Important vertebrate faunas occur in fissure deposits of Late Triassic–Jurassic age in SW Britain. Although the faunas are well described, their age and palaeoenvironment remain poorly understood. One such fissure system was documented in detail during quarrying operations at Tytherington and has yielded in situ palynomorphs that add much information concerning its age and palaeoenvironment. Significantly, the Tytherington fauna is of the sauropsid type that has generally been dated as Norian or pre-Penarth Group transgression and was also regarded as representing a distinct upland fauna. The palynomorphs, which include a significant marine component, demonstrate that the Tytherington Triassic fissures are infilled with Late Triassic (Rhaetian) sediments that match specific levels in the Westbury Formation. In addition, many of the Tytherington solutional fissures probably formed during the Rhaetian and are consistent with a fluctuating saline to freshwater environment. There is no prima facie evidence of solutional formation and infilling of the reptile-bearing deposits before the Rhaetian trangression. The fissure reptile fauna, which includes the early dinosaur Thecodontosaurus, inhabited a small fire-swept limestone island in the Rhaetian sea. The features of the herpetofauna are entirely consistent with this island model which has Quaternary analogues.