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partial fin spine.
Moulds of bivalves.
(Field of view 60mm)
partial fin spine.
The fossils shown here have been collected from various locations in Somerset and South Gloucestershire; they are typical of what can be found by anyone prepared to put in a little time searching. Before considering the fossils in general, it's interesting, and useful, to take a closer look at the geology in order to know exactly which rocks are fossiliferous. The Westbury formation's position in the Triassic period is shown in a table at the bottom of the page; most useful are the formation names on the far right as each, in this case, can be very distinctive when seen in a cliff section, and that can be applied, of course, to loose material at beach level derived from that cliff. Perhaps the most recogniseable feature is the red coloured mudstone of the Twyning Mudstone Formation which gives way to a band of grey coloured mudstone of the Blue Anchor Formation just below the base of the Penarth group.
The rocks below the Penarth Group do not generally contain fossils in the South Gloucesterhire area, however on the Somerset coast fossils similar to those found in the Westbury Formation are present in the Blue Anchor Formation, but not in such numbers. The point of main interest for the fossil enthusiast is the Westbury Formation above. This is composed, mainly, of dark shales containing a few thin bands of limestone and sandstone. Some of these bands are extremely rich in fossils and are often referred to as bone-beds. Of particular note is the lowest of these units that is present in certain areas of South Gloucestershire. Here, the bone bed sits directly on the Blue Anchor Formation, and so is quite easy to see in the cliff section. Close-up, it's easily recogniseable, too, being a distinctive conglomerate of bone fragments, coprolites, and clasts of the Blue Anchor Formation. The shales do contain fossils, but they are less abundant, and not so well preserved; it's still well worth a look, though!
The photograph on the right, near the page bottom, shows a typical section of the formations discussed as seen in South Gloucestershire. The Somerset coastal exposures have a better developed Blue Anchor formation, which is over 30m thick. The red and light grey material in the cliff is believed to have been deposited in a generally hot, arid, environment that occasionally experienced heavy rains; this cycle is believed to be responsible for the minerals such as gypsum which are present; although the Blue Anchor Formation on the Somerset coast has experienced a marine influence towards the top of the formation as is evident from the type of fossils present. The Westbury Formation is believed to represent a period of deposition in shallow marine conditions, although how the basal bone bed was formed which is also rich in fossils of terrestrial, as well as marine, organisms is debated.
So what fossils are found in the Westbury Formation, you may ask? Well, just about anything that was swimming, walking or crawling in the Late Triassic, with the qualification that what you find will generally only be bits of, rather than whole, specimens. Perhaps the most easily recognised, and most collected, would be skeletal elements of plesiosaurs and ichthyosaurs: vertebrae, limb bones and teeth often being encountered. However, these are not the most abundant and easily collected fossils present.
Fish teeth are numerous, though often less than 1cm in length. However, those of certain species of shark are often exquisitely shaped, this being best appreciated by viewing with a hand lens. The two most abundant fossils likely to be encountered are teeth of Severnichthys acuminatus and Lissodus minimus. The former is a bony fish, and the latter a shark. Other shark teeth are often found, too. Those of Hybodus minor are fairly common, though not as prolific as the ones first mentioned. Other relatively common bony fish teeth are those of Sargodon tomicus, the most frequently encountered being the rounded molariform teeth. The largest evidence likely to be found of bony fish are the tooth plates of Ceratodus latissimus, these are generally over 5cm in length. Ceratodus latissimus was a freshwater lungfish, so the preservation of its teeth along with a predominately marine fauna in the Westbury Formation can be used, along with other evidence, to infer a number of possibilities about the environment the formation was deposited in.
Apart from teeth, fin spines of sharks are particularly abundant. These vary in length from a few centimetres to over 15 centimetres, and often carry elaborate ornamentation on their surfaces. As well as the aforementioned teeth, a typical block of bone bed found on the beach will contain a number of fin spines, although often these will be broken. The spines of Hybodus minor have parallel ribs running the length of the spine, with a hollow portion at the base that is open to what would have been the rear. Nemacanthus monilifer spines are generally flatter in section and have patches of tubercles part way along the spine's length.
Having discussed the fish fossils present, now's a good time to mention coprolites. These are extremely abundant, and believed to have been produced by both fish and the reptiles that will be looked at next. The coprolites can be divided into a number of categories, and their form can also give clues as to the anatomy of the creature that produced them. Furthermore, many contain fossil bone, scales and teeth that indicate the prey on which its producer was feeding.
For the casual fossil hunter perhaps the most prized find would be part of a large reptile or dinosaur. The chances of the latter are fairly remote, although isolated teeth, claws, limb bones and vertebrae have been recorded. Reptile remains, however, are fairly common, particularly parts of plesiosaurs and ichthyosaurs, as well as those of a small reptile known as Pachystropheus rhaeticus. Unfortunately, the fossil elements of the first two mentioned are quickly picked up by collectors! Having said that, by careful searching it is possible to find teeth of both plesiosaurs and ichthyosaurs, and these are usually several centimetres long. Elements of Pachystropheus rhaeticus are a little more common at certain localities; the most frequently encountered being sections of pelvis, limb bones, vertebrae and ribs. These are fairly small, though, compared to elements from the larger reptiles as Pachystropheus was believed to have been generally only around 1 metre long.
Invertebrates figure, too; particularly bivalves, although they are often most abundant at horizons different to those at which most of the vertebrate fossils are found. Rhaetavicula contorta is particularly common at some levels and is recognised by its arched shell with closely spaced ribs. Some beds contain abundant moulds of bivalves, the calcite that formed their shells having been dissolved. Sometimes the more elongate specimens on a bedding plane are all aligned in one direction as a result of the influence of a current that flowed over them 200 million years ago. For a detailed account of bivalves from the Westbury Formation, and Penarth group in general, the book "Fossils of the Rhaetian Penarth Group", published by the Palaeontological Association gives descriptions of around fifty different species. It is also an excellent guide to the numerous other fossils that may be encountered, including those from the Lilstock Formation higher in the Penarth Group.
It's also worth mentioning that you're likely to see structures preserved on some bedding planes in the Westbury Formation that although they may appear superficially like normal fossils, are in fact trace fossils formed by the burrowing or feeding of organisms in the sediment. Also, at some localities, ripples that were formed by wave or current action are preserved; these may cause some confusion when seen for the first time.
That briefly describes some of the most common specimens likely to be encountered in a sequence of beds that span a relatively short space of geological time at the end of the Triassic. In the field, it's likely that material found on the beach from the Westbury Formation will be mixed with that from the Lilstock Formation and from the base of the Blue Lias Formation above. But that's another story...
Basal bone bed from South Gloucestershire.
Westbury Formation within the Triassic.
Benton, M.J et al. 2002. Permian and Triassic Red Beds and the Penarth Group of Great Britain, Geological Conservation Review Series, No.24, Joint Nature Conservation Committee, Peterborough.
Dineley, D.L et al. 1999. Fossil Fishes of Great Britain, Geological Conservation Review Series, No.16, Joint Nature Conservation Committee, Peterborough.
Storrs, G.W et al. 1993. The earliest possible choristodere (Diapsida) and gaps in the fossil record of semi-aquatic reptiles, Journal of the Geological Society of London, vol 150 99 1103-1107.
Swift, A. et al. 1999. Fossils of the Rhaetian Penarth Group. The Palaeontological Association.
Sykes, J.H. et al. 1970. The stratigraphy and palaeontology of the Rhaetic beds of Barnstone, Nottinghamshire. Mercian Geologist, 3.
Sykes, J.H. 1974. On elasmobranch dermal denticles from the Rhaetian Bone Bed at Barnstone, Nottinghamshire. Mercian Geologist, 5.