Patagonichnus: A new trace fossil from the Miocene of Patagonia. A probable fodinichnion of gregarious polychaetes

Miocene tidal deposits from Las Grutas, Patagonia, Argentina, superbly preserve cylindrically laminated burrow systems including Asterosoma radiciforme von Otto and three compound ichnospecies of the newly proposed ichnogenus Patagonichnus; P. calyciformis n. isp.; P. stratiformis n. isp.; and P. thalassiformis n. isp. Together with large, palmate burrows, probably produced by crustacean decapods, they are elite trace fossils. The Patagonichnus burrow systems consist of many distinctive concentrically laminated elementary components including: horizontal and vertical, conical and helicoidal bulbs and swellings; vertical shafts; straight and paired branches; and cone-in-cone structures. Hollow, branched, straight or helicoidal inner lined tubes connect these structures. According to current definitions, isolated components could be misidentified as Asterosoma von Otto, Rosselia Dahmer, Cylindrichnus Toots in Howard, Gyrolithes De Saporta, Helicodromites Berger, or Chondrites von Sternberg. The concentrically laminated bulbs of Patagonichnus represent the feeding behavior of a worm-like organism, spiraling or circling its anterior end within the sediment while its posterior end was temporarily fixed at the terminal position of the inner tube. Spiraling or circling movements, associated with peripheral backfilling and central migration of a probing tube, result in discrete, discordant groups of laminae. In cross-section, these laminae appear as alternate half-spirals, continuous spirals, or concentric circles, which represent the plan view of a helicoidal or conical whole structure. The complete structure is interpreted as a fodinichnion. The connecting, hollow inner tube is interpreted as a dwelling structure and the associated backfilled bulb as a feeding structure, probably made by gregarious polychaetes. Analogies in size, form, geometry, and behavior suggest polychaetes as the trace-makers of the Patagonichnus burrows.