New chitinozoan taxa from the lower Wenlock (Silurian) of the Welsh Borderlands, England

One new chitinozoan genus, Salopochitina, and three new chitinozoan species Salopochitina bella, Eisenackitina varireticulata and Eisenackitina spongiosa, all of early Wenlock age, are described from two British Geological Survey boreholes at Lower Hill Farm, Shropshire, and Eastnor Park, Hereford and Worcester. The new taxa have short stratigraphical ranges, are relatively abundant, and may be useful for correlation in successions from Wales and the Welsh Borderlands.


INTRODUCTION
Chitinozoans from the Wenlock of Wales and the Welsh Borderlands were first described by Eisenack (1977Eisenack ( , 1978, who illustrated forms from the Much Wenlock Limestone Formation at Dudley in the West Midlands. From the Wenlock and Ludlow type areas, Dorning (1981) recorded the ranges of 35 chitinozoan taxa of stratigraphic value.
In 1973, the British Geological Survey (B.G.S.) drilled the Lower Hill Farm Borehole at (National Grid reference: SO 5817 9788). 1690m at 092" from Hughley church near Wenlock Edge, Shropshire (Text- fig. 1). This locality is on the outcropt of the upper part of the Coalbrookdale Formation, at 485 ft OD and about 1.3km east south-east of the standard section for the base of the Wenlock series in Hughley Brook. The borehole was cored to 247.50m in a sequence comprising partoftheCoalbrookdale Formation, theBuildwasFormation, and part of the Purple Shales (Text- fig. 2). Detailed lithological and biostratigraphical data from this borehole, has been used to complement the generally poor surface exposures in the area, which include the international stratorypeforthe Wenlock Series (Bassettetal., 1975). In particular, graptolites recovered from the borehole were used to establish the presence of the Wenlock graptolite biozones (Bassett et d . , 1975, pp. 5-8 (Mabillard, 1981) is consistent with the Llandoveryfrlrenlock boundary being placed above the base of the Woolhope Limestone Formation, at a level where calcareous sediments become more apparent in the section (Dr. R. J. Aldridge pers. comm.). Standard palynological processing techniques were used with methods similar to those described by Neves 8t Dale ( 1 963) and Sarjeant (1 974). Material was prepared for the scanning electron microscope by initially strew mounting residues on cover slips. The cover slips were then attached to small Cambridge stubs with high vacuum wax. Conducting paint was applied to improve electrical contact between the cover slips and the stubs, prior to gold coating. The cover slips were logged by systematically traversing from top left to bottom right.

RESULTS
The organic residues recovered from the samples include both chitinozoans and acritarchs. The material studied is diverse, as is typical for open marine shelf sediments (Aldridge et al.. 1981), with the a diversity index (Fisher et al., 1943) for palynomorphs in the range 12.6 to 30.2 for the Lower Hill -. . . .

I."
Lower maroon shales and soft mudstones.
aheles a n d a r e e n s l l t s t~n e s . Farm Borehole and 6 to 25.3 for the Eastnor Park Borehole. Absolute abundances determined, from the Lower Hill Farm borehole range from 6 to 136 chitinozoans /g and 500 to 2300 acritarchs /g. and in the Eastnor Park Borehole these range from 1 to 16 chitinozoans /g and 20 to 1000 acritarchs /g. The Lower Hill Farm Borehole residues include new chitinozoan taxa (Salopochitina bella gen. et sp. nov., Eisenackitina varirvticulata sp. nov., and Eisenackitina spongiosa sp. nov.), from the Eastnor Park Borehole Salopochitina hella gen. et sp. nov. has also been recorded. Preservation of these chitinozoans is excellent, and detailed SEM and optical microscope work was carried out. The stratigraphical distribution of these new taxa is shown in Text- fig. 3. The apparently short ranges, (acritarch biozones W1 and W2 of Dorning & Bell, 1987) and relatively high abundance (the new taxa account for up to 30% of a chitinozoan assemblage in a single sample) of the new chitinozoan taxa show that they are of biostratigraphical use, at least locally, in the lower Wenlock of Wales and the Welsh Borderlands.

Terminology
Descriptive terminology used is that introduced by the commission internationale de micr-oflore (Combaz et al.,I967), and later refined by Laufeld (1 974 Diagnosis. The vesicle is subconical to ovoid with a broadly rounded base. The flexure, when present, is shallow and concave. The short neck widens orally and possesses a collar. There are one to three solid elongated appendices attached to the centre of the base or the basal margin of the vesicle; these appendices are up to twice as long as the vesicle and are widest where they join the base. The vesicle is unomamented. Description. The appendices are not branched, and apart from broadening proximally to their connection to the vesicle, are of equal width for their entire length, distal terminations of the appendices are generally blunt and rounded. When one appendix only is present, it is attached to the centre of the base; when two or three are present, they are attached to the basal margin of the vesicle. The appendices may possess a rugose ornament. Remarks. The differing number of appendices can be accounted for by intraspecific variation; this is indicated by a morphotype (PI. 2, fig. 1) that possesses one incipient appendix and two longer appendices. A similar stratigraphical occurrence for all the different types supports this idea.

Dimensions. (in microns
The most similar species are Ancyrochitina longicornis and Ancyrochitina nodosa Taugourdeau & De Jekhowsky, 1960. Ancyrochitina longicornis has one long appendix attached to the centre of the vesicle, and A . nodosa has three which are attached to the basal margin. In both species the appendices are also of the same width for their entire length. The taxa differ from Salopochitina gen. nov. in the shape of the vesicle which in S. hella sp. nov. is subconical and in A. longicornis and A. nodosa is cylindrospherical. Further difference between these forms is that in A. longicornis and A. nodosa the appendices are hollow, whereas in S. hella sp. nov. they are solid.

Diagnosis.
A wide subcylindrical to subconical chitinozoan with straight flanks, no neck and no flexure. The base is generally flat but may be slightly concave or convex, it possesses an angular margin. The vesicle has two wall layers, the inner whole, the outer forming areticulate network which is joined to the inner by columns. The reticulate ornament is only present on the chamber, theoral tube being unornamented. Dimensions. (in microns) length of vesicle 85-135 (holotype 1 I5), maximum width ofbase 66-145 (holotype SO), width of Swire aperture 45-88 (holotype 5 5 ) . maximum length of unornamented oral tube 27-52 (holotype 30). Specimens measured -40. Description. In some specimens the ornament is damaged and is not clear; this effect could be preservational or may be due to damage during processing. The operculum when present, possesses a raised central 'boss'. Remarks. The reticulate ornament of Eisenackitina varireticulata sp. nov. is similar to that of Acanthochitina harhatu Eisenack,193 1; the latter differs, however in having a vesicle that is much longer and thinner, and an ornament that is entire.
This species is placed in the genus Eisenackitina, because of the subcylindrical vesicle shape, straight flanks, flat or convex base, and double walled vesicle. The presence of a reticulate ornament clearly distinguishes it from other species of Eisenackitina, with the exception of Eisenackitina spongiosa sp. nov. which in contrast possesses a reticulate ornament that covers the entire vesicle. The reticulum of E. Spongiosa sp. nov. is denser with the individual 'cells' of the reticulum being smaller in diameter than those of E. varireticulata sp. nov. due to a longer length of the supporting columns and therefore greater separation of the two wall Eiseirackitina spmgiosa sp. nov. is placed in the genus Eisenuckitina because of the subcylindrical vesicle shape, straight flanks, flat or convex base, and the two-layered vesicle wall. The entire reticulate ornament over the vesicle surface of Eisenackitinu spongiosa sp. nov. is denser than in E. ~~ur.ir.etic~ulata sp. nov., and also distinguishes it from all other species of that genus. No chains have been found.