TRACE FOSSILS FROM THE PRECAMBRIAN AND BASAL CAMBRIAN

T R A C E FOSSILS F R O M T H E PRECAMBRIAN A N D BASAL CAMBRIAN

M A R T I N F. G L A E S S N E R

GLAESSNER, M.F. : Trace fossils from the Precambrian and basal Cam- brian. Lethaiu, Vol. 2, pp. 369-393. Oslo, October 15th, 1969.

Certain worm-like configurations on rocks are recognized as shrinkage- crack infillings. Some genuine Precambrian trace fossils are briefly described. The early Cambrian contains a richer assemblage, including some distinctive and widespread form genera. The study of early trace fossils leads to conclusions not only on facies, but also on the evolution of behaviour and functional morphology in soft-bodied organisms.

Precambrian Metazoa lacked mineralized skeletons, and trace fossils are therefore likely to be particularly important for the understanding of their early history and subsequent rapid diversification. As in all studies of Pre- cambrian fossils, a clear distinction is essential between configurations of organic origin, and others which resemble trace fossils, but result from abiogenic physical or chemical processes. For that reason, a critical invest- igation was made of certain reported ‘worm burrows’ and similar structures from very ancient sediments. I t led to the conclusion that they are shrinkage- crack infillings. There are, however, other possible trace fossils in rocks which may be 1000 million years old or older. They will require careful further study as they may constitute evidence of the first appearance of Metazoa in the geological record. The Late Precambrian Ediacara fauna of South Australia contains a number of trace fossils which provide evidence of benthonic life at a site where most body fossils have been transported to their place of burial. The Ediacara fossil beds are followed disconformably by the Parachilna Formation with abundant D$locraterion and other burrows. This formation has been assigned to the Lower Cambrian. Its trace fossils have not yet been examined. The age of the Arumbera Formation in Central Australia ranges from Late Precambrian to Early Cambrian. Its upper part contains a great variety of trace fossils, among them form genera and species known only from the Lower Cambrian. The use of trace fossils for inter-continental stratigraphic correlation is justified only when there is evidence of very specific behaviour and morphology of the originator which could not be repeated by convergent evolution in unrelated organisms. These criteria seem to apply to PZugiogmus from the Lower Cambrian of the Baltic region Central and South Australia, and probably also California. Its analysis suggests an animal moving in the sand with the aid of a differentiated foot, hence with at least some characters of molluscs. Similar characters of

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370 MARTIN F. GLAESSNER

the locomotive organ are displayed by a surface trail from beds which are considerably older than the Ediacara fauna. Locomotion trails with characters seen in trails of living gastropods have commonly been ascribed to the Class Gastropoda, but the common occurence of such trace fossils in the Lower Cambrian and their probable presence in the -Precambrian suggests a more cautious approach. Such trace fossils may provide evidence of soft- bodied proto-molluscs, with a foot and probably with a mantle, which diverged in Precambrian time from common ancestry with the annelids.

The study of ancient trace fossils has threefold significance. As Seilacher has shown in many publications, it leads to the recognition of more or less well defined biotopes. In favourable circumstances it can support stratigraphic correlations. In the Precambrian it gives indications of the first appearance and differentiation of certain types of behaviour, particularly locomotion and feeding, which may be linked to distinctive developments of the functional morphology of otherwise unknown, soft-bodied organisms. Fur- ther intensive collecting and study of Precambrian trace fossils should lead to significant conclusions about the evolution of early Metazoa.

ACKNOWLEDGEMENTS. - This research was supported by an Australian Research Grant. Some specimens from Central Australia were received on loan from the Bureau of Mineral Resources in Canberra, whose geological staff also provided guidance in the field. Members of the Geology Department, University of Adelaide, helped in collecting specimens.

Shrinkage crack infillings resembling trace fossils Bedding planes of Precambrian sandstones or quartzites, particularly those with ripple marks, frequently show raised structures which may be short and fusiform, resembling worm castings (Fig. lA), or long and sinuous, resembling casts of worm burrows (Fig. 1B). Such bedding plane structures are known from many areas and from sediments ranging in age from Lower Proterozoic to Eocene. Sinuous markings on rocks of Proterozoic age have attracted much attention, some authors considering them to be trace fossils and others disputing this interpretation. The available material, together with abundant published data, shows conclusively that transitions exist from seemingly discrete fusiform to more continuous casts, and from these to networks of unquestioned mudcrack casts. This proves that such bedding plane features are not of organic origin. Further evidence suggests that some of their unusual characters are due to their origin from the cracking of clay layers under water or during compaction rather than by subaerial dessication. Conclusive proof of this suggestion can only be provided by experimental production of shrinkage cracks in clays to match each of the disputed fossil examples. Such experiments would involve many variables such as composi- tion of clays, sedimentation rates, compaction, bed surface configuration, water chemistry and others. Time and available facilities did not permit me to carry out such experiments. It appears, however, that the morphology of the new and previously described examples available and of some experi-

PRECAMBRIAN AND CAMBRIAN TRACE FOSSILS 371

mental work reported in the literature, give overwhelming support to argu- ments against the ‘worm-cast’ or ‘worm-burrow’ hypothesis for the structure considered here.

Observed transitions from worm-like casts to undoubted drying crack patterns were the main- reason for the rejection by Wheeler & Quinlan (1951) and Cloud (1968, pp.29-32) of the hypothesis of biological origin of the casts. Cloud also states, referring to Hofmann’s (1967) work: ‘Unsolved problems include how corrugated symmetrical structures of this nature could be formed by purely physical processes and how sandy sediments happen to undergo contraction cracking’. Actually, only the problem of corrugation of the spindles figured by Hofmann remains unsolved. Cloud hints at a possible answer, observing that many of the corrugated spindles are partly covered with clay and saying that ‘it is possible that clay sheaths have something to do with the origin of the corrugations’. They are unlike biological segmentation in being chevron-shaped, with apices of the chevrons on the spindle ends opposed and directed away from the ends. Traces of minor corrugations were also found on worm-shaped casts from Precambrian quartzites in Finland by Lauerma & Piispanen (1967). The apparent cracking of sandy sediments is not a real problem. The crack infillings are found only on bedding planes which are partings between sandy layers. Observations in the South Australian Ediacara fossil beds and the Pound Quartzite generally indicate that clay which was present on bedding planes may be reduced to insignificant thickness by compaction and totally lost by weathering, which separates the quartzite slabs. Thin argillaceous layers separating the quartzite strata were mentioned by Wheeler & Quinlan (1951, p. 144) and others.

Several significant features of worm-like crack infillings do not conform with expected shapes of worm trails or bodies. One is the tendency to form ‘8’-like configurations (Dawson 1890, Fig. 14; Hadding 1929, Fig. 34; Schindewolf 1956, P1. 31, fig. 1 ; Frarey & McLaren 1963; Barnes & Smith 1964; Picard 1966; Cloud 1968, Fig. 6A). It is an extreme development of the more general tendency of cracks to delimit rounded areas which must have been mud flakes (Fig. lc). Loops and branches of curved structures are frequently occurring stages in this development. Picard observed that in his material ‘oriented, linear-shrinkage cracks’ rarely develop into ‘3, 4 or 5 sided, complete, shrinkage cracks . . . The sides of these cracks are generally curved and they may grade into unusual, sub-spherical or nearly spherical cracks’ (Picard, 1966, pp. 1052-3, Fig. 4. The term ‘sub-circular’ would be more appropriate). The transitional sequence from ‘incomplete’ (Fenton & Fenton 1937, Shrock 1948) to ‘complete’ mud crack systems in which coherent networks surrounding mud flakes are formed, and from rectilinear to curved to subcircular or sinusoidal cracks, has been amply demonstrated. There seems to be no reason for postulating a biogenic origin for the geometric end members of this series of configurations, either for totally or partially ‘incomplete’ crack infillings such as Manchuriophyczts ( M . yamamotoi Endo, 1933; M . sawadoi, Yabe, 1939, M.sp., Maslov, 1963) or

3’72 MARTIN I;. GLAESSNER


for curvilinear forms. Some peculiarities of the infilling of these cracks or crack systems distinguish them from normal drying cracks and have been thought to indicate biogenic origin. They all seem compatible with the as- sumption of infilling of synaeresis cracks (Jungst 1934, White 1961). Such drying cracks develop under water (see van Straaten 1954, p. 38, for a geological example). White’s experiments included the production of incomplete, branching, curved and sinuous cracks. Richter (1941) has pointed out that cracks may develop not only under water (as in White’s experiments) but also after the clay has been covered by another sediment layer. Subaerial cracks in thick clay layers are wedge-shaped and are always filled from above. Synaeresis cracks may be filled from above or from below (Van Houten 1964, p. 512) or from two wet sand layers enclosing a clay bed. Infilling sand originating from above may not be in continuity with the overlying bed if increasing current velocity has stripped the surface without disturbing the fill, or it may be in continuity with sand both above and below, in the manner of small sandstone dykes. This is well shown in samples from the Pound Sandstone of Red Range near Beltana, South Australia, where compaction and slumping has disturbed ‘incomplete’ crack fillings in obviously soft sediment, producing occasionally separate worm-like sand casts ‘float- ing’ in shale. Some soft-rock deformation has also affected what has been described as the holotype of the type species of Manchuriophycus(Endo 1933, P1. 7, fig. 2). The compaction of sand casts may transform the originally prismatic fill to produce a flattened shape and a median groove or ridge, depending on the manner in which the less compressible sand fill yielded in the more compressible clay matrix.

The morphological distinction between subaerial and intraformational mud cracking is not yet completely clear, and the mechanism of the formation of sinusoidal cracks requires further experimental work. Not all diagnostic characters given by Roniewicz (1965, p. 218) are constant. It seems, however, that synaeresis cracks are more likely to lead to the formation of worm-like casts, particularly in combination with compactional or gravita- tional soft-rock deformation.

A specimen of Precambrian Heavitree Quartzite from central Australia has been figured (Glaessner 1966, Fig. 2) as showing mud flake impressions afid worm-like markings. These are now shown (Fig. 1D) in another photo- graph of part of the lower right-hand corner of the specimen as illustrated earlier. The sediment is cross-laminated. The surface illustrated appears to

~- Fig. 1 . Shrinkage crack infillings (casts) on bedding planes of Late Precambrian quartzites. A. Attached and detached casts, Heavitree Quartzite, Trephina Gorge near Alice Springs, central Australia. B. Sinuous casts on ripple-marked surface, top part of Bitter Springs Formation, 5 miles west of Ellery Creek near Alice Springs, central Australia. C. Heavitree Quartzite, Trephina Gorge near Alice Springs, central Australia. D. Displaced shrinkage crack casts with mud flake casts. Heavitree Quartzite, Undalya Gap, east of Alice Springs, central Australia. E. Dickinsoniu costata Spring, external mould of an annelid worm, on lower surface of Pound Quartzite bed with shrinkage crack infillings, Brachina Gorge, Flinders Ranges, South Australia. A-C 0.5, D-E =’ 1,

374 MARTIN I?. GLAESSNER

be the lower surface of a quartzite bed. Some of the worm-like casts have a minute sericitic lining and can be separated from the rock. They are 4-S mm wide and 1 mm thick. Sinuosity and branching conform with what has been described in mud cracks. Their association with mud flake casts suggests that the worm-like casts are in fact mud crack infillings since mud cracks must have been abundant on this bedding plane. The separation of the flakes from each other and the separation of the cylindrical casts from the sand above and below was probably caused by movement of a thin layer of sediment rather than by a current of water acting on a cracked clay surface. The suggestion of synaeresis rather than sub-aerial mud cracking in this formation will have to be further investigated by field studies, but whatever their outcome, it seems no longer likely that there is any evidence of animal activity preserved in this rock which may be 1000-1100 million years old.

A specimen of the Late Proterozoic quartzitic Pound Sandstone found recently in Brachina Gorge (Fig. 1E) demonstrates clearly the origination and infilling of synaeresis cracks. A smooth bedding plane shows a number of irregularly fusiform ridges up to a few mm in height, varying in width from 1-9 mm, and adhering firmly to the rock. One of them crosses an external mould of a fossil annelid worm (Dickinsoniu costata Sprigg). The distinctive segmentation of the worm is seen faintly to extend obliquely across the ridge and the segments appear to be wider where they cross it. The ridges represent sand casts of cracks in a clayey or silty layer separating the sandstone beds. The soft body of the animal must have been deposited and covered before the cracks formed, as it is stretched across the infilling.

‘Bvooksella’ canyonensis and other fossils from the Grand Canyon Series Cloud (1968, p. 27) considered that the supposed ‘jellyfish’ from the Nanko- weap Group of the Grand Canyon Series resembles ‘structures that have been produced by gas-erosion from sediments or by compaction around compressible or soluble objects such as gas domes or crystals. He described an additional fragmentary specimen as showing ‘that the marginal lobation is a result of small-scale jointing, probably due to compaction of fine sands deposited over a compressible but otherwise undefinable subcircular structure, possibly a small gas blister’. He considered ‘the curious but imperfect resemblance between this structure and one of the Ediacaran medusoids (Glaessner, 1962, p. 479, P1. 1, figs. 1, 2)’ as fortuitous. Having found no further specimens at Ediacara which would resemble the Grand Canyon fossil, I am now inclined to agree with Cloud that the resemblance is both imperfect and fortuitous. I find it difficult, however, to accept his ‘gas blister’ hypothesis, mainly because the fossil shows radiating lobes, of which most if not all terminate with a distinct, arcuate distal edge. In this respect they resemble the lobes of stellate structures on the surfaces of Juras- sic strata which Farrow (1966) described (in the explanation to Fig. 3) as


‘stellate faecal mounds’ and figured (Pl. 3, fig. 3 ; P1. 6, figs. 1, 3) under the name Asterosoma, with a remark (p. 138) that the originator may have been ‘either crustacean or annelid’. Considering the great height of some of these mounds in relatively uncompacted sediments, the fracturing in the compacted Grand Canyon fossil. specimen is understandable. The lobate shapes with distinct distal margins are explicable as overlapping separate faecal extru- sions and in this connection Farrow’s demonstration of a difference between tide-affected asymmetric mounds in the supralittoral facies and symmetrically radiating lobate mounds formed under deeper-water is instructive. It is questionable whether these mounds are correctly named Asterosoma von Otto, 1854 (a little-known Cretaceous trace fossil, Hantzchel 1962, p. W184). A fossil described recently by Altevogt (1968) as Asterosoma radiciforme von Otto from the Upper Cretaceous of Westphalia is unlike those mentioned here. It resembles internal moulds of medusae). Asterophycus Lesquereux, 1876, a name given to very similar objects from the Carboniferous and probably also the Silurian, could be applicable. Dawson (1890, p. 604, Fig. 9) figured a ‘radiating burrow’ of that age and referred to ‘grooves radiating from the orifice’ of worm burrows. The presence of radial grooves on the lobes was included in Hantzschel’s diagnoses of Asterosoma and Astero- phycus, but it was apparently not considered as important by Farrow as his specimens do not show radial grooving. Most of the lobes of the Grand Canyon fossil show only few distinct radial grooves. Its interpretation as a trace fossil which may be referred to as Asterosoma ? canyonensis (Bassler), 1941 seems justified. The important find of a second fragmentary but otherwise identical specimen by Webers and Cloud supports the conclusion that it is not an accidental mechanical effect but more likely the result of a metazoan life process. All that can be said about its possible originator is that it was a sediment feeder, approximately 5-7 mm in diameter, and able to burrow into the sediment. It was probably worm-like in shape, and judg- ing by its likely morphology and feeding habits compared with living invertebrates, it would be considered to be of coelomate grade, probably an annelid.

The age of the Grand Canyon Series is not definitely known. The Apache Group of Central Arizona, which is now known to be about 1100-1300 million years old (Livingston & Damon, 1968), has been tentatively cor- related with the Grand Canyon Group.

Other fossils from the Grand Canyon Group have been collected by Alf (1959). Cloud (1968) was impressed by their resemblance to imprints caused by falling drops. Alf’s material (Fig. 2) differs from experimentally produced imprints in the common occurrence of pairs and triplets surrounded by marginal outlines, but Cloud considers their absence in the known drop impressions as possible ‘accidents of replication’. This feature, which is clearly shown in specimens, casts, and photographs kindly presented to me by Professor R. Alf as well as in Cloud’s Fig. 4B, cannot be formed regularly by falling drops. Instead, these produce intersecting circles as seen in Cloud’s


Fig. 2. Casts of algal colonies on bcd- ding plane of red siltstone, Late Prec- ambrian, Bass Formation, Grand Canyon Arizona, U.S.A. Photo of plaster cast presented by R.M. Alf. >: 1.

Fig. 4A, but not in Alf’s specimens. Straight boundaries between two adjacent ‘objects’ surrounded by a common outline may be expected in the junction of gelatinous sheaths of two algal colonies. Joined disc-shaped colonies, in this case of living diatomaceous algae, were found by M.R. Walter & W.V. Preiss near Robe in south-eastern South Australia (Fig. 3). This confirms that the gross morphology of Alf’s fossils can be matched in algal growth. The fossils are preserved in red siltstones or quartzites, without any organic matter. The ‘vermiform markings’, described by Alf (1959, p. 63, Fig. 4) are mudcrack infillings while the ‘sponge-like nodule’ figured by this author was considered by Cloud (1969, p. 56) as a silica nodule. Cloud (1969, p. 54) also lists as ‘probably nonvital’ what Seilacher (1956, p. 165) had described from the Hakatai Shales under the name of ‘Rusophyus’ as trace fossils of burrowing bilaterally symmetrical animals. I cannot comment on these specimens which I have not seen.

A new trace fossil from the Upper Precambrian This very distinctive trace fossil (Fig. 4, see also Glaessner 1968, P1. lb), was found in situ on a freshly exposed upper surface of a bedding plane of finely laminated (partly cross-laminated) dark purplish micaceous siltstone of the Brachina Formation in Bunyeroo Gorge, South Australia. The counterpart cast was also found. A further short section of the same track, not in continuity with it, was recovered, together with its counterpart cast.


Fig. 3. Colonies of living diatomaceous algae. Lower surface. Collected May 1967 at Lake Eliza, near Robe, South Australia, by W.V. Preiss and M.R. Walter. 1.

The track curves in a smooth arc through almost 90” over a length of about 160 mm. At one end it is subparallel with a faint, straight, current lineation on the bedding plane. At this end the total width of the track is 3 3 mm, at the other end it is about 21 mm wide, both ends being at the broken edges of the rock slab. A submedian ridge which is 2-2.5 mm wide at the ends but narrower in the middle is about 7 mm distant from the convex side of the trail at its narrow end and about 12.5 mm from it at its wide end. The trail is less distinct along its concave margin, which is 21 mm distant from the median ridge at its wider and 12 mm at the narrower end. Between it and the median ridge lies a fainter and less regular, low, ridge-like feature which becomes unrecognizable at both ends. Between the margins the trail is formed by more or less distinct but regular transverse grooves which are about 2 mm long and separated by longer rises. They end at the convex margin in distinct, pit-like depressions which seem to be partly surrounded by raised lunate rims. The margin on the concave side is more like a continuous rim rather than scalloped by the transverse depressions. It forms a rise even where the depressions are seen to cross the median ridge.

This is a locomotion trail made by a bilaterally symmetrical animal whach used rhythmic muscular contractions rather than discrete appendages for propulsion. It had a marked mid-ventral groove and was deformable so that the distance of this groove from each of the margins could change, as well as the total width of the contact area with the substratum, within certain limits. The observed ‘contraction’ of the parts of the body concerned in making the trail is of the order of S O Y , of its maximum width. The difference between the markings on the convex and concave margin indicates the separation of the structural elements making discrete impressions on the convex side, apparently to achieve the observed change of direction, while the ele-


Fig. 3. Bunyerichnus dalgarnoi gen. nov., sp. nov. Late Precambrian, Brachina For- mation, Bunyeroo Gorge, Flinders Ranges, South Australia. Holotype. x 1 .

ments which were in contact with the substratum were trailing passively on the concave side where they produced a double ridge. These ridges are not transversely subdivided. Apart from them the trail shows throughout its length a distinct strictly transverse (not chevron-shaped) ridge-and groove sculpture. This does not prove segmentation of the animal but only rhythmic transverse pressure against the substratum. This occurs in the foot of molluscs as well as in the body wall of annelids.

The crawling locomotion of living annelids (as distinct from burrowing or swimming) is assisted or effected by parapodia. The fossil track was made by a transverse part of the body, not by lateral appendages, although the


lateral ends could be alternatively more firmly impressed in the ground or passively trailed along. The application of a transverse terminal zone of the body to the ground surface, presumably with some anterior zone or zones extended to produce forward locomotion, and a wave or waves of extension and contraction moving back through the body is compatible with the movements of the foot in some gastropods. A ventral groove (which must have been present to produce the distinct and continuous median ridge) occurs among the living mollusca mainly in the Aplacophora, Order Neomen- ioidea, whose locomotion, however, is not known to produce a ribbon-like trail. On the other hand, the separation of a foot, pallial grooves and ventral mantle on the lower surface of the animal, as in the Polyplacophora, could produce the ridges seen on the concave margin of the trail. It is possible, from the viewpoint of functional morphology of the moving parts which made the impression, that the animal was related to primitive mollusca without mineralized shells of which there is no trace in sediments of that age. It would not be safe to speculate further on the nature of its originator, on the basis of a single specimen.

The fossil was found in the Brachina Formation in Bunyeroo Gorge, west of Wilpena Pound, Flinders Ranges, South Australia. This formation, which is 4000 feet (1300 m) thick, occurs in the lower part of the Wilpena Group, some 6000 feet (2000 m) below the top of the Pound Quartzite (top of the Precambrian) and only a short distance above the ‘Upper tillite’, from which it is separated only by the thin Nuccaleena Formation.

The surface locomotion trail differs from those in previously named form genera by its close-set straight transverse rises, its median ridge, and its variable width. Although only one specimen was found, it is named for reference Bunyerichnus dalgarnoi nov.gen,. novsp. The generic name refers to the locality and the specific name records the mapping of the area (Sheet Parachilna, 1 :250,000, 1966, Geol. Survey of South Australia) and descrip- tion of its geology by Mr. C.R. Dalgarno.

Trace fossils from the Ediacara fossil beds Most of the body fossils in the Ediacara fauna appear to have been transported to their places of burial (Wade 1968). The physical conditions of the environment have been examined by Goldring & Curnow (1967). Large numbers of animals lived in this environment, leaving trace fossils of their varied activities. None of them can be related, with any probability, to any of the known bodily preserved animals. In addition to some less common and more obscure kinds of trails, at least six different types are clearly distinguishable. Most of them are represented by a number of almost identical specimens. It is unlikely that any of these distinctive trace fossils were produced by different activities of the same kind of animal. A number of different soft-bodied invertebrates must have been present in addition to the previously described organisms. No formal names will be proposed and


PREC.AMBRIAN AND CAMBRIAN TRACE FOSSILS 381

only brief descriptions will be given, to complement the existing data for a palaeoecological evaluation of the Ediacara fauna. The trace fossils will be grouped provisionally and those previously figured will be listed first.

FORM A. - Trails of ovoid serial structures on lower surface of sandstone layers, with evenly spaced transverse grooves oblique to the lateral margins of the trails, were compared (Glaessner, in Glaessner & Daily 1959, p.395, P1. 45, fig. 3) with foam trails described earlier by Hantzschel. Specimens found subsequently, in which individual bodies of sedimentary material between the transverse grooves have become detached (Fig. 5A), prove that they are not casts of features of the underlying surfaces but trails of pellets which usually merge with the overlying sediment. However, they occasionally separate from it just as they are separated by grooves from each other, and from the sediment on the sides of the trail. They are considered as fecal pellets of sediment feeders. Their average transverse diameter is about 10 mm. The name Cylindrichnus, recently proposed by Bendel (1967, p. 6), who gave a similar interpretation to slightly larger trails of ball-like structures from the Upper Pennsylvanian of Kansas, could be applied to these fossils.

FORM B. - These surface trails (Fig. 5B, see also Glaessner 1961, p. 75, Fig.) may be described as ‘concave exogene semireliefs’ according to Seilacher 1964, or as ‘epichnial grooves’ according to Martinsson 1965, whose terminology will be followed here (see also Hantzschel 1966, p. 15). They are invariably sinuous, occasionally branching, 3 4 mm wide, and often have both sides more or less distinctly raised as ‘levees’. In some places they show distinct transverse arcuate structures across the groove suggesting mud- pellet stuffing (as in Nereites McLeay). They occur fairly commonly on the upper surfaces of layers, some of which show ripple marks.

FORM C. - Meandering grazing patterns, 1-2 mm wide and close-set (thigmotactic), on lower surfaces, i.e. hypichnial casts. This form is represented only by a few slabs in the collection, but these are well covered with the trails (Fig. 5C, D).

FORM D. - Large (up to 25 mm wide) slightly to markedly sinuous trails, (Fig. 5E). They are either hypichnial casts or occur in full relief on lower surfaces. Some show longitudinal or transverse wrinkling. T o explain this, the following comment by Seilacher (1962, p. 227) seems relevant: ‘of the sand-living animals only a few leave lasting galleries or other distinct traces in the sand itself, while all of them can leave sharp impressions when they touch, or crawl along, an underlying clay bed.’ The animal is interpreted as a sediment- feeding worm, different in size from all others. This animal,

Fig. 5 . Trace ‘fossils from Late Precambrian Pound Quartzite. Ediacara, South Australia. A. CyZindrichnus sp. €3. Surface trails. C, D. Meander trails. E. Large sinuous trail. F. Worm trail. A, E x 0.5, B-D, F 8 1.


with different behaviour, could also have produced a unique spiral cast with a diameter of the spiral of up to 12.5 cm and a width of the coiled, infilled track of 20-30 mm.

FORM E. - Small to medium-sized (2-6 mm. wide), mostly bottom trails, i.e. hypichnial casts but also including one surface trail (epichnial ridge) (Fig. 5F).

FORM F. - Short (20-30 mm long), 5 mm wide, concave moulds on upper or lower surfaces, often with clay films adhering to them. These are probably moulds of trails of a sediment feeder which preferred more argillaceous sediment.

These animals were all worm-like sediment feeders or detritus feeders, either living in the sand, or on the mud-sand interface, or grazing on the mud surface. Form C is represented on natural casts of such surfaces. Larger worms seem to have left traces at the bottom of the thicker beds more than smaller worms. The assemblage represents Seilacher’s (1963) Cruxiana facies but without Cruniana which is a trilobite trail. This ‘ichnofacies’ was interpreted by Seilacher as representing the zone of wave action. Associated with its distinctive forms are others such as the meander trails, and ‘Nereites’-like forms which are usually found in Seilacher’s ‘turbidite zone’. Their anom- alous appearance in a shallow water sequence is due to the temporary occurrence of quiet mud flats between current and wave-agitated areas. These mud flats were presumably covered with algae and bacterial growth and organic detritus. We note the absence of representatives of the intertidal Skolithos zone and of the Taonurus (or Zoophycos) facies which represents deep water. Suspension-feeders as well as mud-mining organisms (Chon& rites) are absent.

Some trace fossils from the Upper Precambrian - Basal Cambrian sequence of central Australia The Arumbera Formation represents a time interval from Late Precambrian to Basal Cambrian in the Amadeus Basin, central Australia. There have been no detailed systematic studies of its lithology and fauna. Some of the numerous trace fossils collected in these dominantly arenaceous sediments are discussed here in advance of such studies because of their unusual interest.

The Precambrian age of the lower part of the Arumbera Formation is documented by one specimen of Rangea cf. Ionga Glaessner & Wade (Fig. 9A), which was found by D.J. Taylor east of Deep Well Homestead, 50 miles (80 km) SSE of Alice Springs. A single, unidentified medusoid fossil with pronounced concentric sculpture was found by G.K. Williams 825 feet (275 m) above the base of the formation at Laura Creek, about 15 miles (25 km) SW of Alice Springs, where it is 1350 feet (430 m) thick. Trace fossils, including many of those described here, are common and varied in and above this horizon, but they are rare and uncharacteristic below. This


fact was observed in a number of sections (Laura Creek, Jay Creek, Ross River) within 30 miles (50 km) east and west of Alice Springs. It can be shown that the abundant trace fossil assemblages of approximately the upper 1/3 of the formation are of Cambrian age.

Among the numerous trace fossils collected, there are only a few which resemble arthropod trails. One specimen (Fig. 6A) shows about six pairs of symmetrically placed impressions resembling trilobite walking trails assigned to Diplichnites Dawson by Seilacher (1955, p. 342). Another specimen (Fig. 6B) is similar to Rusophycus but does not show the typical symmetry of this form. There are also hypichnial casts of comb-like structures with some resemblance to some of those described by Seilacher (1955, p. 367, P1. 21). They are, however, much larger and more convex, in the manner of Rusop- hycus, but not paired.

The resemblance of these fossils to tracks and burrows of trilobites suggests, but does not prove, the existence of this Class during Arumbera time. The specimen figured here as D$Iichnites is certainly the locomotion track of an arthropod but no diagnostic characters of trilobite legs (Seilacher 1962a) can be seen.

One of the most common trace fossils in the upper part of the Arumbera Formation (Fig. 6C-E) proved to be identical with Ph-?codes pedum, a form from the Cambrian Neobolus Beds with Redlichia noetlingi(Red1ich) of the Salt Range in Pakistan and the Middle Cambrian of the Grand Canyon. I t was described and analyzed in detail by Seilacher (1955, p. 386, Figs. 4,5: PI. 23, figs. 6, 7 ; PI. 25, fig. 3). Where a main burrow is developed, it can vary from straight to almost circular, as in Seilacher’s material, with close-set, infilled side branches which are directed mainly upward, but which cannot be recognised in the matrix of the bed on the base of which Phycodes pedum forms hypichnial casts. Its originator was a sediment-feeding worm-like animal.

Another identifiable trace fossil was found by private collctors in the Arumbera Formation in the Ross River Gorge, 100 feet (30 m) below the the top of the formation, which is here 1100 feet (370 m) thick (Figs. 7A-D). It represents the form genus Plagiogmus Roedel, 1929, of which Hantzschel (1962, p. W210) gives the following diagnosis: ‘Smooth, flat, concave ribbons, 1.5 to 2.0 cm wide, slightly curved, with pronounced transverse bulges at irregular intervals or closely crowded, slight longitudinal furrow.’ The longitudinal furrow is visible in only one of Roedel’s specimens and even there not on all three observed tracks. The transverse bulges do not reach the sides of the smooth ribbon of the track. This fossil was figured but not named by Nathorst (1897), Hogbom (1925) and Roedel(1926), and subsequently referred to by Hantzschel(l964, p. 96, Fig. 6 ; 1965, p. 72). The available material also includes specimens (Fig. 7E) collected by Dr. B. Daily and myself at the northern end of the Mt. Scott Range northwest of Copley, South Australia, in beds which were subsequently assigned to the Lower Cambrian Parachilna Formation (Dalgarno 1964). The area has since been



re-examined by Dr. Daily who collected abundant trace fossils, mainly Plagiogmus (Fig. 7F-H).

Previous authors had considered Plagiogmus as a surface trail (epichnial groove), but the new material proves it to be an endichnial burrow which is roughly parallel to the bedding and which invariably has a distinct lower surface and an indistinct upper surface. Fig. 8 shows a block diagram of the trail as reconstructed from various observed sections. In two of the specimens from the Arumbera Formation, the distinctive transverse ‘bulges’ within a smooth ribbon trail exposed on a bedding plane are seen to pass in vertical section into a narrow, obliquely textured band, where the infilling and overlying matrix is preserved. This band, the ‘backfill’ of the trail produced by the organism, is compressed to a thickness of about 1 mm in one specimen to 7 mm in another where the sediment was coarser and consequently the rock was less compacted. The band of backfill consists of oblique sedimentary laminae arising from the transverse bulges; weathering makes it impossible to determine whether compositional or textural differences account for the boundaries between them. Similar infilling laminae are seen in at least one of the specimens from the Mt. Scott Range. These are generally more arenaceous and less compacted, and the cross section of a trail 18 mm wide is 8 mm high. Obliquely laminated infilling of a trilobite trail was illustrated by Seilacher (1955, Figs. 5/9). Plagiogmus shows no scratch marks of appendages or claws. There are on some specimens broadly arcuate markings across the bottom trail (Fig. 7A, G) which support Roedel’s view that the animal moved by extension and contraction and also his reference to a resemblance with the trail known as Climactichnites Logan. The transverse ridges or bulges of Plagiogmus are, however, mostly straight, not arched or V-shaped, and they do not extend to the sides of the trail. Its smooth lower surface must be due to the secretion of mucus which facilitated locomotion of the animal and held the grains together until lithification set in. The mechanism of the production of the often regularly spaced ridges and the reason why most of them adhere firmly to the surface of the trail remains problematic. In trilobite trails as figured by Seilacher the infilling sand was transported backward by the legs of the animal. In the absence of any traces of appendages it can be concluded that the originator of Plagiogmus was not an arthropod. Worms produce the backfill of their burrows from ingested and excreted sediment. The cementation by mucus of only the smooth sole of the burrow, the curious facultative periodicity of ‘excretion’ if that was indeed the process which fashioned the transverse ridges, and their upward extensions, remain without parallel among annelid trace fossils or living worms.

Fig. 6. Trace fossils from Lower Cambrian sandstone, Arumbera Formation, about 50 m below top. Laura Creek, WSW of Alice Springs, central Australia. A. Diplichnites sp. B. Rusophycus-like markings. C-E. Phycodes pedum Seilacher (E shows also aligned smaller burrow casts). A, D, E 1 , B 0.5, C .I 2.



Fig. 8. PZagiogmus arcuatus Roedel. Block diagram explaining the fossil as an endichnial burrow. Filling shown by cross hatching.

In gastropods (Seilacher 1955, p. 375, Fig. 1) the backfill fabric is produced in a complex manner: when they move through the sediment below its surface, the sand is transported backward in a conveyor belt action by the expanded foot while the sole profile is formed by its locomotory contraction waves. They leave lasting imprints on the surface when the posterior end of the foot is lifted off and moved forward. The lower part of the trail is infilled by sand lamellae which slope backward and upward as the result of backward pressure from the crawling animal. A close spacing of the typical transverse ridges, occasionally seen on Plagiogmus, makes it resemble Psnmmich- nites Torell, which Seilacher and later Hantzschel considered as a gastropod trail (endichnial burrow). The complexity and variability of behaviour, the gradation towards Psammichnites, the differentiation of the ventral surface suggesting a foot, possibly also the frequent crossing of trails which contrasts with the ‘avoidance reaction’ in the trails of sediment-feeding worms, suggests a more highly differentiated animal. There is no evidence of shell- bearing gastropods of appropriate size in Lower Cambrian rocks. It is likely that the originator of Plagiogmus was an ancestral mollusc with a foot and mantle. It could feed on organic matter in sand and backfill its partly mucous- lined trail with rejected sediment.

Hantzschel selected P. arcuatus Roedel as the type species of Plagiogmus. This was distinguished from P. simplex by Roedel (1929) mainly because of more closely set transverse ridges and the presence on most of them of a fine, curved groove. There is also a weak median groove in at least two of the three tracks on the type specimen of ‘arcuatus’. Hantzschel(l964) expressed doubt about the justification for the distinction between two form species. The spacing of the transverse ridges is variable and median grooves occur rarely. The curved grooves on the ridges have not been observed in the Australian material. Specimens from Mt. Scott agree entirely with P. simplex. Roedel explained correctly the apparent chevron-shape of the ridges in trail

Fig. 7. Plagiogmus arcuatus Roedel. Lower Cambrian. A-D. Arumbera Formation, upper part, Ross River Gorge, W of Alice Springs, central Australia. E-H. Parachilna Formation, N of Mt. Scott, Flinders Ranges, South Australia. E. Crossing trails, coll. B. Daily and M.F. Glaessner. F. Part of two parallel trails. Less than one-half of width of the right-hand trail is preserved. G. Part of trail with weak transverse ridges and strong arcuate impressions. H. Part of trail with close-set transverse ridges and faint longitudinal grooves. F-H coll. B. Daily. All = 1.


Fig. 9. Body fossil and trace fossils. Arumbera Formation, central Australia. A. Rangea cf. Ionga Glaessner & Wade, lower part of Arumbera Formation, east of Deep Well Homestead, SSE of Alice Springs. Coll. D.J. Taylor, No. A2-23, in Bureau of Mineral Resources collections, Canberra. B-C. Molluscan trails from upper part of Arumbera Formation, William Bore, 25m E of Alice Springs, coll. A.J. Stewart. D. Molluscan trail upper part of Arumbera Formation, Laura Creek, WSW of Alice Springs. E. Gordia sp., on weathered surface of red silstone, upper part of Arumbera Formation, 38 m WSW of Alice Springs. A, B, D, E \ 1, C x 0.5.


3 of his specimen (1926, p. 23) as due to juxtaposition of two trails. The Arumbera material is preserved in a finer-grained rock. The ridges are less regular in one (Fig. 7A) and more regular in another trail (Fig. 7B), weaker in a third (Fig. 7D) and spindle-shaped in a fourth (Fig. 7C). Specific distinctions are clearly not justified.

A curious hypichnial cast of a trail which shows fine transverse wrinkling and coarser arcuate transverse bulges (Fig. 9D) resembles Oliz~ellites Fenton & Fenton but lacks the median ridge of that form and of Psammick- nites Torell. Its arcuate markings are similar to those of Plagiogmus and Climacticknites. Hantzschel, following Seilacher, has merged both Oliz~ellites and Psammicknites with Scolicia de Quatrefages. He proposes to use this name for ‘various trails presumably made by gastropods’. As far as those from the Lower Cambrian are concerned, I suggest that reference should be made not to gastropods but to mollusca, as shell-less forms on the level of Polyplacophora or Monoplacophora or even more primitive ancestral molluscs could have existed at that time as benthonic sediment feeders.

Another trail which should be included in this group was probably made by another mollusc-like organism (Fig. 9B-C). Such smooth, double-ridged hypochnial casts have been repeatedly described from the Lower Palaeozoic as Rouaultia de Tromelin, but this name is invalid (Hantzschel 1965, p.80). Well-preserved specimens from the Arumbera Formation (Fig. 9B) have an additional pair of gently sloping smooth outer ribbon-like surfaces.

The Arumbera Formation also contains numerous trace fossils produced by worm-like organisms. A simple but distinctive form (Fig. 9E), charac- terized by smooth sinusoidal curvature, may be placed in the form genus Gordia Emmons. There are others which are common but not distinctive enough to be named. The name Planolites Nicholson is available for the very common non-branching infilled endichnial worm burrows, which are more or less parallel to the bedding. They occur throughout the Arumbera Formation and also in the Parachilna Formation. One of the less distinctive trace fossils of the Ediacara fauna, the epichnial Form B, described above (p. 381) is represented in the collections from the upper part of the Arumbera Formation by a single specimen.

Seilacher (1965, p. 174) has argued strongly that trace fossils, generally poor time markers, show explosive differentiation at the beginning of the Cambrian Period. He considered this as particularly valuable where the Cambrian begins with transgressive sandstone deposits which are unfavour- able for the preservation of body fossils. In such circumstances, he claimed, the base of the Cambrian can be extended downward on the basis of occurrences of trace fossils. These views were based on a review of available data and on Schindewolf’s theories of sudden profound changes of the biota at Era boundaries. All those who have made collections and observations in the Arumbera Formation agree that trace fossils become more varied in its upper part. The occurrence of Plagiogmus arcuatus and Pk-vcodes pedum, together with probable arthropod trails resembling Diplicknites and Ruso-


phycus, and ‘Scolicia’-like molluscan trails in these beds, is a clear indication of their Cambrian age. Plagiogmus was first described from glacial erratics in Germany and traced to the island of Oland. It was questionably referred to the Middle Cambrian ‘Tessini’ Sandstone, at a time when the Lower Cambrian of that area was not well know‘n. Martinsson (letter dated 8th Feb. 1968) has found two further ‘drift specimens on the west coast of Oland in the parish Hogsrum a few km N of the harbour of Stora Ror. The rock agrees very well with the lithology of the Lower Cambrian in the Kalmarsund region (and Nathorst’s specimen) and has nothing in common with the Middle Cambrian “Tessini” (now Paradoxissimus) Siltstone (Mar- tinsson 1965). Both this locality and the one about 10 km to the south from which Nathorst obtained his specimen are situated in such a position that the occurrence of morainic “Tessini” Siltstone is extremely unlikely. You can safely date this rock Lower Cambrian’. This is an unusual trace fossil in that a number of its distinctive characters reflect distinctive morphological features of its originator and are unlikely to be due to convergent behaviour of different animals. I have observed (Glaessner 1968) that the fossil described by Cloud & Nelson (1966) as comparable with Pteridinium from the Middle Deep Springs Formation of the White Mountains, Cali- fornia, lacks the diagnostic characters of Pteridinium and is similar to Plagiogmus. It is larger (25-27 mm maximum width) than most Scandinavian and Australian specimens but equal to at least one. It is preserved in a thinner-bedded and more compacted rock and hence lacks the depth of tracks preserved in more massive and less compressible sandstone. Otherwise it is similar and I agree with Cloud and Nelson that its age is Lower Cam- brian.

The greater diversity of trace fossils in the Lower Cambrian part of the Arumbera Formation supports Seilacher’s thesis of a rapid differentiation of soft-bodied benthonic organisms at the beginning of the Cambrian. It will be necessary, however, to carry out a complete sedimentological analysis of this formation in order to find out what, if any, change in environmental conditions occurred during Arumbera sedimentation. The Lower Cambrian trace fossil assemblage corresponds to Seilacher’s (1963) Cruxiana-facies generally, indicating deposition between low tide and wave base. This agrees with the observed gross sedimentary characters. The preceding sediments of presumably Late Precambrian age contain Planolites-like worm burrows which are not indicative of any specific environment at the present stage of our knowledge. It is possible that there may have been a change of salinity in the basin during the deposition of the Arumbera Formation. Its uppermost beds contain halite pseudomorphs.

Conclusions Trace fossils are less common and less varied in the Precambrian than in the Lower Cambrian, but they are neither generally rare nor insignificant. I agree with those who eliminate from the organic record of the distant

PRECAMRRIAN AND CAMBRIAN TRACE FOSSILS 391

past the sinusoidal and spindle-slaped structures which occur without significant change on bedding planes of sediments as old as Lower Protero- zoic and as young as Eocene. We cannot, however, disregard a fossil from the Grand Canyon Series which may be more than 1000 million years old. I consider it as a trace fossil of a worm-like organism, not as a coelenterate. Some other apparent trails of a similar age described from the Hakatai Shale (Grand Canyon) by Seilacher and from the Belt Supergroup by Wal- cott have to be re-examined. It may be noted briefly that specimensofcf. Syringomorpha and Scolithus-like tubes from the tillitic Areyonga Formation of the Amadeus basin and its equivalents, reported during the mapping of that area, have been re-examined and found to be inorganic sedimentary features. However, Skolithus from the Buckingham Sandstone of the Wessel Group in Arnhem Land, Northern Australia, was apparently correctly identified and is stated (K.A. Plumb, pers. coinm.) to occur below a glau- conite-bearing bed with a reported age of 790 m.y. This trace fossil and Bunyerichnus dalgarnoi nov.gen., novsp., which is not much younger than the Late Proterozoic Upper Tillites of the Adelaide Geosyncline, are among the oldest known trace fossils. The possibility that the originator of Bunv- erichnus was mollusc-like rather than worm-like in its locomotive behaviour may be significant for the study of rates of metazoan evolution in the Late Precambrian.

The Ediacara fauna contains traces of the activity of various worm-like sediment- and detritus-feeders but no burrows of suspension-feeders. Its ‘ichnofacies’ determined according to principles proposed by Seilacher agrees with the sedimentological facies interpretation by Goldring & Curnow (1967). The Ediacara fossil beds are disconformably overlain by the Lower Cambrian Parachilna Formation with abundant Diplorraterion (Dalgarno 1964), indicating littoral conditions during a transgression.

In central Australia, the fauna of the Arumbera Formation includes Rangea cf. longa, one of the Ediacara fossils, in its lower part and a number of distinctive Lower Cambrian trace fossils in its upper part. Apart from worm-like and arthropod-like organisms there are abundant trails of organisms which appear to indicate molluscan rather than annelid functional morphology. It is considered inappropriate to interpret their resemblance with gastropod trails as evidence of the presence of abundant gastropods in the Lower Cambrian, which contains only small and questionable gastropod shells. An earlier and more primitive grade of (proto-) molluscan organization could be postulated on the basis of trace fossil occurrences. Under favourable conditions of preservation, Precambrian and early Cambrian fossils may be useful in three different ways: in stratigraphic correlation, as markers for certain facies conditions, and as indicators of the existence of the existence of morphological and functional levels of organication in soft-bodied organisms which may be highly significant for the study of metazoan evolution.

Department of Geology, [Tnierersity of Adelaide, Adelaide, South =lustralia 5001, April 9th, 1969.

392 MARTIN 12. GLAESSNER

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TRACE FOSSILS FROM THE PRECAMBRIAN AND BASAL CAMBRIAN