The collection and study of microvertebrate fossils is one of those disciplines that are currently right at the cutting edge of palaeontology – and with good reason. It reveals so much about the flora and fauna in different ecosystems that would otherwise be ignored and lost if all we concentrated on was macrovertebrate collecting.
This increased knowledge of diversity in any given palaeoenvironment also enables a more accurate estimation of climatic conditions and increases the taphonomic value of the sites. And now, because of the increase in the amount of time and effort being put into the study of these microsites, more new species are being identified every year. The most high profile example of this in recent years was Steve Sweetman’s work on the Isle of Wight which received international exposure in 2009 as he had apparently identified over 50 new species including dinosaur, mammal, bird, pterosaur, crocodile and many other taxa.
But what exactly is a microvertebrate fossil? In simple terms they are typically exactly the same fossils that you might find in any macrovertebrate bearing strata – except that you very rarely see them or, indeed, they are invisible to the naked eye. The best way to collect this type of fossil is to remove large quantities of sediment from the exposed fossiliferous beds in preparation for screen washing.
However, this is not as simple as shovelling lots of sediment into buckets and thinking it’s a job well done. On the contrary, as with any quarry, the relevant taphonomic and stratigraphic detail need to be taken into account and the site itself must be quantified before collecting since each site is likely to be different and collection methods will vary.
Screen washing, or wet sieving, has been utilised since at least 1847 by Theodore Plieninger in continental Europe and then by Charles Moore in the UK during 1858, who sorted sedimentary matrix removed from Rhaeto-Liassic beds near Holwell, Somerset. The first record of screen washing in North America is by J.L. Wortman in 1891 for the American Museum of Natural History and later followed by Barnum Brown, also of the AMNH, who, in 1906, wet sieved sediments from the Hell Creek Formation in Montana. Brown commented that “This material gives evidence of a much greater Laramie fauna than has been heretofore described” (McKenna et al 1994).
Screen washing is a relatively simple process. Initially, water is passed through the sediment via a couple of screens of different mesh sizes. The first mesh is always of a coarser grade and this traps larger debris as well larger bone and plant fossils. The second mesh is of a much finer grade and sorts out the much smaller matrix and microfossils - and there are even finer meshes if so required.
Nowadays a lot of this matrix is then processed utilising heavy liquid separation which uses chemicals to separate fossil material from the debris due to fact that the specific gravity of fossil bone is heavier than the surrounding particles it is buried with. Using the correct chemical with a specific gravity less than the fossil material would mean that the fossils would sink in the solution and can be removed whilst the unwanted sediment would float. This is a very simplified description of a somewhat complex process which, just as with any other part of preparation, has to take a multitude of factors into account before separation can begin.
This prepared concentrated matrix can then be sorted out which is intensive, time consuming and, at the same time, compulsive and beguiling. Of course, different grades of concentrated matrix will require a different approach. The coarser grades can be sifted through comfortably with no other aids other than, perhaps, a simple hand lens but when it comes to fine and ultra-fine concentrate then a binocular microscope or, getting more common these days, a good quality USB microscope is essential.
A couple of my colleagues and I have been getting to grips with these microscopes and they take a bit of getting use to mainly because of the slight time delay between actually moving material and seeing it translate onto the screen. This comes with practice as does knowing your left from your right which, although is plainly obvious on a day-to-day basis, can be a nuisance depending on how the scope is set up as a movement left may actually register as a movement right on the screen.
The matrix is best spread thinly and not in a little pile since this makes it difficult to discern which particles have been looked at before, then after, thus avoiding wasting time by looking at them again. When you first begin sorting you tend to check every granule because sometimes they look like they may be a fossil and you want to be sure but you soon realise they are not and the vast majority of fossils are clearly what they appear to be and are seldom difficult to recognise. Indeed, so many of them are exact copies of their larger brethren that there is no doubt as to their animal origins.
When you find a fossil it is nigh on impossible to pick them up with tweezers since you may damage them or they ping out never to be found again *cough*. Far better to use a piece of tape to gently secure it and then carefully ease it into a holding receptacle – masking tape is best since this is not very sticky at all and fits the bill nicely. McKenna et al (1994) recommend a wet brush to do the same job.
I guess this appears a lot of trouble on the face of it but this is extremely important work to aid our understanding of the palaeoenvironment, as an indication of temporal constraint and dating rock units, and just how diverse life was throughout prehistory. For me, I always amazed at how something so small and barely visible can be so perfectly preserved after millions of years have passed.
|A superbly preserved jaw recently sorted from some Maastrichtian concentrate|
McKenna, M.C., Bleefield, A.R. & Mellett, J.S. 1994. Microvertebrate collecting: Large-scale wet sieving for fossil microvertebrates in the field; pp. 93 – 111 in P. Leiggi and P. May (eds), Vertebrate Paleontological Techniques, volume 1. Cambridge University Press, Cambridge, UK.