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When did Thecodontosaurus live?
How Ancient is the Bristol Dinosaur?
The Bristol dinosaur Thecodontosaurus obviously lived long ago - it is a dinosaur after all. Dinosaurs are known worldwide during the Mesozoic Era, which is divided into the Triassic, Jurassic, and Cretaceous periods. The appropriate rocks in the Bristol area are of Triassic (251-200 million years, Myr) or Jurassic (200 to 150 Myr) age. Where does Thecodontosaurus fit?
It's actually hard to give a straight answer. The best we can say is that Thecodontosaurus is either Late Triassic or Early Jurassic in age, and almost certainly Late Triassic. But this still includes quite a long span of time, from say 230 to 170 Myr ago, or more probably somewhere between 220 and 200 Myr. Why the problem in dating these fossils?
It's all to do with where they are preserved. The bones were found in cave or fissure fillings formed in more ancient limestones, of Carboniferous age. So the age can be bracketed as some time between the Carboniferous (360-285 Myr) and the present day. From comparisons with similar cave systems around Bristol, it is known that the cave-making episode began in the Late Triassic, and ran into the Early Jurassic, when the Carboniferous limestones were uplifted to form a rolling hilly landscape. In fact the hills around Bristol today are pretty much as they were then. But sea levels were higher, so the hills, including Bristol city itself, formed islands in a tropical sea.
The Durdham Down fissure fillings are now known to be distinct from the nearby Dolomitic or Magnesian Conglomerate. Riley and Stutchbury (1840:349) mistakenly regarded the two sets of sediments as equivalent and representing the 'lower divisions of the new red sandstone series', hence Permian. On the other hand, Etheridge (1870) treated both as equivalent to the German Muschelkalk or Lower Keuper (Middle Triassic), while Huene (1908a, b, c) preferred a lower assignment, equivalent to the German Muschelkalk or Buntsandstein, based on the assumption that the Bristol reptiles were very like some archosaurs reported from the Middle Triassic of the English Midlands.
Moore (1867, 1881) ascribed a Rhaetian (latest Triassic) age to the Dolomitic Conglomerate, on the basis of its topographic position and on similarities of several fissure faunas to those of unequivocal Rhaetian bone bed deposits of the region. Moore (1867) described 'the tableland of Durdham Down, where numerous veins, one of them 18 ft. in thickness, traversed the Carboniferous Limestone. One of these, near the Zoological Gardens, was proved to be of the age of the Lower Lias, - Ammonites, Echini, Foraminifera and the other remains of that age being exhibited by the author, taken from between the walk of the Carboniferous Limestone 30 feet from the surface. In another vein near the Suspension Bridge he had found numerous scattered fish-remains of Rhaetic age, including Saurichthys and Achrodus [sic], mentioned above, found also with the Thecodontosaurus and Palaeosaurus on the Mendips; and he could therefore come to no other conclusion than that the deposits were equivalents in time, and that the Durdham Down reptilia must be referred to the Rhaetic age.'
The fissures in the Bristol area appear to have been formed by a major pluvial (= rainy) episode during the Carnian (Simms, 1994), and the infilling sediments range in age from Late Triassic (?Carnian) to Early Jurassic (Hettangian). The presence of the primitive prosauropod Thecodontosaurus suggests a Late Triassic age, perhaps late Carnian or early Norian (say, 225-215 Myr) for the Durdham Down deposit. Diphydontosaurus avonis, known also from Tytherington and Slickstones (Cromhall) quarries, might indicate a younger age: some of the Tytherington deposits at least are dated palynologically as Rhaetian (Marshall and Whiteside, 1980), and they have also yielded abundant dinosaur bones, probably of Thecodontosaurus. The presence of a phytosaur, Paleosaurus platyodon, at Durdham Down at least guarantees a Late Triassic age, since phytosaurs are not known after the end of the Triassic, and pre-Carnian records are equivocal.
The Bristol dinosaur Thecodontosaurus obviously lived long ago - it is a dinosaur after all. Dinosaurs are known worldwide during the Mesozoic Era, which is divided into the Triassic, Jurassic, and Cretaceous periods. The appropriate rocks in the Bristol area are of Triassic (251-200 million years, Myr) or Jurassic (200 to 150 Myr) age. Where does Thecodontosaurus fit?
It's actually hard to give a straight answer. The best we can say is that Thecodontosaurus is either Late Triassic or Early Jurassic in age, and almost certainly Late Triassic. But this still includes quite a long span of time, from say 230 to 170 Myr ago, or more probably somewhere between 220 and 200 Myr. Why the problem in dating these fossils?
It's all to do with where they are preserved. The bones were found in cave or fissure fillings formed in more ancient limestones, of Carboniferous age. So the age can be bracketed as some time between the Carboniferous (360-285 Myr) and the present day. From comparisons with similar cave systems around Bristol, it is known that the cave-making episode began in the Late Triassic, and ran into the Early Jurassic, when the Carboniferous limestones were uplifted to form a rolling hilly landscape. In fact the hills around Bristol today are pretty much as they were then. But sea levels were higher, so the hills, including Bristol city itself, formed islands in a tropical sea.
The Durdham Down fissure fillings are now known to be distinct from the nearby Dolomitic or Magnesian Conglomerate. Riley and Stutchbury (1840:349) mistakenly regarded the two sets of sediments as equivalent and representing the 'lower divisions of the new red sandstone series', hence Permian. On the other hand, Etheridge (1870) treated both as equivalent to the German Muschelkalk or Lower Keuper (Middle Triassic), while Huene (1908a, b, c) preferred a lower assignment, equivalent to the German Muschelkalk or Buntsandstein, based on the assumption that the Bristol reptiles were very like some archosaurs reported from the Middle Triassic of the English Midlands.
Moore (1867, 1881) ascribed a Rhaetian (latest Triassic) age to the Dolomitic Conglomerate, on the basis of its topographic position and on similarities of several fissure faunas to those of unequivocal Rhaetian bone bed deposits of the region. Moore (1867) described 'the tableland of Durdham Down, where numerous veins, one of them 18 ft. in thickness, traversed the Carboniferous Limestone. One of these, near the Zoological Gardens, was proved to be of the age of the Lower Lias, - Ammonites, Echini, Foraminifera and the other remains of that age being exhibited by the author, taken from between the walk of the Carboniferous Limestone 30 feet from the surface. In another vein near the Suspension Bridge he had found numerous scattered fish-remains of Rhaetic age, including Saurichthys and Achrodus [sic], mentioned above, found also with the Thecodontosaurus and Palaeosaurus on the Mendips; and he could therefore come to no other conclusion than that the deposits were equivalents in time, and that the Durdham Down reptilia must be referred to the Rhaetic age.'
The fissures in the Bristol area appear to have been formed by a major pluvial (= rainy) episode during the Carnian (Simms, 1994), and the infilling sediments range in age from Late Triassic (?Carnian) to Early Jurassic (Hettangian). The presence of the primitive prosauropod Thecodontosaurus suggests a Late Triassic age, perhaps late Carnian or early Norian (say, 225-215 Myr) for the Durdham Down deposit. Diphydontosaurus avonis, known also from Tytherington and Slickstones (Cromhall) quarries, might indicate a younger age: some of the Tytherington deposits at least are dated palynologically as Rhaetian (Marshall and Whiteside, 1980), and they have also yielded abundant dinosaur bones, probably of Thecodontosaurus. The presence of a phytosaur, Paleosaurus platyodon, at Durdham Down at least guarantees a Late Triassic age, since phytosaurs are not known after the end of the Triassic, and pre-Carnian records are equivocal.
