© 2003 by The Society for Integrative and Comparative Biology
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The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives1
1 Department of Earth Sciences, University of California, Riverside, California 92521
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The increase in trace fossil diversity across the Neoproterozoic-Cambrian boundary often is presented in terms of tabulations of ichnogenera. However, a clearer picture of the increase in diversity and complexity can be reached by combining trace fossils into broad groups defined both on morphology and interpretation. This also focuses attention on looking for similarites between Neoproterozoic and Cambrian trace fossils. Siliciclastic sediments of the Neoproterozoic preserve elongate tubular organisms and structures of probable algal origin, many of which are very similar to trace fossils. Such enigmatic structures include Palaeopascichnus and Yelovichnus, previously thought to be trace fossils in the form of tight meanders.
A preliminary two or tripartite terminal Neoproterozoic trace fossil zonation can be be recognized. Possibly the earliest trace fossils are short unbranched forms, probably younger than about 560 Ma. Typical Neoproterozoic trace fossils are unbranched and essentially horizontal forms found associated with diverse assemblages of Ediacaran organisms. In sections younger than about 550 Ma a modest increase in trace fossil diversity occurs, including the appearance of rare three-dimensional burrow systems (treptichnids), and traces with a three-lobed lower surfaces.
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INTRODUCTION |
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SYNOPSISINTRODUCTIONCAVEATS OF TRACE FOSSIL...NEOPROTEROZOIC TRACE FOSSILSPROBLEMATIC "TRACE FOSSILS"DISCUSSIONReferences |
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There was an increase in the diversity and complexity of trace fossils across the Neproterozoic-Cambrian boundary (Seilacher, 1956
). Broadly speaking, Neoproterozoic trace fossils are typically horizontal, unbranched trails or burrows made close to the sediment surface. In the Cambrian, animals probed deeper into the sediment and produced more complex burrows, including branching forms, with a concomitant expansion in the range of sizes. The bulk of the Neoproterozoic–earliest Cambrian trace fossil record comes from siliciclastic strata deposited in shallow subtidal settings. Also other environments are represented including evidence for colonization of the continental slope (e.g., Crimes, 2001). Examination of sections world wide led to the realization that a reasonably consistent order exists in the first order of appearance of several trace fossil types, which permitted erection of trace fossil based zones (Crimes, 1987, 1992, 1994; Narbonne et al., 1987; Walter et al., 1989). Based on successions in Newfoundland, Narbonne et al. (1987) recognized three trace fossil zones straddling the Neoproterozoic-Cambrian boundary—the Harlaniella podolica, Treptichnus pedum, and Rusophycus avalonensis zones—each matching a global trace fossil zone erected by Crimes (1987). The Treptichnus pedum Zone is characterized by the first appearance of an assemblage of trace fossils including complex three-dimensional burrows (e.g., Treptichnus pedum), the first arthropod-type scratch marks (Monomorphichnus), and several ichnotaxa of plug-shaped burrows (Bergaueria, Conichnus) (Narbonne et al., 1987; Narbonne and Myrow, 1988). It was eventually decided to select the base of the Treptichnus pedum Zone at a section at Fortune Head, Burin Peninsula, Newfoundland, as the basal Cambrian GSSP (Brasier et al., 1994; Landing, 1994). Subsequent studies have corroborated and expanded the broad-scale stratigraphic application of the trace fossil based zonation in the Lower Cambrian (e.g., MacNaughton and Narbonne, 1999), although there has been some modification in the stratigraphic ranges of some key taxa at the Burin Peninsula stratotype section (Gehling et al., 2001).
Trace fossils not only are useful in earliest Cambrian stratigraphy, but their importance has been recognized in addressing questions related to the appearance of animals and the interpretation of the Cambrian ‘explosion’ (Seilacher, 1956; Bergström, 1990; Conway Morris, 1993, 1998; Valentine, 1995; Erwin, 1999). The importance of early trace fossils is particularly great if one favors the view that key bilaterian features could only have developed in a moderately large benthic animal (e.g., Valentine, 1994, 1995; Budd and Jensen, 2000; Collins and Valentine, 2001). There have been numerous reports of trace fossils older than 600 Ma, but upon critical evaluation these traces have all proven to be misidentified inorganic structures or metaphytes, or have been misdated (e.g., Crimes, 1994; Hofmann, 1992; Fedonkin and Runnegar, 1992; Runnegar, 1992a, b). Prominent recent reports of trace fossils more than 1 Ga BP (Seilacher et al., 1998; Rasmussen et al., 2002) also are questionable (Conway Morris, 2002; Budd and Jensen, 2003). In fact, all undoubted trace fossils postdate the Marinoan (or Varangerian) Ice Age, which ended about 590–570 Ma BP.
The purpose of this paper is to briefly examine the Neoproterozoic trace fossil record both in terms of its broad patterns and its relation to the Cambrian trace fossil record.
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CAVEATS OF TRACE FOSSIL NOMENCLATURE |
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Discussions of Neoproterozoic and Cambrian trace fossils have largely been conducted at the level of ichnogenera or ichnospecies. Indeed, compilations of trace fossil diversity largely consist of tabulations of ichnotaxa. Naming of invertebrate trace fossils is largely based on morphology, and carries no implication of a commonality of trace producer (e.g., Magwood, 1992
; Pickerill, 1994; Bromley, 1996). A problematic aspect of trace fossil naming is that more often than not, only a limited portion of a trace fossil is visible or preserved. Furthermore, the same trace preserved at different levels, such as preservation at the interface of two contrasting types of sediment, can result in morphologically distinct traces. What is in effect a single biogenic structure therefore can be assigned to different ichnogenera depending on what portion of the trace is visible (or preserved) to the observer; this is particularly problematic for forms that are three-dimensionally complex. It can be argued that naming each trace purely by its preserved morphology is the most objective approach, and that subjective lumping may lead to loss of information. Nevertheless, particularly for the study trace fossils at the Neoproterozoic–Cambrian, I find it preferable to look for similarities, and to critically evaluate the taphonomy of each morphological feature. A benefit of this approach is that it better allows attempts to look for links between Neoproterozoic and Cambrian trace fossils.
Below, selected Neoproterozoic trace fossils will therefore be discussed under broad groupings, defined partly by criteria of morphology but which also incorporate significant interpretative elements (cf., Zhu, 1997; Jensen et al., 2000).
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NEOPROTEROZOIC TRACE FOSSILS |
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Horizontal unbranched trace fossils
The archetypical Neoproterozoic trace fossils are unbranched irregularly meandering to sinuous burrows and trails typically a few millimeters in width, probably formed within less than 10 mm to the sediment-water interface (Fig. 1). Depending on their general pattern, these traces have been assigned to the ichnogenera Planolites, Helminthoidichnites, Gordia, Cochlichnus, Helminthopsis and Helminthoida (e.g., Hofmann, 1990
; Narbonne and Aitken, 1990). In Neoproterozoic examples of these ichnotaxa, traces can typically be followed along bedding planes or cleavage surfaces for less than a few tens of centimeters, complicating ichnogeneric assignment. The Neoproterozoic Helminthoidichnites, Helminthopsis and Gordia are all broadly comparable in that they represent transient movement through the sediment, perhaps as feeding/scavenging traces. These traces are found in sandy sediments and appear to have been formed as temporary tunnels as the animal forced itself through the sediment. The subsequent fate of the burrow depended on the properties of the sediment above and below the tunnel, and microbial binding may have played an important role in preservation (Gehling, 1999). Specimens preserved in negative relief on the sole of beds demonstrate that this type of trace formed within the sediment (Seilacher, 1999), but probably close to the sediment water interface. Bedding planes may be densely covered with these traces, often giving the impression of true branching. In no case can it be proven that these are not created by intersection of two traces or by secondary successive branching (cf., D'Alessandro and Bromley, 1987), where an animal has followed the course of an older trace. In principle, all of these forms may have been created by the same type of animal. The absence of ornamentation (i.e., scratch marks) must be understood as a preservational artefact and does not imply original absence of such structures.
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Of particular interest is the occurrence of rare moderately regular meanders (Narbonne and Aitken, 1990
). These can be included in Helminthorhaphe (sensu Uchman, 1995) though they do not have the closely guided meander systems of some Phanerozoic forms. In the Ediacara Member in South Australia, Helminthorhaphe occurs sparsely on surfaces densely covered by Helminthoidichnites and shares the same style of preservation. These guided meanders are not spurious because an exceptionally preserved specimen from the Flinders Ranges contains a Helminthorhaphe, which grades into an involute/evolute spiral (Runnegar, 1992a). The spiral is similar in shape and size to the holotype of Spirorhaphe involuta (Stefani, 1895) from the lower Tertiary of Karpathos, Greece (Fig. 2A). The next oldest occurrence of Spirorhaphe is from the Ordovician of New Brunswick (Pickerill, 1980).
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Dwelling burrows
The case for Neoproterozoic permanent or semi-permanent dwelling structures is problematic. Neoproterozoic globular to plug-shaped forms variously called Beltanelliformis, Beltanelloides, Nemiana and Hagenetta often show similarity with Phanerozoic reports of Bergaueria (Fedonkin and Runnegar, 1992
; Crimes, 1994). The latter is generally interpreted as a cnidarian burrow, formed by sea anemones (e.g., Alpert, 1973). The Neoproterozoic forms likely are body fossils rather than trace fossils, though this distinction may be difficult to recognize in practise (cf., Crimes and Fedonkin, 1996).
The case for Neoproterozoic dwelling burrows of bilaterian origin is problematic. Glaessner (1984, p. 66), compared short grooves with pointed ends from the Ediacara Member of South Australia to the base of U-shaped dwelling traces. These, however, are preserved in negative hyporelief with no additional extension into the sediment and therefore more likely are external molds of an organism (unpublished observation). Narbonne and Aitken (1990) figured specimens of Palaeophycus tubularis from the Mackenzie Mountains which are reasonable candidates for broad, flat U-shaped burrows, which may have been at least semi-permanently occupied. A similar observation can be made for some small traces often referred to Planolites ballandus and Planolites montanus (see Walter et al., 1989).
There appears, however, to be no Neoproterozoic evidence for accommodation to sediment influx, resulting in vertical sediment stacking (spreite). Also, the case for Neoproterozoic Skolithos (Crimes and Germs, 1982; Fedonkin, 1985) is doubtful. For example, Crimes and Fedonkin (1996) now consider that the Namibian specimens proposed to be Skolithos more likely represent body fossils. Grazhdankin and Ivantsov (1996, p. 677) maintained the trace fossil identification of Skolithos declinatus, inclined structures from the late Vendian of the White Sea area (Fedonkin, 1985), reporting densities of 245 burrows/dm2. More detailed information on this interesting form is needed.
Horizontal trace fossils with levees; Archaeonassa and Psammichnites
An animal that pushes itself through the upper portion of a sandy layer will displace sediment to either side. The morphology of this type of trace fossil will depend on how deeply the animal is submerged in the sediment (Fig. 2B), the sediment properties, as well as morphology of the animal. In sediments with high tensile strength, displaced sediment may rupture to form segmented levees (cf., Knox and Miller, 1985). Neoproterozoic trace fossils of this type have been reported as Aulichnites and Nereites (Narbonne and Aitken, 1990; Crimes and Germs, 1982; Jenkins, 1995), but they can all be included in the broadly defined Archaeonassa (cf.,Yochelson and Fedonkin, 1997). Reports of Neoproterozoic Nereites (Crimes and Germs, 1982; Jenkins, 1995) appear to be based on material in which the segmented lobes formed through rupture of sediment with higher tensile strength (cf., Knox and Miller, 1985). By contrast, the lateral lobes of Phanerozoic Nereites are constructional, formed by repeated probing in the sediment (e.g., Orr and Pickerill, 1996).
A slab from the Ust Pinega Formation, along the White Sea (Fig. 3A) shows several specimens of Archaeonassa that have a wide flat central area flanked by relatively narrow raised lobes. Other specimens demonstrate the great importance of trace morphology as a function of trace depth relative to the sand surface (Fig. 3B).
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The Neoproterozoic Archaeonassa are of particular interest in that they provide probable links to the Lower Cambrian "Taphrhelminthopsis" circularis (Fig. 4). Seilacher (1997)
, Zhu (1997) and Seilacher-Drexler and Seilacher (1999) explicitly, or implicitly, included the lower Paleozoic reports of Taphrhelminthopsis in Psammichnites, interpreting it as a collapsed top surface above the actual burrow. In the model of Seilacher (1997) these are internally complex trace fossils that are morphologically distinct, depending on what part of the trace fossil is exposed. However, most Lower Cambrian "Taphrhelminthopsis" circularis have been described only based on upper surface morphology with no information on internal structures (but see Hofmann and Patel, 1989; Hagadorn et al., 2000). Related Lower Cambrian forms include Protospiralichnus, Planispiralichnus and Qipanshanichnus (Crimes et al., 1977; Fedonkin, 1985; Luo et al., 1994).
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Valentine (1995)
hypothesized that most horizontal Neoproterozoic trace fossils were produced by mollusk-like animals. The flat surface between the levees in Archaeonassa from northern Russia (Fig. 3A), certainly is consistent with such a producer. Conway Morris and Peel (1995) derived molluscs and annelids from halkieriids or their ancestors. The body fossil Kimberella has recently been re-interpreted as a mollusc-like animal (Fedonkin and Waggoner, 1997). There has even been reports of Kimberella found in association with structures interpreted as radular scratch marks (Seilacher, 1997; Fedonkin, 2001). There have been no detailed documentation of the nature of this interesting association and the purported radular marks, but halkierids or proto-halkierids make one attractive search pattern for the producer of Archaeonassa-type trace fossils. |
PROBLEMATIC "TRACE FOSSILS" |
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Purported Neoproterozoic tightly guided meanders and fecal rows
Somewhat surprisingly there are reports of tightly guided Neoproterozoic meanders (e.g., Crimes and Fedonkin, 1994
). Such trace fossils contrast to models that depict evolutionary optimization of meander traces during the Phanerozoic with increasingly effective coverage starting from rather simple Cambrian specimens (Seilacher, 1974). Subsequent findings (see Crimes and Fedonkin, 1996) have shown that relatively closely guided meanders were present also in the Cambrian, but these are more loose than the Neoproterozoic forms. It now is clear that this interpretation of Neoproterozoic forms should be abandoned and that they probably are not trace fossils at all (e.g., Jensen, 1996; Seilacher, 1998; Gehling et al., 2000).
The most widely reported of these purported meander traces is Palaeopascichnus Palij, 1976, consisting of closely positioned wide or short, elongate bars or grooves arranged in straight or winding rows (Fig. 5B). In most cases the width of the segments remain about the same. Palaeopascichnus has a wide distribution including Russia (Fedonkin, 1981), the Ukraine (e.g., Palij, 1976), South Australia (Glaessner, 1969; Jenkins, 1995), Poland (Paczesna, 1986), and Newfoundland (Narbonne et al., 1987; Gehling et al., 2000).
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Palij (1976)
compared Palaeopascichnus to the larger Phanerozoic meander Helminthoida but noticed that Palaeopascichnus differs in that no turning points could be observed. Indeed, Fedonkin (e.g., 1981) has suggested that it was caused by an animal with a feeding organ that operated transversely to the animals' direction of movement.
Subsequently, "classic" meanders from the Neoproterozoic were reported as Yelovichnus gracilis (Fedonkin, 1985). On closer examination, however, there appear to be no examples in which the meander limbs are actually connected by turning points. A typical specimen (Fig. 5A) consists of closely juxtaposed flattened oval structures with narrow raised rims. Although not always apparent, in well preserved areas these rims form closed ovals (Fig. 5B). The apparent meanders are evidently an artifact arising from close juxtaposition of the oval units. A similar effect of raised rims occurs in Palaeopascichnus in the places where the elongate bars are missing (Fig. 5B). Identical rimmed units also are found scattered on sediment surfaces. Notably the same range of structures occur in Neoproterozoic strata of both the White Sea area and in the Ediacara Member of South Australia (cf., Jenkins, 1995, pl. 1d,i, pl. 2c). It therefore seems that Palaeopascichnus and Yelovichnus are related, and reflect different preservational styles of the same structure.
A further likely preservational variety of Palaeopascichnus occurs as low-relief elongate depressions in calcisiltites of the Wonoka Formation in South Australia (Haines, 2000).
Neoproterozoic simple and complex rows of pellet-shaped objects that have been assigned to Neonereites and interpreted as fecal pellets (e.g., Fedonkin, 1981), should also be compared to Palaeopascichnus. Important to this argument is the ‘metaphyte’ Orbisiana simplex Sokolov, 1976, from the Neoproterozoic of Russia. Specimens from the Neoproterozoic of the Kunevichi-4 core near Ladoga, western Russia, consist of rows or aggregates of objects delineated by a pyritic rim (Fig. 5C, D). There is a range of organization of the form, from biserial strings forming long chains to aggregates with an irregular outline. The exact shape of the objects are not clear at the present, but they appear to consist of aggregated spherical or hemispherical bodies. Size differs between specimens but appears to remain more or less fixed within a specimen (Fig. 5C, D). In the examined samples there are larger specimens with a diameter of spheres 0.4–0.9 mm, and smaller specimens with a diameter of 0.2–0.3 mm. Sediment within the spheres appears to be identical to that surrounding them. Forms that appear identical to Orbisiana have also been described by Chistyakov et al. (1984) from Neoproterozoic outcrops near lake Onega, western Russia. They noted that strands may be sequentially subdivided up to five times and consist of elements about 1 mm wide (Chistyakov et al., 1984; fig. 3B).
There is a strong resemblance between Orbisiana simplex and the various Neoproterozoic forms reported as Neonereites. For example, in Neonereites biserialis, figured by Fedonkin (1981, Pl. 14, 1–5; 1994, Fig. 6A) the sediment filled objects appear to have a narrow sediment-free zone separating each other. Orbisiana simplex with pyritized walls occurs in green fine siltstone, whereas Neonereites-type forms are found in fine sandstone, and so the two may be different preservational aspects of the same structure.
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An assignment of these objects is problematic. Haines (2000)
suggested that Palaeopascichnus from the Wonoka Formation should be compared with brown algae such as Padina rather than to trace fossils, and Runnegar (1995) suggested that they are stratiform stromatolites. Probably related, at least in terms of taphonomy, are Mesoproterozoic bedding plane markings from the Belt Supergroup, USA and the Bangemall Group, Australia, which have been interpreted as probable megascopic algae (Grey and Williams, 1990; Horodyski, 1993; for further discussion of these forms see also Fedonkin and Yochelson, 2002). Grey and Williams (1990) reconstructed these as hollow or fluid-filled spheres with toughened walls of probable algal affinity. A similar interpretation seem likely for Palaeopascichnus and Yelovichnus. More recently Leguta and Seilacher (2001) suggested that these are xenophyophoran protists.
For the purpose of this paper, exact affinities are not at issue, but it is clear that a trace fossil origin must be abandoned for Palaeopascichnus. Fedonkin's (1981) suggestion of vertical probes may need further consideration for certain forms, but the apparent gradation among the objects discussed above strongly argues for a non trace fossil origin for all of these "ichnogenera."
Harlaniella and other cork-screw shaped forms
From several Neoproterozoic sections, rope-like twisted worm-shaped objects have been reported. This structure, known as Harlaniella Sokolov, 1972, is interpreted by most specialists as a horizontal spiral trace fossil, somewhat comparable to such Phanerozoic ichnotaxa as Helicolithus and Helicorhaphe (e.g., Müller, 1971). In the basal Cambrian stratotype section at Fortune Head, southeastern Newfoundland, Harlaniella podolica is the nominative taxon for a Neoproterozoic trace fossil Zone (Narbonne et al., 1987). Other occurrences of Harlaniella podolica include Podolia, Ukraine (Kiryanov, 1968; Palij, 1976; Palij et al., 1979); Lublin, and eastern Poland (Pacze
na, 1986).
The spiral nature of Harlaniella appears to be based on inferences of the three dimensional structure from the rope-like surface morphology. However, Palij (1976, p. 73) noted that Harlaniella podolica is similar to Palaeopascichnus delicatus, and further (Palij, 1976, p. 74) that in Podolia, the two forms often are found next to each other and may be transitional. A similar relation appears to exist also in Newfoundland, where Palaeopascichnus and Harlaniella occur on the same bedding surfaces (Narbonne et al., 1987, fig. 6B). Palij's significant observation appears not to have been investigated further, and Harlaniella appears to be accepted as a spirally twisted trace fossil.
The trace fossil interpretation of Harlaniella, may need further consideration. This point may be illustrated with a specimen of Harlaniella from the Neoproterozoic of the Pasha core, south-east of lake Ladoga, western Russia (Fig. 6A, C), which was previously figured and described as Harlaniella sp. by Palij et al. (1979; pl. 63:3). This specimen is similar to occurrences of Harlaniella podolica from Podolia illustrated by Palij et al. (1979, pl. 50:1–3). In two regions, this specimen has inclined segments typical of Harlaniella, but between these are transverse segments identical to those seen in Palaeopascichnus (Fig. 6C). In one of the outer regions, the inclined segments turn sharply into an orientation nearly parallel to the structure. Overall, it is difficult to reconcile these observations with a spiral burrow.
A symmetrical spiral will follow a predictable course and so fossil examples can be tested against the expected model (Fig. 7). Application of this test to specimens of Harlaniella shows that there are problems with the interpretation of Harlaniella as a helical spiral. A specimen figured by Palij et al. (1979, pl. 50:3) has in its central part segments that are imbricated at about 45°. The width of the trace is about 1.1 mm. Assuming the extreme case that the width of spirally coiled burrow is 0.55 mm, the distance between successive turns in a symmetrical spiral will be about 1.73 mm. The distance between segments on the fossil range from 0.4–0.7 mm. Considering that the actual width of the burrow in the fossil probably was less than 0.55, it becomes even more difficult to make this a spiral. Measurements for the type material of Harlaniella figured by Sokolov (1972, 1973), yields similar or larger discrepancies between actual and predicted measurements (Fig. 7).
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Although compaction can be invoked, it would not result in a consistent change in angle. It must therefore be considered that Harlaniella is not a trace fossil. Its close association with Palaeopascichnus (see above) lends weight to this notion.
Large, apparently cork-screwed forms from South Australia (Form A of Glaessner, 1969, fig. 5A) are probably body fossils rather than trace fossils (Fedonkin and Runnegar, 1992).
Tubular body fossils
Preservation of tubular fossils as casts and molds on clastic bedding planes appear to have been locally common in the Neoproterozoic. These are easily confused with trace fossils. Glaessner (1977) discussed this problem in relation to whether Archaeichnium is a trace or body fossil, but this problem appears to be more extensive than has been realized previously. This includes forms reported as Taenidum and Muensteria from Namibia (Germs, 1972, pl. 2:2–3) which are casts of a tubular fossil identical or similar to Cloudina. Other examples of probable tubular body fossils are forms from South Australia that Jenkins (1995) identified as Taenidum isp., and Palaeophycus tubularis.
The difficulties involved in distinguishing trace fossils from elongate tubular body fossils can be illustrated by a specimen that has been previously identified as Planolites (Palij et al., 1979, pl. 63:3). This form shows a very gradual change in width which cannot be explained solely by a trace changing level (Fig. 7A). In addition, parts of the structure show complex internal fractures that are more consistent with fracture of a brittle body wall than breakage of trace fossil fill or lining (Fig. 7B). This structure is more readily explained as an elongate, gently tapering body fossil, than as a trace fossil.
In the examination of latest Neoproterozoic tubular structures preserved as casts or molds in siliciclastic sediment, more attention should probably be directed to the increasing diversity of tubular organisms recovered from the Doushantou Formation and other pre-Ediacaran biotas (e.g., Xiao et al., 2002).
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The view of Neoproterozoic trace fossils presented above removes some of the peculiarities of certain other schemes. With the re-interpretation of forms such as Palaeopascichnus there is no "extinction" of ichnotaxa at the end of the Proterozoic. This also leads to a decrease in the number of Neoproterozoic ichnotaxa. Furthermore, it is possible to relate Neoproterozoic trace fossils more easily to Cambrian forms, thus strengthening the case that they were produced by metazoans. Arguable some of the more simple Neoproterozoic trace fossils could have had non-metazoan producers (cf., Conway Morris, 1998
), but this cannot be the case for the more complex forms. At this point must be considered the problems involved in reading behavioral complexity from the morphology of trace fossils. There exist no simple extrapolation of behavior from trace fossil morphology, and even less a correlation to neurological sophistication of trace producer. It is, however, reasonable to argue that a complex trace fossil represent a more complex behavior than does a simple one. Miller (1998) suggested the useful terms "incidental" and "deliberate," where incidental denote a simple structure likely of one function and short occupation, whereas a deliberate trace fossils has a more complex architecture likely representing multiple functions and an extended time of occupation. One type of complex trace fossils are those that consist of numerous interconnected parts, of which Treptichnus is a relatively simple example. Closely guided regular meander traces is another type of trace fossil that requires a moderately complex sensory systems (e.g., Seilacher, 1967). The presence of trace fossils such as Spirorhaphe occurring with diverse Ediacaran forms suggests that bilaterian evolution was well under way at this stage. The Neoproterozoic trace fossils represent the initiation of a rapid but gradual build-up of infaunal activity, which increased markedly in the Cambrian.
Problems in precise correlation of sections means that only tentative suggestions can be made regarding the evolution of Neoproterozoic trace fossils. A preliminary attempt at such a correlation was presented by Budd and Jensen (2000; fig. 6).
Narbonne et al. (1987) erected the Harlaniella podolica Zone, corresponding to the upper part of the Neoproterozoic trace fossil zone 1 of Crimes (1987). This was defined by the occurrence of Palaeopascichnus and Harlaniella. As discussed above these probably are not trace fossils.
As previously discussed (Jensen et al., 2000), treptichnid trace fossils range below Treptichnus pedum in Namibia and Spain. Treptichnids recently were found below the first Treptichnus pedum in the Newfoundland GSSP, though the precise implications of this remain unclear (Gehling et al., 2001). Treptichnids overlap the range of Cloudina in both Namibia and Spain and thus far treptichnids have not been found in localities yielding diverse Ediacaran assemblages. The treptichnids in Namibia are bracketed by radiometric ages of 545 Ma and 548 Ma (Grotzinger et al., 1995) and so are very close to currently accepted age for the base of the Cambrian (e.g., Grotzinger et al., 1995). Other trace fossils that first appear at approximately this level are trace fossils with a three-lobed lower surface (see Jensen and Grant, 1998). This suggest a latest Neoproterozic trace fossil zone that may be correlative with a low diversity Nama association (Gehling and Narbonne, 2002).
Sections with diverse Ediacaran body fossils are dominated by simple horizontal forms and by the appearance of Archaeonassa. Radiometric dating of sections in the White Sea area suggest that these are younger than about 555 Ma (Martin et al., 2000). There are reports of simple trace fossils of Planolites type that are considered by some researchers to be older than the main Ediacaran assemblages in Australia (e.g., Walter et al., 1989; Jenkins et al., 1992). This correlation requires further confirmation but may suggest an even older zone with short simple trace fossils.
Tentatively it is therefore possible to recognize two, possibly three, latest Neoproterozoic trace fossil zones (Fig. 8). New finds of trace fossils and further re-evaluations of old material coupled with rapidly increasing new radiometric dates and revised correlations will test the validity of this scheme. It is probably safe to argue that trace fossils will continue to provide essential information on the early evolution of animals and that it will ultimately be possible to devise a more detailed and reliable Proterozoic zonation.
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ACKNOWLEDGMENTS |
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My sincere thanks to Graham Budd and Kevin Peterson for inviting me to this SICB Symposium, which ultimately triggered me to publish ideas that had been in manuscript form for years. For generously sharing their material, time in the field, and stimulating discussions my sincere thanks to Graham Budd, Simon Conway Morris, Mary Droser, Mikhail Fedonkin, James Gehling, Kaisa Mens and Bruce Runnegar. The majority of the research on which this paper is based was conducted when I was at the Department of Earth Sciences, University of Cambridge. I gratefully acknowledge funding from the Leverhulme Trust and NERC (grant GR3/10713 to Simon Conway Morris). During my stay at Riverside I have benefited from grants from National Geographic and the National Science Foundation (grant EAR-0074021 to Mary Droser).
Robert Gaines read an earlier draft and significantly improved the readability and language of this contribution. James Hagadorn and an anonymous reviewer are thanked for their insightful reviews.
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FOOTNOTES |
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1 From the Symposium The Cambrian Explosion: Putting the Pieces Together presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 2–6 January 2002, at Anaheim, California.
2 Present address: Area de Paleontologia, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain; E-mail: soren{at}unex.es
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SYNOPSISINTRODUCTIONCAVEATS OF TRACE FOSSIL...NEOPROTEROZOIC TRACE FOSSILSPROBLEMATIC "TRACE FOSSILS"DISCUSSIONReferences |
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