Research paper

**The annelid Rotularia spirulaea (Lamarck, 1818) (Polychaeta, Serpulidae) from the Eocene marls of the Pamplona Basin (Navarre): microstructure, taphonomy and palaeoecology**

JAVIER ELORZA
Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Apartado de correos 644, 48080 Bilbao, Spain. josejavier.elorza@ehu.eus
Corresponding author

HUMBERTO ASTIBIA
Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Apartado de correos 644, 48080 Bilbao, Spain. humberto.astibia@ehu.es

ABSTRACT

This work recognizes the presence of numerous fossils of the annelid serpulid Rotularia spirulaea (Lamarck, 1818) in the Ardanatz Sandstone and Ilundain Marl formations from the Eocene (Bartonian and (?) Priabonian) of the Pamplona Basin (Navarra). The different calcitic layers of the prismatic microstructure of its tube are determined. It is formed by two shell layers: 1) a thick outer layer (1) composed of three zones in continuity among them; and, 2) another extremely thin inner layer (2) that delineates the tube cavity (lumen). R. spirulaea shells show a remarkable ecophenotypic plasticity. The trochospiral, trochospiral-planispiral and planispiral coiling of the specimens could be related to the characteristics of the substrate –without cohesion, moderately cohesive or cohesive, respectively–, which would condition the ability to support the organism during its ontogeny. The arrangement of the R. spirulaea tube during the trochospiral stage would be a stable horizontal resting position. Once this stage had been overcome, it could pass to a planispiral coiling, with the formation of a stabilizing keel to acquire gradually a vertical position within the muddy substrate. It is from this stage, when the unrolled part of the tube would form a chimney-like structure, that it would be partially overhang from the sediment. The lumen is usually ﬁlled by sediment and/or by three distinct stages of calcite crystal growths, and by geopetal textures, which would support the idea of a vertical life position in the maturity stage of individuals.

Key words: Rotularia spirulaea, morphology, taphonomy, autoecology, Paleogene.

How to cite: Elorza, J. & Astibia, H. 2017. El anélido Rotularia spirulaea (Lamarck, 1818) (Polychaeta, Serpulidae) de las margas del Eoceno de la Cuenca de Pamplona (Navarra): microestructura, tafonomía y paleoecología. [The annelid Rotularia spirulaea (Lamarck, 1818) (Polychaeta, Serpulidae) from the Eocene marls of the Pamplona Basin (Navarre): microstructure, taphonomy and palaeoecology]. Spanish Journal of Palaeontology, 32 (2), 343-366.

Received 25 February 2017, Accepted 22 May 2017, Published 31 December 2017

https://doi.org/10.7203/sjp.32.2.17048

(Spanish only)