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The Ediacaran Biotas in Space and Time1
1 Department of Biology, University of Central Arkansas, Conway, Arkansas 72035-5003
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The "Ediacaran organisms," which preceded and overlapped the Cambrian radiation of metazoans, include many fossils whose systematic positions remain contentious after over fifty years of study. It might seem that nothing particularly useful can be learned from a biota full of oddballs. However, analyses of the distribution of the Ediacaran organisms in time and space can be carried out without having to guess at the systematic position of the organisms. Combining these results with data on paleotectonics, paleoenvironmental parameters, and the ages of various assemblages sheds light on the origins, ecology, and even the systematic positions of the Ediacaran organisms. Parsimony Analysis of Endemism (PAE) confirms earlier studies in grouping Ediacaran biotas into three major clusters: the Avalon, White Sea, and Nama Assemblages. The available radiometric and stratigraphic data suggest that the Avalon is the oldest, the White Sea is next oldest, and the Nama extends to the base of the Cambrian. The "frondlike" Ediacaran taxa, and to a lesser extent the "medusoids," collectively show significantly longer stratigraphic ranges, broader geographical and paleoenvironmental ranges, and less provinciality than "bilaterian" and tubular taxa. Almost all tubular Ediacarans appear to be confined to equatorial areas, whereas other Ediacaran organisms show weak or no latitudinal diversity gradients. I conclude that the Ediacaran organisms show a diverse range of responses to various environmental parameters. There is no basis for classifying them all as having a single body plan and mode of life, as has often been done in the past.
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INTRODUCTION |
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The "Ediacara biota" is an assemblage of enigmatic fossil organisms, mostly unmineralized but including a few calcareous and agglutinated taxa. Ediacaran fossils (Fig. 1) are known from nearly thirty localities on all six continents. Named for their most famous locality, the Ediacara Hills of southern Australia, the Ediacara biota first appeared in the late Neoproterozoic about 600 million years ago, and Ediacara-type fossils extend into the Cambrian. However, most Ediacaran organisms are between 565 and 541 million years old. (For recent reviews see Runnegar, 1995
; Waggoner, 1998; Narbonne, 1998; and papers in this issue.)
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These fossils have become notorious for attracting a great deal of taxonomic speculation. They were once generally considered to be metazoans in extant phyla, mostly cnidarians (e.g., Glaessner, 1984
). Various workers have reinterpreted some or all Ediacarans as metazoans in extinct phyla, lichens, unicellular protists, multicellular and colonial protists, and bacterial colonies. On at least three separate occasions, the Ediacarans have been grouped in a single extinct kingdom-level taxon. Even among those who support metazoan affinities, there is controversy. Some of the "frondlike" Ediacaran taxa, for example, have been theorized to be the ancestors of complex bilaterians (Dewel, 2000), while others have argued that the same fossils are conventional crown-group anthozoans (e.g., Jenkins, 1985, 1992) or unusual benthic ctenophores (Dzik, 2002).
With little secure knowledge of the systematic affinities of most Ediacaran organisms, biologists might prefer to simply ignore them. Conway Morris (1993) described the Ediacaran fossils as being "in imminent danger of elevation to a classic status as evolutionary enigmas," and a reviewer of a recent book wrote that "there is now an intellectual free-for-all on the Ediacarans, unconstrained by much in the way of data" (Palmer, 1998). However, it would be a mistake for neontologists to ignore the Ediacaran organisms. There are a number of cases in the history of paleontology in which problematic fossils were extensively used in ecological and evolutionary studies before anyone could be reasonably certain of their systematic affinities, even at the kingdom or phylum ranks. Conodonts and archaeocyathids, for example, provided phylogenetic, stratigraphic and ecological information long before their taxonomic position was determined (e.g., Hass, 1962; Sweet, 1988; Rowland, 2001). Acritarchs continue to be used in studies of Proterozoic paleoecology and stratigraphy, even though their systematic affinities are still not well understood (e.g., Butterfield, 2001). It is equally possible to apply data from Ediacaran fossils to broader issues in the evolution of the Earth and life before the Cambrian.
One way of studying Ediacaran fossils without having to resolve their systematic position is to search for patterns in the distribution of Ediacaran taxa over space and time. Although taphonomic and collection biases can and do obscure these distribution patterns, they remain a key source of information about the ecological context in which the Cambrian radiation took place. Biogeographic, systematic, and paleoecologic hypotheses based on Ediacaran fossil distribution patterns can be continually tested as more fossils are described and more localities are opened. The pattern that I will focus on in this paper is the pattern of similarities and differences among different Ediacaran biotas, represented by a biogeographic area cladogram. Ultimately this pattern has three major sources: the tectonic history of the Earth; speciation and extinction in Ediacaran lineages over time; and ecological responses to environmental factors, ranging from global patterns such as latitudinal biodiversity gradients to smaller-scale patterns resulting from community-level structuring (e.g., Clapham and Narbonne, 2002). All three of these influence the final form of an area cladogram. Two areas might have similar Ediacaran assemblages because they shared a unique tectonic history (such as being on the same continent, or on separated parts of a continent that had recently split). However, two areas might also have similar assemblages, and cluster on a biogeographic cladogram, simply because they had similar paleoenvironments, even if there is no geographic signal in the dataset (for instance, if all Ediacara taxa were limited by certain environmental factors but were otherwise extremely widespread geographically, with no regional endemism at all). Two areas might also have clustered together because their biotas date from the same time period. Previous biogeographic analyses of the Ediacaran organisms have focused primarily on relating distribution to tectonics (Waggoner, 1999); the other two sources of biotic similarities have not received equal attention. This paper is an attempt to begin disentangling the effects of ecological, temporal, and historical controls on Ediacaran biogeographic patterns. With an area cladogram in hand, it becomes possible to plot various variables on the cladogram and look for correlations. It is possible to test questions such as: Does one area clade consist predominantly of localities with one paleoenvironment? Is one grouping significantly older than another? Which Ediacaran taxa are restricted to particular biogeographic assemblages?
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METHODS |
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Data
I drew up a binary data matrix showing the presence or absence of 70 Ediacaran genera or presumed genus-level taxa at 15 different localities. In my previous analysis, assemblages from multiple time horizons and/or geographically close localities were lumped together. In the present data set, biotas were subdivided geographically and stratigraphically, giving 21 terminal taxa in all. This data set grew out of the "compromise" data set presented in Waggoner (1999)
, with a number of additions and updates made (Jenkins, 1995; Gehling et al., 2000; Grazhdankin, 2000; Grotzinger et al., 2000; Hagadorn and Waggoner, 2000; Hofmann and Mountjoy, 2001; Narbonne et al., 2001; Dzik and Ivantsov, 2002). (The data set and full annotations are available on the Internet at http://faculty.uca.edu/~benw/data/, or on request from the author.) For many of the analyses, I partitioned the data set into four major groups: discoidal medusoids, flattened fronds, bilaterians, and tubular taxa. A few genera were not included in any of these groups. (Table 1) These names, which are fairly commonly used in the literature, should not imply anything about the organisms' systematic affinities: the medusoids are not necessarily cnidarians, and the bilaterians are not necessarily animals. Nor do these groupings necessarily represent clades of organisms; they represent "functional groups" that shared an outwardly similar form, growth pattern, and set of functional constraints.
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Using the paleotectonic reconstruction of Smith (2001)
for the "Early Vendian" (Fig. 4), I estimated the paleolatitude of each Ediacaran locality to the nearest five degrees. I tested for simple linear correlations between the absolute value of the paleolatitude and the number of genera in each locality, using both total genera worldwide and the number of genera in each partition.
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: 3 = Summer Coast, White Sea; 4 = Winter Coast, White Sea (Members 1, 9, and 11); 5 = Urals; 6 = Podolia, Ukraine; 7 = Finnmark, Norway; 8 = Olenëk Uplift, Siberia; 9 = Wernecke Mountains, Canada; 10 = Ediacara Hills, Australia (lower member and main member); 11 = Central Australia. Nama Assemblage 
: 12 = Namibia (Kuibis and Schwartzrand Groups); 13 = Mojave Desert; 14 = British Columbia; 15 = South ChinaAnalyses
I used the computer program PAUP 3.1.1 (Swofford, 1992
) to draw up an area cladogram from the entire data set, treating localities as taxa and the presence or absence of taxa as characters. This method, Parsimony Analysis of Endemism (PAE), does not depend on having a phylogeny of the organisms themselves. A drawback is that it cannot distinguish between similarity among localities caused by vicariance, and similarity among localities caused by several taxa dispersing along the same route (geodispersal; see Lieberman, 2000). However, although PAE has less power than phylogeny-based methods, it remains useful in cases where the organismal phylogeny is unknown. I used bootstrapping and decay indices to assess branch support; decay indices were calculated using the program TreeRot (Sorenson, 1996). The shortest trees were imported into the computer program MacClade 3.08 (Maddison and Maddison, 1999) and compared with data on the estimated paleolatitude, depth, lithology, and other paleoecological factors for each locality. I deleted uninformative taxa and then created a set of 5,000 random trees using the program MacClade 3.08. I calculated the mean consistency index and standard deviation for each Ediacaran taxon over the full set of random trees, and compared these with the actual consistency index for each taxon. An Ediacaran genus with a consistency index (CI) on the true tree that is not significantly different from its mean CI over the random tree set (CIrandom) is essentially randomly distributed. On the other hand, a genus with a non-random distribution should have an actual CI significantly greater than its CIrandom. In this analysis, a genus with a CI greater than two standard deviations above its CIrandom was considered to be significantly non-randomly distributed, whereas a genus with a CI less than one standard deviation above its CIrandom was considered to be randomly distributed.
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RESULTS |
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Cladistic analyses
Parsimony analysis yielded 211 trees, and the consensus did not resolve very well (Fig. 2). I reweighted the data by the rescaled consistency index and reran the analysis; this yielded two most parsimonious trees, which were identical with two of the original 211 shortest trees. Reweighting a second time gave a single most parsimonious tree (Fig. 3), which was identical to one of the 211 most parsimonious trees from the unweighted data set and to one of the two shortest trees from the first reweighting. This tree was used in all analyses presented here. Repeated reweighting and searches did not change the tree topology.
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The pattern of area relationships in this cladogram is substantially identical to that documented in earlier analyses (Waggoner, 1999
; Gehling, 2001). The biotas of Australia and eastern Europe make up one cluster. The relatively understudied assemblage from the Olenëk Uplift of Siberia goes with this cluster, along with at least one of the assemblages from northwestern Canada. All these biotas have been grouped as the White Sea Assemblage by Gehling (2001). Characteristic fossils of this assemblage include the bilaterally symmetrical forms, such as dickinsoniids, Kimberella, and Spriggina; the annulated concentric forms Kullingia and Ovatoscutum; and all but one of the triradially symmetrical discoidal forms, such as Tribrachidium. The biotas from Namibia are quite different, clustering with the assemblage from the Mojave Desert of the southwestern United States, and with the biota from south China. New discoveries in British Columbia now link that biota with those of Namibia as well (Hofmann and Mountjoy, 2001). This assemblage, named the Nama Assemblage by Gehling (2001), is dominated by mineralized and agglutinated taxa, notably Cloudina, Archaeichnium and Namacalathus, and by the endemic frondlike fossils Ernietta and Swartpuntia. Finally, the biotas of Newfoundland and central England make up the very distinctive Avalon Assemblage, characterized by the lobate "medusoid" Ivesia and by unique frondlike fossils such as the bushlike Bradgatia and the unnamed "spindle." The deep-water biota from Sekwi Brook, northwestern Canada, is the only biota that does not clearly belong to any of these three assemblages. The Sekwi Brook biota is dominated by medusoids, most of which have been synonymized (Gehling et al., 2000); thus it has very little cladistic signal. None of the fossils reported so far would disqualify the Sekwi Brook biota from membership in the White Sea Assemblage, which might be predicted on paleogeographic grounds; in fact, the Sekwi Brook biota includes Windermeria, a probable relative of Dickinsonia (Narbonne, 1994), which further suggests that its closest affinities are with the White Sea assemblage.
When plotted on a recent reconstruction of continent position in the early Vendian (Fig. 4), the localities form a consistent pattern. The Avalon localities are close to each other, whereas the White Sea assemblage is much more widespread. The White Sea assemblage appears discontinuous, with the biotas of Australia well separated in space from those on Baltica. Waggoner (1999) pointed out a similar disparity between continent position and biotic similarity during the Vendian, and suggested that it reflected a much earlier origin and relict distribution for the Ediacaran organisms. However, relatively few White Sea Assemblage localities are located in equatorial waters; the assemblage seems to be distributed in the temperate latitudes of both hemispheres. The Nama Assemblage is centered on the Equator. This is especially interesting because the Nama Assemblage is the only one known to include biomineralized taxa—both calcareous forms such as Cloudina, Wyattia, and Namacalathus, and agglutinated tubular taxa such as Archaeichnium and Onuphionella.
It should also be noted that the Ediacaran assemblages are quite different from Cambrian biogeographic provinces. The Mojave Desert and British Columbia, for example, share a Nama-type Ediacaran assemblage with southwest Africa, but share "olenellid realm" Lower Cambrian trilobites with the rest of Laurentia, Baltica, and Siberia. Australia has a White Sea Ediacaran assemblage in common with Baltica, but a "redlichiid realm" Cambrian trilobite assemblage shared with China and peri-Gondwana, and an archaeocyathid biota shared with South Africa (Waggoner, 1999; Brock et al., 2000; Lieberman, 2001). This may reflect an earlier origin for the Ediacara organisms than the current fossil record can show, with their distribution reflecting a pre-Ediacaran continental arrangement (Waggoner, 1999).
Ecology of Ediacaran taxa
The distribution of geologic facies on the cladogram is at least partially correlated with the clustering scheme of the cladogram. At the Avalon Assemblage localities, fossils are associated with volcanic ash laid down in tectonic basins, either in deep water (Newfoundland) or in water of uncertain depth (Charnwood Forest). All localities in the Nama Assemblage include mineralized fossils found in carbonates; Ediacaran fossils are otherwise found in carbonates at only one other locality, on the Olenëk Uplift of northern Siberia. This makes it difficult to tell whether the cladogram's branching pattern is created by tectonics or by environmental similarity; two localities could have similar taxa because they had similar environments, not because they were close to each other. Confounding the problem is the fact that the paleoenvironment affects fossil preservation; it is always possible that a fossil taxon was present in a given locality but not readily preserved under those environmental conditions.
However, environmental factors cannot be the sole reason why these localities cluster the way they do. The Nama Assemblage, for instance, includes unique taxa found predominantly in carbonates (e.g., Cloudina, Namacalathus) but also taxa found in clastics (Ernietta, Swartpuntia, and other tubular fossils such as Archaeichnium). If paleoenvironment were the sole determinant of what fossils are found where, we might expect that the sandstone beds of Namibia should contain forms such as Dickinsonia and Kimberella, which are common in sandstone event beds in the White Sea Assemblage. The fact that taxa unique to the Nama Assemblage are found in several facies, and the fact that a few taxa are found in more than one facies within the Nama Assemblage (e.g., Cloudina, "smooth tubes"; see Hagadorn and Waggoner, 2000) suggest that paleoenvironment is not the most important determinant of what fossils are found in an assemblage. This is confirmed in the White Sea Assemblage. The biotas on Baltica do not form a single monophyletic clade; this is probably partly due to insufficient sampling at some localities (such as the Ural Mountains and Finnmark) but also partly due to paleoenvironmental factors. The assemblages preserved on the undersides of sandstone beds in relatively shallow-water facies (Ediacara Hills, White Sea Members 9 and 11) group together, apart from the assemblages preserved in deeper-water, fine-grained shales (Podolia, White Sea Member 1). This suggests that paleoenvironmental factors such as depth and sediment composition did affect the composition of local biotas. However, many taxa are shared between sandstones and shaly facies on Baltica, such as Tribrachidium, Dickinsonia, and Kimberella. The Baltica localities still form a clade with each other and with Australia, Siberia, and northwest Canada. Paleoenvironment influences the branching pattern of the area cladogram, but it does not determine the large-scale pattern of relationships.
When we examine the distribution of individual taxa in different sedimentary environments, other patterns emerge. Bilaterian and tubular taxa tend to be restricted to particular sedimentary environments. For example, there is only one bilaterian fossil from a slope setting (Windermeria from Sekwi Brook, Canada) and none known from carbonates, whereas Namacalathus, included in the tubular taxa, is currently known only from thrombolitic-stromatolitic carbonates (Grotzinger et al., 2000). Common medusoids and fronds, however, do not generally show this restriction. The frondlike Pteridinium, for example, is found in relatively shallow-water sandstone event beds (Namibia) and in deep-water shales (North Carolina; see Gibson et al., 1984). Charniodiscus ranges from deep-water ash beds (Newfoundland) to fine shales below wave base (White Sea Member 1) to shallow-water sandstone beds (Ediacara); Charnia is found in all of these settings as well as in platy carbonates (Olenëk). The medusoid Aspidella is virtually ubiquitous; Hiemalora, Nemiana, Eoporpita, and Tribrachidium also are distributed in a wide range of facies.
Temporal development of the Ediacara biota
Simple Ediacaran organisms are known from about 600 million years ago (Hofmann et al., 1990), but the earliest complex Ediacaran organisms and diverse biotas have been dated to 565 million years (Benus, 1988). Ediacaran biotas persisted up to the base of the Cambrian (Narbonne et al., 1997), with a few holdover taxa carrying on into the Cambrian (e.g., Jensen et al., 1998; Crimes and McIlroy, 1999; Hagadorn et al., 2000). Unfortunately, many key localities have not yet been radiometrically dated, and there is as yet no complete framework for understanding how the Ediacara biota changed with time. However, when the available radiometric dates are placed on the cladogram, together with dates estimated from chemostratigraphy or other correlations, a pattern emerges. The Avalon Assemblage includes the oldest biotas, with known ages of 565 million years (Ma) for the Mistaken Point assemblage of Newfoundland (Benus, 1988) and 559 Ma for fossiliferous rocks at Charnwood Forest, England (Compston et al., 2002). The Nama Assemblage includes the youngest biotas, ranging from 548.8 Ma up to the base of the Cambrian at 543 Ma (Grotzinger et al., 1995; Hagadorn and Waggoner, 2000). This has recently been confirmed by radiometric dates from the Ara Group of Oman, which has the Nama-type fossils Cloudina and Namacalathus extending just above a bed dated at 543.2 Ma (Grotzinger et al., 2002). The biotas of the White Sea Assemblage are of intermediate age, with known radiometric dates of 555.5 and 551 million years for Baltica (Martin et al., 2000) and an estimate of about 550 million years for the northwestern Canada fossils, based on isotope correlations (Kaufman et al., 1997). This creates another problem in interpreting the cladogram: We cannot tell whether the pattern of area relationships is generated primarily in time, or in space. In other words, the pattern of the cladogram could be generated by independent evolution in three biogeographically separate regions; or it could result from a single, globally homogeneous biota that changed over time, with each of the three assemblages representing that biota in a different time period.
Checking the cladistic branching order of the biotas against their actual or relative ages allows us to test the hypothesis that similarity between biotas is caused by closeness in time; if the branching order is not correlated with age, then it is likely that biotas are clustering because of geographic proximity and/or ecological similarity. A complete test is not yet possible, but there are three cases in which sets of three or more biotas can be placed in temporal order, because they are part of one stratigraphic sequence and/or because they can be correlated independently. The biotas of Namibia and the Mojave Desert show a branching order on the cladogram that is fully consistent with the temporal order: the oldest biota (Kuibis Group) branches before the younger biotas (Schwartzrand Group and Mojave Desert, both of which appear just before the Cambrian). The Avalon biotas are not stratigraphically consistent; the youngest biota, Charnwood Forest, branches before the older biotas from Newfoundland. In the case of localities on the White Sea coast of Russia, the branch order is not perfectly stratigraphically consistent, with the lowest biota (Summer Coast) more derived than the next oldest biota (Member 1, Winter Coast). However, the younger biotas on the Winter Coast of Russia (Member 9 and Member 11) are in the most derived position. The Wernecke Mountains biota of northwestern Canada makes up a more basal branch than its estimated age of about 550 million years would suggest. However, the Wernecke biota is less accessible and has not been collected as thoroughly as the biotas from Ediacara and the White Sea coast of Russia, and its age is not as closely constrained. The partial consistency between cladistic and stratigraphic order suggests that the area cladogram partially reflects the real pattern of biota formation over time, but that environmental factors and geographic proximity also influence the topology of the area cladogram.
Taxon partition analyses
Comparing the character support indices across various partitions showed that the fronds and medusoids are more homoplastic than the mineralized taxa or bilaterians. When autapomorphic taxa (with CIs of 1.0) are discarded, fronds and medusoids have mean CI values that are lower than the mean CIs for bilaterian and tubular taxa (Table 2). One-tailed unpaired t-tests revealed that fronds are more homoplasious than bilaterians, but this result was only weakly significant (P = 0.08, df = 5). Medusoids are also more prone to homoplasy than bilaterians, but again, the result is not strongly significant (P = 0.08, df = 5). Other differences in CI partitions were not statistically significant, but the number of taxa may be too small for an accurate assessment. The same pattern is reflected in the percentages of taxa endemic to single assemblages: 25% (5/20) of the frondlike genera are found in more than one assemblage, as compared to 15% (3/20) of the medusoids and 14.3% (1/7) of the tubular taxa. None of the bilaterian genera (0/8) is found in more than one assemblage.
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Analysis of consistency indices of taxa over random tree sets confirms that the frondlike taxa are more homoplasious than the norm. When autapomorphic genera were excluded, seven frondose genera out of twelve, or 58.3%—Glaessnerina, Rangea, Charniodiscus, Pteridinium, Phyllozoon, Inkrylovia, and Ernietta—were essentially randomly distributed, whereas only three fronds, or 25%—Bradgatia, Swartpuntia, and the unnamed "spindle"—were significantly non-randomly distributed. Four medusoids out of fourteen (35.7%) were indistinguishable from random: Anfesta, Chondroplon, Aspidella, and ‘Aspidella hatyspytia,’ whereas six (42.9%) were significantly non-random (Tribrachidium, Ovatoscutum, Kullingia, Conomedusites, Hiemalora, and Ivesia). One bilaterian out of four (Spriggina) and one tube out of five (Namacalathus) were randomly distributed; all other bilaterians and tubes were significantly non-randomly distributed. This confirms that fronds are the most homoplasious group on the tree: over half of the frond taxa that are not unique to one locality are randomly distributed on the cladogram. All taxa with CIs indistinguishable from random also had RIs (retention indices) of zero, confirming that this is a suitable test of how phylogenetically informative a taxon is.
Many extant marine taxa show latitudinal diversity gradients, in which the highest diversity is found in lower latitudes—although there are a number of interesting exceptions, and the causal factors are not always certain (see Rohde, 1992; Roy et al., 1998). I plotted the diversity of the four groups (Fig. 5) against the absolute value of the estimated paleolatitude. The only group that showed the typical latitudinal diversity gradient was the tubular fossils. With one exception—the organic-walled sabelliditids, which were not subdivided into genera for purposes of this study—tubular fossils are restricted to within 20 degrees of the equator, and this gradient is statistically significant even if tested with just a linear correlation. Medusoid and bilaterian taxa show the opposite gradient; they tend to be most diverse in higher latitudes. However, this is not statistically significant for the bilaterians and only weakly significant for medusoids. Finally, the frondlike taxa show no diversity gradient at all: the plot of frond diversity versus paleolatitude is pure scatter.
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One reviewer of this paper pointed out that localities that I have placed in the White Sea Assemblage contain a number of "Avalon-type" taxa (e.g., Charnia, Charniodiscus) and "Nama-type" taxa (e.g., Pteridinium, Rangea). However, it is precisely these taxa whose distribution patterns appear essentially randomized: they are known from a wide range of paleoenvironments, span a wide time range, and show no evidence for latitudinal diversity gradients or other ecological controls on distribution. They should not be thought of as "Avalon" or "Nama" taxa, but rather as part of a widespread "core Ediacaran assemblage," whose distribution is not influenced by the same factors—whether tectonics, time, or environment—that affected the distribution of other Ediacaran organisms.
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CONCLUSIONS |
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Tectonics, environmental factors, and successive evolutionary developments all affected the distribution of Ediacaran taxa. It is not possible to completely disentangle how all three of these affected the evolution and distribution of Ediacaran organisms. However, tectonics appears to have set the broad-scale pattern for Ediacaran biogeography; both temporal succession and paleoenvironments have had important effects within this broader pattern.
The older fronds and simple medusoids have the widest distribution, the widest niches, and the lowest provinciality. They appeared first, in interglacial or post-glacial environments in deep water, between 600 and 560 million years ago. They later colonized environments around the world in a variety of facies, both clastic and carbonate, in both shallow and deep water; they also show little geographic endemism. So far as is known, they are also the last to disappear, with a number of Ediacara-type fronds and medusoids known from unequivocally Cambrian rocks. (Note that some medusoid taxa, possibly many, may be the basal holdfast organs of fronds; see Gehling et al., 2000.)
Bilaterians and tubular taxa, on the other hand, appear later in the fossil record, exclusively in the White Sea and Nama assemblages; the oldest dated bilaterians are 555 million years old. These are much more restricted in their distribution, possibly because they did not have as much time to disperse as the fronds and medusoids, but probably also because of environmental restrictions on their range. This implies that they responded to their surroundings in different ways from the more tolerant medusoids and fronds. A few medusoid and frond taxa, such as Ivesia, Tribrachidium, Swartpuntia and Ernietta, show similar geographic and temporal restrictions.
Several Ediacaran bilaterians show signs of motility, muscular contraction, and behaviors such as epifaunal grazing (Seilacher, 1997, 1999; Ivantsov and Fedonkin, 2001; personal observations). The "tubular" fossils also show evidence for behaviors more typical of Phanerozoic marine animals: predation (Bengtson and Zhao, 1992), epibiosis (Germs, 1972), and a positive latitudinal gradient. Evidence for these phenomena is lacking in Avalon biotas. Correlated with this is the fact that trace fossils are rare or absent in the Avalon Assemblages; clear evidence for a vagile benthos is only present in the White Sea and Nama Assemblages. This means that later Ediacaran biotas in the White Sea and Nama assemblages were ecologically more like Phanerozoic marine biotas than the Avalon assemblage.
Until fairly recently, the "default option" has been to place all the Ediacaran fossils in one kingdom, whether extant or not, and to see them all as reflecting the same fundamental Bauplan. However, my results imply that the Ediacara biota was heterogeneous in both time and space. The community structure and ecological processes of, say, the Mistaken Point biota of Newfoundland would not necessarily have applied to the biota of the Ediacara Hills of Australia, the Winter Coast of the White Sea, or the Schwartzrand Group of Namibia. This implies that the organisms were not taxonomically a homogeneous group, either; it should not be assumed that all shared a single basic structural plan. Whatever the Ediacaran organisms actually were, various taxa were distributed differently, responded differently to their environments, and appeared and disappeared at different times. It is inaccurate to treat the "Ediacara biota" as a single biota, except in the very broadest sense.
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ACKNOWLEDGMENTS |
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I thank R. Dewel for organizing this SICB symposium and for inviting me to participate, and two anonymous reviewers for their helpful comments. An earlier version of this work was presented at the 2001 Geological Association of Canada / Mineralogical Association of Canada meeting (St. John's, Newfoundland); I thank G. Narbonne and J. Gehling for organizing the special session and field trip at the meeting, and all participants for the stimulating discussions during the meeting and field trip.
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FOOTNOTES |
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1 From the Symposium New Perspectives on the Origin of Metazoan Complexity presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 2–6 January 2002, at Anaheim, California.
2 E-mail: benw{at}mail.uca.edu
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References |
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SYNOPSISINTRODUCTIONMETHODSRESULTSCONCLUSIONSReferences |
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Bengtson, S., and Y. Zhao. 1992. Predatorial borings in Late Precambrian mineralized exoskeletons. Science, 257:367-369.
Benus, A. P. 1988. In E. Landing, G. Narbonne, and P. Myrow (eds.), Trace fossils, small shelly fossils, and the Precambrian-Cambrian boundary. Bulletin 463, New York State Museum, Albany, New York.
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