preservation of soft-bodied animals in precambrian sandstones at ediacara, south australia

PRESERVATION O F SOFT-BODIED ANIMALS IK PRECAMBRIAN S A N D S T O N E S A T EDIACARA, S O I J T H AUSTRALIA

WADE, MARY: Preservation of soft-bodied animals in Precambrian sandstones at Ediacara, South Australia. Lethuiu, Vol. 1, pp. 238-67. Oslo. 15th July 1968.

With only two exceptions, this soft-bodied fauna can he grouped naturally into two categories, ‘non-resistant’ and ‘resistant’. Non-resistant animals (mostly medusoids) collapsed or decayed before diagenesis had set the enclosing sediment; fossils occur in convex relief on the depositional bases of sandstone laminae. Resistant animals (most annelids, pennatulids, and unique groups) supgprted the.covering sediment until it had set; fossils occur in negative relief on the bases of sedimentary laminae. Where there was little clay between adjacent quartzitic laminae, counterpart moulds or counterpart casts are also found on tops of underlying laminae.

’l’he peculiarity of the fossilization of a large and varied fauna of soft-bodied animals in relatively coarse, shallow-water sediments has been briefly dis- cussed several times by M.F. Glaessner, particularly in Glaessner & Daily (1959), and by Glaessner & Wade (1966). The purpose of this paper is to relate the various types of preservation to the sediments in which they are found, and to discuss the conditions of deposition and burial and the effects of diagenesis. As certain preservations are characteristic of species, genera, or larger taxa, their differing behaviour under similar conditions indicates the relative toughness, softness, or susceptibility to decay, of the organisms.

Specimens with registered numbers prefixed T or F are deposited in the collections of the Geology Department, University of Adelaide. Those with registered numbers prefixed P are deposited in the collections of the South Australian Museum.

Whilc studying this material I have profited from many discussions with Professor M.F. Glaessner. I hanks are due to him for his critical reading of the manuscript, and also to Dr. R. Goldring and Mr. C.N. Curnow for readily providing a typescript of their paper on the facies of the fossiliferous sediments at Ediacara Range. The South Australian Mines Depart- ment made three samples of diamond-drill core from bore E3, Ediacara, available for petrological investigation, and I had the advantage of studying a petrological report by D. Smale and N..4. l‘rueman, Australian Mineral Development Laboratories, on these and three surface specimens from the fossiliferous beds at Ediacara Range.

l‘his research was partly supported by an Australian Research Grant to Prof. M.F. Glaessner.

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PRESERVATION OF PRECAMI%KIAN ANIhlALS 230

The fossiliferous sediments The fossiliferous sediments, as stated when they were first recognized (Sprigg, 1947), are in the upper portion of the Pound Quartzite, the youngest formation of the Adelaide System, presently defined as entirely Precambrian in age (Thompson et al. 1964). The stratigraphically lowest occurrence of fossils in the formation is that at Red Range near Beltana (Fig. l), some 180 m below the top of the Pound Quartzite. It is not of significantly greater age than the beds at Ediacara Range, 19 km to the WNW, and is in fine-grained quartzitic rock, intensely weathered, now the consistency of a siltstone.

At Ediacara Range the earliest fossils known are several specimens of Pteridinium cf. simplex (Gurich) found in boulders of a massive fine-grained quartzite (Glaessner & Wade, 1966). These are approximately 45 m below the top of the Pound Quartzite and 18 m below the main fossiliferous unit, but outcrop is bad and the dip of the beds differs in the vicinity, making extrapolation doubtful. Search of possible lateral equivalents of these beds has been unfruitful.

The best exposure of the main fossiliferous unit is at Greenwood Cliff, at the diagonally opposite side of the synclinal outcrop. Here the outcrop is cut off by Gap Creek Fault only 2 or 3 m below the base of the main fossiliferous unit. T h e change upward from barren, white sandstones to the red-brown sandstones and red-brown or greenish clayey siltstones of the lower beds of the main fossiliferous unit is quite abrupt; at first, thin layers of clay separate rather massive sandstones up to about 5 cm thick, but the proportion of sandstone dwindles rapidly while that of clayey siltstones and clays increases. T h e sand fraction amounts to half or more of the total sediment in the 8 m thickness found here but many of the layers are so thin that few comparable pieces remain in more weathered parts of the outcrop. Trace fossils, in the form of the short casts mentioned by Goldring (Goldring & Curnow 1967), are found just above the earliest clayey silts, and are the first of the common fossils.

Throughout these lower fossiliferous beds the surfaces of sandstones are stained red-brown with haematite, though it is rare for more than traces to be present within the sandstone layers. In a petrological report on behalf of Australian Mineral Development Laboratories, Smale & Trueman sug- gested that this ubiquitous surface staining might be due to iron being ex- pelled from around sand grains in the sandstones during the growth of authigenic quartz. They also noted that where present, clay had tended to inhibit growth of authigenic quartz. They commented upon the much greater amount of authigenesis in the surface samples (with localization of the ferruginous staining) than in diamond drill core samples, but clay-rich core samples had been supplied to them to allow analysis of the clay which was dominantly illite. In the core samples the ferruginous staining was more dis- persed around the sand-size quartz grains, whether these were cemented by clay or not. These observations imply that these sands were initially iron-

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MARY WADE

T E R T I A R Y - R E C E N T BASINS

C A M B R I A N

P O U N D Q U A R T Z I T E

WONOKA F O R M A T I O N

TRIANGULATION / FAULT A STATION

0 T E R T I A R Y - R E C E N T BASINS

@ DlAPlHlC CORES

ADELAIDE S Y S T E M ROCKS

/ FAULT

A TRIANGULATION STATION

I 4 2 J

Pig. 1 . L,ocality plan: A shows the extent of Adelaide System rocks in the Adelaide Geo- syncline and related shelf areas; the vertically lined area is figured in B where known diapiric cores are indicated in black. T h e triangulation stations of Mt. James and Randell Lookout in Ediacara Range, and Beltana Hill in Red Range, are shown by open triangles. Lake Torrens Sunkland and the adjacent Beltana Sub-basin are cross-hatched as in C, where the Pound Quartzite is shown enclosing fossiliferous Cambrian deposits in the southern half of Edi- acara Range inlier. The hatched line near its top indicates the main fossiliferous member, and, east of the Southern Workings, the lowest fossiliferous outcrop known at Ediacara.

coated and must have undergone much less penecontemporaneous submarine reworking than most of the Pound Quartzite. This implication is in accord with conclusions from the character of the sediment. The sedimentary setting of the main fossiliferous unit has been investigated in detail by Goldring ik Curnow (1967). They view it as differing from the main sedimentary facies of the Pound Quartzite by having been deposited in an environment of tem- porarily lessened hydrodynamic intensity.

These fossiliferous sediments are not uniform, however ; at Greenwood Cliff they change very rapidly from 'relatively unsorted, haematite-stained sandstones with plentiful silty and clayey interbeds, upward to clean, white sandstones. These lack the haematite-stained surfaces and the large amounts

PRESERVATION OF PRECAMBRIAN ANIMALS 241

of clay- to silt-size particles present in the lower beds, and are less flaggy as a result. This upper ‘fossiliferous’ bed is locally unfossiliferous near its base due to reworking, and to rather vigorous hydrodynamic conditions (Goldring & Curnow 1967), but is relatively rich in fossils a little higher. Its fauna is unchanged from that of the lower fossiliferous beds. Still higher in the section evidence of vigorous water-movement again becomes dominant (Gold- ring & Curnow 1967) so that bedding planes which show little sign of lateral transport are rare; few are even potentially fossiliferous as most breaks were formed by conditions prohibitive of the preservation of soft-bodied animals. The highest horizon from which fossils have been collected in situ is 4.6 m above the base of the upper fossiliferous beds. The known measured thickness of the main fossiliferous unit at Greenwood Cliff is thus 12.6 m, the upper limit being transitional, as Goldring & Curnow have shown, to the normal cross-stratified and flat-stratified facies of the Pound Quartzite.

Goldring found the upper fossiliferous bed much better sorted than the lower (Goldring & Curnow 1967) and containing larger-scale sets of bedding. He attributed this to shallowing of the water, and compared some structures to those now formed at depths not less than 6 m. He found no positive evidence of emergence during depositi0n.h either the lower or upper beds of the main fossiliferous unit, or, indeed, of the enclosing portions of the Pound Quartzite.

This lithostratigraphic division holds for the entire northwest exposure, from the north side of Gap Creek, southward to where it is lost under alluvium southwest of Greenwood Cliff. I t is also valid for the most distant exposures, those extending from near the Southern Workings to the southeast side of the outcrop. The northeast exposures are not good but several narrow bands of clean, white sandstone are interbedded in the more silty, clayey flagstones of the lower beds. The upper fossiliferous beds here are also white sandstones but weathering or sand cover obscures the nature of the change from the underlying sediments. By contrast, the western outcrop is unique in lacking coarse sediments. Sandstones in both the lower and upper fossiliferous beds are fine-grained. Those of the lower fossiliferous bed (which is locally unfossiliferous in its middle part) are light pinkish-maroon, while the upper beds are white, but there is a zone of transition at least 2 m thick with re- peated gradational colour-changes from pink to white along the strike.

The isolated position of the Ediacara Range inlier (Fig. 1) denies us knowledge of the position and nature of the barrier that sheltered the area at the time of deposition of the main fossiliferous unit. The position of the western shore is unknown. Flat-lying sandstones, underlying fossiliferous Cambrian limestones and extending 200 km west of Lake Torrens, may prove to be the middle Marinoan ABC Quartzite; if this is so, the western shore was probably within 40 km of Ediacara. I t could have been relatively close, but Gold- ring & Curnow (1967) did not indicate the west as a source of sediment. The Pound Quartzite outcrops intermittently for several hundred km to the north and south, and easterly for 300 km.

242 MARY WADE

Seventeen kilometres east of Ediacara Range the core of the Beltana Diapir now outcrops (Fig. lb). This is one of many diapiric cores of Willouran (lowest Adelaide System) rocks which intrude younger Adelaide System and Cambrian rocks in the FIinders Ranges. Several were eroded and contributed coarse sediments to surrounding Sturtian (middle) and Marinoan (upper) Adelaide System rocks, and even more have effected neighbouring Cambrian sediments (Dalgarno 1964; Thompson 1965 ; Forbes 1966; Walter 1967). The Beltana Diapir (Leeson, in Leeson & Nixon 1966) contributed local con- glomerates to both the mid Marinoan Brachina Formation, and the late Marinoan base of the Pound Quartzite (at the north end of Red Range); it also overturned Cambrian rocks at its southwest end. This intermittent his- tory of its uplift suggests that, though apparently not emergent during th t deposition of the top of the Pound Quartzite, it may well have caused a submarine shoal capable of breaking the force of larger waves. It alone is not likely to be the full explanation of sheltered*conditions at Ediacara but the occurrence of a number of persistent shoals, with occasional emergence, may do much to explain the lack of wave-action commensurate with the minimum known fetch of the sea, and the consequent deposition of-beds with fine- grained sediments and without significant reworking at Ediacara.

The Pound Quartzite varies greatly in the degree of silicification that has taken place. In the Ediacara Range it is predominantly an indurated sandstone but weathering of feldspar (and possibly, of clay cement) has rendered it friable in places whilst secondary silicification has converted it to quartzite elsewhere. For uniformity, beds of sand-grade sediments, predominantly sandstone, are called sandstones here, though for individual flags ‘sandstone’ or ‘quartzite’ is used to fit the specimen described.

The lower surfaces of sandstone layers are formed of sandgrains which were packed against the smooth-textured surfaces of the fine material that weathered away to form the flaggy partings. As a result, these lower surfaces are themselves characteristically smooth-textured, even when of irregular shape or large grainsizes. The upper surfaces, whether buried as deposited or stripped by water-movement before burial, were not forced into con- formity with the finer sediments and are generally rougher in texture, though very fine-grained upper surfaces are relatively smooth. Extreme thinness of the clay separating adjacent sandstone layers may lead to the base of the upper sandstone layer partly conforming to the top of the lower sandstone layer; in F17322 (Fig. 2 A and R) for example, scattered large quartz grains on the top of the lower lamina of sandstone are reflected by moulds on the base of the overlying lamina. As there is much less clay in the upper than in the lower beds, the bases of sandstone layers tend to be rougher in the upper beds but this is only a modification of the general trend just described. The original sand-grade sediments of the upper beds are finer and better sorted than those of the lower beds (Coldring & Curnow 1967) so that the tops of these sandstones are often smoother than is common in the lower beds. However, even when these two characters of relatively rough lower and relatively smooth

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Fig. 2. F17322a, h , approx. 0.5, sandstone slab split to reveal ripple-marked fossiliferous bedding plane. A: hase of upper portion showing numerous low casts of medusoids, and (at pointer) the furrows of Pseudorhiaostomites and the external moulds of some of the large sandgrains which occurs intermittently on the surface of the underlying portion. B: Top of lower portion shouzing moulds of medusoids and counterpart cast of Pseudorhizostomitcs (at pointer).

244 MARY WADE

upper surface occur on one flag, its orientation can usually be told by the surface textures alone. Failing that, cross-bedding is usually present at some level and can be used for determination of ‘facing’.

Specimens illustrated In the description of specimens, directional terms such as lower or below, upper or above, are used consistently to indicate relative positions at the time of deposition. As the most common fossils are of medusoids which disintegrated before diagenesis compacted the covering sediments, typical examples of fossils of these non-resistant animals are mentioned first. They are fol- lowed by examples of the fossils resulting from the burial of resistant animals, which were able to support the overlying sediments until diagenesis had set them in relatively permanent moulds.

The most common medusoid fossils are low to almost flat casts on the bottom of sandstone flags. Several are shown on slab F17322 (Fig. 2) and to the right side of slab F17323 (Fig. 3C). The underlying lamina was com- pletely weathered away from the latter slab, but F17322 had ?o %e split to reveal the shallow moulds on the top of the lower portion, which match the low casts on the bottom of the upper portion. The centre of its right side shows a poor example of Pseudorhizostomites Sprigg, a common trace fossil made by escaping products of disintegration, presumably of another medusoid (Glaess- ner & Wade 1966). This fossil consists of furrows on the base of the upper block; corresponding ridges occur on the top of the underlying block. Better examples of Pseudorhixostomites are shown on Figs. 4 and 5 ; the latter is sketched to half natural size (Fig. 6). Figs. 7A-D and 8 A-D are based upon slab F17322 and typify the preservation in the upper fossiliferous beds, while in the lower fossiliferous beds where the lower lamina is clay it weath- ers away, as did the sediment underlying the small fossil at the right side of F17323. Some specimens have considerable elevation, and to illustrate these, Fig. 9 has been based upon the much-illustrated specimen Reltanella gilesi Sprigg (T3 ; 2056).

Fossils formed by several sets of bedding are relatively rare. One of the simplest of these is slab F17324 (Fig. lo), which consists essentially of two sets of bedding separated more or less sharply by a bedding plane from which an irregular, thin, layer of clay must have weathered, as the sets can be partially separated. The lower set is a rather thin lamina of sandstone, incomplete

Fig. 3. F17323a-e, ‘x. 1 , arkosic sandstone slab; A : top view showing circular inlier of sandstone at left. B: vertical section, sets of bedding numbered in order of deposition. C : lower surface showing, at right, flat cast of a small medusoid and at left, the multiple cast of a large medusoid (central annular structure at pointer). The outer ring of the large medusoid is imprinted on the base of set 1 while its disc is cast by set 4; set 5 onlaps (see cross-section Fig. ZB), but remaining fragments of set 6 do not reach to the cast.

246 MARY WADE

Fig. 3 and 5. Pseudorhizostomites howchini Sprigg, 4 (upper figure): F17030, I’ 1.25 oblique view showing furrows on the base of the fossiliferous slab focusing on a narrow fun- nel through the bedding to the top of the slab. 5 (lower figure): F17332, I 1, base of the slab sketched in oblique view as Fig. 6.

Fig. 6. Psetrdorhizostomites howchini Sprigg. Sketch in oblique view of the specimen figured in Fig. 5, showing double furrows marking escape routes of decay products and downfolding and partial obliteration of bedding above the trace fossil, prior to trunction of the surface.

toward the centres of three imprints of unidentifiable medusoids. The base of the upper set projects down above each medusoid imprint to fill the gaps in the lower set. This is reconstructed in cross section in Fig. 11, based upon the fossil medusoid in the top central position (Fig, 10). More complex examples of this type of preservation are seen in Fig. 3 (the large specimen on the left) and F17327 (Fig. 12). In these examples 5 and 4 sets of bedding respectively are involved in the covering of the medusoid and the infilling of the space left where it disintegrated. These examples are all of a conventional medusa shape. All could represent Ediacaria Sprigg.

A somewhat similar effect is apparently produced by the collapse of a Cyclomedusa davidi Sprigg (F17325, Fig. 13 A and B). Though the original shape of this animal is thought to have been broadlyturbinate(see F17326, Fig. 15). The cast shows it flattened, dorsal side down, partly buried in little more than 1 mm of clayey silt which still partly adheres to the sandstone slab and cast. On the upper side of the slab a hollow agrees rather closely with the

PRESERVATION OF PRECAMBRIAN AiXIILlALS 247

central part of the Cjlclomeduw (Fig. 16). This is believed to indicate that the central portion of the animal still bulged on its free side, from being flattened against the substrate, when it was buried, and that disintegration soon after burial allowed the sand lamina above to sink without fracturing. lhlanp specimens bear witness to the thin and flexible outer portions of this species, and to its less compressible inner portion (e.g. P12702, Fig. 14). F17325 is not a unique specimen, though such fossils are naturally rare. They depend not only on the chance of quick burial of the animal but also on the upper surface not being eroded after sagging, and on its being ultimately covered

A A

' _ . .-. ' . .. . .- . . _ . . . . B B

C C

D D

Figs. 7 (I&/ and 8 ( r ight ) . Based on the specimens in Fig. 2 A and B. 7A-D : Formation of Psetrdorhizostomites howchini ; (A) deposition, (B) burial, (C) disintegration before consolidation of the sediment, in this example with incomplete subsidence of the overlying sediment, (D) formation of a counterpart cast by less competent sediment of underlying lamina. 8A-D: Formation of a medusoid cast, (A) deposition, (B) failure to withstand burial allows formation of a very shallow cast which persists unaltered save for consolidation and (1)) is able to pre- serve the mould which originally formed it through the late diagenetic changes which formed the counterpart cast 7D.

248 MARY WADE

. . . . . . . . . . . . . A

. . . . . . . f ; . . ' . . . '. . . . . . . . . . . . . : . . . . . . . . . . . . . . . B

Fig. 9. Based upon Beltanella gilesi Sprigg, holotype. A: Burial within clayey sediment of variable depth, the more resistant parts of the substrate cause flattening. B: Burial by sand and gentle subsidence of the covering layer of clay result in a positive composite mould.

Fig. 10. F17324a, b, approx. 0.7; base of sandstone slab showing a thin lamina of fine sandstone hearing traces of the outer rings of three medusoids hut incomplete over all or part of the disc of each. It has been broken away at the right side and shows the lamina above (lined in white) projecting downward to fill the spaces in the lower laminae.

by a sediment prone to weathering, such as the sandy siltstone laminae (x in Fig. 13 B).

Even when the sinking of the overlying sediment during the disintegration of a buried medusoid did not cause disruption in the overlying lamina (e.g. Fig. 13), tension must have developed around the circular flexure at the edge of the saucer-shaped depression. Curved fractures delimiting edges of elevated casts, or of elevated structures on such casts (Fig. 14), are extremely common when the slabs bearing the casts are not thick. I t has not been possible to trace the bedding of a slab across the edge of the casts with certainty in any of these specimens. This is partly due to the formation of cracks and


other secondary weathering features but probably mainly to differential compaction of the enclosing sediment locally destroying minor bedding features, as the medusoids disintegrated and the casts were formed. Two of the factors that must have influenced the relation of these curved fractures to the thinner flags are, first, the tendency for the settling movements to be distributed more widely the thicker the layer of sand and thus to be dissipated so that the rock was relatively unstressed, and secondly, the tendency for the thicker beds to be in the upper member where there is and was less fine material between sandstones, so that specimens forming normal moulds were mostly quite flattened against the substrate and received no support from partial enclosure in it but were exposed to early destruction by a relatively heavy load of coarse sediment.

Resistant animals include pennatulids, polychaets and most of the unique taxa of the Ediacara fauna. They are usually found as impressions in the bases of sandstone slabs-but both Dickinsonia Sprigg and Tribrachidium Glaessner have been collected in situ from the upper fossiliferous beds with the impressions filled by the top of the underlying lamina which has arched up to form a cast. Besides casts of orthoquq5zitic siltstone, two casts are of medium-

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. . . * . * * - . . * . . . . . . . . . . . . . . . . .

A

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ’ . . . ”

C Fig. 1 1 . Based upon F17324, cf. Fig. 10. (A) Deposition and (B) partial burial of medusoid. (C) Disintegration of medusoid and the infilling of the space it had occupied. The plug of sediment is in continuity with the set of bedding that makes up the upper slab. In this example the sharp edges of the slumped remnants of the partial cover shown in (B) suggest collapse under conditions where the slumped sediment was quickly covered (probably collapse at the start of addition to the sedimentary load). There are other examples in which the medusoids disintegrated prior to the formation of plugs which are featureless and blunt-edged.

PRESERVATION OP PRECAMBRIAN A N I M A L S 25 1

Fig. 13. F17325. approx. 0.7, lines mark sections measured to compose Fig. 16. A : base of slab bearing Cyclornedusa davidi Sprigg. B: upper surface of slab showing hollow above central of fossil and remnants of clayey silt ( I ) which originally covered the slab. Figs. 14 and 15. C. cf. davidi, = 1, bases of slabs showing casts. 14: P12702, two specimens, one superimposed on the other and deforming it. At top left the fracture delimiting the slab follows the edge of an annular furrow in the fossil. 1 5 : F17326, showing truncate apex of originally turbinate cone ; overlapping folds show flattening of central area.

FiE. 12. F17327, 1 1 , multiple cast, possibly of Ediacuria, sets of bedding numbered in order of deposition; the oldest set in the slab does not appear here. A : top view, fifth set of bedding surrounded by older sets. B: oblique edge and top view. C: base of slab showing multiple cast composed of sets 2-4. Sets 2 and 3 cannot be distinguished at left.

17 - Lethaia 1 : 3

252 MARY WADE

grained sandstones that would be expected to behave as competent rocks but they are overlain (and one at least is also underlain) by still more competent sandstones. Numerous examples are known of the filling of an impression from below by the arching of thin laminae of clay and silt, e.g. D. costata Sprigg (T53: 2004, Fig. 17) has fragments of fine silt laminae still adhering

Fig. 16, Diagram of tilted slab (portion of specimen F17325, shown in Figs. 13) , showing relation of depression on upper surface to fossil on lower surface, viewed obliquely from below.

.. . -., I

Fig. 17. Dickinsonia costata Sprigg, * 1, anterior ends uppermost. T53; 2004, impression in base of slab, large segments (particularly at right anterior) bear small ridges of orthoquartzitic siltstone. Slight faults cross the specimen. Underlying laminae of orthoquartzitic siltstone adhere to the inner surface of the mould and show a counterpart cast of it when removed.

PRESERVATION OF PRECAMBRIAN AN1MAI.S 253

to the inside of the mould as well as to the bedding-plane surface. I t is in an exceptionally fine-grained sandstone, which is little more than silt grade at its base. As is usual, the ends of the segments of D.costata are convex out- ward and many of its segmental boundaries are marked by furrows reaching the margins between these crenations. Most of its larger segments bear an additional ridge of orthoquartzitic siltstone running approxiniately parallel to the intersegmental boundaries, often along the centre of the segment. As the closest living form - Spinther - bears elongate notopodial-elytral ridges which could trap fine sediment in this position, the presence of such ridges in Dickinsonia is inferred. Less obvious structures on many other specimens are also simply explained by this assumption, and very difficult to explain without assuming a closely similar structure.

The pennatulid Rangea longa Glaessner & Wade is also known as counterparts, both in fine-grained sandstone, from the upper fossiliferous beds, but this was not collectkd in situ. Pteridinium cf. simplex Gurich, from massive, fine-grained sandstones, is also found as positive and negative counterparts.

A

Fig. IS. Based upon P12716 (Glaessner, 1959, P1. 45, fig. l ) , frond of Rangea, (4) partly buried in the top of a sandstone bed. (B) Burial of frond and sandstone surface by clayey silt, etc. The fossil was presumably buried in situ as considerable detail is present on the mould. (C) Final disintegration of the fossil, and compaction of the incompetent clayey siltstone some time after the sandstone had consolidated. Even after compaction the fine sediments were thick enough to cushion the mould from flattening against the next higher sandstone bed.

€3

C

The only known example of current-aligned fossils (Glaessner, 1959, P1. 40, fig. 1) is several Rangea on slabs forming parts of one bedding plane, an upper surface which has been hollowed out by the current in places, and had

254 MARY WADE

sand irregularly heaped below the frond-shaped animals. This forms the basis of Fig. 18 A-C. The more common preservations found are summarized in Figs. 19 and 20.

Preservation At Ediacara, fossils were formed from bodies in the following conditions and positions :

(1) Non-resistant animals in any position in which they could be wholly or partially buried and then cast by relatively competent material. Casts etc. are not observed unless the inter- action of competent and incompetent beds allows flaggy partings to form.

(2) Decaying animals leaving in the sediment a record of the escape of products of disintegration. .. -

( 3 ) Resistant animal lying on top of fine sediments and covered by sediment which formed a competent rock.

(4) Resistant animals (very small examples unknown) partly buried in the upper surface of competent beds and covered by less competent beds.

(5) Relatively large and tough animals in massive sediment (only known example, Pteridi- nium cf. simplex). The condition appears to have been that the a n i m z h a s resistant enough to allow the sandgrains pressing against it to be ordered into smooth surfaces which could not subsequently interlock, though compression forced the opposed sides into contiguity. The size requirement is the practical one that a massive rock will split to reveal a fossil only if there is a sufficiently large plane of weakness in it.

The series of Figs. 6-9, 11, 16, 18-22 shows fossils as they now appear in vertical section; 7-9, 11, 18-21 also show inferred burial positions. Disre- garding the trace fossil Pseudorhizostomites (Figs. 6 and 7), the types of preservation may be described as follows :

Figs. 8, 9, and 11. A non-resistant animal: a medusoid deposited on a layer of mud over silt or sand may be flattened to an outline, particularly if small. On the other hand, if wholly or partly buried in the mud or in silt, collapse forms a cast (Fig. 8 A and B). If a layer covering the upper surface was present and not disrupted by the collapse, or if there were internal resistant structures, the resultant fossil is termed a positive composite mould (McAlester, 1962; see also Fig. 9). The above possibilites are the same whether the underlying or enclosing material is silt (Fig. 8) or mud (Fig. 9) but what is observed is influenced by the usual loss of the one-time niud layers by weathering and the preservation of the silt layers as an orthoquartzitic siltstone (Pettijohn) containing moulds - counterpart moulds in the ter- minology of Glaessner & Wade 1966. Fig. 11 A-C illustrates a rather rare occurrence, a medusoid which did not decay until sand had been deposited around it; the space it had occupied then being filled by sediment continuous with one or more later laminae of sand. This structure is here termed a multiple cast (Figs. 3 , 10, 12).

Any animal tough enough to resist being abraded to destruction against sandy substrate by hydrodynamic forces capable of moving and depositing


sand, could be buried in or at the tops of sand laminae (Fig. 18). The resulting imprints (so far only those of resistant animals or undeciphered markings) are normal external moulds.

Figs. 19-21. A resistant animal such as Dickinsoniacostata (Fig. 19 A - 21A) buried on, or in, silt or mud remained until early diagenesis had caused the

A A A

B B

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. . . . . , . . . . . . . .

C C C

Figs. 19 ( l e f t ) , 20 (middle) , and 21 (vight) . Resistant animals (typified by Dickinsonia costata Sprigg). Deposition on (19A) quartz silt, (20A) mud, (21A) in or under silt or mud. Upon burial (19B-21B) all specimens support the overlying sediment till it has set (i.e. through early diagenesis). During late diagenesis compaction forces the finer, less competent, beds into any available spaces (moulds) in the enclosing competent beds. Thus counterpart casts are made, of (19C) orthoquartzitic siltstone resistant to weathering and (20C) clayeq siltstone or clay, prone to weathering and seldom seen. Neither claystone, clayey siltstone, nor orthoquartzitic siltstone which has been compressed between unfossiliferous surfaces of competent beds (21C) shows any fossil remains at Ediacara. Such beds are known almost exclusively from Greenwood Cliff and Gap Creek.

sediment to set, and therefore formed an imprint on the base of the overlying lamina (Fig. 19 B - 21 B). This imprint is either a normal externalmould, or, if the animal was modified by death and burial so that it revealed structures not seen at its surface in life, a negative composite mould (McAlester 1962). During late diagenesis the siltstone or mudstone of the underlying lamina was squeezed into the mould above, forming a counterpart cast now visible on the top of the lower layer (Fig. 19C, 2OC). These are usually weathered away

256 MARY WADI?

unless they are of quartzitic material (Fig. 19C). Counterpart casts attest the plasticity during late diagenesis of the fine-grained layer that formed them. The presence of counterpart casts occurring on the same surfaces as counterpart moulds (e.g. Pseudorhizostornites and medusoids, Fig. 2; Figs. 7 and 8) also emphasizes that the counterpart moulds (Fig. 8D, lower lamina) owe their presence now to the more competent sandstone casts they originally shaped. In contrast, Fig. 22 shows a counterpart mould in a layer ‘m’ below the clay layer in which the original medusoid was deposited (see p. 259).

The plasticity or incompetence of the fine-grained layers enclosed between more competent layers (now sandstone to quartzite), is the most probable explanation of the total lack of fossils corresponding to resistant animals buried in silt or mud. (Fig. 2lC). Not only medusoids but some pennatulids and Dickinsonia tenuis are included in the non-resistant animals that were wholly or partly buried in clayey beds and preserved as positive composite moulds and casts; it therefore seems unlikely that the exclusion of fossils of resistant animals from the finer sediments was due to ecologic or depositional factors causing their absence at the time of deposition of the fine sediments. Compaction sufficient to form counterpart casts by moving the fine material into spaces in the sandstone (Fig. 19C, 20C) would also eliminate spaces in the fine material. With only one known exception, every piece of fine material examined has upper and lower surfaces concordant with the coarser and more competent beds on either side. The exception is the one specimen of Trih- rachidium heraldicum which has both oral and aboral sides (see Glaessner & Wade 1966) but even this shows incipient counterpart casting of the most deeply impressed arm. Another specimen from the same slab was accom- panied by a counterpart cast.

T o summarize the results of burial in incompetent sediments: when decay takes place after early diagenesis has set the sediment firm, the space once occupied by the animal is almost invariably obliterated in late diagenesis (Fig. 21A-C). Only non-resistant animals buried partly or wholly in or on fine sediments (see Figs. 8-16) are likely to decay fast enough to allow subsequently deposited sediment to subside before early diagenesis into the space they have occupied. The infilling forms a positive composite mould, a cast or a multiple cast. If of competent material, this can be exhumed by weathering.

The only massive bed which is known to contain fossils is a very well- sorted, fine-grained quartzite which contains Pteridinium cf. simples. These fossils are of leaf-shaped bodies that were sometimes buried in a contorted position. Regardless of their attitude they consist of concave moulds each exactly filled by a convex cast. The sandgrains forming cast and mould are in contiguity but not interlocking, and of identical material. Bedding is not visible. As sediment load acting upon a soft-bodied animal could not explain the even filling of the contorted casts, the moulds must be normal external moulds enclosing counterpart casts formed by compaction - possibly as late in diagenesis as the counterpart casts occurring in the well-bedded rocks.

PRESERVATION OF PRECAMBRIAN A N I n u L s 257

Responses to fossilization From a very early stage in the study of the Ediacara fauna it has been obvious that most medusoids were preserved as casts, .while most other fossils were moulds. As knowledge of the fauna and understanding of the preservation has increased, these differing responses to fossilization have become more sharply defined.

Table 1 lists the identified, bodily preserved fauna, giving the total number of individuals in each species and the number of specimens of each preservation. The figures are approximate for groups having large numbers of frag-

Table 1. Approximate number of specimens known from Ediacara, listed under thpir presewations. The pwsence of counterparts sometimes makes thepresewa- tions total more than the number of indiziduals. ( N o t e : Rimberia Claessner & Wade, 1967; non ,Cotton & Woods, 1935. )

I I

Form of fossil

I- Undetermined medusoids . . . . . . . . . . Pseudorhizostomites howchini . . . . . . . .

'I(imheria' quadrata . . Cyclomedusa d a d i s . .

Cyclomedusa radiata . .

Rugoconites enigmaticus Lorenzinites rarus. . . . . . Conomedusites lobatus . . Ovatoscutum concentricu

Cyclomedirsa plana . . . .

Mawsonites spriggi . . . .

.4vborea arboren . . . . . . . .

Praecambridium sigillum . . . . . . . . . . . . Tribrachidium heraldicum . . . . . Pawancorina minchami . . . . . . . Spriggina jloundersi . . . . . . . . . . . . . . Spriggina ovata . . . . . . . . . . . . . . . . . . Dickinsonia costata . . . . . . . . . . . . . . . . Dickinsonia elongata . . . . . . . . . . . . . . Dickinsonia tenuis . . . . . . . . . . . . . . . . . . Unnamed dickinsoniid worm . . . . . .

many - 80 1

;> 40 - 20 6

N 60 8

- 1 5 4

18 1 5 1

12 1

34 ?7 7

40 - 60 15 4

' 300 - 30 8 6

- 80

17 1

1 12

5 7

3 5 - 60 15 4 - 300 - 30 6 6

m * z 0 -

many

40 - 20

- 20 8

4

5

1

2

2

w + w m

.-

.- 0 a $

... ;G

% E o .z '3

__

few

1 few

6 ? - 40

- 15

32

5

1

m c Lli m

e, e a 3

.- .-.. 4-

E ~

fen

2 ?

2

1

mentary specimens ~ notably Dickinsonia - or with few distinguishing characters, e.g. llledusinites and Ediacaria. Multiple casts occur in Arhorea arborea (Glaessner), Dickinsonia tenuis Glaessner & Wade, and in medusoids. Those of A. arborea are specimens which had their rhachis partly infilled with sand before they were transported to their place of burial. As modern pennatulids with similarly large rhachis have very considerable canals in them, it is probable that the sand entered preexistent spaces in slightly morn stems. Other, similar, sand-filled stems are known, specifically and generically indeterminable because no frond is attached, The holotype of D. tenuis has its dorsal and ventral surfaces separated and cast by a thin layer of very fine sand, at least at its outer edges. These were damaged.

The multiple casts of medusoids are of interest for the evidence they supply of fast sedimentation. In oxygenated sea-water (Hertweck 1966) a recently dead medusa ‘collapsed to a film’ in ten days. Under natural conditions at Ediacara the water cannot have remained as still as in the laboratory, but thc multiple casts (Figs. 3, 10, 12) show several sets of bedding piled around and over the medusoids prior to the subsidence of the ‘roof’. Several specimens like those on Fig. 10 are known from loose slabs or observed in situ, in which from one to three layers of quartzite, separated by smooth beading planes from which the originally intercalated clay layers have been weathered, form collars around a circular plug of quartzite which projects downward from the base of a quartzite slab. Sprigg (1947, p. 220, P1. 6, fig. 2) figured such a plug and said in the text ‘The impression may be that of a simple discoid jellyfish or of a hydroid float’, and in plate explanations ‘Discoid Scyphozoan or Zooidal float’. In some specimens the lowest collar and less frequently the (depositional) base of the plug may bear traces of a medusoid (Fig. lo), indicating that it is a multiple cast (Fig. 11C); in others the plug is featureless, as if clay (now weathered away) had entered the cavity once occupied by a medusoid and covered its imprint on the lower surface, before the entry of the sand that now forms the featureless quartzite plug. Similarly the absence of the imprint of the outer ring of a medusoid on the overlying collar of quartzite is probably due to clay covering the outer ring before the deposition of the sand that now forms the collar.

I t is clear that all the composite moulds, multiple casts and Pseudorhi- zostomites result from the burial of the original animals. I t is extremely probable that the subsidence-structures (e.g. Fig. 16) including the tendency for the slabs to fracture around casts (p. 11) also indicate the burial of the bodies rather than the infilling of empty moulds in the substrate. Nevertheless, there are large numbers of casts which give no evidence of whether their originating medusoids disintegrated prior to the introduction of fresh sediments (Glaessner 1959, p. 392-393), or under load. A very few casts represent moulds which have filled gradually (since the depositional base of the cast is not all one bedding plane) and the animals forming these must have disintegrated prior to the filling of the mould.

Recent medusae lying on their exumbrellar sides on a firm substrate tend


to flatten to correspond to the sediment surface, even under seawater, and to bulge on the free (subumbrellar) side. If the Ediacara medusoids had approximately the same texture as modern forms, we could expect to find that normal casts, being formed in the uppermost lamina of mud or silt, were much flatter than multiple casts which, in contrast, should give some indication of the true shape. Most of the multiple casts and plugs represent Ediacaria flindersi or are indeterminable specimens of dimensions that make Ediacaria their most probable originator. The remainder are doubtfully assigned to Medusinites. Though normal casts of Ediacaria tend to be very flat (even a specimen of 21 cm radius rises only 0.4 cm from outer ring to disc, each of which is flat), multiple casts are known to reach a height of about 2 cm. From this we can conclude that these discoidal medusoids were of approximately the texture of modern medusae in the same size-range.

Mawsonites spriggi Glaessner & Wade and Cyclomedusa davidi Sprigg (which includes Spriggia annuZata (Sprigg), see Glaessner & Wade 1966) were the most distinctly conical of all the ‘medusoid’ forms. C. daeidi was also very flexible and compressible and folded almost flat (Figs. 13,14, and 15).

Some well-preserved casts of medusoids, mostly of Ediacaria and Medu-

\ a b ........ . . .

..... C

Fig. 22. Based upon Medusinites asteroides (Sprigg), (Fig. 24). (a) Fossil as it occurs, a cast on the depositional underside of a quartzite slab in lithological continuity with the slab ‘k‘ but partly surrounded by the uniformly thin silt layer ‘m’. The small, central disc of the medusoid penetrates layer ‘m’. (b) An exploded diagram showing the rclations of layers ‘k’, ‘m’, and ‘n’ (an uncollected layer below ‘m’). Thin layers of mud (weathered shale) on either side of layer ‘m’ are omitted. Layer ‘k’ is seen from below, ‘m’ and ‘n’ from above. (c) Diagrammatic cross section, the mud layers are represented by spaces. After hardening of the cast (i.e. during late diagenesis) compaction has forced its most elevated portion through the underlying orthoquartzitic siltstone lamina while the less elevated portions formed the siltstone lamina into a counterpart mould. Layer ‘n’ may have been involved in the counterpart mould where the outline of the medusoid was transmitted through ‘m’.

260 MARY WADE

Figs. 23-15. Fig. 23 (top): Ediacaria cf. Pindcrsi Sprigg, F17328, I 1, a natural cast on base of main slab but with central disc protuding downward through the next lamina, a wedge shaped layer of fine sandstone which is separated from the overlying main sandstone slab by a clearly defined parting. Fig. 24(lower right) : Medzisinites asteroides Sprigg, P13786, I 1, a natural cast on the base of the slab with the central disc'protruding through an orthoquartzitic siltstone lamina which underlied the slab. Fig. 25 (lower left): F17329, I 1, latex cast of Trihvachidium heraldicurn Glaessner, strongly compressed specimen showing three hullae equally spaced and the inner ends of the arms between them. The outer parts of the arms have merged with a slightly raised, annular, marginal zone almost twice the radius of the area it encloses.

PRESERVATIOM OF PRECAMBRIAN ANIMALS 261

sinites from the upper fossil bed, have the central disc protruding through a layer of silt, or even of coarser sediment, which may still adhere to the outer ring and to depressions in the surface of the cast (Fig. 22; see also Glaessner & Wade, P1.97, fig. 2 ; this paper, Figs. 23 and 24). They differ from the plug- and-collar arrangement of multiple casts in tapering downward so that the more central parts appear on the lower bedding plane and progressively peripherad structures appear higher. They look as if the animal had been buried in, or forced through, up to 3 laminae of clay and coarser sediment. Apart from difficulties of anchoring the animals for burial, or their question- able durability, the convexity of the medusoids would ensure that the spot beneath a medusoid would usually carry a sediment load lighter than its surroundings. I t also seems unlikely that a freshly-formed cast could sink into the substrate without distortion or obliteration of fine structures. The possibility that these medusoids actually lived on their aboral sides in hollows in the substrate; like Cassiopea and Pol?/clonia (Walcott 1898, p. 6-7), was therefore seriously considered. There is, however, no sign of the distinctive- ness of even fine laminae of mud and silt becoming disturbed next to the medusoid represented by the cast (e.g. Glaessner & Wade 1966, P1. 97, fig. 2; this paper, Figs 22-24), as would have been the case if a living, growing crea- ture had nudged out a hollow. A comparison of Fig. 22c with 8b and 9b illustrates another possibility - that the most protruding part of the sandstone or quartzite cast (or positive composite mould) was driven through, or into, less competent underlying layers during late diagenesis when counterpart casts and counterpart moulds were formed. This preservation is more evident in the upper fossiliferous beds where the less competent layers now border upon orthoquartzitic siltstone than in the lower beds where they are usually weathered away. The frequent occurrence of each of E. jindersi and M. asteroides in groups on one bedding plane indicates that they were gregarious in life like many Recent medusae (Glaessner & Wade 1966). Thus the fre- quency of counterpart moulds in probable Ediacmia and in Medusinites re- flects the gregarious habit of these species and the fact that large slabs bearing groups have been collected mainly from the upper fossil bed, rather than their life position.

Composite moulds show structures characteristic of more than one surface or region in the original animal at one level in the fossil. They may be positive or negative, dominated by the cast or by the external mould. For example a positive composite mould of E. j i n d m i , formed by a probable filling of the gastric cavity and a cast of the exumbrellar surface, is the specimen described as ProtodiPleurosoma wardi Sprigg (1949, PI. 9, fig. 2; see also Harrington & Moore 1956, Fig. 64). Glaessner (1959, PI. 45 fig. 2) illustrated composite moulds of Arborea urborea (Glaessner) and a medusoid, and an exumbrellar cast of a medusoid on one slab. A. arborea is found in raised relief or flattened on the bases of quartzite flags. Its tissues appear to have been unable to hold up any sediment-load but its spicules appear as impressions on the convex surfaces. Sometimes, also, the oultine of the rhachis is impressed through

262 R ~ ~ A I ~ Y WADE

the frond in ventral view (Glaessner 19S9, pl. 45 fig. 2; Glaessner & Wade, 1966, PI. 6 Fig. 2). Tribrachidium heraidicum, when compressed almost flat, shows (in latex casts) a raised resistant, marginal zone not evident in external views of either side (Fig. 25). The list of positive composite moulds is com- pleted by mention of a specimen of Dirkinsonia costata with some of its ali- mentary caeca preserved in raised relief (Glaessner & Wade 1966, P1.5 fig. 4).

Dickinsonia was broad and flat in life. Its strongly-marked segmental boundaries follows the same plan as those observed in the Recent Spinther and, though the presence of ridges in the position of notopodial-elytral ridges can only rarely be demonstrated in Dickinsonia (Figs. 17 and 26), they can be inferred as a constant feature of the dorsal side. All specimens showing traces of such ridges also show an axial discontinuity or off-setting of the segmental boundaries (the ‘median furrow’ of Sprigg (1947, 1949) and Harrington & Moore (1955, 1956)). I t is believed that other specimens clearly showing such axial structures are dorsal sides too. ’specimens with segmental markings which are continuous across the axis are considered ventral sides (see Sprigg 1949, PI. 21, fig. 4; Fig. 10F; refigured by Harrington & Moore 1956, p. F26, Fig. 15 (lb)). An axial line is often present on the immature segments of specimens with otherwise continuous segmental markings; ahd more rarely on some mature segments. This is here regarded as interference from the dorsal side of the body which was pressed against the ventral side by the weight

I I I

I

- f - - -LD I

I I I I

I c -E

Fig. 26. Dickinsonia costata Sprigg. Surface profiles from latex casts, anterior to right. Solid lines indicate structures seen in the latex casts or (F) in the fossils. Dashed lines are restoration. (A) Diagram showing region of sections B-F (either left or right side), G is a little posterior to F on the same specimen. (B) Dorsal view, external mould of one of the least flattened specimens, coupled with (C) a somewhat flattened ventral side of a specimen of similar size and segment number (T ; 2001, Sprigg 1949, PI., fig. ; refigured Harrington 8z Moore, 1956, p. F26, Fig. 15 ( lb)) . There is no control for the restoration of thickness but all the moulds are very shallow, even when the segments (as in B) retain convexity. ( D and E) Similar flattening in probable dorsal (D) and in ventral sides. (F and G), T53; 2004, Fig. 17; left side of a cast, (F) showing the surface of a cast ,and cross-section of orthoquartzitic siltstone ridges, restored as sediment trapped between the dorsal surface and notopodialelytral ridges. The position of the intersegmental furrows is partly extrapolated from the edges but is unequivo- cal in (G) toward the rear where the body seems proportionately less flattened.

PKESEKVATION OF PRECAMBRIAN ANIMALS 263

of the overburden. The resultant fossil is a negative composite mould. These impressions are dominated by the dorsal or ventral side, whichever was uppermost at burial.

Fig. 26 shows by section the surface relief of various latex casts of D. rostata. In each drawing the structures shown in solid lines are real, those shown in dashed lines are inferred. Fig. 26A shows the approximate position which the sections figured as 26B, C, E, and F occupied; 26G and possibly 26D were situated a little more to the posterior. In Fig. 26B, a dorsal side, the external niould P13610 is combined with the external niould of a ventral side T50; 2001 (Fig. 26C) to show the probable proportions of the worm but there is no basis for a close estimation of its thickness. Most fossils are more compressed than P13610, and several other cross-sections are shown. The strongly compressed dorsal side T53; 2004 (Fig. 17 and Fig. 26F and G) bears, on its larger segments, thin ridges of silt spanning the width of the segment, approximately parallel to the intersegmental boundaries. They are shown stippled in text-Fig. 26F, and are explained as silt caught between the body of the animal and dorsal lamellae (i.e. notopodial-elytral ridges). The positions of the intersegmental ,s.epta shown in Fig. 26F are extrapolated from the crenations of the margin between segments. Reference to the somewhat flattened specimens which coniprise the majority shows that, whether the dorsal or ventral side is observed, the most elevated portions of a cast are usually adjacent and parallel to the intersegmental furrows (Figs. 2, 3). If the resistant material causing this ‘high’ is positioned beneath the intersegmental furrow during fossilization as in parts of T53 ; 2004, the intersegmental furrow tends to be obscured by flattening; more often it is emphasized by a ridge in front of it (Fig. 26D, E). The more complete the flattening of a specimen the more closely the highs approach to a thin, sharp ridge exactly delimiting the intersegmental boundaries (Fig. 28). The ultimate is reached when only the pattern of these ridges can be seen (Sprigg 1949, PI. 20, figs. 1, 2 right side only). The latter figure was reproduced by Harrington & Moore (1956, p. F26, Fig. 16) to illustrate the holotype of D. sprkgi Harrington & Moore. The fact that the septa1 regions form ridges on the vast majority of specimens indicates that most specimens ate negative composite moulds rather than simple external moulds.

The rather rare species Dickinsonia tenuis Glaessner & Wade is known from both external moulds and casts but only doubtfully from negative composite moulds. An undescribed Dickinsoniid worm is known only from impressions which, when cast in latex, show the surface ridges and furrows draped over an axial ridge which could be due to the intestine. Without knowledge of the original profile, or any other evidence to suggest that the specimens are negative composite moulds, it seems better to regard them as external moulds, for external moulding certainly predominated in the preservation.

Pteridinium cf. simplex (Gurich) is also represented by both external moulds with counterparts and by normal casts. They occur, however, in different deposits: two flattened casts in the bedded deposits where the animals lay on

264 MARY WADE

mud, and external moulds with counterpart casts where they were buried in unbedded sand. In this deposit their preservation is identical with that described in detail from the Nama Series in Southwest Africa by Richter (1955) and Glaessner (1963).

Figs. 27 ( le f t ) and 28 (right). Dickinsonia costata Sprigg, ~ 1, latex casts of natural negative composite moulds. Anterior ends up. 27: F17331, showing two concentric shrinkage marks. 28: T5.1; 2050, showing variable flattening which, at its most extreme at the anterior of the right side, has resulted in fine ridges just anterior or posterior to the segmental sutures.

Only Kangea longa is identified from external moulds on upper depositional surfaces. Five of 7 specimens known in this preservation are from one spot, from scattered slabs that can be partly reconstructed as one bedding plane. These slabs show a surface quite strongly scoured by current action which has gullied among the pennatulid fronds and heaped the sand below them, prior to burial, so that the detail of the external moulds was impressed on anuneven surface (Glaessner 1959, P1. 45, fig. 1; this paper, Fig. 16). ‘The other two specimens are from widely separated localities but show similar sedimentary features.

PRESERVATION OF PRECAR'IRRlAN ANIMALS 265

Fig. 29. A medusoid settling through water moving from right to left may either land dorsal side first and adhere to the substrate (2 lefthand diagrams), or touch bottom as at the right and be turned over by the current, or - if its exumbrella happens to be facing the current when it touches (a less stable attitude) - it may land sub-umbrella down.

Evidence of provenance Modern pennatulid species are often gregarious. The presence of a group of R. Zonga on an upper surface, all orientated in the same way, and all stretched out lengthwise though the current has eddied around them and scoured the sediment, indicates the possibility that these individuals might have been growing where buried. Unfortunately, the base of the frond is not preserved in any specimen, and the lower part of the rhachis is unknown.

The fact that medusae are normally stranded exumbrellar side up has been implied or recorded many times (e.g. by the authors mentioned by Walcott 1898; Schafer 1941). I t contrasts strongly with the preservation of the Edi- acara medusoids which were mostly preserved exumbrellar side down (Glaessner 1959) ; consequently an experiment on the deposition of medusae in sea-water was carried out at a local beach using nearly dead specimens of the semaeostome Chrysaora - an undescribed species. Three normal specimens and one with aberrant, very short, mouth arms were available. These were placed in a pool, and the water was agitated by the addition of more water and allowed to quieten. They settled as if stranded when the water was shallow, but when the depth of the pool was mors than about $ of their diameter they either settled directly on their aboral sides, or, if sinking ventral side down, when one margin or the trailing mouth-arms touched the substrate and checked lateral movement, the free edge would rise in the current and turn the medusa over (Fig. 29). Slightly deeper water was required to allow those with mouth-arms longer than their radii to turn over; otherwise the response of those with long mouth-arms was the same as that of the aberrant specimen. Thus the comparison should be valid for the Ediacara medusoids which did not have large mouthparts. In water of more than the critical depth, the specimens settled exunibrellar side down a total of 15 times in 20.

The position in which the majority of the hundreds of Ediacara medusoids (identified and indeterminable) were deposited indicates that the deposit was continually under water at all times when the animals now fossilized were deposited and buried. The sedimentologic work of Goldring (Goldring PE Curnow 1967) shows even more conclusively that there was no emergence of the beds at Ediacara during sedimentation. He considered the upper beds of

266 MARY WADE

the main fossiliferous unit formed under more shallow conditions than the lower beds (p. 5).

A number of specimens of Dickinsonia costata show shrinkage marks all around the body (Fig. 27) indicating that they contracted from their maximum expansion after deposition where buried. Duplicate parallel markings around some specimens indicate that they expanded and contracted at least twice after deposition but no indication of locomotion of these animals has been observed. It seems most probable that these were near death when deposited in an environment in which they did not live. D. elongata Glaessner & Wade, a ribbon-shaped form, was often intensely contorted and these individuals appear to have been dead before deposition.

With the possible exception of Rangea longa (p. 28) there is no indication of any bodily-preserved animal living in situ. There are numerous trace fossils as yet underscribed, some of which have been mentioned by Glaessner (1959, 1961) and Goldring & Curnow (1967). These do indicate quite a varied, mainly in-sediment, local fauna. The constancy of constituents of the bodily preserved fauna throughout the hydrodynamic changes pictured by Gold- ring & Curnow suggests strongly the introduction of this,fauna to the environment of deposition. In general, Glaessner’s view (1961, etc.) that most of the animals were transported to the spots where they were buried is con- firmed.

Department of Geology, University of Adelaide, South Australia, 5001, February 28th, 1968

R E F E R E N C E S

DALGARNO, C.R. 1964: Report on the Lower Cambrian stratigraphy of the Flinders Ranges, South Australia. Trans. Roy. Soc. South ilustralia 88, 129-149, Adelaide, South Austr- alia.

FOHBES, B.G. 1966: Features of Palaeozoic tectonism in South Australia. Trans. Roy. Sor. South Australia 90, 45-56, Adelaide, South Australia.

GLAESSNER, M.F. 1961 : Pre-Cambrian animals. Scienfz$c ;Imerican 20-1, 72-78, New 170rk, U . S. A.

GLAESSNER, M.F. 1963 : Zur Kenntnis der Nama-Fossilien Sudwest-Afrikas. rlnn. Natur- hisf. A4’zrs. Wien 66, 113-120, Wien.

GIAESSNER, M.F. & DAILY, B. 1959: T h e geology and Late Precambrian fauna of the Edi- acarn fossil reserve. Rec. South rlusf. Mus. 1313, 369-401, Adelaide, South Australia.

GLAESSNER, M.F. & WADE, M. 1966: T h e Late Precambrlan fossils from Ediacara, South Australia. Palaeontology 91-1, 599-628, London, England.

GOLDRING, R. & CURNOW, C.N. 1967: The stratigraphy and facies of the Late Precambrian at Ediacara, South Australia. J. Geol. SOC. Australia l d / Z , 195-214. Sidney, Australia.

HHRRINCTON, H.J. & MOORE, R.C. 1955: Kansas Pennsylvanian and other jellyfishes. Kansasgeol. Surw. Bull. 11315, 153-63, Topeka, U.S.A.

HERTWECK, G. 1966: Moglichkeiten des Fossilwerdens von Quallen - im Experiment. Natzrr und Museum 96 ( 1 1 I , 456-62, Frankfurt am Main.

LEESON, B., & NICON, L.G. 1966: Beltana. 1 :63,360 map series. Geol. Surv. South Australia, Adelaide, South Australia.

PRESERVATION OF PRITAMBRIAN ANIMALS 267

MOORE, R.C,, [ed.] 1956: Treatise on Invertebrate Paleontology F, Coelenterata. Geol. Sor. Amer., New York, U.S.A.

RICHTER, R. 1955 : Die altesten Fossilien Siid-Afrikas. Senckenbergiana Lethaea, .?6, 243-89, Frankfurt am Main.

SCHAFER, W. 1941 : Fossilisations-Bedingungen von Quallen und Laichen. Senckenhergiana 23, 189-216, Frankfurt am Main.

SPRIGG, R.C, 1947: Early Cambrian ( 7) jellyfishes from the Flinders Ranges, South Aus- tralia. Trans. Roy. Soc. South Australia 71, 212-24, Adelaide, South Australia.

SPRIGG, R.C. 1949: Early Cambrian ‘jellyfishes’ of Ediacara, South Australia, and Mt. John, Kimberley District, Western Australia. Trans. Roy. Soc. South Australia 73, 72-99.

THOMPSON, B.P. 1965 : Geology and mineralization of South Australia. ( In MCANDREW, J. [editor]: Geology of Australian ore deposits, 2nd ed., 1965). Eighth Comm. Min. M e t . Congress. Melbourne, Victoria.

THOMSON, B.P., COATS, R.P., MIRAMS, R.C., FORBER, B.G., DALGARNO, C.R., S- JOHNSON, J.E. 1964: Precambrian rock groups in the Adelaide Geosyncline: A New Subdivision. Geol. Surv . South Aiistvalia, Quart. Geol. Notes, 9 , 1-19, Adelaide, South Australia.

WALCOTT, C.D. 1898: Fossil medusae. Monogr. U.S. Geol. Surv. 30, 1-201, Washington, U.S.A.

WALTER, M.R. 1967: Archaeocyatha and the biostratigraphy of the lower Cambrian Hawker Group, South Australia. J. Geol. Soc. Az~stralia l d / l , 139-52, Adelaide, South Australia.

18 1,ethaia 1 : 3

preservation of soft-bodied animals in precambrian sandstones at ediacara, south australia

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