The rosetted trace fossil Dactyloidites ottoi (Geinitz) commonly has been associated with shallow-water, high-energy marine environments, although detailed sedimentological analyses of the host deposits are unfortunately few. The Aptian–Albian Otoio Formation (Basque-Cantabrian Basin) provides an excellent opportunity to test the paleobathymetric and paleoenvironmental controls on the restricted distribution of D. ottoi. The Otoio Formation records a great variety of shallow-water marine environments, but D. ottoi occurs only in intervals interpreted as fluvial-dominated deltas (Gilbert-type and mouth bar-type deltas). Moreover, detailed facies analysis and geometric relationships of the deltaic deposits suggest the following main controls on the distribution of D. ottoi: (1) paleodepth between 0 and 3 m, quantified from the foreset heights of Gilbert-type deltas; (2) siliciclastic and organic matter-rich sandy substrate; (3) high and discontinuous sedimentation rates; and (4) reduced salinity, responsible for the small size of the trace from the Otoio Formation. These observations are in agreement with published data from other localities.

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