Palaeoecology of silurian crinoids of Gotland (Sweden)

Palaeogeography, Palaeoclimatology, Palaeoecology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

P A L A E O E C O L O G Y OF SILURIAN CRINOIDS OF G O T L A N D (SWEDEN) A. A. MANTEN 1

Utrecht (The Netherlands) (Received March 17, 1967) (Resubmitted February 13, 1968) SUMMARY

The crinoids which occur in the Middle and Upper Silurian of Gotland (Sweden) have grown in shallow, rather clear, well-aerated and warm water. They reached their optimum development on the flanks of reefs. The size which the crinoids attained was strongly influenced by water agitation. In the course of the Silurian, in the area studied, a gradual increase in average stem diameter took place, which suggests an increasingly better adaptation to the conditions that prevailed in the reef environment. INTRODUCTION

The Swedish island of Gotland, situated in the Baltic, is widely known for its succession of Silurian (Gotlandian) strata. The local stratigraphical names, used in describing the deposits, are shown in the left-hand part of Fig.3. The Visby Beds may correlate with the Upper Llandoverian, the H/Sgklint, Slite and Halla-Mulde Beds with the Wenlockian, and the Klinteberg, Hemse, Eke, Burgsvik and HamraSundre Beds with the Ludlowian. Limestones and marlstones are the most common sediments in the Silurian of Gotland. Organic reefs form an important element of most of the stratigraphical units. THE FOSSIL REEFS OF GOTLAND

Three major types of reefs can be distinguished (MANTE~, in preparation). Their main characteristics are given in Table I. The small Upper Visby-type reefs are the oldest reefs found in Gotland. They are found abundantly in the Upper Visby Beds. Most widespread are reefs of the Hoburgen type. These are common in the H~Sgklint, Slite, Klinteberg, Hemse and Hamra-Sundre Beds. In the Hemse and Hamra-Sundre Beds reefs of the Holmh~iller type are also found. All reefs were evidently formed in shallow water. OCCURRENCE OF CRINOIDS

Crinoid remains from Gotland have been described by ANGELIN (1878), 1 Private address: Cortezlaan 9, Utrecht (The Netherlands).

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A.A. MANTEN

TABLE I M A I N C H A R A C T E R I S T I C S O F T H E T H R E E REEF T Y P E S O F G O T L A N D

Enclosed in a profile of

Most common shape

Upper Visby type

Hoburgen type

Holmhiiller type

marlstone and marly limestone in alternating layers knoll; lens; inverted cone

marly limestone and limestone

rather pure limestone

inverted rightelliptical cone; very elongated fiat lens about 100 m ~

crescent

Average size of reef limestone section

less than 10 m 2

Ratio height/length of the reefs Organic composition of reef frame Algae Number of fossil species per reef as a whole (incl. reef dwellers)

1 --

1 1 - -

5

rather variable

more than 1,000 m 2

1

1

1

50

15

75

generally variable

rather uniform

absent or very rare common moderate in number many (20-200) (10-60) for small reefs

very common not many (5-40) for large reefs

Dominant reef builders

corals

stromatoporoids

Average size of reef builders Average shape of stromatoporoid colonies Matrix

relatively small

stromatoporoids; corals larger

rather flat

lenticular

round or irregular and high little marly

large

very strongly marly

strongly marly

Weathered surface

conglomeratic; sometimes bed-like

Surrounding sediments

narrow mantle of stratified marly limestone

generally conglomassive meratic; partly brecciated or bed-like reef detritus; large reef detritus; crinoid amounts of crinoid limestone (?) limestone

BATHER (1893), SPRINGER (1920) a n d UBAGHS (1956a,b, 1958). Emphasis is laid by these authors o n morphological descriptions of isolated well-preserved specimens, rather than on the stratigraphical a n d palaeogeographical distribution a n d palaeoecology. This paper contains a few notes, set in perspective by some p r e m a t u r e conclusions, which aim to show that a study of the palaeogeography a n d palaeoecology of crinoids m a y lead to interesting observations.

Crinoids in reef-surrounding sediments C r i n o i d limestones are present a r o u n d m a n y of the reefs in G o t l a n d . They are so characteristic of the direct s u r r o u n d i n g s of fossil reefs, that one c a n almost

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PALAEOECOLOGY OF SILURIAN CRINOIDS OF GOTLAND

173

be sure that if crinoid limestone is exposed somewhere, reef limestone will also be found in its close vicinity. The crinoid limestones are generally built up, for the main part, of small and large disarticulated crinoid-skeletal remains and a calcareous mud which fills the interspaces and cements the whole. Embedded in the deposit there is generally a varying amount of reef debris. The larger crinoid fragments, generally strongly recrystallized, have retained their original forms. In both transverse and radial sections the remains may show a fine porosity, a net-like structure. Crinoid sand is also a very common constituent of the crinoid limestones. It is generally most abundant at the original seaward side of the reefs, i.e., usually the southeastward side. The most characteristic crinoid limestone, which is a real crinoid breccia, is often better developed around the higher parts of the reefs than around the lower parts. There, and also around the crinoid breccia higher up, the more usual crinoid limestone or otherwise a limestone with crinoid remains, and reef debris is generally present. Some reefs possess a thin, but distinct talus mantle, consisting predominantly of coarse reef material, which mantle is intercalated between the reef limestone and the crinoid limestone or limestone with reef debris. Around the reefs of Hoburgen type, crinoids developed abundantly on all sides. By no means, however, did this lead in all cases to the development of a real crinoid breccia. Many reefs are only surrounded by crinoid limestone with reef debris or with more regular stratified limestone with crinoid and reef material. In a crinoid limestone with reef debris in the Klinteklint (Boge Parish, Slite Beds), the vertical distributions of the larger crinoid stem fragments have been studied (Fig.l). It appeared that there is a rough correlation with the number of pieces of reef debris in the same rock. At the base of the section, neither is abundant; at the top, both decline. Crinoid material of smaller size is still abundant there, but the larger stem fragments decrease in number. Both the crinoid breccia and the crinoid limestone with reef debris are generally thick bedded. In some instances they are cross-bedded and in a few cases they show evidence of wave sorting. In the Upper Visby Beds, which consist of marlstone and marly limestone, no crinoid limestones are found to envelop the reefs. Around the reefs of Holmh~illar type they were also present, but in Recent time they were eroded from most of the localities where these reefs are exposed. In the stratified limestones and marly limestones which were deposited at greater distances away from the reefs, crinoid remains are found scattered at r a n d o m through the sediment, together with other marine invertebrates, such as brachiopods, bryozoans, corals, and occasional moluscs. In conclusion, the distribution of crinoid remains in the limestones of Gotland shows that these organisms could grow almost everywhere on the sea floor at the time that these rocks were laid down, but that they were only abundant Palaeogeography, Palaeoclimatol., Palaeoecol.,

7 (1970) 171-184

174 Reef debris >2cm8 >0.Scm O

A.A. MANTEN Crinoid stem fragments >0.Scme

10 20 30 40 50 60 10 20 Number of pieces per dm 3 of rocks

30 40 50

Fig. 1. Vertical distribution of reef debris and crinoid stem fragments in a section through crinoid limestone with reef debris in the Klinteklint (Boge Parish). Only the material with a size, and/or diameter of more than 5 mm has been counted. An additional curve also shows the number of pieces of reef debris larger than 2 cm. The section is about 4 m high. The rock has been deposited at the southwestern side (lateral) of a Hoburgen-type reef, the material exposed at the base presumably at 4-5 m distance from the reef, the material at the top at 6-7 m from the reel The highest part of the reef extended about 1-5 m above the top of the crinoid-limestone section. in the direct vicinity o f reefs. W h e n reef g r o w t h e n d e d in a p a r t i c u l a r locality, a p p a r e n t l y the conditions for crinoid d e v e l o p m e n t also b e c a m e less suitable.

Crinoids in reef limestone The sediments s u r r o u n d i n g the reefs o f H o b u r g e n type always c o n t a i n m a n y Y m o r e orinoid fragments t h a n the reef limestones themselves, which m a y even be p o o r in such fossils. C o m p a r a t i v e l y speaking, m o s t c o m m o n are crinoids represented in the reef limestones within the K l i n t e b e r g a n d Eke Beds. Generally the reefs o f Holmh~illar type contain m a n y m o r e crinoid remains t h a n those o f H o b u r g e n type (Fig.2). The central parts o f the reefs m a y consist for 1-6 % o f crinoids. T o w a r d s the periphery this percentage increases, a n d close to the m a r g i n crinoid remains m a y f o r m 1 0 - 2 5 % o f the rock volume. The highest percentages are f o u n d t o w a r d s the ends o f the larger, crescent-shaped reefs. In the surface o f the developing reef depressions occurred which were either filled with

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175

PALAEOECOLOGY OF SILURIAN CRINOIDS OF GOTLAND W-NW

ao

E-SE |

N-NW

S-SE

Be

8 ._ 60 . . . . . . . . . . . . . . . .

~..

60

-

-

"~ 40-

~

.

-

-

....

-40

--...--.--...--..--...--.:-==':'-.--:"--"~'~

stromatoporoids compound

corals

.......... crinoids ..... calcareous Algae and m a t r i x Fig,2. Volumetric composition of reef limestone of the crescent-shaped reef at HolmhAllar, Hamra-Sundre Beds (see sketch in right bottom comer of illustration). The diameter of the reef is approximately 650 m. The left graph represents 70 m of reef limestone near the west-northwestern end of the reef. Note the increase in crinoid volume towards the reef end, and the notable decrease in stromatoporoids. The right graph represents 72 m of reef limestone, from the centre of the reef, perpendicular to the reef axis in the direction of its original seaward margin. Note that the increase in crinoid volume towards the outer side of the reef is less marked here than towards the reef end of the left graph.

debris or became pools in which a different fauna developed, particularly corals and bryozoans. In some of the debris-filled depressions crinoid stem fragments are extremely abundant; in others they are much less numerous. In general, there seems to be a certain relationship between the amount of crinoid remains in a depression and the distance of the depression from the reef margin. In the reefs of Hoburgen type debris-filled depressions can also be found, but there they are generally less characteristically developed and nowhere do they contain large amounts of crinoid remains. As will be discussed in rather more detail later in this paper, the aboveoutlined distribution of crinoid remains in the reef limestones suggests that crinoids lived on the reef flanks rather than on the reef top. AVERAGE DIAMETER OF CRINOID STEM FRAGMENTS

There are notable differences in the size of the crinoid stem remains found in the various localities. In an attempt to investigate whether some general lines could be detected in crinoid development, average diameters of stem fragments were determined. A total of 84 samples was studied and per sample 100 diameters were

Palaeogeography, Palaeoclirnatol., Palaeoecol., 7 (1970) 171-184

7'

,~

"~

3

•~-~

Hemse Beds

Khnteberg Beds

Hal=a- Mulde Beds

Slit® Beds

Hemse group

Klinteberg limestone

Mulde marlstone Halla limestone

Slit® group

×

~

x

x-®

I

~

® @

~

110

®

t

120

Fig.3. Graphical representation of variations in the average diameter of crinoid stem fragments over the stratigraphical units in Ootland.

+ limestone with reef debris around Hoburgen-type reefs

within Hoburgen-type reefs

x~ tilted depression

(~

I

e crinoid limestone around Hoburgen-type reefs

+

®

~

I

L~ filled depression within Holmhdllar-type reefs

e

®

x(~ ~

®

~,~

~y, ® (38

®

~ x

x ~x

xx+ ~®

~

xx

x

+

X ;(X

I

Holmhallar-type

x

~ ®

•

x

l

I

(ram) 100

X reef limestone,

x

Qx~-~

++

I

Average thickness of crindd stem fragments 50 60 70 80 90

X reef limestone, Hoburgen-type

Upper Visby marlstone Visby Beds Lower Visby mar Istone

H6gklint Beds

Eke Beds

Eke group

Tofta limestone H6.qKlint limestone

Burgsvik Beds

Hamra- Sundre Beds

Manten (in prep,

Eburgs~cik sandstone and oolite

Hamra l i m e s t o n e

Sundre limestone

Hede (1921 and later)

Stratigraphical succession

>~

>

¢~

177

PALAEOECOLOGY OF SILURIAN CRINOIDS OF GOTLAND A 30'

20

10

0

10.

2

4

G

8

B /

Diameter of crinoid stem fragments

(ram)

Fig.4. Frequency distribution of crinoid stem diameters. The percentages have been calculated for the fractions between each two successive whole-milimetre values. A. Crinoids in two samples taken from Upper Visby reef limestone. B. Crinoids in five samples of crinoid limestone from around Hamra-Sundre reefs of Hoburgen type (Hoburg "marble"). T A B L E II VARIATION IN STEM DIAMETER IN CRINOIDS OF GOTLAND

Age

Average diameter of crinoid stem fragments (ram) and number of samples (in brackets) reef limestone

H a m r a - S u n d r e Beds 7.14(7) (Holmh~Ular reef type) H a m r a - S u n d r e Beds 6.84(5) ( H o b u r g e n reef type) 1 Eke Beds 7.86(1) H e m s e Beds 7.27(8) (Holmh/illar reef type) H e m s e Beds 7.15(5) ( H o b u r g e n reef type) Klinteberg Beds 6.46(2) Slite Beds 6.61(3) H6gklint Beds 4.60(2) U p p e r Visby Beds 5.03(2) ( U p p e r Visby reef type)

depression within reef

crinoid limestone

limestone with reef debris

9.58(3)

--

--

--

10.25(5)

8.07(3)

-9.80(3)

7.79(2) --

7.72(2) --

8.32(1)

7.55(3)

6.87(7)

--5.46(1) --

7.55(3) 7.80(4) 4.91(4) --

6.35(3) 7.14(1) 4.62(2) --

1 Also two s a m p l e s o f stratified algal limestone with crinoid remains, f o u n d directly u n d e r n e a t h reef limestone with average diameter of crinoid stem f r a g m e n t s of 7.16 ram.

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A.A. MANTEN

measured. The average values which were found are shown graphically in Fig.3 and are summarized in Table II. Fig.4 gives a picture of the size-frequency distribution. The main conclusions are: (1) The average diameter is larger in crinoid stem fragments found in depressions within the reefs than in those found scattered in the reef limestone proper, and presumably also than in those found in the crinoid limestones around the reefs. (2) The average diameter of crinoid stem fragments is distinctly larger in crinoid limestones than in the reef limestone which they enclose or in limestone with reef debris which often occurs somewhat farther away from the reefs than the real crinoid limestones. (3) There is no significant difference in the diameters of crinoid stem fragments found in reef limestone and in limestone with reef debris. (4) In reef limestone of Holmh~illar type the crinoid stem fragments are, on the average, thicker than in reef limestone of Hoburgen type. (5) From the Upper Visby Beds up to the Hamra-Sundre Beds, the crinoid stems tend to attain greater average thicknesses. From the Hemse Beds upwards, this trend is more pronounced in the figures from the reef-surrounding sediments than in those from the reef limestones. (6) The averages found for the thickness of crinoid-stem fragments in the Klinteberg Beds are below the values that would be expected on the basis of the increase noted under (5). (The Klinteberg Beds were, however, deposited in shallower water than the majority of Slite and Hemse Beds.) (7) The difference between average crinoid diameters in reef limestone and surrounding deposits is greatest in the Hoburgen-type reefs in the Hamra-Sundre Beds. (This may be caused by the origin of the samples; the reef-limestone samples were collected low in the reef-sections of Hoburgen, except for one with an average diameter of 7.38; the samples from the surrounding deposits are of younger age; during the formation of these reefs, the water depth, initially very shallow, increased.) (8) The average diameter of crinoid stem fragments in the reef limestone is closest to that in the surrounding sediments with the small reefs in the Eke Beds. (9) The difference between the sample with the highest and that with the lowest value of average crinoid stem diameter, as obtained from the reef limestones, is greater for the reefs of Holmh~illar type than for the nearest comparable reefs of Hoburgen type (the latter usually occupied a smaller area of the sea floor). In the Hemse Beds those differences amount to 2.25 and 0.85 respectively, in the Hamra= Sundre Beds to 1.84 and 1.26. Additional observations on crinoid-stem diameters are: (10) In the deposits which envelop the reef limestone, the average diameter

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of crinoid stem fragments is, as a rule, somewhat smaller at the original seaward side of the reefs than at the original landward side. (11) The average crinoid diameter in the reef-surrounding deposits generally decreases with increasing distance from the reef. (12) Crinoid remains with notably large diameters (over 20 mm) occur in scattered fashion from the Slite Beds upwards. They are most common in the crinoid limestone of the Hamra-Sundre Beds (Hoburg "marble") and in the filled depressions within the reefs of Holmh~illar type. FACTORS INFLUENCING THE DEVELOPMENT OF CRINOIDS

Different crinoid taxa A major problem in the study of crinoids is the loosely articulated skeletal structure of the living animal. Minor agitation of the water over the sea floor has presumably already resulted in major disarticulation of the skeletal parts of dead individuals. Well-preserved crowns are rarely found in the reefs or on their flanks. Some are found locally in limestone with crinoid remains and reef debris, which was laid down as inter-reef deposits. The taxonomic information which the author has obtained from these crowns is rather scattered and one-sided. Therefore, he is unable to prove whether some or all of the variations found in crinoid development should be attributed to the occurrence of different taxa. That different taxa have been present, however, is s u r e (MANTEN, in preparation, table IX) and that the ecological requirements and average sizes of each of these were identical is unlikely. Taxonomic identifications of crinoids cannot be made on the basis of stem fragments alone. There are some morphological differences between the stem remains, even within one sample, but it is known that these may occur even within one species. Thus the generally cylindrical columnals of Crotalocrinus species passed into somewhat more pentagonal forms down the stem. (In earlier literature, these pentagonal Crotalocrinus stem fragments are incorrectly described as

Cyathocrinites pentagonalis

GOLDF.).

For the Niagaran (Middle Silurian) reefs in the North American Great Lakes region, LOWENSTAM(1948,1950,1957) found that in the development from the quiet to the semi-rough and rough water stages of reef development, more and more camerate crinoids, with their massive box-like calices, began to occur in addition to the more-fragile inadunate crinoids.

Linkage to the reef environment In Middle and Upper Palaeozoic times crinoids were generally associated with reef structures (LAUDON, 1957, p.961). In Gotland it is only in very close connection with reefs that crinoids appear to have lived in tightly-knit gregarious coenoses. The question can be put here as to whether the crinoids lived predominantly Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 171-184

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h.A. MANTEN

on the reef surface, on the reef flanks, or in the immediate reef surroundings. The author believes that it is the second environment which has been the most important. If the majority of crinoids lived on the reef surface, one would expect to find many more crinoid remains in the matrix of the Hoburgen-type reefs. The different hydrodynamic properties of the crinoid remains, due to the very porous nature of the crinoid skeletons, compared to other fossil material of comparable size, should, of course, be taken into account. But where other, smaller fossils could be embedded in between the reef builders, together with terrigenous debris, why should not many more crinoid fragments have been preserved in reef interstices if many crinoids had grown and become disarticulated on the reef surface? The fact that the average diameter of crinoid stem fragments is larger in the crinoid limestones around the Hoburgen-type reefs than in the reef limestones also suggests that the crinoid limestones are not formed by the washing off of crinoid material from the reef surface. The increase in crinoids from the central parts of the Holmh~illar-type reefs towards the margins suggests that also these crinoids were by far the most abundant on the reef flanks, even though several may also have been growing on the surface of these larger reefs. On the other hand, the presence of the most characteristic crinoid limestones directly against several reefs of Hoburgen type, or in some cases against their talus mantles, shows that the densest communities of crinoids must have been located very close to the reefs. Much reef debris even moved down from the reef over the crinoid thanatocoenoses to a final position somewhat further away from the reef. In the further reef surroundings, also, many crinoids presumably grew, but nowhere do they seem to have been so particularly abundant as on the reef flanks, irrespective of whether these flanks consisted of the reef frame proper or of a mantle of coarse talus material deposited around the actual reef. The crinoid curve in Fig.1 suggests that the conditions for crinoid development became less favourable with the death of the reef around which they grew. The typical situation of a reef with forests of crinoids on its flanks, developed particularly in the case of the larger reefs. The smaller the reefs were (Klinteberg Beds, Eke Beds) the less distinction there apparently was between the fauna of reef surface and reef flanks.

Water depth Abundant information on the fossil reefs of Gotland shows that these reefs developed in shallow water, usually less than 50 m deep (MANTEN, in preparation). In contrast to most crinoids of the present day, which are deep-water forms, Palaeozoic crinoids thus flourished abundantly in shallow water. Even in very shallow water deposits, crinoid remains are found in large numbers. One of the most characteristic crinoid limestones of Gotland is the Hoburg Palaeogeography, PalaeoclimatoL,PalaeoecoL, 7 (1970) 171-184

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"marble", which is linked to the Hoburgen-type reefs of the Hamra-Sundre Beds in southwesternmost Gotland, particularly to the younger parts of these reefs. It was there that the highest average crinoid stem diameters were found. There are indications (MANTEN, in preparation) that these younger reef parts were formed in slightly deeper water than the older parts of these reefs and than most of the other Hoburgen-type reefs. The next-largest average stem diameters are found in the reefs of Holmh~illar type. Crinoids are also much more abundant in these reefs than in the other reefs of Gotland. It is likely that at least some of the most characteristic reefs of the Holmh~illar type developed in somewhat deeper water than the majority of Hoburgen-type reefs. There are, thus, some indications that in comparatively deeper shallow water (probably deeper than 10-15 m), the crinoids became larger and presumably also more abundant than in very shallow water. However, the degree of, and variations in, water mobility may have been of greater influence on the size of the crinoids, whose remains are found in these reefs, than water depth in itself. Mobility of the water There are several further indications that the crinoids grew larger in less agitated water than in strongly agitated water. This is understandable if we think of the rather fragile skeletons of these organisms. The larger average diameters of crinoid stems at the original landward side of the reefs as compared to the original seaward side, and in filled-in depressions within the reefs as compared to the reef matrix, no doubt are functions of water mobility. The same may be true for the greater variation found in crinoid diameters within reef limestone of Holmh~illar type. In larger reefs there is more variation in local environmental conditions between the various parts of the developing reef. The presence of unusually large amounts of crinoid columns in the reef environments and the marked scarcity of skeletal remains of the crowns of crinoids in these places may perhaps also be attributed to the destructive action of agitated water. Another possibility is that predators fed on the crowns of the crinoids, allowing only the other skeletal parts to accumulate on the sea floor. Sediment content of the water Against the widespread belief that crinoids could only live in a clear sea, AGER (1963, p.132) demonstrated that at least some crinoid taxa could also very well thrive in muddy seas. He observed that in the Mississippian of Indiana (U.S.A.) autochthonous crinoid remains (long stems, calices, holdfasts) commonly occur embedded in shales. The fossil crinoids of Gotland, however, belong to taxa which preferred relatively clear water. Some crinoid remains are found in stratified marlstone deposits, but are only randomly scattered. In stratified limestones in reefless areas, Palaeogeography, Palaeoclimatol.,PalaeoecoL, 7 (1970) 171-184

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A . A . MANTEN

crinoid remains are more commonly found than in the marlstones, though they are nowhere abundant there either. The stratified deposit in which they are most common is the algal limestone at the base of the Hamra-Sundre Beds. The marly to somewhat clayish matrix of some reef-like intercalations in this algal limestone indicates that there was a certain supply of terrigenous debris. On the other hand the general abundance of Algae in the algal limestone suggests that this deposit has been laid down while the water was not truly muddy. At least not to such a degree that a silt or clay deposit sedimentated from it, such as was the case in the Mississippian of Indiana. As far as the reefs of Gotland are concerned, the matrix of the reefs of Hoburgen type, also generally shows that the water in which the reefs grew contained terrigenous debris, but the mobility of the water prevented substantial deposition of this material. Crinoids are least common in and around the reefs with the strongest marly matrix, those of the Upper Visby type. The reefs with the purest matrix, those of the Holmh~llar type, are the richest in crinoids. Water temperature In the assessment of ancient climatic conditions, reliance may be placed on comparisons of ancient life with its modern equivalents. Both ancient and recent reefs show a great variety of organisms with calcareous parts. As early as half a century ago, VAUGHAN (1911) already argued that the physiology of organic calcium-carbonate precipitation is governed by physico-chemical processes which probably will have remained similar over the years. Hence, the high rate of calciumcarbonate precipitation in stromatoporoids, corals, crinoids, calcareous Algae, and other contributors to the reefs of Gotland, suggest warm-water conditions similar to those which occur in tropical regions of the present day. Oxygen content o f the water The general characteristics of the sediments of the reefs and of the associated sediments, combined with the high density of organisms present in these deposits, show that the water was well-aerated during the formation of the Palaeozoic reefs of Gotland and the crinoid limestones and other sediments which envelop them. Oxidizing conditions must have prevailed. Better adaptation to the environment in the course of time It is difficult to give a satisfactory explanation for the increase in average diameter of the crinoid stems in the course of geological time, as found in Gotland. As this trend does not directly continue in Devonian and younger formations in other areas, it is apparently a regional phenomenon. The reefs of the Upper Visby Beds are the first which formed in the western part of the Baltic Sea of Palaeozoic times. In the following younger geochronological unit, the H6gklint Period, crinoids began to occur in crowding communities around reefs. From H6gklint time on-

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wards, the crinoids appear to have become increasingly adapted to the reef environment, as appears from both their increase in size and in abundance. Since the sizes of the H~Sgklint, Slite, Hemse (Hoburgen type), Eke, and Hamra-Sundre (Hoburgen type) reefs and their elevation with respect to sea level were rather similar, it is not likely that differences in environmental conditions, such as currents were a major cause. N o r is there any indication that differences in the nature of the sites may have played a part; for instance that some samples were taken from sites which had more in situ growth of crinoids and, therefore, contained more large stems. The various stratigraphical units abound in exposures of reef limestone and surrounding sediments and the impression that an increase in the average crinoid diameter had taken place in the course of the Gotland Silurian was obtained from intensive field work, even before the measurements reported here were made to substantiate this impression. Care was taken that representative sites were chosen. It would be interesting to know whether the increase in average size took place within the same taxa, or whether in the course of time new taxa evolved in the area which were more specialized towards this environment, or whether perhaps other taxa migrated into the area from elsewhere, to find in the reef environment their optimum living conditions. The author is unable to answer this intriguing question at his present stage of knowledge. CONCLUDING REMARKS

This paper is certainly not the last word on the subject of the palaeoecology of the Palaeozoic crinoids of Gotland. It is only a hesitant first attempt. The great number of factors which have been of influence on the development of these crinoids makes it still difficult to evaluate their individual importance. Much detailed work has to be carried out to refine the general picture now obtained. More taxonomic information is badly needed. Also many more measurements of stern diameters are required to determine more exactly the influence of each relevant environmental factor on crinoid development. Volumetric studies on crinoid distribution are desirable as well. It is hoped that the preceding pages will stimulate palaeoecologists to undertake such studies on crinoids in whatever area is suitable for this kind of work. The results promise to be rewarding. REFERENCES ANGELIN, N. P., 1878. Iconographia Crinoideorum in Stratis Sueciae Siluricis Fossilium. Roy. Swedish Acad. Sci., Holmiae, 64 pp. (Opus postumum). AGER, O. V., 1963. Principles of Paleoecology. McGraw-Hill, New York, N.Y., 371 pp. BATHER, F. A., 1893. The Crinoidea of Gotland, 1. The Crinoidea Inadunata. Kgl. Svenska Vetenskapsakad. Handl., 25(2): 1-200.

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LOWENSTAM, H. A., 1948. Biostratigraphic studies on the Niagaran interreef formations in northeastern Illinois. Sci. Papers Illinois State Mus., 4: 1-146. LOWENSTAM, H. A., 1950. Niagaran reefs of the Great Lakes area. J. GeoL, 58: 430-487. LOWENSTAM, H. A., 1957. Niagaran reefs in the Great Lakes area. In: H. S. LADD (Editor), Treatise on Marine Ecology and Paleoecology, 2. Paleoecology. GeoL Soc. Am., Mem., 67(2): 215-248. MANTEN, A. A., in preparation. Silurian Reefs of Gotland. Typology, Palaeoecology and Stratigraphical Implications. Elsevier, Amsterdam. SPRINGER, F., 1920. The Crinoidea Flexibilia. Smithsonian Inst. PubL, 2501: 1-486. UaAGHS, G., 1956a. Recherches sur les crinoides camerata du Silurien de Gotland (Su6de). Introduction g6n6rale et partie I: Morphologie et pal6obiologie de Barrandeocrinus sceptrum ANGEUN. Arkiv ZooL Kgl. Svenska Vetenskapsakad., Ser. 2, 9(26): 515-550. USA~HS, G., 1956b. Recherches sur les crinoides camerata du Silurien de Gotland (Su6de). Pattie II: Morphologie et position syst6matique de Polypeltes granulatus ANGELIN. Arkiv Zool. KgL Svenska Vetenskapsakad., Set. 2, 9(27): 551-572. UBAGHS, G., 1958. Recherches sur les crinoides camerata du Silurien de Gotland (Su6de). Partie III: Melocrinicae. Avec des remarques sur l'6volution des Melocrinidae. Arkiv Zool. Kgl. Svenska Vetenskapsakad., Ser. 2, 11(16): 259-306. VAUGHAN, T. W., 1911. The physical conditions under which Paleozoic coral reefs were formed. Ball. GeoL Soc. Am., 22: 238-252.

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