Microfossils

Microfossils:

Microfossils

This is a term used to describe a wide diversity of groups that are basically too small to find in the field. The boundary is somewhere around a millimetre, although some are very much smaller than this. The technique used to study them is generally bulk sampling and dissolution: take some lumps of promising rock to the lab, drop it in acid, and look at what comes out in an electron microscope.

Broadly, they can be divided into organic and phosphatic microfossils. There are also siliceous ones, but sponge spicules are treated under ‘sponges,’ radiolarians are unknown from Builth, and silicified calcareous fossils have also not yet been found in an extractable state. In fact, the microfossils recorded here have largely been found by chance, on the surfaces of slabs, and a thorough search for them is yet be to be undertaken. This is a huge gap in the record of Builth Inlier fossils. Does anyone want to do something about it?

Organic microfossils

Although they have not been studied in detail, there are two broad types of organic microfossil present in the area: chitinozoans and acritarchs. Acritarchs are the resting cysts of probably unicellular algae or protists (quite possibly relatives of dinoflagellates), and can reach a millimetre or so in diameter, but are usually much smaller. They typically have a reticulate pattern on the surface, sometimes with spiny ‘appendages.’ There is likely to be quite a high diversity, but their study is a specialised field involving very nasty acids and powerful microscopes. I have occasionally seen some on the surfaces of pale shales under a simple microscope.

Chitinozoans are usually less than two millimetres long, but rarely less than a tenth of millimetre, so many of them are visible as tiny reflective grains on the surface of mudstones. They are flask-shaped, sometimes with a lot of fine detail (more spiky appendages) under an electron microscope, but again, have effectively not been studied from here. They are believed to be the eggs of an unknown animal, and this is borne out by occasional discoveries of articulated arrays of lots of chitinozoans inside an organic envelope. Such things have also been found at Llanfawr, but not yet perfectly preserved. Better specimens are the path to fame and glory, for the chitinozoans are among the last great palaeontological enigmas…

Conodonts

Except for what seems to be a single fish scale, and a couple of enigmatic phosphatic spines, the only vertebrates known from the Inlier are the tiny, primitive conodonts. The only mineralised (and therefore ‘preservable’) parts of them are their teeth, which in general are less than a millimetre long. Each animal was an eel-like creature with large eyes at the front, and no jaws (we know this thanks to extraordinary fossils from Edinburgh and South Africa). The teeth make a grasping array, and include up to seven or eight different shapes in one animal. Some are simple cones, but others are extravagant affairs of numerous spikes along one or more axes. Very rarely, you might find a collection of several conodonts in a nearly-symmetrical array; these represent their positions in the living animal, and are vital to understanding which teeth belong to which species.

Usefully, the colour of conodonts changes from pale yellow, through brown and black, to white and finally transparent, depending on the temperature that the rocks have experienced. White indicates around 400-500 C. Black ones are often difficult to distinguish from scolecodonts (see worms), except to the specialist.

[2]Acritarch indet. Up to 1 mm diameter, usually much less.