Palaeoenvironmental and functional interpretation of Rhizocorallium jenense spinosus (ichnosubsp. nov.) from the lower Jurassic of Asturias, northern Spain

The new ichnosubspecies Rhizocorallium jenense spinosus from the Late Sinemurian (Early Jurassic) of Asturias (northern Spain) is described. This ichnotaxon occurs in three outcrops (El Puntal, Punta Rodiles and Punta La Llastra), near the Villaviciosa estuary, belonging to the Buerres Member of the Rodiles Formation. Rhizocorallium jenense spinosus is similar in shape and size to the ichnospecies Rhizocorallium jenense (herein ichnosubspecies R. jenense jenense) in comprising short and more or less straight U-shaped spreite burrows, commonly obliquely oriented with respect to the bedding plane, but is characterized by the presence of spine-shaped protuberances on the outer margin of the arm. Number of observed spine-shaped protuberances per specimen is variable, being commonly 2–4. Protuberances form obtuse, equilateral or acute triangles. The apical end of the spine-shaped protuberance varies from pointed to rounded. The specimens were emplaced in soft or slightly stiffer substrates to firmgrounds. Recurrent distribution of the spine-shaped protuberances reveals a well programmed behavioural pattern of the tracemaker, previously unknown in Rhizocorallium. Several strategies are envisaged: (a) a cache strategy, implying that the tracemaker collected organic matter during favourable times and subsequently stored material in the spine-shaped protuberances for resource-poorer times, (b) a farming strategy, the spine-shaped protuberances being used for concentration of micro-organisms (bacteria, fungi), and (c) a brooding strategy of the tracemaker, revealing a relationship between brood care and oxygen provision, with the location of the spine-shaped protuberances on the external margin of the arms related to a better water circulation.