Single or partially preserved elements of concentrically laminated burrows, as exemplified by the ichnogenera Asterosoma, Rosselia, and Cylindrichnus, all share an apparently simple structural plan consisting of regular to irregular concentric laminae defining swollen bulb-like and/or cylindrical to conical structures, surrounding a central or eccentric lined tube. In general, interpreted ichnotaxobases include spatial orientation, form, and degree of connection of laminated structures, comprising mainly three basic plans: a) several connected bulbs, horizontal or inclined, with radial or branching fan-like arrangements ( Asterosoma ); b) mainly single, vertical or inclined, spindle-shaped or conical bulbs ( Rosselia ); and c) single, mainly inclined, cylindrical or conical forms ( Cylindrichnus ). In spite of this apparent simplicity -or perhaps because of this- the ichnotaxonomy and ethology of concentrically laminated burrows are problematic ( cf. Uchman and Krenmayr, 1995; Goldring, 1996; Schlirf, 2000; Bromley and Uchman, 2003). Seilacher (1969) and Chamberlain (1971) considered Rosselia a synonym or close relative of Asterosoma , but with different interpretations on the origin of the concentric lamination, which is interpreted either as the results of the radial pressure exerted internally by the trace maker (Seilacher written com., 2003; see also Nara, 1995) or of spiraling movements of the organism within the sediments while searching for food (Chamberlain (1971). Frey and Howard (1970; 1982; 1985) and McCarthy (1979) interpreted intergradational forms among Asterosoma , Rosselia and Cylindrichnus . Another problem is that in shallow marine and estuarine environments these ichnogenera are frequently associated and tend to occur in dense numbers (Uchman and Krenmayr, 1995; Nara, 1995, 2002; Schlirf, 2000; Bromley and Uchman, 2003, and references therein), and as a result in most cases it is difficult to understand or reconstruct a complete burrow structure. Moreover, the original definition of Asterosoma includes the mention of closely associated cylindrical branched tubes (? gabelnde Cylinderchen ?, von Otto, 1854), and a similar association is probably recognized as Asterosoma -? Chondrites ? in the literature ( e.g . Müller, 1971; Schlirf, 2000, Pl. 3, Fig. 9).
Estuarine deposits in the Upper Cretaceous of Antarctica and the Eocene of Tierra del Fuego (Olivero and López C., 2001, 2002) bear distinct horizons fully bioturbated with concentrically laminated burrows, which may be loosely classified either as Asterosoma or Rosselia . They are frequently associated with Chondrites -like delicate branching tubes and partially preserved, conical to cylindrical, laminated shafts similar to Cylindrichnus . The general impression is that these trace fossils could be part of more complicated structures but the dense bioturbated fabric in homogeneous very fine sandstones forming the host rock, prevents the reconstruction of a complete burrow structure.
Luckily, a similar but superbly preserved ichnocoenosis was found in heterolithic tidal deposits of the Patagonia Formation (Early Miocene) at Las Grutas, Río Negro Province (figure 1). In this case, the preservation of whole burrow structures in friable mudstone and sandstone clearly shows that they could consist of large, complicated burrow systems composed of a variety of connected elementary components. Consequently, the new ichnotaxon Patagonichnus n. igen., including three new ichnospecies, is proposed for these complicated burrow systems.
The geometry of the Patagonichnus burrow system is variable and closely related to a particular granulometry of the substrate, but in general it consists of an intricate system of vertically and horizontally branched lined tubes surrounded by cylindrically laminated structures. Straight or helicoidal, vertical to horizontal short branches of this tube system are associated with bunches of vertical, horizontal or helicoidal, concentrically laminated bulbs or shafts. If the intricate connecting tube system is not recognized, the elementary bulbs or shafts could be classified by presently recognized ichnotaxobases in part as Asterosoma , Rosselia or Cylindrichnus . The horizontal to vertical helicoidal components of the inner tube could be misidentified as Helicodromites or Gyrolithes , respectively. Moreover, if isolated from bulbs, the branched inner tube could be misidentified with Chondrites . Thus, the main objectives of this paper are to describe and interpret the complicated Patagonichnus n. igen. burrow system from Las Grutas and to briefly discuss the probable biology and gregarious mode of life of the producers. In addition, an associated large, palmate, and concentrically laminated burrow, probably produced by crustacean decapods, is briefly described.