The late Triassic and early Jurassic fissure faunas from Bristol and South Wales: Stratigraphy and setting

Wales were traditionally referred to “upland” environmental settings, and assigned an overall time range of some 60 Myr; we reject both of these viewpoints. In terms of the environmental setting, based upon the evidence of palaeogeography, sediments, and fossils, we show that all the Triassic-aged fissures were near-coastal and embedded in karst formed on palaeo-islands in a subtropical archipelago. Further, the dates of these fissure fills in both regions do not span the Carnian to Rhaetian, as commonly supposed, but instead they are probably all Rhaetian. We update information on the Charles Moore Holwell “Microlestes” fissure assemblage and demonstrate that the marine fauna confirms a Rhaetian age. As proposed by previous authors, younger fissure fills around the St. Brides area of South Wales were also formed on an island, but during the Hettangian–Sinemurian. The youngest fissure deposit in the region is Pliensbachian. The age range of the fissures thus shrinks to c. 20 Myr, and the first episode of tetrapod fissure infilling shifts from the time of the Carnian “pluvial” episode, to the early Rhaetian, when western Europe was undergoing major rifting associated with the beginning of the opening of the North Atlantic, and when the Rhaetian sea flooded over landscapes that had long been purely terrestrial and arid. We recognise clear changes in the faunal composition of the terrestrial vertebrate biota from the early Rhaetian, dominated by reptile taxa, with some mammaliamorphs, that then declined rapidly in diversity before the Hettangian. This herpetofauna was followed by three faunal assemblages in the Early Jurassic of St. Brides which became increasingly diverse following the end-Triassic mass extinction; each had prominent mammaliamorphs as well as high numbers, but few species, of rhynchocephalians.