Microvertebrates from multiple bone beds in the Rhaetian of the M4-M5 motorway junction, South Gloucestershire, UK

The Rhaetian (latest Triassic) is best known for its basal bone bed, but there are numerous other bone-rich horizons in the succession. Boreholes taken around the M4–M5 motorway junction in SW England provide measured sections with multiple Rhaetian bone beds. The microvertebrate samples in the various bone beds differ through time in their composition and in average specimen size. The onset of the Rhaetian transgression accumulated organic debris to form a fossiliferous layer high in biodiversity at the base of the Westbury Formation. The bone bed at the top of the Westbury Formation represents a community with lower biodiversity. The bone beds differ in their faunas: chondrichthyan teeth are dominant in the basal bone bed, but actinopterygians dominate the higher bone bed. These differences could be taphonomic, but are more likely evidence for ecological-evolutionary changes. Further, a change from larger to smaller specimen sizes up-sequence allows rejection of an earlier idea that the successive bone beds represented multiple reworkings of older bone beds.