Taphonomic alteration processes in Bouleiceras (Jurassic Ammonitina) of the Iberian Range. Paleoenvironmental and paleobiogeographical implications

Abstract

The genus Bouleiceras (Ammonitina, Bouleiceratinae) appears in the Iberian Range from the Tenuicostatum Zone to the Serpentinum Zone (Lower Toarcian, Jurassic). It displays a lower frequency in number of specimens (approximately 1%) in relation to other groups of ammonoids at the same levels within the studied sections. The studied species of this genus are B. nitescens Thévenin, B. elegans Arkell, B. arabicum Arkell, B. tumidum Arkell, B. marraticum Arkell, B. ibericum Martínez and B.? betetensis Martinez. They correspond to incomplete internal moulds and the record includes a scarce number of specimens per species. By identifying different types of taphonomic alterations within the specimens (biostratinomic and fossildiagenetic) it was possible to recognise the removal processes (resedimentation or reelaboration). Most of the specimens present different mechanisms of taphonomic alteration that denote prolonged processes of necroplanktonic drift and resedimentation or reelaboration such as colonisation, facet abrasion, distortion, disarticulation in internal moulds, etc. Despite the scarcity of material they can be classified as taphonic populations of type 3. Taphorecords point to a shallow marine environment with low sedimentation rates and episodic high-energy events. None of the data seems to correspond to areas linked to favourable environments for the ontogenic development of these species.