The Rhaetian transgression marked a major change in landscape. The Permian and Triassic had been a time of terrestrial conditions across Europe, including much of mainland UK, as well as the North Sea and Irish Sea, represented by red bed clastic successions. Seas flooded across Europe at 205.7 Ma and the shift from terrestrial to marine environments is marked in the UK by the switch from the red beds of the Mercia Mudstone Group to the black mudstones and shelly limestones and sandstones of the Penarth Group. The area around Bristol was marked by a complex landscape in which an archipelago of islands of Carboniferous limestone was formed in the new shallow seas. The application of new methods in geographical information systems allows a detailed exploration of a number of conformable surfaces, the unconformity between the underlying Paleozoic rocks and the overlying Mesozoic strata, as well as levels within the latest Triassic sediments, marking the advance of the sea and interactions with the coeval tectonics, which caused some islands to rise and some basins to descend. The new geographical information system models show a sequence of palaeogeographical reconstructions of the archipelago and relate this to the island tetrapod faunas, which show strong evidence of the species–area effect.

Supplementary material: Supplementary tables S1-S6 and 2D island map GIS files are available at https://doi.org/10.6084/m9.figshare.c.5273256

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