Muellerisphaerids from the Llandovery of western mid-Wales

Muellerisphaerids are described and illustrated from Britain for the first time. The mechanical processing method used in their extraction is described. They are preserved as pyrite internal moulds and thus lack the necessary morphological detail (i.e. spines) to be identified to family level. Other microfossils occurring in the samples are also recorded.


INTRODUCTION
applied the informal name mazuelloid to a group of phosphatic, spinose microspheres of unknown affinities from the Silurian of Greenland and Czechoslovakia and the Devonian of Germany. Kozur (1984) created the Order Muellerisphaerida to accommodate these specimens and similar material from the Silurian and Lower Devonian of Hungary. Burrett (1989, Holmer (1987, Priewalder (1987) and Norford and Orchard (1985) have subsequently recognised muellerisphaerids from the Ordovician of Tasmania, the Ordovician of Vastergotland (Sweden), the Silurian of Austria and the Silurian of northern Canada respectively. This paper reports the occurrence of muellerisphaerids, preserved as pyrite internal moulds, from Llandovery mudstones from western mid-Wales. For comparison with these moulds, Figure 1 illustrates the main features of a typical, chemically isolated, complete muellerisphaerid. The central body is hollow and spherical, with a surface which may be smooth, granular or covered with pillars up to 10pm long and 2pm in diameter (see Aldridge and Armstrong, 1981, Fig. 1). Hollow spines are always present, the density of which over the sphere's surface varies considerably between species, as also does their shape, from parallel-sided to conical. Spine terminations are often broadly rounded, but in some species they are irregularly forked. Aldridge and Armstrong (1981) and Kozur (1984) have described the tripartite wall structure, consisting of two outer layers, each less than 1 pm thick, composed of crystallites arranged perpendicular to the sphere's surface, and an inner, organic, layer.

MATERIAL AND METHODS
The material examined consisted of the residues from three mudstone samples collected from the following localities and horizons : (i) Rheidol Gorge. 0. T. Jones (1909) fig. 5 ) illustration of an unnamed specimen from the Ludlow of the Carnic Alps), X 300.
The following mechanical processing method was Dry lumps of rock were crushed using a steel with a surface broken by a continuous pattern of dome-shaped protuberances (Pl. fig. 9). Whilst these bear a superficial resemblance to framboids, used : pestle and mortar. Fines were sieved through a 30 mesh onto a 240 mesh. Any material remaining on the 30 mesh was crushed again. When all the residue was on the 240 mesh sieve the sample was washed slowly and the material on the sieve rubbed between finger and thumb to remove fine grained matter. If any fine grained material still adhered to the pyrite residue the sample was boiled with Quaternary Ammonium Detergent and the rubbing process repeated. (For the particularly indurated sample from 0. T. Jones LOC. F17 this process of boiling and rubbing was repeated several times.) The residues were examined under the light microscope and in the S.E.M.

DESCRIPTION
The Aberystwyth Grits Formation sample yielded nothing but fragments of burrow and pyrite framboids (PI. fig. 1).
The samples from the Derwenlas Formation, however, yielded a large number of microfossils all preserved as pyrite internal moulds. These fall into three morphological groups :  [4][5][6][7][8]. These domes represent infillings of the hollow lower portions of the muellerisphaerids' spines. 3. Spheres, of a similar size to the muellerisphaerids, they may be internal moulds either of muellerisphaerids with a rather denser ornament of spines (cf. Aldridge and Armstrong (1981), fig. If), or possibly of radiolaria.

DISCUSSION
This is the first record of muellerisphaerids from the British Isles and the first ever account of this order recovered from mechanically processed Lower Palaeozoic rocks. Previous workers have used chemical isolation methods, for example Aldridge and Armstrong (1981) processed samples using dilute acetic acid, whilst Kozur (1984) used hydrofluoric acid.
In this form of preservation the muellerisphaerids may be distinguished from acritarchs principally by their greater size. The muellerisphaerid calcium phosphate wall, described by Aldridge and Armstrong (1981) and subsequently by Kozur (1984) and Burrett (1985), has not been seen on any specimen. Whether it was lost prior to, or during, processing is not known.
It is interesting to note that all records of muellerisphaerids have so far been from deep-water sediments. Whilst it may be that they were restricted to the planktonic environment of the open sea, it may be that their preservation potential in shallow-water sediments is very low (due to the delicate nature of their skeletons). Kozur (1984) subdivided the order Muellerisphaerida into two families, the Aldridgeisphaeridae and the Armstrongisphaeridae, the former with broad spines and the latter with narrow spines. Obviously, on the basis of internal moulds it is impossible to decide as to which family the described material belongs as several armstrongisphaerids have broad spine bases before tapering to a much narrower spine termination. Thus it is not possible to augment Kozur's preliminary biostratigraphical observations.

Explanation of Plate
All x300 Housed at U.C.W. Aberystwyth