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Acid resistant vertebrate fossils from the Westbury Formation

Pseudodalatias barnstonensis

Pseudodalatias

barnstonensis.

FOV=3.5mm

Hybodus minor

Hybodus minor.

FOV=3.5mm

Lissodus minimus

Lissodus minimus.

FOV=3.5mm

Lepidotes

Lepidotes sp.

FOV=3.5mm

Chondrichthyan (shark) scale

Chondrichthyan (shark)

scale.

FOV=3.5mm

Gyrolepis albertii

Gyrolepis albertii

scale.

FOV=3.5mm

Sargodon tomicus incisiform tooth

Sargodon tomicus

incisiform tooth.

FOV=3.5mm

Sargodon tomicus molariform tooth

Sargodon tomicus

molariform tooth.

FOV=3.5mm

Some fossils that occur in the Westbury Formation bone beds are best extracted using acetic acid. This dissolves the calcite cement of the conglomerate, releasing fossilised teeth and scales with no observable damage as long as not too high a concentration is used. Generally speaking, though, larger skeletal elements need to be treated differently to the method outlined below. It's necessary to frequently remove, wash, and seal the exposed parts of the specimen to prevent damage by the acid. I don't intend to discuss that latter process, here. My reasons for trying the method described below were to explore the variety of evidence left of late Triassic fish in the form of teeth and scales, and to compare my findings with those I'd read about in various publications.

Over the last year I've collected a good quantity of bone bed from South Gloucestershire specifically for the purposes outlined below. Most of this is in the form of fragments from walnut to fist sized; much being the left-overs from other collectors after they've removed the choice specimens. This is no concern, though, as many of the fossils I intend to look at are generally best observed with a handlens or low-power microscope. Other larger blocks of bone bed have been collected, too, but they await a different fate, more of that another time.

This initial look at the subject to hand was intended to find a suitable system for extracting the specimens, and to get a general feel for the diversity of species represented, so I have not attempted any quantative examination at this stage. Instead, the picture on this page represent some of what I found.

An 8% solution of acetic acid was used for this process; I'd previously read that solutions upto 20% could be used without significant damage to the specimens, but thought it best to err on the side of caution. The solution was prepared from one of 80% purchased from a chemical supplier. It's important to note that this stuff is dangerous at that concentration, and I diluted it down to the working concentration outside, with good ventilation, using goggles and protective gloves. I do not recommend anyone who's not used potentially harmful chemicals before to try this; instead, vinegar purchased in bulk will do a similar job, although it will take much longer. Vinegar is an approximate 4% solution of acetic acid.

A plastic container with a lid, approximately 450mm by 300mm by 200mm deep was filled to a depth of 100mm. Approximately 1kg of walnut sized pieces of bone bed were placed in a plastic garden-centre-purchased riddle with a 20mm mesh and immersed in the solution. This was left for two days before removing the riddle, then sieving the residue in the container through a 3mm, 1mm then 0.125mm sieve. This was repeated every two days over a period of two weeks until all the original material was dissolved. The 3mm sieve was, again, purchased from a garden centre, the 1mm and 0.125mm from a specialist supplier. However, good results can also be obtained using a flour sieve from the local supermarket. You can also get the vinegar in 2.5l containers there, as well.

And now for the fun bit -- picking through the residues is very exciting, though as the sieve size gets smaller, so the task becomes more demanding. The material left on the 3mm sieve is easily picked through and usually contains large bone fragments, coprolites, and clasts of insoluble, or partly digested, conglomerate. The bits that look like they may break down more can be returned to the acid. A few larger teeth of hybodont sharks, and the bony fish Severnichthys acuminatus are often present, too. The 1mm sieve produces mainly teeth, scales, coprolites, and pieces of broken fin spine or bone mixed with non-soluble mineral grains. This can just about be picked through with the naked eye, removing the specimens of interest with a piece of kitchen towel twisted to a point and moistened with water. When applied to an object of interest it will stick to this, and can then be transferred to a suitable receptacle. The 0.125mm sieve-retained material needs to be picked through using a microscope with low magnification, though a hand lens or magnifying glass could also be used. I used a x20 magnification on a stereo microscope and this gives an excellent 3d quality image of the sample. The same species are represented by teeth, which are particularly abundant, but also apparent are a number of small skeletal elements of what I assume are fish. I've not been able to identify any of these apart from one which appears to be a vertebrae. The specimens of interest can be removed in a similar manner to that used above.

So far, although I've looked through all the coarser samples, I've still only picked through round 25% of the material left on the 0.125mm sieve -- there's quite a lot of it. Anyway, I've found evidence of the following species, mainly in the form of teeth:

2006

1mm sieve used above

1mm sieve used above.

3mm sieve used above

3mm sieve used above.

Bibliography

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.

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. 1971. A new dalatiid fish from the rhaetic bone bed at barnstone, Nottinghamshire. Mercian Geologist, 4.

Sykes, J.H. 1974. On elasmobranch dermal denticles from the Rhaetian Bone Bed at Barnstone, Nottinghamshire. Mercian Geologist, 5.

Sykes, J.H. 1979. Lepidotes sp.: Rhaetian fish teeth from Barnstone, Nottinghamshire. Mercian Geologist, 7.