Onset of the Rhaetian Transgression in deep waters at Lilstock, North Somerset: Microvertebrate faunas

Rhaetian seas in the latest Triassic transgressed from west to east over the southwest of the UK, reaching parts of South Wales and the North Somerset coast first. Evidence comes from marine conditions in the pre-Rhaetian Williton Member, a unit not seen further east. Here, we confirm this hypothesis with reports of diverse, Rhaetian-style fish faunas in the Williton Member, as well as evidence that the Westbury Formation bonebeds are from deeper waters than most others in the region. Our study focuses on the classic coastal section at Lilstock, which shows the entire Penarth Group and the Triassic-Jurassic boundary. The Williton Member fossil beds yield Rhaetian-type chondrichthyans (Lissodus, denticles), osteichthyan teeth (Gyrolepis, Sargodon, Saurichthys), and bivalves. The basal and higher bone beds of the Westbury Formation are dominated by osteichthyans (86.8 %, 84.7 %), with chondrichthyans relatively rare (13.2 %, 15.3 %), the opposite of what is seen at other locations in the southwest of the UK (16–59 % osteichthyans; 41–84 % chondrichthyans). The similarity of the faunal composition in the basal and higher Rhaetian bone beds is also unusual, and the dominance by bony fishes can be interpreted as evidence for deeper water than further to the east.