Arenaceous foraminifera from the Llandovery/Wenlock boundary beds of the Wenlock Edge area, Shropshire

Several thousands of specimens of arenaceous foraminifera have been collected from two sections in Shropshire that span the Llandovery/Wenlock boundary. Preservation in limestone samples is superior to that in shales, but both lithologies yielded abundant specimens. The uppermost Purple Shales, of Llandovery age, contain numerous Ammodiscus exsertus, together with Webbinelloidea tholus, Psammosphaera cava, Hemisphaerammina and Hyperammina. A faunal change occurs close to the base of the Wenlock, with the lower beds of the Buildwas Formation containing rather fewer foraminifera; assemblages here are dominated by Hyperammina, Lagenammina and Lituotuba? This event probably reflects a change in sea-floor environment, with habitats favoured by epifaunal encrusters and browsers contracting at the end of the Llandovery.


INTRODUCTION
Literature on Silurian foraminifera from Britain is very sparse. They were first recorded by Smith (1881), who found "Lagena-like" bodies in ostracod collections from the Wenlock of Shropshire. Brady ( 1 888) subsequently described Smith's material together with additional specimens from the Woolhope Limestone Flormation of the Malvern Hills, referring them all to the genuslagena. Cummings (1952) restudied the specimens and, on the basis of the composition and structure of the test wall, assigned them to the parathuramminacean genus Saccumminopsis ; from Brady's illustrations it appears that the textulariid genus Lagenammina may also be represented. Other collections of Silurian foraminifera have been reported by Ireland (1958Ireland ( , 1967 and by Aldridge et a/. (1979), but these records are very brief and lack description o r illustration of the assemblages.
In the course of a detailed micropalaeontological study of the Llandovery/Wenlock boundary beds of the Welsh Borderland we have recovered many thousands of specimens of arenaceous foraminifera. The vast majority of these are from the type area of the Wenlock Series, along Wenlock Edge in Shropshire. In this paper we present an outline of these foraminifera1 assemblages and illustrate the common taxa.
T h e Leasows section is the International Stratotype for the base of the Wenlock Series (Bassett et a / . , 1975;Holland, 1980) and the Domas section exposes a parallel sequence. A t both localities there is a conform-able transition from soft maroon shales and mudstones of the Llandovery Purple Shales to olive-grey silty shales of the Wenlock Buildwas Formation.
Samples were collected as closely as possible and included all lithologies at each locality. Sample horizons and lithologies are shown in Fig. 2. Soft shales were disaggregated with petroleum spirit followed by hot water, limestones were digested with acetic acid; all residues were washed through a 7 5 p m sieve. All samples, except the bentonitic clay (24/12), yielded diverse microfossils, including abundant arenaceous foraminifera.

Abundance and preservation
Absolute abundances have not been determined, as many samples yielded thousands of specimens. Estimates vary from tens of thousands per kilogram in the Purple Shales to several hundreds of specimens in the Buildwas Formation. A t some horizons, assemblages arc dominated by a profusion of o n e o r more species, particularly Ammodiscus exsertus Cushman. Similar significant increases in abundance have been reported by Amsden etal. (1981) from the Silurian of Oklahoma; they refer t o these events as "floods".
The quality of preservation and the relative frequency of species vary considerably with changes in lithology. Specimens with inflated chambers are compressed o r completely crushed in shales, but excellently preserved, undistorted material occurs in the acid-insoluble residues of limestones. Low diversity assemblages, composed almost exclusively o f A . exsertus were recovered from shales in the Purple Shales, whereas carbonates from these beds yielded more varied faunas. This appears to be due to selective preservation, as the differences are even apparent between limestone nodules and shales from the same bed.

The foraminifera1 assemblages
The distribution of foraminifera through the two sampled sections is shown in Fig. .3. Llandovery shale samples at both localities are characterised by A . exsertus, in association with species of Hyperarnrnina. Limestone samples from the Purple Shales contain abundant A . exsertus together with Webbinelloidea tholus (Moreman), Psarnmosphaera cava Moreman and Hernisphaerarnrnina. Limestones from the Buildwas Formation d o not contain these species, a n d A . exsertus disappears in the lowermost Wenlock beds. Assemblages from the Buildwas Formation contain smaller numbers of foraminifera than the Purple Shales and are characterised by representatives of the genera Hyperarnrnina, Lagenarnrnina and Lituotuha?
By far the most abundant species in the collections is A . exsertus. The populations of this species include a number of distinct morphotypes, some of which have previously been given separate specific status. Similarly wide variation in A . exsertus has also been recognised by Amsden et al. (1980), who concluded that many forms are the result of aberration and d o not merit taxonomic separation. In our samples, planispirally coiled specimens with an exsert neck (PI. 2, fig. 6) account for more than 50% of the total. About 30% are considered to be damaged or immature specimens which lack an exsert neck (PI. 2, fig. 7) and approximately 20% may be regarded as aberrant forms. These include multiplechambered, planispirally coiled forms, previously referred t o A . furcus Moreman (PI. 2, figs. 8-10), forms with irregular coiling, previously assigned to Lituotuba exsertus Moreman (PI. 2, figs. 15-16), and forms with a partially detached final whorl previously designated as L . recurva Mound (PI. 2, figs. 11-14). Simple, straight or branching, tubular species are also abundant, although almost all o u r specimens were recovered from shales. Straight or slightly curved tubes of  Specimens referred to Lagenammina occurred only in residues from shale samples, particularly those from the Buildwas Formation. The genus is represented by shortnecked and long-necked forms (PI. 1, figs. 11-12), the latter resembling the illustrations by M.cClellan (1966) of L . cumberlandiae (Conkin). Specimens tentatively referred to Astrorhiza? sp. (PI. 1, fig. 1) and Thurammina? sp. (Pl. 1, fig. 14) are uncommon and poorly preserved. Small numbers of specimens assigned to Lituotuba? (PI. 2, occur in shales and limestones of the Buildwas Formation. The species Turritellella workmani Dunn (PI. 2, fig. 17) and Thurammina irregularis Moreman (PI. 1, fig. 13) are represented by single specimens only.
It is not easy to assess the environmental significance of the changes recognised in arenaceous foraminifera1 assemblages through our sections. In Recent seas, arenaceous foraminifera are most abundant in marginal environments (Phleger, 1960), but during the Silurian they probably filled most benthic niches occupied today by calcareous species. Palaeoecological analysis of Lower Palaeozoic forms has rarely been attempted, but recently Toomey (in Amsden et al., 1980) has interpreted the modes of life and habitats of several species of Wenlock age from Oklahoma. The assemblages he records are similar to ours in many respects and the same ecological groups appear to be present. Thus, benthic encrusters with hemispherical tests are represented by W. tholus and Hemisphaerammina, and vagile benthic browsers by A . exsertus, P. cava and Lagenammina. Tubular species, such asH. harrisi, probably lived interstitially within the sediment, but close to the surface.
All three ecological groups occur in the Purple Shales, with the epifaunal A . exsertus and W. tholus being particularly abundant. In the Buildwas Formation, interstitial forms are dominant, with Lagenammina representing the benthic browsers. Modem agglutinated browsers and encrusters are intimately associated with marine plants, and Toomey (in Amsden etal., 1980)  that algae with a grass-like habit may have formed a comparable habitat on the Silurian sea floor. If so, environmental changes influencing marine plants, themselves not preserved, would be manifested in changes in the associated foraminifera1 faunas. Thus, the floods of epifaunal browsers in the Purple Shales and their relative demise in the Buildwas Formation may reflect expansions and contractions of plant habitats.
Too little is known about Silurian foraminifera for us to detect any species of potential biostratigraphical value. Murray (1981) has suggested that the likelihood of foraminifera being good index fossils for the Lower Palaeozoic is poor, and there is nothing in our study that allows contradiction of this view. Although several species, including H . harrisi, T. irregularis, W . tholus and P. cava are known only from the Silurian, their distribution patterns seem to reflect environmental, rather than evolutionary, controls.