Morphology and ecological significance of Zoophycos in deep-sea sediments off NW Africa

Palaeogeography, Palaeoclimatology, Palaeoecology, 32 (iL980/1981): 185--212

185

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MORPHOLOGY AND ECOLOGICAL SIGNIFICANCE IN D E E P - S E A S E D I M E N T S O F F NW A F R I C A

OF ZOOPHYCOS

ANDREAS WETZEL and FRIEDRICH WERNER

Geologisch-Paliiontologisches Institut der Universitiit Kiel, 2300 Kiel (German Federal Republic) (Received November 13, 1979; revised version accepted April 15, 1980)

ABSTRACT Wetzel, A. and Werner, F., 1981. Morphology and ecological significance of Zoophycos in deep-sea sediments off NW Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 32: 185--212. Eighty-six large-diameter (15--50 cm) ,sediment cores taken off the NW African continental slope have been examined for their bioturbation structures. The Zoophycos "spreiten" -- burrows appearing m profile as series of curved arcs -- were found to be particularly useful for environmental analysis, because they are (1) widely distributed and numerous in Quaternary sediments, (2) easily identified dffe t o ' t h e characteristic structures in vertical and horizontal core sections, and (3) extraordinarily well preserved. Zoophycos is found off NW Africa in water depths >2,000 m, in sediments with Corg contents of 0.3--1.8%. It is possible to recognize in the core sections all of the morphological spreiten types of Zoophycos which were described from ancient sediments. All of these spreiten are formed with two modifications classified as "U-type" and "J-type", according to the shape of the basic tube. The distribution of U- and J-types is significantly related to Corg content and probably depends on the available oxygen content in the respiration water. Morphological analysis leads us to assume that sipunculid animals are the creating organisms. Vertical sediment mixing can be measured by using tracer particles derived from single "event" layers. It is explained by excursions of the creating organism and]or by its active movement of the respiration water. Excellent preservation is a typical feature of Zoophycos, which is evident all over the fossil record. This is due to the considerable depth in the sediment at which the organism producing Zoophycos lives. Core sections wi~h maximum frequency of Zoophycos, indicating optimum living conditions, can be correlated over great distances. For Lhese horizons, it is possible to estimate roughly the maximum population density of Zoophycos-creating organisms, yielding 1 animal]100 m 2 . •

"x.

INTRODUCTION

Zoophycos "spreiten" -- burrows containing disturbed back-fill sediment a p p e a r i n g as a series o f c u r v e d arcs i n c r o s s - s e c t i o n s - - r e p r e s e n t t h e m o s t c o n s p i c u o u s t r a c e fossils i n m a r i n e s e d i m e n t s ( F i g . l ) . T h e y are k n o w n f r o m O r d o v i c i a n t o U p p e r T e r t i a r y s e d i m e n t s a n d t h e r e f o r e it w a s n o s u r p r i s e 0031-0182/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

186

Fig.1. S p r e i t e n - c r e a t i n g o r g a n i s m in t h e basic t u b e ( s c h e m a t i c ) .

when Zoophycos were found also in l~ecent sediments. Because its spreiten can hardly be overlooked in sediment cores, several authors described the burrows, but failed to define them as Zoophycos (i.e. Bouma, 1964; Griggs et al., 1969; Donahue, 1971; v. Stackelberg, 1972; Kudrass, 1973). Seilacher (1967) was the first to describe this trace fossil in y o u n g deep-sea sediments. Reineck (1973) studied Zoophycos in surface sediments from the Indian Ocean by means of three
Fig.2. Vertically oriented thin section through a Zoophycos feeding spreiten, core 310, 228 c m s e d i m e n t d e p t h , a = lamellae w i t h fecal material, partially p e l l e t e d o r h o m o g e n e o u s ; b = lameUae w i t h selected grains, a s s u m e d as n o n - e a t e n . Spacing o f scale 1 ram.

187

Fig.3. Fossil Zoophycos specimen from Apennine flysch deposits (from Bellotti and Valeri, 1979, fig.8).

Zoophycos is normally seen as a body with a sheet-like extension (Fig.3). This habitus has often led to neglecting the vertical dimension. The advantages in studying recent examples are as follows: (1) Morphology: In spite of the spatial restrictions due to the core crosssection, the quantity of individuals found in our core material allows a rather complete reconstruction of the whole burrow. (2) Internal structures: Because of the good preservation which is not affected by diagenetic alteration, or compaction, m a n y details can be observed better than in consolidated sediments. (3) Ecology: The relationship between burrows and ecologic parameters, including water depth, depth of burrowing, organic carbon content, and sediment type, can easily be determined. It has to be emphasized that the Zoophycos spreiten are not only conspicuous in size and structure, but are also very well preserved. This is due to a very important ethologic--ecologic feature: The creating organisms burrow deeper than nearly all others and the Zoophycos are therefore n o t destroyed by bioturbation.

188 MATERIAL AND METHODS

Zoophycos burrows are frequently found in continental-slope sediments off West Africa. Fig. 4 shows the location of the sediment cores for which biogenic structures have been examined and in which Zoophycos occurred. This material was collected during several cruises with R.V. Meteor and R.V. Valdivia (Seibold, 1972; Seibold and Hinz, 1976). Additional sediment cores were investigated from the continental slope off Portugal and Morocco and the Indian Ocean (earlier Meteor cruises), Sulu Sea (Valdivia) and the Pacific Ocean ( Valdivia). The sediment cores were taken mainly with a box corer (KSgler, 1963), with a cross-section of 15 X 15 cm. In a few cases 30 X 30 cm box cores and occasionally piston cores (cross-section 10 cm) were used. For the study of sedimentary structures, X-radiographs of the whole core sections have been prepared using 8 mm thick sediment slices. The bulk of the bioturbation structures have been analysed in this fashion. To accomplish the threeKiimensional representation of certain structures, additional Xradiographs were taken in serial sections or in various orientations, including stereopair sections. Zoophycos can readily be observed on the fresh sediment section itself. For certain analytical problems microscopic examinations of textures and structures have been useful. For this purpose large-sized thin sections (6 X 4 cm) were prepared from plastic-impregnated sediment (Werner, 1966). Organic carbon was determined using the m e t h o d described by Hartmann et al. (1971). MORPHOLOGY OF ZOOPHYCOSBURROWS

Shape The m o r p h o l o g y of Zoophycos varies widely, although less in the pattern of the spreiten than in size and shape. This complexity has led to a large a m o u n t of descriptive literature and to a confusing nomenclature. Voigt and H~intzschel (1956) and H~intzschel (1965) resolved the nomenclature problem by combining all these trace fossils (sixteen ichnogenera and -species) under the name Zoophycos. The material investigated in this study likewise shows a high morphologic variability, although it occurs in a short geological period and in rather uniform sedimentary facies. This variety, however, is not recognized immediately, because normally Zoophycos appears in limited vertical sections only (Fig.9). The reconstruction of the complete burrow, as well as the recognition of the morphological variability, can be accomplished only when the relationship of the single section to the whole burrow is known (e.g. H~intzschel, 1965; Simpson, 1970; Fordyce, 1976; Valeri, 1976; Seilacher, 1977).

189 20 °

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Fig.4. Location of investigated sediment cores. • = cores containing Zoophycos; w i t h o u t Z o o p h y c o s . D e p t h c o n t o u r s in m e t e r s .

• = cores

Th e basic concepts necessary to solve this problem could be obtained b y using structural observations on our material in c o m b i n a t i o n with the corresponding literature. It includes:

190

(1) Spreiten back-fill displaying a typical crescentic structure formed by a lateral shifting of the t u b e in which the creating organism normally stays ( Fig. 1). The sediment volume moved b y the animal is approximately balanced, i.e. the sediment volume reworked by the organism is found more or less completely within the spreiten without being considerably transported horizontally. The fecal sediment c o m m o n l y is seen to consist of layers of pellets, usually alternating with sorted material (assumed to be non-fecal). (2) The marginal parts of the spreiten possess a typical structure which is partly b o u n d e d b y an open tube and partly by a stuffed tube. These concepts are the basis for distinguishing different morphological forms which we group roughly into three main categories (Fig.5) based on the position of the more or less horizontal feeding parts to the vertical central part. These three types are: (i) helicoidal; (ii) trumpet-like; and (iii) tongue-like. Morphological analysis proceeded by comparing the geometric relations of single constructional elements, as explained in Figs.6, 7 which show an example of one special Zoophycos (type iii). We define the shaft spreiten (a) as the axial part of the burrow; the deviation spreiten (b) as transitional from this part to the external feeding spreiten (d), and the regression spreiten (c) formed by the reverse m o v e m e n t from the feeding spreiten (d) back to the shaft spreiten. The elements (a)--(c) belong to the central part and (d) belongs to the feeding part of a Zoophycos burrow. All these constructional elements (Fig.8) result from a spreiten formed by the shifting of a curved tube ("basic tube"), where the animal is burrowing. The different marginal structures in our material, i.e. an open or actively filled (1) " s t u f f e d " tube, led us to distinguish two different relationships b e t w e e n spreiten and the basic tube: (A) U-form. In this type the back-fill is b o u n d e d b y an open tube on b o t h sides, suggesting the whole b u r r o w to be ventilated by a continuous t u b e system which is connected with the sea floor by two holes. (B) J-form. Only one side of the back-fill is b o u n d e d by an open tube with one hole at the sea floor. Thus, an open tube system is lacking. Examples from X-radiographs for these relationships are shown in Fig.9.

(i)

(ii)

(iii)

Fig.5. Main categories of morphological types of Zoophycos: (i) helicoidal, (ii) trumpetlike, (iii) tongue-like.

191

Fig.6. Constructional elements of Zoophycos, demonstrated on three examples of tonguelike burrow types. Indices refer to these three individuals. Photograph of core 211, 68--81 cm sediment depth, a = shaft spreiten; b = deviation spreiten; c = regression spreiten; d = feeding spreiten; t = open marginal tube. T h e f o r m o f t h e basic t u b e - - U-shaped or J - s h a p e d -- influences t h e shape o f the d i f f e r e n t c o n s t r u c t i o n a l e l e m e n t s (Fig.6, a - d ) as described in t h e following. T h e spatial relationships b e t w e e n these e l e m e n t s are s h o w n in Fig.7.

Shaft spreiten (a). T h e vertical o r s o m e t i m e s o b l i q u e l y o r i e n t e d back-fill is b o u n d e d b y t w o t u b e s in t h e case o f t h e U - t u b e , and b y o n e t u b e in the case of the J-tube.

Deviation spreiten (b). In certain intervals h o r i z o n t a l l y o r i e n t e d feeding spreiten are a t t a c h e d to t h e shaft-spreiten. T h e transitional m o t i o n f r o m t h e shaft spreiten to t h e f e e d i n g spreiten creates t h e d e v i a t i o n spreiten. T h e d e v i a t i o n p a r t is always f o r m e d as a w e d g e - s h a p e d b o d y d u e t o t h e p r o t r u s i v e shift o f t h e basic t u b e f r o m t h e shaft spreiten p o s i t i o n t o t h e lateral d i r e c f i o n (Fig.6). In t h e case o f t h e J-shaped basic t u b e , we observe an o p e n t u b e o n l y

192 Block diagram

Scale

Serial sections

A

Fig.7. Structure of connection between feeding and shaft spreiten, demonstrated on a tongue-like Zoophycos burrow after core 289, 660-672 cm sediment depth. Capital letters indicate position of serial sections in the block diagram; serial sections spacing 1 cm. Arrows indicate direction of motion of the animal, a--d = constructional elements as in Fig.6: a = shaft spreiten; b -- deviation spreiten; c = regression spreiten;d = feeding spreiten. in t h e axial p a r t o f t h e b u r r o w . If t h e basic t u b e shows a U - f o r m , t h e deviat i o n s p r e i t e n are b o u n d e d o n b o t h sides b y an o p e n marginal t u b e . This is t r u e o n l y in t h e u p p e r m o s t spreiten, whereas t h e d e e p e r ones are n o t distinguishable f r o m a J - t y p e . T h e m o r p h o l o g i c a l t y p e s (i--iii) d i f f e r in t h e spatial e x t e n s i o n o f t h e i r deviations in t h e f o l l o w i n g way: (i) T h e b u r r o w assumes a helicoidal shape d u e t o t h e helical shift o f t h e basic t u b e . T h e d e v i a t i o n is linked o b l i q u e l y with t h e axial p a r t and turns a r o u n d it several times. T h e d i a m e t e r o f t h e central p a r t is 5--6 times t h a t o f t h e basic t u b e . (ii) Like t y p e (i), b u t the d e v i a t i o n has o n l y o n e t u r n which n o r m a l l y e x c e e d s 270 ° a n d resembles a t r u m p e t . (iii) T h e d e v i a t i o n comprises o n l y a small s e g m e n t o f a circle. T h e distal f e e d i n g s p r e i t e n t h e n assumes a t o n g u e - s h a p e .

Regression spreiten (c). T h e s e are f o r m e d w h e n t h e creating animal m o v e s b a c k t o t h e m a i n shaft a f t e r having f o r m e d a b r a n c h e d or l o b a t e d feeding s p r e i t e n (Fig.7). In this case t h e basic t u b e is shifted f r o m distal t o p r o x i m a l relative t o t h e surface. In c o n t r a s t t o t h e d e v i a t i o n spreiten, t h e r e t u r n t o t h e vertical p o s i t i o n n e x t t o t h e shaft spreiten is carried o u t w i t h o u t g e n e r a t i o n o f a wedge~shaped back-fill s t r u c t u r e . Regression s p r e i t e n were o b s e r v e d o n l y in tongue-~ike b u r r o w s with a J-shaped basic t u b e . Principally regression spreiten c o u l d exist also in t r u m p e t like b u r r o w s with a U- or J-shaped basic t u b e ; h o w e v e r , n o such e x a m p l e c o u l d be f o u n d in o u r material.

193

Constructional elements

Form of the basic tube U-Form (A) I J-Form (B)

I

Shaft spreiten Block diagram ~2crn

Deviation spreiten Block diagram ~ 4 cm

L

Regression spreiten Block diagram ~3crn

Feeding spreiten Continuously formed Top view > 15 cm, Apex line

Discontinuously formed Top view

Fig.8. Constructional elements of Zoophycos burrows.

Feeding spreiten (d). The feeding spreiten are the largest and most important parts of a Zoophycos burrow. The effects of the basic tube form are analogous to those of shaft and deviation spreiten. The formation of the marginal tube depends on the shape of the basic tube. The shape of the feeding spreiten is controlled additionally b y the direction o f ' t h e spreiten progression. Principally, t w o possibilities exist: (1) In the more simple case, the reworking o f a certain sediment volume proceeds so that a more or less uniform spreiten is formed. Its apex line, as defined in Fig.8, runs continuously around the central part (Fig.8). The helicoidal and trumpet-like types (i) and (ii) include this principle. (2) In the more complex cases, there are several feeding spreiten. Each feeding spreiten is connected with the proceeding one (Fig.8). The apex line of a single spreiten is curved, b u t in a different way with U- and J-shaped basic tubes, respectively. With the U-shape, its course is approximately

194

Fig.9. E x a m p l e s for Zoophycos s p r e i t e n in X - r a d i o g r a p h s (negatives). A. Core 239, 1 2 8 . 5 - - 1 3 4 c m s e d i m e n t d e p t h . B. Core 310, 3 0 9 - - 3 2 4 c m s e d i m e n t d e p t h . L e g e n d : a = active filled m a r g i n a l s t r u c t u r e ( J - s h a p e d basic t u b e ) b o u n d i n g feeding s p r e i t e n ; b = o p e n m a r g i n a l t u b e ( U - s h a p e d basic t u b e ) b o u n d i n g f e e d i n g s p r e i t e n ; c = o p e n m a r g i n a l s t r u c t u r e b o u n d i n g a s h a f t s p r e i t e n ; d = p a r t s o f feeding s p r e i t e n w i t h h o m o g e n e o u s back-fill; e = p a r t s o f feeding s p r e i t e n w i t h p e l l e t e d back-fill; f = v a r y i n g h e i g h t o f o n e feeding s p r e i t e n ; g = h a l o , i.e. z o n e o f d e n s e r m a t e r i a l a r o u n d b u r r o w s .

195

Fig.10. Process of discontinuous spreiten formation (J-shaped basic tube). Left: first spreiten, right: successively formed spreiten. concentric in its proximal part, but radial in its distal part (with respect to the central part of the burrow (Fig.8 (A)). With the J-shape, however, a reverse relationship is evident (Fig.8 (B)). The number of feeding spreiten which are constructed successively in this way varies widely. If only one or a few feeding spreiten are built, before the creating organism continues with prolongation of the shaft spreiten, the tongue-like burrow types (iii) of Fig.10 (left) are generated. With a continuous construction of coherent spreiten, the lobate forms of the helicoidal and trumpet-like types (i) and (ii) are generated (Fig.10, right), depending on the number of single feeding spreiten. The combination of all these constructional elements leads to the m o t phological types shown in F i g . l l .

Size The size of the Zoophycos burrows could normally not be determined directly in our core material, because the individuals were mostly larger than the sample cross-section. On the other hand, a variation of sizes is strongly suggested by the varying dimensions of different constructional elements. The burrow size is assumed to be an important parameter in ecological, facies analytical, and stratigraphical problems (Marintsch and Finks, 1978). Several measurements referring to the burrow dimensions have been used (Table I). The values comprise observations on all morphological types because it was not possible to distinguish these in a continuous section. The measurements are: (1) Diameter of central parts. This could be determined in a few cases by serial sections. (2) Diameter of marginal structures. This dimension can be measured easily and is closely related to the diameter of the burrowing organism, and therefore it is considered as the favoured size measurement. All of the

196

Helicoidol (1)

(2)

Trumpet- like (1)

(2)

Tongue-like (1)

(2)

(1) Continuously formed (2) Discontinuously formed

U-types Block diagnams not observed

1

1

Four and more callings of feeding spreten

!

l 1 1

Three and more flats of feedingspreten

J-types

~

\'- L-'L 1

;-_F-J1

Possible variotions in verticol sections (schematic diogrorns)

Three and more fiats of feeding spreiten

Fig.11. Morphological types of Zoophycos, as derived from the investigated material. Top of qolumns indicating main morphological categories, subdivided in continuously and discontinuously formed spreiten. U-types with open tube system, J-types with blindending tube system. Lower row showing variation in vertical axial section, as partially found in the investigated material or known from the literature.

TABLE I Ranges of dimensions of Zoophycos burrows

Vertical extension of burrows Vertical extension of shaft spreiten Diameter of central parts short axis long axis Vertical distance between feeding spreiten (distal parts) Height of feeding spreiten Diameter of marginal structure Horizontal extension of burrows (longest axis)

Minimum values (ram) (observations on small spreiten)

Maximum values (ram) (observations on large spreiten)

200 100

1,100 400

4 40

12 90

20 3 2

120 12 10

150

1,000

197 observed marginal structures had circular cross-sections, indicating that no appreciable compaction had occurred in our material. (3) Height of feeding spreiten. This can be readily measured, but varies more than the diameter of the marginal structure. The ratio between the parameters (2) and (3) varies within the limits 1:2 and 5:6 (fig.9). (4) Vertical distance between feeding spreiten. This distance can also be measured easily in vertical core sections. It has to be measured, however, in the distal parts of the spreiten, because only there is it approximately constant. As shown in the diagram (Fig.12), a good correlation exists between the spreiten distance (distal parts) and the organism size as expressed by the marginal structure diameter. It should be noted, however, that this correlation m a y be only valid in the area of investigation. (5) Horizontal extension. The horizontal extension of Zoophycos burrows could be measured in a few examples where the spreiten were smaller than the core diameter. The extension of larger burrows could be calculated in two ways: (a) From assuming (as exemplified recently by Marintsch and Finks, 1978), that individual size and burrow extension are narrowly related. From our measured examples a ratio of ~ 1 / 1 0 was established. (b) From noting the relationship between the number of spreiten sections cut by the core wall on both sides and the number of sections with the marginal structure preserved within the core, working within the same spreiten height, and assuming random distribution of the core position relative to the Zoophycos burrows. (6) Vertical extension (= depth of burrowing). The vertical extension of the whole Zoophycos burrow may be subdivided into a section between the sediment surface and the uppermost feeding spreiten, composed of the shaft [cm]

Vertical distance of feeding spreiten

10-

Q

•

Height of feeding spreiten • ~ [mm]

o

5

lO

Fig.12. Relation between vertical distance of feeding spreiten (distal parts) and organism size (expressed by the diameter of the marginal tube).

198

spreiten, and a lower section, comprising the feeding spreiten. The upper section could never be measured directly in its total length as is also the case in the fossil examples. This is mainly due to the fact that the surface layer is strongly affected by bioturbation. For the investigated material, it was possible, however, to determine the burrowing depth of nearly all of the associated burrows. As the lower part of Zoophycos is very rarely disturbed b y other burrowing organisms, we can calculate a minimum extension for the shaft spreiten as ~ 3 0 - - 4 0 cm. In a few cases where the shaft spreiten could be observed in longer sections, 30 cm was obtained as minimum value. In contrast to the upper section, the lower section can be measured directly in many examples. Accordingly, vertical extensions of up to 60 cm were not unusual (Table I).

Internal structures The Zoophycos spreiten show a typical rhythmic lamellae structure (Fig.9) which has been described in detail and ethologically interpreted by m a n y authors, including Sarle (1906), Simpson (1970) and Ekdale (1977). Most of the Northwest African forms display a subdivision of the lamellae structure into major and minor lamellae, as analysed by Simpson (1970). Thin-sections of the spreiten cross-section showed clearly that one member o f the lamellae rhythm consists mainly of fecal pellets (Figs.2, 9, 13), as observed also b y Reineck (1973) and Ekdale (1977). The pellets are partially preserved as single, ellipsoidal grains, b u t are partially deformed and amalgam a t e d to coherent, homogeneous masses, with all transitions in between (Figs.2, 9). These transitions were observed also within one burrow and even

Fig.13. Internal structure of feeding spreiten. X-radiograph horizontal section (negative), c o r e 3 1 0 , 78 c m s e d i m e n t d e p t h , a = m i n o r l a m e l l a e w i t h fecal m a t e r i a l ; b = m i n o r l a m e l l a e w i t h s e l e c t e d g r a i n s ; c = m a j o r lamellae.

199 within one feeding spreiten. Therefore, the lamellae structure seems to be inadequate for characterizing morphological burrow types, as was done by Ekdale (1977). The other m e m b e r of the lamellae rhythm consists of material sorted b y the animal, assumed to be non-fecal. ECOLOGY

Adaptation to the biotope In the sediments of the Northwest African continental slope, the primary sedimentary structures are often destroyed by bioturbation. Among the associated burrows, Zoophycos is the deepest. The sediment reworked by the creating animals has been previously eaten several times by epi- and infauna. Therefore these sediments are relatively deprived of nutrients. A number of ethological and anatomical adaptations of the (Recent) Zoophycos-creating organism m a y be used to identify features of its ecological niche: (1) The behaviour is characterized by an economic relationship between locomotion energy and reworked sediment volume. This is self-evident from the feeding spreiten characteristics. (2) As discussed below the rhythmic motions generating the spreiten do not necessarily imply the passing of the whole organism through the basic tube, b u t only its frontal part (see also Simpson, 1970) (3) The nutrients still present in the sediments are enriched by an active grain-size sorting. The fine-grained material, richer in organic matter, is eaten preferentially. (4) The eaten material is used for back-fill of the generated cavities, thus reducing tranport energy to a minimum. (5) The question of the ventilation for respiration is, of course, important for an organism living so far away from the sediment surface. It is curious that two different solutions are realized. Burrows with a U-shaped basic tube obtain a continuous ventilation system with two holes at the sea floor. On the other hand, the J-shaped t y p e lacks such a system, and the organism is connected to the sea floor by only one pipe in the axial part. Very probably the exchange of water necessary for respiration is provided b y pulsatory motions of the whole animal or parts of it. Obviously the ventilation performed with a U-tunnel system is more effective. It could be assumed, therefore, that U-type burrows are related to environments with a certain deficiency in oxygen, whereas J-type burrows would be sufficient in connection with oxygen-rich b o t t o m waters. The distribution of U- and J-types in the investigated material is found not to be random. This relation to other sediment parameters is discussed below. (6) Vertical sediment transfer. The construction of feeding spreiten with a balanced budget should not normally imply an active vertical exchange of

200

sediment particles over larger distances. There exist, however, many observations o f significant particle transport within Zoophycos burrows. Transport downwards from a layer with characteristic particles overlying Zoophycos spreiten has been observed in several cores in the investigated material. Chough (1978) has observed similar evidence for transport in a Zoophycos b u r r o w from the Labrador Sea. Differences in colour and/or composition between the Zoophycos spreiten and surrounding sediments are c o m m o n l y observed, without indications of the transfer direction. On the other hand, there is also evidence for an upward-directed transfer over considerable distances. Fig.14 shows an example based on analysis of benthic foraminifera brought in b y a turbidity current. For a material transfer the following mechanisms are possibly involved: (a) Down- and upward transport within feeding spreiten. Material is moved within each of the feeding spreiten. The spreiten normally have a certain inclination proximal to the axial part, and vertical transfer of material results as the sediment is transported through the gut of the animal. (b) Transport through the ventilation system. The water m o v e m e n t within the tunnel induced by the organism for respiration transports particles downward. (c) A certain a m o u n t of material always enters the funnel end of the burrow. This process should be most effective when an organism is crossing the upper part of the tunnel. The more frequently this occurs the more material is transported downward. In extreme cases the tunnel m a y be clogged completely, and the Zoophycos-creating organism may be forced to leave its basic t u b e in order to repair the funnel. (d) The Zoophycos animal itself may carry sediment in its gut in an upward direction. 0 ,oo

50

.

.

.

.

.

.

.

Zoophycos JE (b "O O ¢J

m

m m

i~;ii;i!ii~TURBD TE 200

:ml Fig.14. Vertical mixing by Zoophycos-creating organism: Percentage o f shallow-water foraminifera related to the total amount of benthic foraminifera (after Haake, 1980).

201 Resulting from these relationships, sediment mixing over larger vertical distances as interpreted from the Zoophycos burrow, is mainly due to occasional mishaps.

Possible Zoophycos-creating organisms The analysis of morphology and internal structure permits some deductions concerning the anatomy of the creating animal. These deductions are based on the premise that its anatomy is finely tuned to a burrowing habitat (Seilacher, 1967). This is evident in the following features: (1) The great length of the tube in relation to its small diameter suggests a vermiform organism. (2) The particle sorting has to be done at the m o u t h by tentacles or similar appendages. The mechanism of this kind of sorting in sipunculids has been studied recently b y Hansen (1978). (3) The observation that the height of the feeding spreiten occasionally varies rhythmically and is larger than the diameter of the marginal structure (Fig.9) suggests peristaltic motions of the creating organism generated by a hydroskeleton surrounded by layers of longitudinal and circular muscles ("Hautmuskelschlauch", Kaestner, 19691. (4) The alternating structure of minor lamellae, consisting of sorted grains and fecal material leads to the conclusion that m o u t h and anus pass along the same section of the tunnel. Optimally, this may be realizea only by an animal with a U-shaped digestive tract. These anatomic properties are combined in two systematic groups of marine animals, the Sipunculida and the Phoronida, as was concluded also by Ekdale (1977). The sipunculids have, in contrast to the phoronids, an extensible introvert ("proboscis"). This is a strong, muscular organ, appropriate for locomotion in the sediment. It allows a to-and-fro motion which seems to be most adequate for generating the lamellae structure in the observed neat and regular way. Therefore, the Zoophycos-creating organism is thought to belong to this systematic group. Previous authors who have discussed Zoophycos burrows have suggested several kinds of organisms as generators. Bradley (1973) discussed Zoophycos as generated b y sea pens, b u t this is not compatible with the presence of fecal material within the spreiten. The suggestion of Simpson (1970), who discussed Arenicola-like animals as possible creating organism, cannot be accepted for the same reason. Bischoff (1968) suggests polychaetes with parapods as creating organisms, interpreting small ovoid moulds within the feeding spreiten as due to parapods. These structures are, however, very probably fecal pellets, and therefore, it is not likely that a polychaete may generate the burrow since it does n o t have a U-shaped digestive tract. On the other hand, the zoological observations on Sipunculida (Kaestner, 1969) are compatible with our requirements for the Zoophycos-creating organism. In particular, the .following points are of interest:

202 (1) Sipunculida are mostly marine animals, living o n - and i n - - m u d d y sediments up to 1 m below the surface in water depths of up to 5,000 m. (2) Sipunculida show decided thigmotactic reaction on their tentacles together with food reception. Kaestner (1969, p. 456) states that these forms are sediment feeders which feed selectively. This was shown by experiments with Dendrostomum which did not ingest sand which was previously cleaned of organic compounds, but did ingest sand when it was mixed with powdered serum-albumine. (3) Respiration in the Sipunculida is done via the cuticle of the organism which appears as an adequate adaptation to the way of living, with peristaltic movements in a m u d d y environment. (4) The Sipunculida are monosexual animals. This favors the possibility of patchy distribution as discussed below. (5) Not much is known on the locomotion within the sediment. Kaestner (1969), however, described observations similar to those of Sch~fer (1962) regarding the significance of the introvert ("proboscis") as a locomation organ.

Construction of the spreiten With the assumption of a "worm-like" animal with the characteristics described above for the creator of Zoophycos, the construction of a spreiten could be t h o u g h t to take place in the following way. The organism is oriented with a distal position of the m o u t h , i.e. downward in the direction of motion within the basic tube as the actual place of activity. The extension of the minor lamellae would correspond to the " t o - a n d - f r o " m o t i o n of the introvert. At forward m o t i o n -- or expansion -- of the introvert, sediment is reworked by the tentacles around the m o u t h , and hereby grains are selected and deposited proximally as back-fill (Fig.15). The rest of the material is ingested. At backward m o t i o n -- or inversion -- of the introvert, the digested sediment is deposited on the selected material as back-fill, thus forming a pair of minor lameUae. While fecal pellets are deposited mainly as lamellae, selected grains are also f o u n d as a disturbed band immediately adjacent to the spreiten ( " h a l o " , Fig.9; Reineck, 1973; Fordyce, 1976).

Regional distribution and biotopical implications Fig.4 shows the occurrence of Zoophycos in the core samples investigated off Northwest Africa. They can be f o u n d all over the area in water depths greater than 2,000 m. However, in other continental margin regions the upper distribution limit has sometimes been found to be higher. For example, off the west coast of India, burrows occurred in cores from 1,200 m water depth (v. Stackelberg, 1972, and pers. comm.), and off Portugal and

203

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sediment

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JO

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,,~ ml~

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~"setect ed grains back fill of J fecal materiQI

Fig.15. S c h e m a t i c process o f spreiten formation. The "proboscis" (introvert) of a fictive organism is shown in action, forming lamellae. Upper part: forward motion of the introvert during selecting and feeding of sediment. Lower part: backward motion of the introvert during digestion and formation o f fecal lamellae. Morocco at 1,700 m. On the ot her hand, within the dept h range of o p t i m u m distribution o f a b o u t 1, 500- - 3, 500 m of f West Africa,occasionally cores do n o t contain Zoophycos. This is due to special environmental conditions rather than to c o n t i n g e n c y of sampling (see discussion, p. 207). A discussion o f these b o u n d a r y conditions for the Zoophycos distribution m ay provide some estimates on t he environmental needs of the creating organism. As some o f these limiting conditions we observed:

(1) Sediment grain-size. N one o f t h e cores o f f Cape Blanc contain Zoophycos: These sediments are characterized by high sedimentation rates ( > 2 0 cm/ 1,000 a) t o g e t h e r with high am ount s o f silt and fine sand fractions (>>70%; B. K o o p m a n n , pers. comm.). On t he ot her hand, the sediments are totally r e w o r k e d by infauna. This shows, t o g e t h e r with t he organic carbon c o n t e n t o f 0.3--1.8% (Miiller, 1975), t h a t t he limiting factor for Zoophycos in this area is neither deficiency of n u t r i e n t supply nor of oxygen. It seems m ore probable th at th e mechanical properties o f the sediments are not favorable for the Zoophycos animal. This hypothesis m a y be s uppor t ed by the observation t hat t he ichnofauna includes burrows typical for coarser sediments, such as Thalassinoides and Scolicia (generated by crustaceans and echinoids). (2) Oxic sediments. A n u m b e r o f cores in t he southwestern part o f the investigation area (Fig.16), located far o f f the coast, contain no or only a few and small Zoophycos specimens. These cores are characterized by a low sedimentation rate and oxic conditions. Because the absolute c o n t e n t of

204

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F i g . 1 6 . C o r e s f r o m w a t e r d e p t h > 2 , 0 0 0 m w i t h Zoophycos, s h o w i n g t h e r e l a t i o n o f b u r r o w s w i t h J- a n d U - s h a p e d b a s i c t u b e . L e g e n d : o = 1 0 0 % b u r r o w s w i t h U - s h a p e d b a s i c t u b e ; • = 1 0 0 % b u r r o w s w i t h J - s h a p e d b a s i c t u b e ; ~ = w i t h o u t Zoophycos.

205 organic carbon is not extraordinarily low, it m a y be assumed that the oxidation of organic matter starts with the more valuable nutrients which are more easily decomposed. Therefore, at the normal Zoophycos living depth the a m o u n t of available organic matter would not be sufficient. All the sediment cores containing well-developed and numerous Zoophycos specimens are characterized by a relatively small coarse silt and fine sand fraction as well as by anoxic conditions below the surface layer. The latter is indicated by a strong decrease of sulfate content (Hartmann et al., 1973) and by the presence of macroscopic pyritized biogenic structures. (3) Content of nutrient substances. In some cores or core sections where Zoophyco~ is missing, we assume that a surplus of n u t r i e n t s - reflected by high amounts of organic carbon -- is the cause for its absence. Such cores are located in three areas (Fig.16): off the m o u t h of the Gambia River, off Senegal, and off Cape Barbas. All these sediments are characterized by high organic carbon contents (>2%) combined with high sedimentation rates (Milller and Suess, 1979) and strongly predominating large-scale biodeformational structures. The best illustrations of these relationships are provided by five cores within a small area covering about some ten kilometres off Cape Barbas. All of them show the same phenomenon: The Holocene section, with a sedimentation rate of 6 cm/1,000 a (Diester-Haass et al., 1973) and organic carbon c o n t e n t of ca. 1% (Mfiller, 1975), displays a burrowing pattern observed as typical for this water depth zone of the continental slope in the whole investigation area. Numerous Zoophycos burrows are associated with Chondrites and Planolites. Below the Holocene/Pleistocene boundary, the sedimentation rate as well as the organic carbon content increase rapidly -- sedimentation rates up to 20 cm/1,000 a and Corg content up to >2% (Hartmann et al., 1973). In these sediments, neither Zoophycos nor other burrows are present, but only biodeformational structures which are intensively developed all over this core section. This evident contrast suggests t h a t a surplus of nutrient supply in the sediment is not evoking a specialized behaviour of infauna organisms (Seilacher, 1977). In contrast, Zoophycos, Chondrites and even Planolites are adapted to a more or less food-restricted environment, while the sedimentfeeding organisms creating biodeformational structures do not show any active grain-size sorting. They are spatially related in a vertical sequence of increasing burrowing depth with simple tunnels (Planolites), burrow systems (Chondrites) and spreiten (Zoophycos). In this sense, the described profiles well illustrate the e c o n o m y of feeding burrows.

Relation of U- and J-types to the biotope As mentioned above, two constructional types of the spreiten, U- and Jtypes, are distinguished. It has to be expected t h a t the specimens with Jshaped basic tubes, having a less perfect ventilation system, should be more

206

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w

0

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. . . .

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Fig.17. Corg c o n t e n t of sediments versus relation b e t w e e n U- and J-Zoophycos types. R e l a t i o n ( U / U + J ) × 100 d e t e r m i n e d o n core sections, each containing a b o u t 10 feeding spreiten with marginal tubes. Corg percentages are m e a n values o f the same core sections. Values f r o m 9 different cores. T h r e e points, closely together, represent different sections o f one core.

restricted in relation to oxygen deficiency t h a n the U-types, because in anoxic sedimentary conditions the sea water available for respiration is assumed to decrease ,rapidly in oxygen content. A more effective ventilation system such as is provided by U-types then becomes necessary. Therefore, it is necessary to ascertain the geographical distribution of U- and J-types to see whether they reflect different degrees of oxic/anoxic conditions in the sediment. Fig.16 represents the quantitative relation between U- and J-types in the investigated cores or groups of cores. In the slope sediments south of the Gambia m o u t h , the U-types dominate strongly. The organic carbon content as well as the sedimentation rates of these samples are high (P. Mfiller and U. Pflaumann, pets. comm., Kiel: 1.5% Corg; 15 cm/1,000 a). Consequently, a high level of the reduction zone, as well as abundant macroscopic pyrite structures formed near to the sediment surface, are observed in these cores. Also the group of cores off the Senegal m o u t h which have similar U/Jratios, are characterized by a high organic carbon content. A third example is related to the sediment cores discussed above in which the moderately high Corg content in the Holocene section is, according to Diester-Haass et al. (1973), due to upweUing conditions. All of these examples with high U/J-relationships are in contrast to a few cores taken from a greater distance from the continent. Here J-types together with low organic c o n t e n t and more oxic conditions in the uppermost sedim e n t layer dominate. A compilation of these relationships, using the organic carbon content as a representative parameter, is shown in Fig.17. Although the a m o u n t of avail-

207 able data is not y e t satisfactory, the good degree of correlation can be considered as support of the present hypothesis.

Burrowing depth and preservation As discussed above, the burrowing depth of Zoophycos may extend to 1 m. This is reflected in the excellent preservation of the Zoophycos feeding spreiten in the investigated material as well as in the fossil records from the continent. It has to be concluded, therefore, that the Zoophycos animal normally burrows deeper than any other of the associated organisms. Apart from a few minor exceptions, the Zoophycos spreiten are disturbed only by other individuals, but not by other burrowing species. In contrast to this, the shaft spreiten are largely disturbed, particularly in their upper parts. Therefore shaft spreiten are seldom found and described as parts of Zoophycos. The great burrowing depth is dependent on the presence of appropriate nutrients in the sediment. In cores where the occurrence ~of nutrients is restricted to shallower depths, Zoophycos is missing~or adapted to that biotope by smaller specimens with a short or even without a shaft spreiten. As an example of this, in sediment cores of the Central Pacific (see M~iller, 1975) we observed in the Miocene section such small specimens which are disturbed by Planolites and Chondrites. This indicates the relatively shallow position of these Zoophycos within the bioturbation system.

Density of population There are two conclusions which permit us to a t t e m p t a calculation of the population density of Zoophycos animals: (1) The complete preservation of the Zoophycos feeding spreiten permits their numerical recording. (2) The distribution of Zoophycos spreiten in the investigated core profiles suggests t h a t sections with marked abundances can be correlated between neighbouring cores from single stations as well as from larger distant locations. Therefore, a continuous, definite population density m a y be assumed. The a m o u n t of the sediment reworked by Zoophycos relative to the total volume is 50%. For the calculation such sections of m a x i m u m Zoophycos occurrence a number of cores with known stratigraphy based on absolute age determinations were used. All the data used for the calculation are summarized in Table II. To evaluate the data listed in Table II, in terms of animals per unit area, the following assumptions had to be made: (1) Non-patchy distribution of Zoophycos-creating organisms. The Zoophycos burrows in a definite sediment volume are distributed equally in time. (2) Number of feeding spreiten per Zoophycos burrow. In order to transfer the counted spreiten structures crossing the cores into individual numbera, a probable number of four crossings per Zoophycos was assumed. Further-

208

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209 more, it was assumed, that one animal is producing only one Zoophycos in its life time. (3) Horizontal dimension of Zoophycos. Normally, the Zoophycos spreiten that are so abundant in the layers studied, consist of large-sized individuals (spreiten height > 8 mm) with diameters of about I m. Therefore, one organism lives on an area of ~ 1 m 2 . (4) Life-span of Zoophycos-creating organisms. This, of course, is purely hypothetical and may be tentatively chosen within 1 and 50 years. This range has been used in the calculation of the population density (Table II). The population densities were calculated in the following way: (1) number of spreiten (N) × 1/4 = number of burrows (2) [number of burrows/time span (t)] × living time of animal (l) = animals/ 1,000 m 2 (A) (3) [reciprocal value of animals/I,000 m 2 ] X 1/1,000 = m2/animal (P) A-4

N'l

• t

P=(1/A).

1 , 0 0 0 [ m 2]

In this way, hypothetical average values of population densities were calculated. In the investigated sediments an upper limit of about one animal per 50 m 2 at a life-span of 50 years has to be considered as a maximum, valid only for o p t i m u m environmental conditions. Therefore, one animal per 100 m 2 appears as a more probable value. Clearly the chance of collecting the Zoophycos-creating animal by b o t t o m sampling is extremely small. The calculation, of course, does not take into account the possible patchiness of the population. Considering our hypothesis concerning the Zoophycoscreating organism and the monosexuality of Sipunculida, we would favour a patchy animal distribution. This is in agreement with the observation of irregular distributions of other deep-sea benthic fauna (Grassle et al., 1975). C ONCLUSIONS (1) Zoophycos burrows have been found to occur in water depths exceeding 2,000 m in the investigated sediments off Northwest Africa. (2) Zoophycos is the best-preserved biogenic sedimentary structure found in deep-sea sediments. This is due to the extreme burrowing depth of the Zoophycos-creating organism, thus excluding destruction by other burrowing animals. (3) Because of its characteristic internal back-fill structure, Zoophycos spreiten can hardly be overlooked in X-ray radiographs of core sections. (4) On the one hand, m a n y morphological types of Zoophycos were often observed in the facies units; on the other hand, different types of internal structures (e.g., pelleted and non-pelleted fecal material) have been f o u n d to occur together in one burrow. Therefore, a taxonomical subdivision of the ichnogenus Zoophycos has been avoided. (5) All of the morphological types of Zoophycos f o u n d in the investigated material may have either a double (U-type) or a single (J-type) ventilation tube.

210

(6) Occasionally, vertical mixing of sediment particles occurs due to excursions of the Zoophycos-creating organism and/or by movement of the respiration water. According to the vertical extension of Zoophycos burrows, this mixing comprises distances of up to 1 m. The vertical mixing may lead to significant errors in the stratigraphic evaluation of sediment cores. (7) The balanced sediment budget and the type of internal structure of the back-fill leads to the assumption of sipunculid animals as the creating organisms. (8) The population of Zoophycos burrows in a distinct sediment volume is representative for the original population density of the creating organisms. For core sections with m~ximum frequency of Zoophycos, a population density of 1 animal/100 m: has been estimated. (9) Core sections with maximum frequency of Zoophycos, representing optimum living conditions, can be correlated over great distances. (10) The distribution of Zoophycos clearly indicates a sensitive correspondence to environmental factors. Distribution frequency and size are probably related to quality and quantity of nutrients, whereas the formation of U- or J-type burrows depends on the oxic/anoxic properties of the sediments. ACKNOWLEDGEMENTS

This study has been possible through the substantial help of various persons. Dr. Fr.-W. Haake, Dr. B. Koopmann, Dr. H. Lange and Dr. P. Mi~ller (Kiel) generously provided unpublished data of the investigated sediment cores. Prof. E. Seibold, Prof. M. Sarnthein and Dr. P. Miiller (Kiel) contributed fruitful discussions. W. Rehder assisted with the preparation of X-radiographs and photographs, and H. Hensel with preparation of the samples. Prof. W. H. Berger and Prof: J. E. Warme reviewed the manuscript and Dr. T. Healy helped with the English text. Financial support was given by the DFG (Deutsche Forschungsgemeinschaft). All these contributions are gratefully acknowledged. REFERENCES Bellotti, P. and Valeri, P., 1979. L'influenza dell'ambiente sedimentario sull'assetto elicoidale dell strutture a Zoophycos. Boll. Soc. Geol. Ital., 9 9 : 1 - - 1 1 . Bischoff, B., 1968. Zoophycos, a polychaete annelid, Eocene of Greece. J. Paleontol., 42: 1439--1443. Bouma, A. H., 1964. Notes on X-ray interpretation of marine sediments. Mar. Geol., 2: 278--309. Bradley, J., 1973. Zoophycos and Umbellula (Pennatulacea): Their synthesis and identity. Palaeogeogr., Palaeoclimatol., Palaeoecol., 1 3 : 1 0 3 - - 1 2 8 . Chough, S. K., 1978. Morphology, Sedimentary Facies and Processes of the Northwest Atlantic Mid-Ocean Channel Between 61 ° and 51 ° N, Labrador Sea. Thesis, MCGill University, Montreal, Que., 167 pp.

211 Diester-Haass, L., Schrader, H. J. and Thiede, J., 1973. Sedimentological and paleoclimatological investigations of two pelagic ooze cores off Cape Barbas, Northwest Africa. "Meteor"-Forsch.-Ergebn., C, 16: 19--66. Donahue, J., 1971. Burrow morphologies in north-central Pacific sediments. Mar. Geol., 11: M1--M7. Ekdale, A. A., 1977. Abyssal trace fossils in the worldwide Deep Sea Drilling Project cores. In: T. P. Crimes and J. C. Harper (Editors), Trace fossils, 2. Geol. J., Spec. Iss., 9: 163--182. Fordyce, R.E., 1976. Zoophycos from the Torlesse Supergroup, North Canterbury, New Zealand. N.Z.J. Geol. Geophys., 19: 289--291. Grassle, J. F., Sanders, H. L., Hessler, R. R., Rowe, G. T. and McLellan, T., 1975. Pattern and zonation: a study of bathyal megafauna using the research submersible Alvin. Deep-Sea Res., 22: 457--481. Griggs, G. B., Carey, A. G. and Kulm, L. D., 1969. Deep-sea sedimentation and sediment-fauna interaction in Cascadia Channel and on Cascadia Abyssal Plain. Deep-Sea Res., 16: 157--170. Haake, F.-W., 1980. Benthische Foraminiferen in Oberfl~ichen-Sedimenten und Kernen des Ostatlantiks vor Senegal/Gambia (Westafrika). "Meteor" Forsch.-Ergebn., C, 32, 1--29. Hansen, M. D., 1978. Nahrung und Fressverhalten bei Sedimenfressern dargestellt am Beispiel von Sipunculiden und Holothurien. Helgol. Wiss. Meeresunters., 31: 191--221. Hi~ntzschel, W., 1965. Vestigia Invertebratorum et Problematica; Fossilium Catalogus I: Animalia pars 108. W. Junk, 's-Gravenhage, 142 pp. Hartmann, M., Lange, H., Seibold, E., and Walger, E., 1971. Oberfl~chensedimente im Persischen Golf und Golf von Oman. I. Geologisch-hydrologischer Rahmen und erste sedimentologische Ergebnisse. "Meteor" Forsch.-Ergebn., C, 4: 1--76. Hartmann, M., Miiller, P., Suess, E., and Van der Weijden, C. H., 1973. Chemistry of Late Quaternary sediments and their interstitial waters from the NW African continental margin. "Meteor" Forsch.-Ergebn., C, 24: 1--67. Kaestner, A., 1969. Lehrbuch der Speziellen Zoologie. Bd. I. Wirbellose 1. Teil. Fischer, Stuttgart, 3rd ed., 898 pp. KSgler, F.-C., 1963. Das Kastenlot. Meyniana, 13: 1--7. Koopmann, B., 1979. Sahara-Staub in Sedimenten des tropisch--subtropischen N-Atlantik wiihrend der letzten 30.000 Jahre. Dissertation, University of Kiel, 107 pp. Kudrass, H.-R., 1973. Sedimentation am Kontinentalhang vor Portugal und Marokko im Spiitpleistoz~n und Holoziin. "Meteor" Forsch.-Ergebn., C, 13: 1--63. Marintsch, E. J. and Finks, R. M., 1978. Zoophycos size may indicate environmental gradients. Lethaia, 11 : 273--279. Miiller, P. J., 1975. Diagenese stickstoffhaltiger organischer Substanzen in oxischen und anoxischen marinen Sedimenten. "Meteor" Forsch.-Ergebn., C, 22: 1--60. Miiller, P. J. and Suess, E., 1979. Productivity, sedimentation rate and sedimentary organic matter in the oceans. I. Organic carbon preservation. Deep-Sea Res., 26: 1347--1362. Reineck, H.-E., 1973. Schichtung und Wfihlgefiige in Grundproben vor der ostafrikanischen Kiiste. "Meteor" Forsch.-Ergebn., C, 16: 67--81. Sarle, C. J., 1906. Preliminary note on the nature of Taonurus. Proc. Rochester Acad. Sci., 4: 211--214. Schiifer, W., 1962. Aktuo-Pali~ontologie. Kramer, Frankfurt, 606 pp. Seibold, E., 1972. Cruise 25/1971 of RV "Meteor": Continental margin of West Africa. General report and preliminary results. "Meteor" Forsch.-Ergebn., C, 10 : 17--38. Seibold, E. and Hinz, K., 1976. German cruises to the continental margin of North West Africa in 1975: General reports and preliminary results from "Valdivia" 10 and "Meteor" 39. "Meteor" Forsch.-Ergebn., C, 25: 47--80. Seilacher, A., 1967. Bathymetry of trace fossils. Mar. Geol., 5: 413--428.

212 Seilacher, A., 1977. Evolution of trace fossil communities. In: A. Hallam (Editor), Patterns of Evolution. Developments in Palaeontology and Stratigraphy, 5. Elsevier, Amsterdam, pp. 359--376. Simpson, S., 1970. Notes on Zoophycos and Spirophyton. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils. Geol. J. Spec. Iss., 3: 505--514. Valeri, P., 1976. Impronte do Zoophycos nei calcari marnosi paleogenici al Valico delle Capanelle (L'Aquila). Boll. Soc. Geol. Ital., 94: 2155--2182. Voigt, E. and H~ntzschel, W., 1956. Die grauen B~nder in der Schreibkreide NordwestDeutschlands und ihre Deutung als Lebensspuren. Mitt. Geol. Staatsinst. Hamburg, 25: 104--122. v. Stackelberg, U., 1972. Faziesverteilung in Sedimenten des indisch-pakistanischen Kontinentalrandes (Arabisches Meer). "Meteor" Forsch.-Ergebn., C, 9: 1--73. Werner, F., 1966. Herstellung yon ungestSrten Diinnschliffen aus wasserges~'ttigten, pelitischen Lockersedimenten m~ttels Gefriertrocknung. Meyniana, 1 6 : 1 0 7 - - 1 1 2 .