The terrestrial environment in which the greater part of the Triassic
succession was formed was generally unfavourable for the preservation
of fossils. An exception is the chance preservation of lizards, reptiles
and dinosaurs in cave and fissure-infills in the Carboniferous Limestone,
as demonstrated by the occurrence of the earliest flying vertebrate, Kuehneosaurus,
at Emborough. However, the gradual advance of the sea across the
Mendip region in the Late Triassic resulted in the deposition of
the marine deposits of the Penarth Group, in which fossils are locally
common. A bone bed is sometimes developed at the base of the Penarth
Group (named the 'Rhaetic Bone Bed'), containing the bones and teeth
of marine and freshwater fish and reptiles (including Ichthyosaurus and Plesiosaurus).
The overlying sediments represent a variety of shallow marine, lagoonal
and near-shore environments, and contain a low diversity fauna that
includes the bivalves Modiolus, Protocardia, Rhaetavicula
contorta and Chlamys valoniensis. A bone of the dinosaur Scelidosaurus in
the succession near Wells shows that land was nearby.