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Trace Fossil Rhizocorallium From The Middle Triassic Of The Betic Cordillera, Southern Spain: Characterization And Environmental Implications

¹ Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n, 18071 Granada, Spain fjrtovar{at}ugr.es

A detailed characterization of Rhizocorallium assemblages from the Middle Triassic sediments (Ladinian, Muschelkalk facies) of the Betic Cordillera, Spain, and their paleoenvironmental interpretation are provided. Rhizocorallium jenense shows variable U-shaped structures: straight and short, slightly arcuate or sinuous, and fan-shaped. Spreiten are protrusive, and vertically retrusive forms are frequent. Scratches are very abundant, more or less continuous and parallel on the marginal tubes, and more prominent and shorter on the spreiten. Rhizocorallium shows a major orientation at 121°E. Rhizocorallium irregulare is scarce and occurs as long, straight-to-slightly-sinuous or arcuate U-shaped forms and usually exhibits scratch marks. The dominance of R. jenense and its orientation, similar to that of gutter casts, reveal colonization during high-energy regimes, mainly related to storm phases. The presence of R. irregulare reflects comparatively quieter intervals, probably interstorm phases. Trace-fossil features reveal firm-ground consistency and Glossifungites ichnofacies. The occurrence of dominant, suspension-feeding R. jenense and scarce, deposit-feeding R. irregulare reflects a high concentration of nutrients in the water column and comparatively organic-poor sediments. The high concentration of Rhizocorallium indicates a large population of trace makers and, probably, opportunistic behavior. Size-frequency curves, however, are coherent with steady-state populations (taphonomic bias?) and with a single generation. Scratch-mark casts could suggest crustaceans as producers, with some appendages in continuous contact with the substrate.