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What was Thecodontosaurus' world like?
How Did the Fossils Come to be Preserved?
The Bristol dinosaur Thecodontosaurus is found in an unusual rock type. The bones are contained in a mass of broken fragments of rock, mainly grey limestone, set in a matrix (= background material) of pinkish fragments and mud. The rock type is quite like the stone from which the older parts of Bristol Temple Meads Station are built. It is called a breccia, since the limestone fragments are sharp-edged, obviously broken up from pre-existing rock, and dumped rapidly, without much transport (transport would have rounded off the corners of the blocks).
The early records show that the 1834 bones came from a limestone quarry in the northern Clifton area of Bristol, and that they were found in an ancient cave or fissure cut into the limestone. Similar fissures are known from many sites around Bristol and in South Wales, where they represent the `Mendip Archipelago', a string of islands located in tropical climates. Picture the scene 220 million years ago, when Bristol was more like the Seychelles or the Caribbean today, and dinosaurs enjoyed warm weather, and could see other white, limestone islands over the hot seas. How do we know this?
In the Triassic, Britain lay just north of the Equator, and it, and the rest of the northern continents, have drifted northwards ever since. Geology and fossils confirm this. British Triassic rocks contain a great deal of evidence for the tropical conditions: ancient sand dunes, tropical soils (`calcretes', or limestone concretions, that show there was monsoonal rainfall), desert dust layers, and red, iron-stained sediments. The fossils too - seed-ferns, conifers, and reptiles - also suggest hot and dry conditions.
How do we know the bones actually came from caves? The early geologists tried to show what the Bristol dinosaur site looked like. Etheridge (1870:189, fig. 6) reproduced a sketch by Stutchbury of the occurrence of the Durdham Down bones, showing a saucer-shaped depression on top of steeply dipping Carboniferous Limestone. Disarticulated bones are shown scattered throughout the conglomeratic infill, with some resting directly on the limestone. Moore (1881:73, fig. 2), however, gave a sketch surveyed by W. Sanders which suggests that the fossiliferous sediment rested on a rather flat, eroded Carboniferous pavement. Halstead and Nicoll (1971) re-interpreted the Durdham Down deposit as a collapsed cavern, regarding the saucer-shaped depression indicated by Etheridge as its base, the rest having been eroded since Late Triassic times. The nature of the brecciated sediment lends support to this idea, consisting as it does of angular fragments of Carboniferous Limestone in a matrix of yellow marl.
Most of the dinosaur bones were not found in articulation and many seem to show signs of transport, perhaps as a result of having been washed by flood water into the cave. Other specimens, however, are articulated - most notably the forelimb in the Peabody Museum, Yale University, USA. Several individuals are represented: Huene (1908) counted 20 femora (= thigh bones) and fragments, indicating the presence of at least ten individuals.
Other fossils from the Durdham Down site are rare. There is the phytosaur Paleosaurus platyodon, based on a tooth (Riley and Stutchbury (1840;pl. 29, fig. 5), as well as other similar teeth in the Natural History Museum, London (hitherto unfigured), with, perhaps, a putative phytosaur humerus named Rileya bristolensis Huene, 1902. In addition, two specimens of the sphenodontid Diphydontosaurus avonis were noted as the partial skeleton of a 'lizard' and a small jaw of the sphenodontid Clevosaurus by Halstead and Nicoll (1971:pl. 23B) - both specimens are in the collections of Bristol City Museum.
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Unprepared breccia rock with Thecodontosaurus material |
The early records show that the 1834 bones came from a limestone quarry in the northern Clifton area of Bristol, and that they were found in an ancient cave or fissure cut into the limestone. Similar fissures are known from many sites around Bristol and in South Wales, where they represent the `Mendip Archipelago', a string of islands located in tropical climates. Picture the scene 220 million years ago, when Bristol was more like the Seychelles or the Caribbean today, and dinosaurs enjoyed warm weather, and could see other white, limestone islands over the hot seas. How do we know this?
In the Triassic, Britain lay just north of the Equator, and it, and the rest of the northern continents, have drifted northwards ever since. Geology and fossils confirm this. British Triassic rocks contain a great deal of evidence for the tropical conditions: ancient sand dunes, tropical soils (`calcretes', or limestone concretions, that show there was monsoonal rainfall), desert dust layers, and red, iron-stained sediments. The fossils too - seed-ferns, conifers, and reptiles - also suggest hot and dry conditions.
How do we know the bones actually came from caves? The early geologists tried to show what the Bristol dinosaur site looked like. Etheridge (1870:189, fig. 6) reproduced a sketch by Stutchbury of the occurrence of the Durdham Down bones, showing a saucer-shaped depression on top of steeply dipping Carboniferous Limestone. Disarticulated bones are shown scattered throughout the conglomeratic infill, with some resting directly on the limestone. Moore (1881:73, fig. 2), however, gave a sketch surveyed by W. Sanders which suggests that the fossiliferous sediment rested on a rather flat, eroded Carboniferous pavement. Halstead and Nicoll (1971) re-interpreted the Durdham Down deposit as a collapsed cavern, regarding the saucer-shaped depression indicated by Etheridge as its base, the rest having been eroded since Late Triassic times. The nature of the brecciated sediment lends support to this idea, consisting as it does of angular fragments of Carboniferous Limestone in a matrix of yellow marl.
Most of the dinosaur bones were not found in articulation and many seem to show signs of transport, perhaps as a result of having been washed by flood water into the cave. Other specimens, however, are articulated - most notably the forelimb in the Peabody Museum, Yale University, USA. Several individuals are represented: Huene (1908) counted 20 femora (= thigh bones) and fragments, indicating the presence of at least ten individuals.
Other fossils from the Durdham Down site are rare. There is the phytosaur Paleosaurus platyodon, based on a tooth (Riley and Stutchbury (1840;pl. 29, fig. 5), as well as other similar teeth in the Natural History Museum, London (hitherto unfigured), with, perhaps, a putative phytosaur humerus named Rileya bristolensis Huene, 1902. In addition, two specimens of the sphenodontid Diphydontosaurus avonis were noted as the partial skeleton of a 'lizard' and a small jaw of the sphenodontid Clevosaurus by Halstead and Nicoll (1971:pl. 23B) - both specimens are in the collections of Bristol City Museum.
