Upper Wenlock miospores and cryptospores derived from a Silurian volcanic island in the Prague Basin (Barrandian area, Bohemia)

Highly-diversified assemblages of dispersed trilete miospores and cryptospores belonging to the Artemopyra brevicosla-Hispanaediscus verrucous Assemblage Zone were recovered from upper Wenlock (Silurian) tuffitic shales and limestones in the Prague Basin (Bohemia). The fact that the numerous sporomorphs have only been found in the region of the Svatý Jan Volcanic Centre supports previous sedimentological evidence that this volcanic elevation was emerged during the uppermost Wenlock. Twenty-nine sporomorph forms were determined. One new genus, Rugosisporites gen. nov., and three new species, Rugosisporiies kozlicus sp. nov., Synorisporites maculosus sp. nov. and Artemopyra rugaticosta sp. nov. are described. The comparison with other coeval assemblages is discussed.


INTRODUCTION
The objective of this paper is a study of upper Wenlock sporomorphs from the Prague Basin. No Silurian sporomorphs have been published From the basin prior to this study. The first trilete miospores described from the basin come from upper Pragian (Devonian) Dvorce-Prokop Limestone (McGregor, 1979;Vavrdov6, 1989).

Facies development and land plant traces in the Silurian of the Prague Basin
During the Silurian the Prague Basin was a part of north Gondwanan shelf seas bordering the Rheic ocean, fairly distant from large emergent lands. Anoxic bottom conditions generally occurred across the shelves. During the Llandovery and lower Wenlock such conditions, represented by black shale facies, existed in nearly the whole Prague Basin (see in KEZ, 1991). In the deeper parts of the basin they persisted until the middle Devonian.
From the mid-Wenlock to the early Ludlow the sedimentation in the Prague Basin was influenced by volcanic activity related to deep synsedimentary faults and their zones of intersections (Kfii, 1991). The accumulation and redeposition of volcanic material caused a considerable shallowing of the central segment of the Prague Basin resulting in the deposition of bioclastic limestones in the region of the Svatg Jan Volcanic Centre (Fiala, 1982;Horny, 1955;Kiii, 1991). These limestones contain within the C. rnurchisoni Graptolite Biozone the oldest trilete miospores (Ambitisporites auitus Hoffmeister and Ambitisporites dilutus Hoffmeister) so far discovered in the Silurian of the Prague Basin (Dufka, 1990). Volcanic activity together with late Wenlock (late Homerian) regression lead to continuous shallowing and consequent emergence of the top of the volcanic elevation ( K E , 1991). The new island may have favoured the rapid development of early land plants.
Numerous dispersed sporomorphs produced by these plants were deposited in tidal flat sediments.
Lack of sporomorphs in a number of samples of lower Ludlow (Gorstian) sediments from the vicinity of the Svaty Jan Volcanic Centre suggests the extinction of the upper Wenlock land flora due to submergence of the island or further volcanic activity.
Sporomorphs in the upper Ludlow and lower P'idoli strata are absent or rare and poorly preserved. However, based on sedimentological evidence, the top of the island was probably still above the sea level during this period (KEZ, 1991). Additionally, occurrences of Cooksonia-like sporophytes from lower P'idoli rocks (Obrhel, 1962) suggest that the recurrence of an island flora could be possible and the absence of sporomorphs may be caused by lack of outcrops within the upper Silurian near-shore deposits.

COLLECTED DATA AND PALYNOLOGICAL TECHNlQUES
This paper on upper Wenlock sporomorphs is the outcome of the palynological investigations carried out within the new palaeontological and biostratigraphic study of the Wenlock/Ludlow boundary in the Prague Basin (Khi, 1992).
The majority of accessible upper Wenlock (Homerian, Motol Formation) and lower Ludlow (lower Gorstian, Kopanina Formation) sections in the Prague Basin have been investigated for organic microfossils. Sporomorphs, however, have been recovered only from late Wenlock strata of the central part of the basin, in the area of Svaty Jan Volcanic Centre (sensu KEZ, 1991, p. 182). The Motol Formation is developed here in facies containing bioclastic and tuffitic limestones and tuffites ('Kuel' facies) associated with basalt and volcaniclastic rocks (Horny, 1955;K i i , 1991). In contrast to lateral deeper parts of the Prague Basin, where the sedimentation of graptolite shales continued, the richness of benthic fauna and dearth of graptolites characterise this volcano-carbonate facies.
Twenty-three samples from five outcrops have yielded some sporomorphs (Figs 1 and 2).
The most significant specimens of sporomorphs yet found come from tuffites, tuffitic shales and limestones forming the eastern part of the large rock outcrop above the Berounka river, near the village of LiStice. General study of these rocks was done by Horn? (1955) and more detailed biostratigraphy was discussed by K E (1992). Ki% defined here two sections (759 and 760; for detail see KM, 1992) separated by layers of tuffitic shales and volcaniclastics, more than forty metres thick, covered with slope debris. Graptolites of the C. lundgreni Biozone (M. priodon flemingii) were found in the lower section no. 759 (KEZ, 1992). These graptolite data together with new intrabasinal correlation of the overlying volcaniclastics (J. K E , pers. comm.) suggest that the tuffites and bioclastic limestones of the section no. 760 may range from the M . dubius purvus Biozone to the M. vulgaris Biozone, Wenlock, Silurian.
All samples were processed using standard palynological techniques (HC1-HF-HCI-heavy liquid separation). No oxidation was applied. Organic residue was sieved through 53 p m and 10 p m nylon sieves. Light microphotographs were made with a Zeiss-Fluoval. Electron Microscope (SEM) photos were taken with a TESLA BS 340 SEM.

CHARACTERISTICS OF THE PALYNOSPECTRA
Based on the relative abundance of sporomorphs and acritarchs, recorded in each sample, two upper Wenlock palynofacies with spores could be distinguished.  HL-I, where the specimens are well-preserved. In spite of the low content of sporomorphs a high specific diversity including sculptured forms was recorded in samples HL-I and SJ-57 (Fig. 2). Acritachs and prasinophycean-like sphaeromorphs dominate the palynospectra. Their absolute abundance, however, is fairly low (<SO specimens/g in sample HL-1). The distribution and number of acritarchs are similar to those of prasinophytes in all samples. Acritarchs are mostly light-yellow, excellently preserved and of high variability. Chitinozoans were recovered only from two samples of section 759 (Fig. 2). Two long-ranging species, Ancyrochitina group ancyrea Eisenack and Conochitina tuba Eisenack, were recognized.

B. Section 760 (all samples)
Trilete miospores and cryptospores clearly predominate over the marine microphytoplankton. Their mode of preservation ranges from well-preserved specimens that exhibit all fine details of the wall ornamentation to specimens destroyed by microbial attack or pyrite crystal grows. Sporomorph colour ranges between light and dark brown, depending on sample lithology and exine thickness. The abundance and species diversity of sporomorphs vary slightly in individual samples, nevertheless they have an approximately uniform character throughout the section. All samples are characterized by dominance of distally verrucate miospores and cryptospores, by the common occurrence of murornate and laevigate miospores and cryptospores and by the scarcity of 'permanently' fused cryptospore dyads and tetrads (Fig. 3). Acritarchs are rare, but well-preserved. Together with prasinophycean-like algae they represent maximally 10% of counted palynomorphs, except for the uppermost three samples in the section where they constitute nearly 25% of recorded palynomorphs.
No chitinozoans have been found in the section.

Sample LS-56
Sporomorphs are scarce, but diversified. Acritachs are also rare, represented by ill-preserved, small spherical forms with short simple processes (?Micrhystridiurn, ?Tylotopallu). No chitinozoans were found. A substantially increased content of amorphous kerogen indicates anoxic bottom conditions. In all studied samples the occurrence of further macerals was noted, but they are not yet classified. No mazuelloids have been found in sporomorph-containing samples. Scolecodonts are common. Nernatothallus-type pseudocellular cuticles are commonly found compared with nematoclasts ('tubes') which are rare. In general, the number of sporomorphs does not increase together with the number of Nernatorhallus-type organoclasts. Similarly, the abundance of macroscopically fragments of organic matter formed by closely packed filaments (cf. Prototuxires Dawson) in upper Wenlock tuffitic shales and limestones can not be correlated with occurrences of sporomorphs.

Dufka
Description. Amb subtriangular to subcircular. Equatorial crassitude narrow (<2 pm). Laesurae distinct, more or less sinuous, simple or with inconspicuos lips, extending from 4/5 to entire spore radius. Curvaturae perfectae either coincident with the equator or slightly to distinctly invaginate. Proximal surface (contact area) rugulate, composed of numerous tiny muri or wrinkles, usually convolute and anastomosing which are somewhat randomly disposed, but in general radially aligned, predominantly near the equator. Distal exine laevigate or hearing scattered minute grana. libycus. p. 617). They dominate among verrucate miospores in section 760 and may be discriminated from Synorisporites cf. verrucatus by lower and more separated verrucae confined within the equatorial thickening. However, the morphological characteristics of both the species from the Prague Basin appear to intergrade (more than those of the Libyan forms).

Indeterminate trilete miospores
Trilete mispore type A Dimensions. 12 specimens measured: equatorial diameter 31(33)38 pm. Description and remarks. A group of trilete miospores characterized by narrow thickenings, more or less parallel to equator which form conspicuous equilateral triangle. Two forms bearing this triangular structure were recorded. 1. Thickennings distal, accompanied with granulate or rugulate ornamentation. Proximal exine smooth; curvaturae straight, extending to the margin and curvaturae coinciding to the equator (Pl. 3, fig. 16).
Trilete miospore type C (Pl. 3, figs 14, 15) Description. Amb subcircular. Laesurae distinct, mostly straight, extending to the spore margin. Distal exine provided with low, chaotically distributed low muri, convoluted and anastomosing to form irregular reticulum. Proximal surface probably smooth. Dimensions. 4 specimens measured: equatorial diameter 19-21 pm. Rodriguez, 1977 has more or less similar distal sculpture, but it differs by its substantially larger size and a thick equatorial crassitude.

RIOSTRATIGRAPHY AND DISCUSSION
In the biozonal scheme proposed by Richardson & McGregor (1986) (1986, p. 8, 9) and Burgess & Richardson (1991,  The assemblage of the spore Zone l a from the lowermost part of the San Pedro Formation in Spain (Rodriguez, 1978) is not controlled by index macrofauna, nevertheless the recorded palynospectrum indicates presence of the A . hrevicostu-H. verrucutus Assemblage Zone. Seven of nine forms from the zone 1 a, excepting Retusotriletes abundo Rodriguez, 1978 and Convolutispora sanpetrense Rodriguez, 1978, are known from the Prague Basin. North American assemblages of the upper Wenlock (Strother & Traverse, 1979;Burgess & Richardson, 1991, p. 621) and Ludlow
In spite of the rare occurrence of trilete miospores and cryptospores in the Prague Basin, there are significant similarities between these assemblages and coeval assemblages from England, Libya, Spain and North America (Fig. 4). A conformity of the species spectra may indicate some communication between those distant areas by means of spore dispersal. Sporomorphs might be transported over a great distance most probably by strong surface currents, similar to the transport of larvae of benthic fauna. However, because of their small size (mean diameter of sporomorphs below 25 pm), wind transport of spores cannot be excluded (for discussion see Fanning et al., 1988). In general, the mentioned resemblance of upper Wenlock/lower Ludlow sporomorph assemblages supports the previous ideas about the uniform evolution of the land flora during the upper Silurian (e.g. Edwards, 1990), at least in the North Atlantic Region (sensu Boucot, 1990, fig. 1).