Bryozoan palaeoecology in the Late Silurian of Gotland

Palaeogeography, Palaeoclimatology, P.alaeoecology, 20 ( 1 9 7 6 ) 1 8 7 - - 2 0 8 © Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s

BRYOZOAN PALAEOECOLOGY IN THE LATE SILURIAN OF GOTLAND

KRISTER BROOD

Section of Palaeozoology, Swedish Museltm of Natural History, Stockholm (Sweden) ( R e c e i v e d J a n u a r y 21, 1 9 7 5 ; revised a n d a c c e p t e d F e b r u a r y 6, 1 9 7 6 )

ABSTRACT Brood, K., 1976. B r y o z o a n p a l a e o e c o l o g y in t h e Late Silurian of G o t l a n d . Palaeogeogr., P a l a e o c l i m a t o l . , Palaeoecol., 20: 1 8 7 - - 2 0 8 . T h e p r e s e n t p a p e r deals w i t h t h e ecology of the B r y o z o a in t h e U p p e r W e n l o c k i a n of G o t l a n d . The U p p e r W e n l o c k i a n s e d i m e n t a r y r o c k s o f G o t l a n d , locally k n o w n as t h e HallaMulde Beds, are d e p o s i t e d in a shallow sea. T h e s e d i m e n t a r y r o c k t y p e s include welld e v e l o p e d reefs o f " b a r r i e r " a n d " f r i n g i n g " t y p e s a n d m a r l y l i m e s t o n e s in b o t h f o r e r e e f a n d b a c k r e e f positions. These s e d i m e n t a r y r o c k s are rich in B r y o z o a as well as in o t h e r shelly fossils. T w e n t y seven b r y o z o a n species have b e e n i d e n t i f i e d f r o m this s t r a t i g r a p h i c s e q u e n c e . T h e b r y o z o a n s t e n d to o c c u r in associations. T h e s e are n o t s t r i c t l y c o m p a r a b l e w i t h the n e o n t o l o g i c a l a n i m a l c o m m u n i t i e s in t h e P e t e r s e n sense, b u t s h o u l d r a t h e r be r e g a r d e d as " e c o z o n e s " . Twelve d i f f e r e n t faunistic associations can b e i d e n t i f i e d in t h e i n v e s t i g a t e d material. On B r y o z o a alone, five m a j o r z o n e s are discernible. INTRODUCTION

The present study considers the ecology of Bryozoa from the Upper Wenlockian Halla-Mulde Beds of Gotland (Hede, 1925). Stratigraphically (Fig.l), this sequence comprises the graptolite zones with Cyrtograptus lundgreni Tullberg and Monograptus ludensis (Murchison) including the nassa-dubius inter-regnum of Jaeger (1959). The present investigation is based on material collected in the field by the writer, museum material obtained from the Swedish Museum of Natural History in Stockholm and from core drillings from southern Gotland made by the Geological Survey of Sweden. During Wenlockian times geological conditions changed significantly in the Baltic--Scandian area, through the effects of the collision of the North American and European continental plates (Harland and Gayer, 1972). This collision resulted in a narrowing of the "Proto-atlantic" (which disappeared entirely in the Late Silurian) and in the formation of the Caledonian Mountain Range. During these geological processes, peripheral effects caused changes in the palaeogeography of the Scandinavian Shield. From a stage during the Early Silurian when the entire Scandinavian Shield was covered by the sea (Walter, 1972), the water gradually retreated towards the south during the Middle and Late Silurian. 187

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Fig.2. Tentative palheogeographic map for the Scandinavian region during Late Wenlockian times. The map is compiled after various sources including St~rmer (1967), Kaljo et al. (1970), Sokolowski et al. (1970), and Walter (1972).

189

According to current interpretations (Kaljo et al., 1970; Sokolowski, 1970; Walter, 1972) the palaeogeography in this area during Late Wenlockian was as indicated in Fig.2. The sea formed a large e m b a y m e n t covering the southern part of Sweden and the present-day Baltic Sea. This e m b a y m e n t opened towards the south across northern Poland to the large sea covering great parts of northern and middle Europe. It appears probable that connections with the British area, which could be used by spreading Bryozoa, became cut off during the Late Wenlockian as the bryozoan faunas of the Lower Wenlockian in Scandinavia are identical with the British, whereas the Ludlovian faunas are wholly different, except for long-ranging species such as Ptilodictya lanceolata (Goldfuss), which evolved in the Wenlockian (Brood, 1976). The nearshore areas of the embayment, from northern Gotland and the eastern part of the Baltic states, mainly received carbonate sediments, whereas the western and central (deeper) parts of the basin had chiefly terrigenous sedimentation of clays and silts (Stbrmer, 1967; Kaljo et al., 1970; Manten, 1971; Walter, 1972). The Halla-Mulde Beds crop out along a narrow belt across the middle part of Gotland (Fig.3). In the northern area, where the rocks are mapped as the Halla Beds according to Hede (1925), the sequence is separated from the underlying Slite Beds by an erosional surface and from the overlying Klinteberg Limestones by another erosional boundary (Fig.4). In the northern part of the area, the Halla-Mulde sedimentary cycle starts with a thin band of oolitic limestone (the Bara Oolite), which generally is interpreted as a littoral deposit (Manten, 1971). Immediately above this there follows a reef limestone. This reef limestone (generally only a few metres thick) probably represents a fringing reef according to standard terminology (Wells, 1957). On the southern side of this area there are bedded, marly limestones. The calcium content of these marly limestones decreases in a southeasterly direction and the limestones grade into a shale towards the southeast. In the western part of Gotland, around the Karlso Islands, the sedimentary rocks contain a group of large reefs of a type that resembles modern barrier reefs (Manten, 1971). This reef-bearing sequence is approximately 40 m thick and contains, besides the reef bodies, bedded crinoidal limestones of interreef origin. Sedimentary rocks which may have been deposited north of this reef area and near the supposed shorelines are not known, due to erosion. To the southeast of this second reef area the sediments are marly limestones of the same type as in the eastern part of the island of Gotland. This variable sedimentary environment provided excellent conditions for a rich marine life, as evidenced by an abundance of species and individuals of shelly organisms. Bryozoa constitute an important part of the fauna in most of the sedimentary rocks of this area. Different approaches exist for the determination of abundance and importance of faunistic components in sedimentary rocks. Commonly, the number of individuals has been used as reference, as in Ziegler et al. (1968). This pro-

190

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cedure, however, can give misleading results in m any circumstances, as many fossils occur as incomplete individuals, even in sediments with well-preserved fossils, f o r example the d i s ~ t i c u l a t e d shells of brachiopods and bivalves and m o u l t stages of arthropods. Some of these problems can be dealt with by

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B Fig.4. A . R e c o n s t r u c t e d section o f Halla Beds of G o t h e m s h a m m a r . The section is constructed from several small sections along the coast. B. Diagrammatic section of the Upper Wenlockian of Lilla Karls6 at the western part of the island.

counting only certain parts or stages of the individuals, but this method does not take the size of individual specimens into account. This leads to difficulties in estimating the importance of different organisms as community members, if whole communities with species of different phyla are considered. The counting of individuals is an especially doubtful method when colonybuilding organisms are investigated, as size differences are commonly great, and colonies tend to disintegrate during sedimentation and compaction. Therefore, a point-counting method, which gives a measure related to volume of rock rather than to individual specimens has been used here. Skeletal grains were counted according to the grain solid measure of Dunham (1962), so the voids enclosed within the fossils were counted separately. The boundary between matrix and skeletal remnants has been drawn at 0.1 ram. However, pore fillings in echinoderms have not been measured separately. The Bryozoa are extremely well suited for palaeoecological investigations as they can be successfully identified in thin sections and are also ecologically

192 variable. The Palaeozoic Bryozoa had calcitic skeletons, which make them comparatively resistant to recrystallization processes (Brood, 1976). This means that Bryozoa can easily be identified in rocks where aragonitic organisms such as stromatoporoids and molluscs are recrystallized b e y o n d identification. This appears to be especially valuable in reef limestones. ZONATION OF BRYOZOA Palaeoecological investigations of the Upper Wenlockian sedimentary rocks of Gotland show that several biological associations can be identified. The term "association" is used here for the fossil asserfiblages found in the present investigation. This term corresponds to the " c o m m u n i t y " of authors such as Ziegler et al. (1968) and Hancock et al. (1974), b u t cannot be strictly compared with the biological c o m m u n i t y in the sense of Petersen {1913). The present writer prefers not to use the term " c o m m u n i t y " for a fossil assemblage, where c o m m o n l y many specimens show signs of having suffered postmortem sorting and where the soft-bodied animals are not preserved. The Bryozoa in the investigated material can be arranged in five different zones, each characterized by its c o m m o n e s t species. The bryozoan zones are arranged here in a postulated pattern from the shore outwards, partly representing differences in depth, b u t local variations in the sea b o t t o m , as a result of differences in currents and b o t t o m sediments, may yield different bryozoan associations within the same water-depth range.

SOFT-BOTTOM ASSOCIATIONS

"Prgsopora" gotlandica zone This bryozoan zone is characterizedby the trepostome "Prasopora"gotlandica Hennig. The biological assemblages dominated by this bryozoan species vary between an algal, a molluscan, a brachiopod, and a trace-fossilassociation, respectively. These associations are all characterizedby a low-speciesdiversity, though individuals are abundant. The algal association (Fig.5B,C) is characterizedby Solenopora gotlandica Rothpletz and Sphaericodium gotlgndicum Rothpletz and the stromatoporoid Stromatopora antiqua (Nicholsonand Murie). The molluscanassociation is characterized by abundant specimens of a Palaeoneilo species (Fig.6). The brachiopod--bryozoanassociation (Fig.7) is dominated by the brachiopods Delthyris elevata Dalman and Microsphaeridiorhynchus sp. and Bryozoa. The trace-fossil association (Fig.5A) is distinguished by "Chondrites" sp. The matrix in all cases consists of a clay-rich calcilutite. The fossils are generally well preserved, b u t many of the bivalve shells are disarticulated and indicate p o s t m o r t e m transportation (Fig.6). This bryozoan association appears to have lived on a clay-rich substrate. Possibly, the water was relatively calm, as clay-sized particles were deposited, though current action must have taken place at least periodically, as is indicated

193

Fig.5. A. Peel o f bed c o n t a i n i n g t h e "Chondrites" sp. a s s o c i a t i o n s h o w i n g b u r r o w s filled w i t h coarser material. G o t h e m s h a m m a r . (x 4). B. Peel of b e d c o n t a i n i n g the Solenopora gotlandica-Stromatopora antiqua association. Note s t r o m a t o p o r o i d e n c r u s t e d by Sphaericodium gotlandica R o t h p l e t z in the u p p e r left a n d f r a g m e n t s of "Prasopora" gotlandica H e n n i g a n d Saffordotaxis gotlandicus Brood. G o t h e m s h a m m a r . (x 4). C. Peel of bed cont a i n i n g t h e Solenopora gotlandica--Stromatopora antiqua association. N o t e S. antiqua ( N i c h o l s o n a n d Murie) t o the left a n d several Solenopora specimens. G o t h e m s h a m m a r . (x4).

194

Fig.6. A. Surface of slab showing numerous specimens of Palaeoneilo sp. Gothemshammar. (× 0.5). B. Peel section of bed containing the Palaeoneilo association showing several specimens of "Prasopora" gotlandica Hennig, gastropods and pelecypods. Gothemshammar. (× 4). C. Peel section of bed with the Palaeoneilo association showing numerous specimens of Palaeoneilo sp. Gothemshammar. (x 4).

195

Fig.7. A. Peel s e c t i o n of bed c o n t a i n i n g t h e Delthyris elevata--Microsphaeridiorhynchus a s s o c i a t i o n s h o w i n g n u m e r o u s s p e c i m e n s of "Prasopora" gollandica Hennig. G o t h e m s h a m m a r . (x 4). B. Slab s h o w i n g Delthyris elevata--Microsphaeridiorhynchus sp. association. A p a r t f r o m the b r a c h i o p o d s , n u m e r o u s s p e c i m e n s of "Prasopora" gotlandica H e n n i g can be seen. G o t h e m s h a m m a r . (× 1).

196 by the orientation of bivalve shells. The presence of different algae and the low species diversity, together with geological evidence (position in the lower part of a transgressive section) suggest that this bryozoan zone represents a shallowwater environment with an approximate depth of 5--20 m. Presumably, the growth of algae on the sea b o t t o m stabilized the soft sediments and made the substrate more suitable for the sessile organisms. Algae are also good settling places for bryozoan larvae. It seems possible that the brachiopod--bryozoan association lived within a zone, possibly r e s e m b l i n g the "eel-grass" or Cyrnodea-meadows of the modern seas. This is suggested by the hollow stems of m a n y specimens of the encrusting "P. "gotlandica, which indicate that they grew around soft, cylindrical objects which have disappeared during fossilization, a growth form which is c o m m o n in post-Palaeozoic forms (Brood, 1972). The molluscan concentrations appear, on the other hand, to be the result of a postmortem transport, as t h e shells occur disarticulated and are orientated with their convex side upwards. Possibly, by comparison with m o d e m seas, these concentrations will develop in areas where there is no cover of algae and the sea b o t t o m is instable so that movements of sedimentary particles takes place.

Saffordotaxis gotlandicus--Ptilodictya lanceolata zone This bryozoan zone (Fig.8) provides a rich bryozoan fauna where species such as S. gotlandicus Brood, P. lanceolata (Goldfuss), Fistulipora mutabilis Hennig and Fistuliporella delicatula Brood are common. These bryozoans are associated with a rich fauna of brachiopods such as Isorthis crassa (Lindstrbm), Leangella segrnentum (Lindstrbm) and Craniops implicata (Sowerby). In addition, trilobites, gastropods and Tentaculites are common. The fossils are excellently preserved but nevertheless some brachiopods are found in a disarticulated condition suggesting slight mechanical sorting. This bryozoan zone appears to be developed in moderately shallow water at possibly 20--50 m. Suggestive of this depth figure is the fact that some of the species that are representative of this zone are also present in the reef areas. Overlapping of species also occurs with the preceding "P. "gotlandica zone. Some specimens of encrusting genera, such as Fistulipora, display erect, hollow stems, which suggests that they have been growing around soft, stick-like objects which have disappeared during fossilization. This would suggest that part of the sea b o t t o m with this fauna was covered by algae and is suggestive of shallow water. Suggestive of a calm-water environment is the presence of erect, bifoliate growth forms of the normally encrusting Sagenella consimilis (Lonsdale). Sedimentary rocks which contain fossils belonging to this zone are present in the Mulde-Fr&jel area. This faunistic assemblage is especially well developed at the now abandoned brick-yard at Frojel.

197

Fig.8.A. Peel o f b e d w i t h the Isorthis crassa--Saffordotaxis gotla~dicus association. A b e n t o n i t e layer can be seen in t h e u p p e r part of t h e figure. Several s p e c i m e n s of Saf[Ordotaxis gotlandicus Brood, Phaenopora lindstr~emi Ulrich and Ptilodictya laJTccolata (Goldfuss) can be seen. FrSjel. (x 4). B. Surface of slab with Isorlhis crassa Safford~ taxis gotlandicus association. Several s p e c i m e n s of Phae~p~ra litldstroemi Uh'ich and Isorthis crassa ( L i n d s t r o m ) c a n be seen. Frojel. (x 2 ).

198

Asperopora claviforrnis zone This bryozoan zone (Fig.9) is dominated by the trepostomes A. claviformis (Hennig) and A. asperum (Hall), but bryozoan species such as Ptilodictya lanceolata, Sagenella consimilis and Fistulipora mutabilis are also present.

Fig.9. Peel section of bed containing the Heliolites interstinctus--Halysites catenularius association showing matrix with high content of small arthropod shells. Bl~h~ll. (x 4). These bryozoan species are associated with brachiopods such as Leptaena depressa (Sowerby) and Dicoelosia biloba (Linnaeus). Corals such as Heliolites interstinctus (Linnaeus) and Halysites cf. catenularius Linnaeus, and stromatoporoids such as Pycnodictyon densum Mori and Actinostromella vaivarensis Nestor are also characteristic. The skeletal material constitutes less than five per cent of the total rock volume. The matrix consists of a clay-rich calcilutite. The fossils are well-preserved and show little indication of postmortem transport. The larger fossils especially are invariably f o u n d in growth position, though some of the smaller forms may be disturbed through compaction processes and biogenetic reworking. The good preservation of the fossils, absence of algae, and the geological position suggest that this bryozoan zone represents an association from a water depth greater than the preceding ones. Possibly, the water depth may have been of the magnitude 50--200 m. Some overlapping occurs between this and the S. gotlandicus--P, lanceolata zone. Sedimentary rocks with this association of fossils are well exposed on the west coast of Gotland, near the small villages of Bl~h/ill and Djupvik. Towards the southeast, away from the postulated shore-line, the fossil con-

199 tent decreases except for the graptolites, which increase. As a result, the calcareous c o n t e n t of the rock decreases and the marly limestone is ieplaced by a siltstone. The fossils are few, but include the bivalve Slava interrupta (Broderip), Gothograptus nassa and monograptids. No Bryozoa have yet been f o u n d in these siltstones. This type of sedimentary rock is not exposed on the Gotland mainland and is found only in the core drillings. HARD-BOTTOM ASSOCIATIONS A large part of the sedimentary rocks of the investigated area consists of reefs and reef-derived rocks. The bedded limestones which surround the reefs generally contain the same fauna as the reefs, as they are formed mainly of debris washed down from them. Palaeozoic reefs may have three growth stages according to authors such as Lowenstam (1957) working on Silurian reefs and Klovan (1974) who studied Devonian reefs. According to current views, reefs may contain a "semi-quiet, non-resistant stage", a "semi-resistant stage" and a "wave-resistant stage". By comparison with modern reefs the "semi-quiet stage" is supposed to be below the base of the storm waves, the "semi-resistant stage" is between the base of the storm waves and the base of the normal waves, and the "resistant stage" is in the upper, constantly turbulent water. The depth figures of the zones depend on the exposure of the reefs to wind and waves, but the wave-resistant stage may be estimated to be between sea level and 10 m, the semi-resistant stage between 10 and 20 m and the semi-quiet stage below 20 m (Klovan, 1974). Such a division can generally be observed in the Wenlockian reefs of Gotland. The semi-quiet stage is characterized by corals, while stromatoporoids are generally absent; the semi-resistant stage is typified by tabular stromatoporoids and corals, and the wave-resistant stage by dome-like stromatoporoids of large size. The wave-resistant stage is poorly developed in the area investigated, however, and the reefs here display chiefly the semi-quiet and semiresistant stages.

Coenites repens--Hallopora elegantula zone This bryozoan zone corresponds roughly to the semi-quiet stage. Coenites repens Wahlenberg (here considered as a bryozoan: Brood, 1970) appears to be one of the pioneer reef builders together with Acervularia conglomerata (Lang and Smith) and Favosites spp. The species diversity is low in this part and the only other c o m m o n bryozoan species are Thamniscus toernquisti Hennig and Hemipachydictya holmi (Hennig). Many specimens of corals and bryozoa attain large sizes and coral colonies up to 1 m 3 in volume are frequently found. The major part of the specimens occur in growth position but overturned specimens are also present.

200

T h a m n i s c u s t o e r n q u i s t i - - F e n e s t e l l a gu tnica z o n e

This bryozoan zone (Fig.10) corresponds approximately to the semi-resistant stage of the reef. The bryozoan fauna is dominated by fenestellid species such as Thamniscus toernquisti, Semicoscinum balticum Hennig, Fenestella gutnica Brood, F. subantiqua D'Orbigny, and Reteporina reticulata (Hisinger). In

Fig.10. A. Peel section of reef limestone from Thecia swinderniana--Labechia conferta stage of reef showing several specimens of Fenestella gutnica Brood and Reteporina reticulata Hisinger. Lilla Karls6. (x 5). B. Peel section of reef limestone with Coenites repens-Acervularia conglomerata association showing n u m e r o u s specimens of Coenites repens Wahlenberg. Lilla Karls6. (× 3).

201 association with these br yozoa, a high diversity is also shown by brachiopods, corals, crinoids and algae. Tabular corals such as Thecia swinderniana (Goldfuss) and s t r o m a t o p o r o i d s such as Labechia conferta (Lonsdale) and Ecclirnadictyon tuberculatum Mori are also characteristic. The wave-resistant stage does not yield any conspicuous numbers of Bryozoa apart f r o m certain encrusting forms, presumably species belonging to the genera Fistulipora, Ceramopora and Spatiopora. These forms encrust the lower surface of the s t r o m a t o p o r o i d s and corals. Identification of these forms is difficult, however, due to recrystallization of these parts of the reefs. The debris f r o m the reefs occurs in large areas around and between the reef bodies ( F i g . l l ) . The bedded limestones, which are f o r m e d as a result of this accumulation of reef debris, consist chiefly of crinoidal limestone, but also contain significant amounts of corals, b r y o z o a n s and brachiopods. The fauna in these sediments is mainly reef-dwelling forms which occur in a more or less c o r r o d e d state. The crinoid ossicles and skeletal debris which constitute the limestone beds must have f o r m e d on a sea b o t t o m of coarse shifting sand, which apparently was difficult for colonization by sessile organisms. In some parts, however, clay-sized material has been allowed to settle and a stabler surface was formed. However, the B r yoz oa are rarely the first to colonize such environments, b u t were preceded by corals and s t r o m a t o p o r o i d s which are generally the first organisms to act as stabilizers of such m u d d y areas. The B r y o zo a f o u n d in these bedded limestones are chiefly the same as in t h e reef limestones. The b r y o z o a n assemblages show representatives from b o t h the F. gutnica--T, toernquisti and C. repens--H, elegantula zones. There seem to be a t e n d e n c y for presence of the T. toernquisti--F, gutnica association in the inter-reef sediments and of C. repens--H, elegantula in the forereef sediments (Table I). ROLE OF BRYOZOA IN THE UPPER WENLOCKIAN SEDIMENTARY ROCKS OF GOTLAND (Figs.12,13) As with m o d e r n Bryozoa, the distribution of fossil species from the Gotland Silurian may have been controlled by environmental factors such as water depth, water turbulence, substrate conditions, salinity, temperature, c o n t e n t of mud and oxygen of the water and pH-value. Such factors together fo rm the e n v i r o n m e n t in which the animals live. Changes in one or several of these factors will cause changes in the e nvi r onm ent and as a consequence also a change of the biological system, which will result in di fferent species associations. As demonstrated, different Late Wenlockian environments were inhabited by different b r y o z o a n associations. These assemblages appear to be well defined when the whole fauna is considered, but considerable overlapping still occurs among single species. The B r y o z o a play an i m p o r t a n t role as organic rockformers in the investi-

202

Fig.11. Surface of slab of reef detritus showing high content of Coenites repens Wahlenberg. Diameter of coin = 17 ram. Lilla Karls/J. B. Peel section of bedded reef detritus showing high content of crinoid debris and coenitids. Lilla KarlsS. (x 5). g a t e d area, especially in t h e reefs a n d r e e f - d e r i v e d s e d i m e n t s (Table II). Occurrences o f B r y o z o a in tossil a n d m o d e r n reefs have b e e n n o t e d b y a u t h o r s such as L o w e n s t a m (1957), Maxwell (1968), Scoffin ( 1 9 7 1 ) and C u f f e y (1973), t h o u g h t h e y s e e m to have b e e n m u c h o v e r l o o k e d . T h e v o l u m e o f B r y o z o a in t h e i n v e s t i g a t e d r e e f l i m e s t o n e s varies b e t w e e n 4 and 8% o f t h e t o t a l r o c k v o l u m e , b u t t h e y c o n s t i t u t e b e t w e e n 20 a n d 40% o f the skeletal m a t e r i a l o f

Microsphaeridiorhynchus s p . Delthyris elevata

"Prasopora" go tlandica Coenites repens-Hallopora elegantula Thamniscus toernquisti-FenesteUa gutnica

Microsphaeridiorhynchus sp.-Delthyris elevata Coenites repens-Acervularia conglomerata Thecia swinderniana-Labechia conferta Clathrodictyon striatellum-Ecclimadictyon macrotuberculatum Pentamerus gotlandicus

Leptaena depressa-Dicoelosia biloba Asperopora claviformis-Ptilodictya lanceolata

Heliolites interstinctus-Halysites catenularius Slava interrupta-(Gothograptus nassa)

Marly limestone

Mudstone

9

Isorthis crassa Saffordotaxis gotlandicus --Ptilodictya lanceolata

Isorthis crassa-Saffordotaxis gotlandicus

Pen tamerus go tlandicus

no autochtonous bryozoans, but reef-derived forms

Rhyncotreta cuneata-Ferganella borealis ?

?

Lingula sp. Microsphaeridiorhynchus sp.

"Prasop.ora" gotlandica "Prasopora" gotlandica

Stromatopora antiqua-Solenopora gotlandica Palaeoneilo sp. "Chondrites" sp.

Marly limestone with brachiopods and bryozoans

Crinoid limestone

Reef limestone, wave-resistant stage

Reef limestone, semi-quiet stage Reef limestone, semi-resistant stage

Marly limestone with trace fossils Marly limestone with bryozoans and brachiopods

Marly limestone with pelecypods

9

Brachiopod association

Lingula sp.

enveloping algae

Oolitic limestone Marly limestone with algae

Bryozoan association

"Prasopora" gotlandica

Biological association

Lithofacies

Correlation of lithofacies and organic associations

TABLE I

¢D

204 TABLE II Percentage of Bryozoa of total rock volume and of skeletal material of rock Figures are mean values obtained from measuring more than 300 thin sections and peels Biological association Stromatopora a n t i q u a Solenopora go tlandica "Chondrites" sp. Palaeoneilo sp. "Prasopora" gotlandica-Microsphaeridiorhynchus sp. Isorthis crassa-Saffordotaxis gotlandicus Heliolites interstinctus-Halysites catenularius Slava interrupta-Gothograptus nassa

Percent Bryozoa of ostracomass 5.06( 0

-18.83)

Percent Bryozoa of total rock volume 1.72(0

-- 5.77)

2.68 0.82 22.34

0 -i7.19) 0 - 4.90) 0.60-42.33)

0.45 (0 -- 3.35) 0.05 (0 -- 0.31) 4.51 (0.18--10.16)

11.36

5.13-14.18)

3.24 (1.12-- 5.82)

20.13 (18.27-21.99)

1.48 (1.38-- 1.57)

0

0

Coenites repens-A cervularia co nglo merato

39.29 (17.58-54.17)

8.08 (7.28--13.15)

Thecia svinderniana-Labechia conferta

19.08 ( 5 . 6 9 - 5 4 . 8 4 )

4.47 (3.45-- 6.30)

Pentamerus go tlandicus

27.27 ( 2 . 4 5 - 4 6 . 9 2 )

7.11 (0.99--23.87)

(Reef detritus)

the G o t l a n d l i m e s t o n e s . In t h e m a r l y l i m e s t o n e s o f n o n - r e e f origin, the B r y o z o a m a y c o n s t i t u t e u p to 5% of the t o t a l r o c k v o l u m e , b u t b e t w e e n 2 a n d 20% o f t h e skeletal m a t e r i a l . On the w h o l e , B r y o z o a f o r m the s e c o n d c o m m o n est c o m p o n e n t o f t h e skeletal m a t e r i a l o f the investigated limestones, outn u m b e r e d o n l y b y the e c h i n o d e r m s . I t m u s t be r e m e m b e r e d t h a t it is d i f f i c u l t t o c o m p a r e ecological d a t a f r o m m o d e r n a n d fossil r e e f areas. In ecological studies o f m o d e r n , living reefs t h e c o m m o n l y u s e d m e a s u r e is b i o m a s s (standing crop), w h e r e a s in fossil reefs t h e c o m p o n e n t s are generally m e a s u r e d b y v o l u m e or w e i g h t o f rock. As t h e m e a s u r e o f b i o m a s s d o e s n o t t a k e i n t o c o n s i d e r a t i o n the p r o d u c t i o n rate (especially o f c a l c i u m c a r b o n a t e ) a n d g e n e r a t i o n length, t h e results f r o m m o d e r n a n d fossil areas are i m p o s s i b l e t o c o m p a r e . T h e B r y o z o a , for instance, have an a p p r o x i m a t e life span o f o n e y e a r ( t h o u g h individual colonies m a y live longer) ( R y l a n d , 1 9 7 0 ) a n d will t h e r e f o r e a p p e a r to be relatively c o m m o n e r in fossil a s s e m b l a g e s t h a n in living e c o s y s t e m s , w h e n c o m p a r e d w i t h corals (with an average life s p a n o f t e n y e a r s or m o r e ) . This m a y be one of the reasons f o r t h e B r y o z o a b e i n g o v e r l o o k e d as an i m p o r t a n t c o m p o n e n t o f the reefs, t h o u g h a geologist (Maxwell, 1 9 6 8 ) n o t e d high c o n t e n t s of B r y o z o a in t h e s e d i m e n t s a r o u n d the G r e a t Barrier R e e f , east o f Australia.

205

/ ~

coen,tes~epe~s

[ ~

Fenestel[o gutmca Thamnlscus toerr~qulst

[~]]~ So'fordot . . . . gotlcnd .... PtHodlctyc

~

/,///

lancecla|a

_~-~ Asperop....lavi...... /

2 '~

o

(D

lO

~o k~

Fig. 12. Map s h o w i n g t h e i n f e r r e d d i s t r i b u t i o n of the b r y o z o a n a s s o c i a t i o n s in t h e O o t l a n d area d u r i n g t h e Late W e n l o c k i a n .

Though the main frame-building organisms in the Gotland reefs are corals and stromatoporoids (Hadding, 1941; Manten, 1971) Bryozoa obviously played an important role as secondary frame-building elements, and especially as sediment-trapping organisms, as can be seen also in modern reefs (Cuffey, 1973). This can be clearly seen from analyses of the constituents of the reef limestones (Table II). A macrohabitual distribution is observed of the bryozoan forms, which are generally found in specific associations. Within these associations, microhabitual preferences of growing and function can be found among species and genera. It is possible here to distinguish between framebuilding, sediment-trapping, sediment-binding and sediment-forming forms of Bryozoa. The main frame-building forms among the Bryozoa studied are the coenitids. These forms are strong and can withstand water turbulence. However, smaller forms such as fenestellids and ramose cryptostomes and trepostomes may also be considered as minor frame-building organisms, especially in the semi-quiet and semi-resistant stages where water agitation is generally low. The Bryozoa here appear to have been successful competitors of the corals and stromatoporoids as can be concluded from their c o m m o n occurrence in the rocks (Table II}, while the corals may have had difficulties in growing due to low light intensity.

206 Oohtes

x

x

x

x x X

/ X x

x

X

x

~ x

x

x

X

x

x X

x

x

X

X "

x

X x

X

x

DO

X

~ ~

x

x

~ ~./

i ~'

:o

oo

TheciaSwindernianaand Coenit. . . . pens Solenoporagotland.... Stromatoporaantiqua Chondrites sp.

I

i

Paleoneilosp.

"Prasopora"gotlondica~ Microsphaeridiorhynchussp. [=-Z1 Pent . . . . . . gotlandicus ],,or

~

X

•

,o,,o do,ox,,

HeliohtesinterstinctusHelysilites catenularius Nonograptussp.-

~

•

hi . . . . . . . .

•

O

10

20km

Fig.13. Map showing the inferred distribution of the organic associations in the Gotland area during the Late Wenlockian.

The major role played by the Bryozoa studied here is as cavity-dwelling organisms and occupants of sheltered areas within the reefs as can be seen in sections showing in situ occurrences, a function suggested also for modern Bryozoa (Cuffey, 1973). Within these sheltered areas, the c o m m o n e s t forms are ramose stick-like stems and reticulate colonies. Presumably, these bryozoan types had a sediment-trapping function after their death, b u t they may n o t be considered as active sediment trappers when alive, as this would directly lead to their burial under the sediment. On the contrary, the ability to remove sediment from the surface of the colony is quite good as shown by studies of living cyclostomes, which are quite efficient at removing sediments from their surface (MacKinney, oral personal communication, 1974). However, the function of the dead colonies was probably important for the reinforcement of the reef body. A function which may also be attributed to the Bryozoa is the stabilization of soft substrates by encrustation. However, this function appears to be of small significance for the forms investigated although encrusting Bryozoa occur c o m m o n l y in the studied material. These encrusting forms, mainly cystoporates such as Spatiopora irregularis, Fist~iliporella delicatula and Fistu-

207

lipora species, b u t also cyclostomes such as Sagenella consimilis and Corynotrypa dissimilis Vine, appear to grow solely on hard substrates in the Wenlockian material. These forms consequently grew mainly on the skeletons of other organisms and may well have been important as cementing agents, but they rarely acted as mud-stabilizing factors. Their aversion from growing on mud is noted also b y Scoffin (1971) for Silurian reef Bryozoa and by Cuffey (1973) for modern reef-dwelling forms. The Bryzoa in the Gotland reefs appear to have played much the same role as in the modern reefs described by Cuffey (1973). However, their framebuilding function and contribution to the rock volume appear to be greater in the Silurian than in modern seas. It deserves mention, however, that few investigations of the quantitative and qualitative importance of modern and fossil Bryzoa are yet available. It seems probable that the c o m m o n l y held opinion that the Bryozoa form a minor group of little interest may well change, since the group apparently contributes greatly to the calcium-carbonate production within a reef and is also both taxonomically diversified and ecologically sensitive. ACKNOWLEDGEMENTS

I am indebted to Mrs. Solweig Jewall and Mrs. Inger Arnstrom for the preparation of the text figures. Comments from Prof. D. V. Ager, Swansea, Prof. A. Fischer, Princeton and Dr. V. Jaanusson, Stockholm improved the paper considerably.

IU A contribution

to

Project E C O S T R A T I G R A P H Y

REFERENCES Borg, F., 1965. A comparative and phyletic study on fossil and recent Bryozoa of the suborders Cyclostomata and Trepostomata. Ark. Zool., 17 (1): 1--91. Brood, K., 1970. The systematic position of Coenites Eichwald. Geol. F6ren. Stockholm F6rh. 92 (4): 469--480. Brood, K., 1972. Cyclostomatous Bryozoa from the Upper Cretaceous and Danian in Scandinavia. Stockholm Contrib. Geol., 26: 1--464. Brood, K., 1976. Cryptostomatous Bryozoa from the Silurian of Gotland. Stockholm Contrib. Geol., in press. Cuffey, R., 1973. Bryozoan distribution in the modern reefs of Eniwetok atoll and Bermuda platform. Pac. Geol., 6: 25--50.

208 Dunham, R. J., 1962. Classification of carbonate rocks according to depositional texture. In: W. E. Ham (Editor), Classification of Carbonate Rocks. American Association of Petroleum Geologists, Tulsa, Okla., pp. 108--121. Hadding, A., 1941. The Pre-Quaternary sedimentary rocks of Sweden, 6, Reef limestones. Lunds Univ. Arsskr., Afd. 2, 37 (10): 1--137. Hancock, N. J., Hurst, J. and Fi]rsich, F. T., 1974. The depths inhabited by Silurian brachiopod communities. J. Geol. Soc. London, 130: 151--156. Harland, W. B. and Gayer, R. A., 1972. The Arctic Caledonids and earlier oceans. Geol. Mag., 109 (4): 289--314. Hede, J. E., 1925. Beskrivning av Gotlands Silurlager. In: H. Munthe, J. E. Hede and L. von Post (Editor), Gotlands Geologi, En Oversikt. Sver. Geol. Unders., Ser. C, 331: 13--30. Hennig, A., 1905--1908. Gotlands Silurbryozoer, 1--3. Ark. Zool., 1905, 2 (10): 1--37; 1906, 3 (10): 1--62; 1908, 4 (21): 1--64. Jaeger, H., 1959. Graptolithen und Stratigraphie des jtingsten Th~ringer Silurs. Abh. Dtsch. Akad. Wiss. Berlin., K1. Chem. Geol. Biol., 2: 1--197. Kaljo, D. et al., 1970. Silur Estonii. Eesti NSV Akadeemia Geoloogia Institut, Tallinn. Klovan, J. E., 1974. Development of western Canadian Devonian Reefs and comparison with Holocene analogues. Bull. Am. Assoc. Pet. Geol., 58 5: 787--799. Lowenstam, A. H., 1957. Niagaran reefs in the great Lakes area. In: H. S. Ladd (Editor), Treatise on Marine Ecology and Paleoecology, 2. Paleoecology. Mem. Geol. Soc. Am., 67 (2): 215--248. Manten, A. A., 1971. Silurian Reefs of Gotland. Developments in Sedimentology, 13. Elsevier, Amsterdam, 540 pp. Maxwell, W. G. H., 1968. Atlas of the Great Barrier Reef. Elsevier, Amsterdam, 258 pp. Petersen, C. G. J., 1913. Valuation of the sea, II. The animal communities of the sea b o t t o m and their importance for marine zoogeography. Rep. Dan. Biol. Stn., 21: 1--44. Ryland, J. S., 1970. Bryozoans. Hutchinson, London, 175 pp. Scoffin, T. P., 1971. The condition of growth of the Wentock reefs of Shropshire (England). Sedimentology, 17: 173--219. StSrmer, L., 1967. Some aspects of the Caledonian geosyncline and foreland west of the Baltic Shield. Q. J. Geol. Soc. London, 123: 183--214. Sokolowski, S. et al., 1970. Geology of Poland, Stratigraphy, part 1. Wydawnictwa Geologuczne, Warsaw, 180 pp. Walter, R., 1972. Palaogeographie des Silurs. Geotektonische Forsch., 41: 1--180. Wells, J. W., 1957. Coral reefs. Geol. Soc. Am. Mem., 67 (1): 609--631. Ziegler, A. M., Cocks, L. R. and Bambach, R. K., 1968. The composition and structure of Lower Silurian marine communities. Lethaia 1 (1): 1--27.