Tempestitic shell beds formed by a new serpulid polychaete from the Bajocian (Middle Jurassic) of the Central High Atlas (Morocco)

Serpulid-dominated shell beds from the Lower Bajocian (Middle Jurassic) of the Central High Atlas (Morocco) are primarily formed by quadrangular tubes of the serpulid polychaete Nogrobs moroccensis sp. nov. The transverse tube ornament consisting predominately of wrinkles, the lack of narrow spirals and the shortness or absence of attached posterior tube portion represent the diagnostic features of this new species. Tubes of Nogrobs moroccensis co-occur with gastropods and oyster–crinoidal debris. We suggest that these serpulids were free-lying in their adult stage and formed benthic meadows on soft-bottom habitats. They frequently show parallel alignment in plane views in both graded and non-graded, few-cm-thick shell beds, corresponding to distal tempestites deposited close to and below storm wave base in an outer-ramp environment. The excellent preservation of serpulids implies short residence time on the seafloor, and the lack of compositional mixing with other communities and their restriction to outer-ramp habitats indicate that they represent parautochthonous assemblages. Analyses of serpulid orientation on the bottom bedding planes of shell beds revealed weak unidirectional arrangement of tubes, whereas orientation on the top bedding planes revealed strong unidirectional arrangement of tubes, with significantly different orientation relative to the bottom planes. This change could suggest a shift from predominantly a current component of the combined storm flow (with uneven seafloor surface and adhesion of skeletal grains to muddy substrate) to significant oscillatory component of the waning storm, as also suggested by bidirectional orientation of gastropods.