Abstract

The first circum-Pacific record of the Bajocian dimorphic ammonite Strigoceras septicarinatum, from the Quebrada San Pedro area, Antofagasta, northern Chile, is described and illustrated. Morphological features of two specimens display macroconch and microconch characters of a single biospecies, usually assigned to separate morphospecies of the West Tethyan genera Strigoceras [macroconch] and Cadomoceras [microconch]. Preservational features of these two specimens, found in situ in the same stratigraphic level at the top of the Torcazas Formation, correspond to resedimented elements of the uppermost Bajocian Dimorphinites Biohorizon, contemporaneous with the sedimentary matrix. These specimens represent dimorphic shells produced by extremely scarce individuals inhabiting the Tarapaca Basin, without evidence of lasting biostratinomic modifications such as sorting by necroplanktic drift or biostratinomic encrusting. In the Tarapaca Basin, the distribution of ammonite shells was taphonomically and ecologically driven by regional changes of relative sea level. Taphonomic, paleoecological and paleobiogeographical observations in the areas of Quebrada San Pedro and Caracoles corroborate the development of an advanced-deepening phase, with episodes of sedimentary starvation, during the uppermost Bajocian, Parkinsoni Zone. This phase characterizes the last episode within a deepening half-cycle of a third order and the maximum deepening of a second-order, transgressive/regressive facies cycle, in the marine, back-arc Tarapaca Basin during the Bajocian-Bathonian interval. These paleontological and paleoenvironmental results represent new criteria to characterize the Dimorphinites Biohorizon in the Tarapaca Basin, allowing the identification the upper Bajocian Parkinsoni Zone in the East Pacific Subrealm, as well as chronocorrelation between the West Tethyan and East Pacific basins at the Bajocian/Bathonian boundary. (C) 2014 Elsevier B.V. All rights reserved.