 27, Figure 4.F); connecting cone-in-cone structures (CADIC PI 28, Figure 5.L); and parts of a linear array of bulbs (CADIC PI 29, figure 4.H). Paratypes: several isolated or welded bulbs (CADIC PI 30). Additional material: many field specimens.
Derivatio nominis. It refers to the calyx or cup-like shape of bulbs.
Diagnosis. Series of dominantly linear arrays of vertical, calyx-shaped, concentrically laminated bulbs, budding from a connecting horizontal lined tube.
Description. The burrow system consists of calyxshaped bulbs, forming a linear array of several welded to isolated bulbs connected by a horizontal, mudlined tube. Bulbs vertically elongated, about 5-7 cm in length, maximum diameter 2-3 cm, generally tapering at both ends, with internal regular concentric and/or irregular spiraling or radiating, muddy laminae, alternating with very thin sandy laminae. External part of bulbs bears delicate impressions of oblique to horizontal cylindrical furrows interpreted as bioglyphs (figure 4.F). Inner tube very thin, its diameter ranges from 0.25 to 3.4 mm (average 1.66 mm). In only two specimens, an unusual structure that consists of vertical, imbricate flower-like bulbs (figure 3.A3) was observed in connection to the normal, horizontally aligned calyces.
The burrow system spreads out horizontally covering large areas, in excess of 1 m2, in mudstone beds. Vertical tubes with associated concentric lamination may connect two or more horizontal burrow systems. When the connecting vertical tube cuts through a sandy bed, a distinct cone-in-cone structure is formed around the vertical tube. The structure consists of elongated, vertical to inclined, cylindrical mud packages, spiraling around the vertical inner tube. In cross section, the spiral bands show internal meniscus-like structures (Figures 4.I, 5.L).
Preservation. Endichnial in mud-dominated beds.
Remarks. The arrangement of the calyx-shaped bulbs and their internal, complex backfilling patterns indicate a fodinichnion structure. However, the vertical cone-in-cone connection with its elaborated muddy cylinders is difficult to interpret as the result of feeding activity. The introduction of mud within a sandstone bed and the spiral arrangement of the muddy cylinders around the inner tube suggest a kind of very elaborated wall made by the trace-maker for protection of the inner tube.
Localities. Very abundant near the top of unit I. Best examples are just to the NE of Bajada Los Acantilados at Las Grutas.
Patagonichnus stratiformis n. isp.
Figures 3.B, 4.E, 5.G-K, 6.A-F
Type material. Holotype: horizontal, concentrically filled branches (CADIC PI 31 Figure 6.A-B). Paratypes: Several concentrically filled branches (CADIC PI 32, Figure 6.E; CADIC PI 33) and helicoidal bulbs (CADIC PI 34). Additional material: many field specimens.
Derivatio nominis. It refers to the tendency of the whole burrow system to cover large, stratiform areas.
Diagnosis. Dominantly sinuous, crowded, long horizontal backfilled branches with successive bulb-like lateral swellings surrounding a ramified, central mud-lined tube. Distal ends could be transitional to helicoidal bulbs and associated helicoidal inner tube. Distinct layers could be connected by straight, vertical, conical to helicoidal, concentrically laminated shafts.
Description. Complicated, compound burrow system dominated by long, sinuous horizontal backfilled branches showing successive lateral bulb-like swellings, 1-3 cm wide, with concentric, conical to spiral, backfilling sandy laminae, surrounding a thin, mud-lined tube, ranging from 1.15 to 7.40 mm (average 3.06 mm) in diameter. Large ( ca. 1.5 mm) mica flakes are generally arranged in a vertical position, parallel to the laminae. The inner mud-lined tube may be divided horizontally, giving way to several, sinuous, concentrically laminated branches that generally do not intersect. When two branches are about to meet, mutual intersection is avoided by upwards or downwards deviation movements of the inner tube and the accompanying bulb-like concentrically laminated structures (figure 3.B1). In this way, a large, extensive, horizontal and stratiform structure, covering at least several square meters, is formed. Distal ends of horizontal inner, sinuous tubes could pass transitionally to helicoidal inner tubes (ranging in diameter from 0.7 to 9.2 mm, average 3.3 mm) and associated helicoidal bulbs (figures 3.B3, 5.K, 6.F). Different layers are connected by vertical, conically laminated structures (figures 3.B2, 5.J, 6.D).
Preservation. Endichnial in sandy beds.
Remarks. Vertical or horizontal elementary components with helicoidal inner tube could be mistaken for Gyrolithes or Helicodromites , respectively. However, the associated helicoidal backfilling readily differentiates P . stratiformis from these ichnogenera (figures 3.B3; 5.K, 6.C,F). Vertical, conically laminated shafts (figure 5.J) are similar to Cylindrichnus or Siphonichnus , but in P . stratiformis these vertical shafts are always connected via the inner tube to the stratiform layer of backfilled branches (figures 3.B2; 6.D).
Localities. Very abundant in sandy layers of unit II.
Patagonichnus thalassiformis n. isp.
Figure 3.C-D, 5.N, 6.G, 7.A-E
Type material. Holotype: Silicone cast and mold of paired, filled branches, associated laminated bulbs, and external tubes of part of the burrow system illustrated in figure 6.G (CADIC PI 35).
Paratypes: several partial filled branches, bulbs, and external tubes (CADIC PI 36). Additional material: many field specimens, partly illustrated in figure 7.A-E.
Derivatio nominis. It refers to the apparent similitude of the burrow system to the galleries of the ichnogenus Thalassinoides .
Diagnosis. Long; dominantly horizontal and straight to curved; single, paired or multiple tubular branches and associated bulbs filled with conical or helicoidal, concentric laminae surrounding an intricate system of lined inner tubes. Horizontal parts could be connected with vertical and shorter backfilled branches. Where the external backfilled structure is missing, the lined inner tube may form intricate, crowded gallery systems with numerous vertical and horizontal branches.
Description. Large, compound, mainly horizontal burrow system, which dominantly consists of up to 1 m long, horizontal, backfilled branches surrounding a complicated system of inner, open lined tubes. Two horizontal, backfilled branches frequently run sideby- side for a long distance and then they detach at acute or right angles, giving the false impression of true branching (figures 3.D, 6.G, 7.B, E). Associated multiple, juxtaposed branches also occur; in most cases they are horizontal and parallel but sometimes they are entangled in joint spiral coils. Crosscutting of the backfilled branches is rare. Horizontal, spindled-shaped, short bulbs could be connected to single, paired or multiple, backfilled branches (figures 6.G, 7.A-C). Backfilling of the tubular branches and bulbs consists of conical or helicoidal laminae. A complex, horizontally and vertically branching network of numerous small tubes, similar to the inner tube, extends outside the backfilled burrow (figures 3.D, 5.N, 7.A-C). In this case, several tubes seem to run in parallel to the surface of the backfilled branches. Short segments of some individual tubes show local expansion of the tube diameter (figures 7.C-D). Single, concentrically backfilled branches are about 1-3 cm wide. The inner open, lined tube ranges from 0.5 to 3.8 mm (average 1.95 mm) in diameter.
Preservation. Endichnial in sandstone beds with abundant fecal pellets of unit II and sandy mudstone beds of unit III. Differentially weathered tuff beds at unit III superbly preserve the complex geometry of the network of lined tubes extending outside the backfilled branches. In part, the external surface of these tubes is covered with ovoid fecal pellets.
Remarks. The geometry of the backfilled branches bears a general resemblance to the gallery system of the ichnogenus Thalassinoides . However, the paired or multiple, closely juxtaposed tubular branches; the concentrically laminated backfilled structures; the associated tapering bulbs; and the inner lined tube are features sufficiently distinctive of P . thalassiformis n. isp. The sub-concentric laminae that may form by passive filling in some galleries of Thalassinoides and Ophiomorpha (Seilacher, 1973; Bromley, 1996) have a different geometry, and are quite different from the delicate concentric lamination of P. thalassiformis n. isp., in which the backfilled material is similar to that of the host rocks. In addition, the associated and connected network of small, branching tubes are not known in Thalassinoides , and this connected tube network is totally different in origin and structure from the composite trace formed by the occupation and reworking of crustacean galleries by polychaetes (Gingras et al ., 2002). In the latter case, the polychaetes used the inner surface of the large crustacean gallery as an extension of the sediment-water interface and thus the threadlike tubes are oriented perpendicular to the gallery inner surface (Gingras et al ., 2002). On the contrary, the tube network in P. thalassiformis tends to be oriented parallel to the surface of the main backfilled branches (figure 5.N).
Localities. Frequent in sandstones bearing abundant reworked fecal pellets at the top of unit II and dominant in sandy siltstones interbedded with thin tuff beds in unit III.
Large, palmate, concentrically laminated burrow
Figure 7.F
Material. Several branched tunnels with retrusive spreite (CADIC PI 37), and many field specimens.
Description. Large, inclined, bundled burrow consisting of a main tunnel from which several minor tunnels branch out in a palmate pattern (figure 7.F). The lower part of the structure consists of an inclined (between 15º-40º) main tunnel up to 40 cm long, about 3-10 cm wide, and with a blind distal end. At its upper part it is divided into several secondary tunnels (minimum 3 and up to 7 observed) forming a broom-like structure. Most of the tunnel wall bears distinctive scratch traces in the form of parallel furrows of similar shape to the bioglyphs made by crustacean decapods (Bromley, 1996). The main and branched tunnels characteristically display one or few sets of retrusive spreite laminae, reaching several centimeters in height, ending with an empty or, more frequently, passively filled, tunnel. The cross sections of these final tunnels are circular, and the passive filling is characterized by concentric laminae with a central or upward displaced, mud-filled tube. The laminae of both the retrusive and the passive fillings consist of different lithologies from that of the host rock. The whole structure is about 70-100 cm in length and 20-50 cm in height.
Remarks. Ethologically the large, palmate burrows are interpreted as both domichnia and equilibrichnia structures. The burrow was probably made by crustacean decapods, which constructed a deep, inclined tube (the main tunnel) and two or more branches connecting to the surface, allowing the animal to maintain water circulation through the main domichnion burrow. The retrusive spreite represents the activity of the producer, packing on the burrow's floor the sediment that accidentally entered the tunnels from outside or from other parts of the burrow. The vertical piling-up of successive, retrusive sets of laminae, probably represents an equilibrium structure, in response to agradational increments of the sedimentary surface and/or to sudden sedimentary infill or collapse of the open tunnel. Near complete infilling of the tunnel promoted the opening of new branches and the abandoned tunnel was passively filled with the concentric laminae. This large, palmate burrow is an elite trace fossil in unit I. It is also present in the layers with fecal pellets characteristic of unit II. Formal systematics of this burrow are delayed, pending results of studies in progress by the authors.
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