The age, fauna and palaeoenvironment of the Late Triassic fissure deposits of Tytherington, South Gloucestershire, UK

Abstract

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 reptilebearing 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.

The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades

“…Terrestrisuchus gracilis Crush, 1984AGE: ?Rhaetian, Late Triassic (Robinson 1957a, 1957b, Whiteside and Marshall, 2008 The abundant taxon is known from dozens of specimens, from articulated and disarticulated crania, and postcrania. A few of the three-dimensionally preserved bones were prepared out of the matrix and formed the basis of the description by Crush (1984).…”

A marine vertebrate fauna from the Late Triassic of Somerset, and a review of British placodonts

“…Importantly, the older fissures, generally filled with red mudstones and marls, were formed in tectonic fissures and solutional caves, and filled during the Late Triassic. Some, such as the Tytherington fissures, are early Rhaetian based on palynomorphs, with a mix of terrestrial and marine influences (Whiteside and Marshall, 2008). Stratigraphically younger are the Type 2.1 fissure fills, comprising grey, clastic limestones with excellent microvertebrate fossils, which are older than the Type 2.2 fills, as shown by cross cutting relations.…”

“…In the Mendip Hills, the Bristol area and South Wales, folded Palaeozoic rocks represented topographic highs that were not completely submerged by the Rhaetian transgression, but persisted above sea level forming an archipelago of limestone islands (Fig. 3) (Robinson, 1957;Whiteside and Marshall, 2008). Karstic weathering and tectonic activity created fissures in the limestone, which were infilled with sediment.…”

“…These infills have attracted a great deal of interest since they have yielded remains of early mammals and dinosaurs, such as Haramiya and Thecodontosaurus respectively (Moore, 1881;Richardson, 1911;Kühne, 1947;Whiteside and Marshall, 2008;van der Berg et al, 2012;Foffa et al, 2014). Dating of the fissures has been problematic, with many being Rhaetian in age, others perhaps older and some younger (Kühne, 1947;Robinson, 1957;Whiteside and Marshall, 2008). In general the faunas of vertebrate-bearing fissures, having formed on the tops of islands, are quite distinct from those of marine, vertebrate-bearing Rhaetian bone beds.…”

The origin and early radiation of dinosaurs

“…Similar assemblages have been reported from Greenland , and Dzik et al (2008) recently reported theropod dinosaurs from the latest Triassic of Poland. Poorly dated fissure fills from western Europe record the presence of sauropodomorphs and possible theropods (e.g., Benton et al, 2000;Yates, 2003a); these are generally thought to be latest Triassic to Early Jurassic in age (Whiteside and Marshall, 2008). Dinosaurs are conspicuously absent from the Late Triassic of Asia, but an extensive sauropodomorph-dominated assemblage is known from the Lower Jurassic Lufeng Formation of Yunnan, China; this assemblage also includes rare ornithischians and theropods (e.g., Luo and Wu, 1994).…”