Paleobiology, 30(4), 2004, pp. 543–560
Relative abundance of Sepkoski’s evolutionary faunas in
Cambrian–Ordovician deep subtidal environments in
North America
Shanan E. Peters
Abstract.—The relative proportions of Sepkoski’s Cambrian, Paleozoic, and Modern evolutionary
faunas in Cambrian–Ordovician benthic marine assemblages from mixed carbonate-shale and
shale lithofacies deposited below normal wave base (herein, deep subtidal) in North America are
strongly positively correlated with global relative genus richness in Sepkoski’s global compendium.
The correlation between local and global faunal proportions is robust regardless of how proportions are calculated, including when local proportions are based on number of specimens. Like the
global pattern, the transition between the Cambrian and Paleozoic evolutionary faunas appears to
occur gradually, in that Lower Arenigian (Ibexian) deep subtidal assemblages contain approximately equal proportions of Cambrian and Paleozoic faunal elements. In agreement with previous
work, an onshore-offshore differentiation of faunas is evident both within Ordovician deep subtidal
communities and across a larger environmental gradient.
Within the deep subtidal assemblages studied here, the Paleozoic fauna tends to have a greater
proportion of individuals for a given proportion of genera than the Cambrian fauna, although both
tend to accrue genera at similar rates with increasing relative abundance. The Modern evolutionary
fauna appears to accrue genera more rapidly with increasing local relative abundance. The extent
to which these differences reflect ecological factors such as biomass, metabolic requirements or
larval recruitment patterns, taxonomic practices stemming from variable morphospace saturation,
or taphonomy-related counting biases remains unclear, but it suggests the possibility that Sepkoski’s evolutionary faunas may share ecological characteristics that influence both local relative
abundance and global rates of taxonomic evolution.
Shanan E. Peters. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637.
E-mail: shananp@umich.edu
Present address: Department of Geological Sciences and Museum of Paleontology, The University of Michigan, Ann Arbor, Michigan 48109
Accepted:
21 April 2004
Introduction
One of the most striking patterns to emerge
from a global compilation of fossil first and
last occurrences is based not on absolute taxonomic counts, which are sensitive to, among
other factors, variation in the amount of exposed sedimentary rock (e.g., Raup 1976; Peters and Foote 2001, 2002; Smith 2001), but
rather on their relative numbers, which are
unbiased by sample size. Sepkoski (1981)
identified the Cambrian, Paleozoic, and Modern evolutionary faunas (EFs) as statistically
defined groups of taxa that exhibit covarying
richness patterns and that sequentially replace
one another as the principal faunal elements
during the Phanerozoic (Fig. 1). As such, the
evolutionary faunas first emerged as little
more than a mathematical description of a
stage-by-class family richness matrix. Despite
this abstraction, Sepkoski’s three great evoluq 2004 The Paleontological Society. All rights reserved.
tionary faunas seemed to resonate with many
paleobiologists’ intuitive understanding of
the large-scale fossil record. Perhaps for this
reason, the ecological and evolutionary significance of the three faunas has been explored
and discussed extensively at many temporal
and spatial scales (e.g., Sepkoski 1991; Ausich
and Bottjer 1982; Sepkoski and Sheehan 1983;
Sepkoski and Miller 1985; Bambach 1985,
1993; Westrop et al. 1995; Westrop and Adrain
1998; Li and Droser 1999; Droser and Finnegan 2003).
Identifying a profoundly interesting temporal pattern is important, but distinguishing
possible underlying evolutionary mechanisms
is often considerably more difficult. Sepkoski
hypothesized that his three faunas formed
macroevolutionarily coherent groups, each
with different carrying capacities and intrinsic rates of diversification. Using this theoret0094-8373/04/3004-0004/$1.00
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SHANAN E. PETERS
FIGURE 1. Sepkoski’s evolutionary faunas and unassigned genera for stages in the Phanerozoic. A, Total global
genus richness, B, relative global genus richness. Data from Sepkoski’s (2002) global compilation of marine animals
and animal-like protists. Because stage-to-stage variation in sample size does not influence proportion estimates,
total richness, rather than boundary-crosser richness, is shown and was used to calculate proportions. Dark area,
Cambrian EF; stippled area, Paleozoic EF; gray area, Modern EF; unfilled area, unassigned genera. Notice that the
proportion of unassigned genera has not changed substantially since the Middle Cambrian and that the Cambrian
and Ordovician Periods witnessed a rapid transition between the Cambrian and Paleozoic EFs. Data are plotted
for stratigraphic stages treated as having unit length.
SEPKOSKI’S EVOLUTIONARY FAUNAS
ical framework, Sepkoski explained the major
features of each fauna’s richness history on the
basis of just a few simple parameters and several perturbations (Sepkoski 1979, 1984). Alroy (2004) questioned the extent to which genera (as opposed to the original level of analysis, families) within each evolutionary fauna
exhibited the predicted negative (and possibly
competitive) interactions specifically predicted by Sepkoski’s three-phased coupled logistic model, but he pointed to ecological factors,
such as the Mesozoic marine revolution (Vermeij 1977), as plausible explanations for some
of the globally apparent genus-level behavior
of the faunas. Indeed, ecological interactions,
both within and between evolutionary faunas,
are implied by Sepkoski’s carrying capacities.
However, quantitatively projecting global
patterns into local communities and positing
ecological interactions as a causal mechanism
for those global patterns can be problematic
(e.g., Gould 2000), particularly without at least
demonstrating the potential for the ecological
interactions in the first place. In some cases,
the link between global richness patterns and
local biotic interaction has been made quite effectively (e.g., Vermeij 1977; Lidgard et al.
1993; Sepkoski et al. 2000). In others, the commonly inferred link has been criticized (e.g.,
Gould and Calloway 1980, but see Sepkoski
1996). Sepkoski certainly recognized the potential importance of local, community-level
observations in buttressing an argument for
ecologically mediated interactions having
global macroevolutionary consequences (Sepkoski and Sheehan 1983; Sepkoski and Miller
1985; Sepkoski 1991, 1996; Sepkoski et al.
2000). Nevertheless, relatively little is known
about how the global relative genus richness
of the three faunas is related, quantitatively, to
their relative richness and abundance in local
communities. Because global relative genus
richness can be manifested in local communities in any number of ways, including those
that demand ecological interactions (or lack
thereof) completely opposed to what might be
predicted on the basis of global data, understanding the local-global relationship among
the faunas is important in constraining the
range of hypotheses that might unite them
evolutionarily.
545
Here I do not attempt to demonstrate ecological interactions between the evolutionary
faunas in local communities, nor do I examine
the extent to which their ecological aspects
might be driving global patterns of relative
richness. Instead, I tackle the rather modest
question of whether interactions between the
faunas, as might be inferred from their relative
richness globally, are consistent with their local relative proportions. That is, this study
seeks to test the extent to which Sepkoski’s
global observations on the relative richness of
the three faunas are quantitatively relevant
within individual fossil assemblages. I do this
by comparing the relative genus richness of
the evolutionary faunas in Sepkoski’s (2002)
global genus database to the relative genus
richness and relative abundance of the faunas
in local Cambrian and Ordovician benthic
communities occupying environments located
below normal wave base (herein, deep subtidal). I also focus on perhaps the more important and interesting relationship between local relative abundance and local relative genus
richness within each of the three faunas. I also
briefly visit the question of their onshore-offshore distribution (Sepkoski and Sheehan
1983; Sepkoski and Miller 1985). The Cambrian and Ordovician periods were chosen for
this study because a dramatic shift between
the trilobite-dominated Cambrian EF and the
articulated brachiopod-dominated Paleozoic
EF, as well as an initial radiation of the Modern EF, occurred during this time and because
the Ordovician represents one of the most important transitions in the history of marine
animal life (Sepkoski and Sheehan 1983; Droser et al. 1996; Miller 1997). Awareness of how
global patterns in the evolutionary faunas are
manifested in local fossil assemblages is important for understanding the Ordovician
community reorganization as well as the relationship between synoptic global patterns
and local communities in general.
Data and Methods
Sixty-three samples from well-preserved
Cambrian and Ordovician assemblages deposited in storm-dominated, mixed carbonate-shale and shale facies located below normal wave base (above to below maximum
546
FIGURE 2.
SHANAN E. PETERS
Map of locations used in this study.
storm wave base in what would be classified
as deep subtidal and offshore environments)
were collected from 37 locations and 30 formations in the United States and southern
Canada (Fig. 2). All told, 20,088 fossil specimens belonging to more than 190 genera were
counted from field collections (median sample
size is 159 benthic individuals). To augment
these field data with samples collected by other workers using different counting and sampling techniques, 13 samples were derived
from published whole-assemblage abundance
data from similar facies and depth zones (see
‘‘Literature Sources’’). The literature-derived
portion of the database comprises 4990 individuals (median sample size 249 individuals).
Results are consistent if literature data are excluded. The total number of specimens used
in this study is therefore 25,078 representing
more than 230 genera from 38 formations.
The purpose of restricting depth zone and
lithological sampling was to control as much
as possible taphonomic variables and to allow
the comparison of relatively consistent environments from the Cambrian to the Ordovician, although precise environmental control
through time is difficult. This approach has
the advantage of minimizing the potential effects of environment and taphonomy on temporal signals, but it also restricts inference to
the chosen environment.
Samples from the target environment were
assigned a more specific depth zone relative to
maximum storm wave base on the basis of
physical sedimentary structures, taphonomic
conditions, and stratigraphic context (e.g.,
Brett et al. 1986; Brett and Baird 1986). Although absolute water depth is an important
component in determining the distribution of
benthic taxa on modern marine shelves (e.g.,
Hill et al. 1982; Ellingsen 2002), substrate
characteristics are likely to be much more important in this regard (Ellingsen 2002). Water
depth is also not the most important taphonomic variable (Powell et al. 2002; Best and
Kidwell 2000). Thus, even if absolute depth assignments differ between locations and time
intervals, maintaining relatively consistent
lithofacies may allow more relevant control
than does a constant depth zone per se.
To increase the geographic scope of this
study and prevent documenting basin-specific effects, samples were obtained from as
many locations as feasible. However, because
of outcrop patterns and geographic restrictions in facies tracts owing to basin evolution
and sea level variation, not all time intervals
are represented over a wide geographic range
in the assemblage data (Fig. 2). For example,
Arenigian assemblages in these data are restricted to the Basin and Range Province, and
no Tremadocian assemblages were field sampled. Because the Tremadocian represents an
important interval in the Cambrian–Ordovician transition, a limited number of North
American faunal lists from below normal
wave-base environments were derived from
the literature (see ‘‘Literature Sources’’). These
SEPKOSKI’S EVOLUTIONARY FAUNAS
TABLE 1.
547
Taxa assigned to each evolutionary fauna globally and locally.
Cambrian EF
Trilobita
Hyolithida
Polychaeta
‘‘Inarticulata’’
Eocrinoidea
‘‘Monoplacophora’’
Paleozoic EF
Articulata
Anthozoa
Stenolaemata
Cephalopoda
Crinoidea
Ostracoda
Asteroidea
Modern EF
Gastropoda
Echinoidea
Demospongia
Chondrichthyes
Bivalvia
Gymnolaemata
Malcostraca
Osteichthyes
data consist only of faunal lists (lists of taxa
found at a location without relative abundance
information) and are not included in any of
the quantitative analyses. When Tremadocian
literature data are illustrated, they are shown
with dashed lines to emphasize their distinction from the rest of the assemblage data.
To achieve a wider range of environments in
a comparative analysis of onshore-offshore
patterns in the evolutionary faunas, 18 supplemental Lower (12 lists) and Late Ordovician (6 lists) whole-assemblage faunal lists
from North American intertidal and shallow
subtidal environments were obtained from
the literature (see ‘‘Literature Sources’’). These
data were included only for comparative purposes in the onshore-offshore analysis and
did not enter into any of the quantitative analyses of the temporal patterns in faunal proportions.
Sampling Methods and Binning Protocols.
Field-samples were collected predominately
by excavating quarries at accessible locations
in sections with appropriate fossil-bearing
lithofacies and by collecting all macrofossils
(.1 mm). A limited number of collections
were also made from contextual float when
quarrying was not possible or practical. Specimens were recovered by finely splitting bedding planes and/or by examining bedding
surfaces. Peters (2004) gives more detailed descriptions of sampling methods.
The following identifiable skeletal elements
were counted as individuals: trilobite cranidia, brachiopod valves, bivalve valves, gastropod spires, echinoderm calyxes or semi-articulated to disarticulated thecal elements, nau-
tiloid phragmacones, coral colonies or solitary
individuals, and ostracode valves. Bryozoans
were counted as discrete colonies when possible, otherwise 1-cm lengths were treated as
individuals. Because bryozoans pose special
taxonomic and counting problems, they are almost certainly underrepresented in these
data. Omitting bryozoans from the analysis
does not, however, substantively change results. Although putatively swimming taxa,
such as agnostid trilobites and nautiloids,
were collected and counted, they were omitted from the analysis, which was limited to
benthic assemblages. Including non-benthic
taxa does not substantively change the results
because swimming taxa constitute less than
7% of the total number of individuals in these
data.
Genera were assigned to an evolutionary
fauna if they belonged to a group designated
as a member of a fauna by Sepkoski (1981).
Both Sepkoski’s data (Sepkoski 2002) and the
assemblage data were parsed into faunas as
shown in Table 1. All other taxa were not designated as belonging to any evolutionary fauna (unassigned genera). Richness for each fauna in each stage in Sepkoski’s data was counted as the total number of taxa known to have
existed during that stage (i.e., total richness
derived from first and last occurrence and
range-through data).
Assemblage samples were grouped by
stratigraphic stages according to the Cambrian subdivisions of Palmer (1998). The Delamaran (9 samples), Marjuman (11 samples),
Steptoean (11 samples), Sunwaptan (2 samples), lower Arenigian (Ibexian, [Webby 1998];
548
SHANAN E. PETERS
13 samples), Caradocian (16 samples), and
Ashgillian (14 samples) stages are represented in the field samples and literature data. Because Sepkoski’s (2002) data use a different
stratigraphic subdivision in the Cambrian, assemblage data for the Cedaria zone and later
Marjuman as well as the Steptoean were combined and compared with Sepkoski’s Dresbachian data. Marjuman assemblages older
than the Cedaria zone and Delamaran assemblages were combined and compared with
Sepkoski’s pooled middle Middle and upper
Middle Cambrian subdivisions. The meager
Sunwaptan samples in this study are from the
Eurekia zone and therefore correspond to Sepkoski’s Trempealeuan data. Because not all of
Sepkoski’s genus data are resolved to the level
of upper or lower Arenigian, only those genera that cross the lower Arenigian stage
boundary (Xbl 1 Xbt [Foote 2000]) or that are
explicitly assigned a lower Arenigian first occurrence were included in the analysis. All Arenigian first occurrences unresolved to substage in Sepkoski’s data were excluded from
the global tabulation.
Each field sample was treated as a paleontological ‘‘grab sample’’ and proportions were
calculated separately for each sample. The assemblage data for each stage were not pooled
to determine a single average community
composition. Rather, proportions for each fauna in each sample were calculated separately
and mean values for stages were calculated by
pooling these individual sample proportions.
Faunal proportions were calculated in four
ways at the assemblage level. The first two
ways are based on the number of genera (relative genus richness) and were calculated by
tallying genus richness in each evolutionary
fauna and then dividing either by the total
number of genera in the sample (all taxa) or
by the total number of genera that could be assigned to an evolutionary fauna (faunas only).
The second two ways of calculating faunal
proportions in assemblages are based on the
number of individuals (relative abundance).
In these cases, the number of individuals representing each fauna was counted and then divided either by the total number of individuals in the sample (all taxa) or by the total num-
ber of individuals that could be assigned to an
evolutionary fauna (faunas only).
Local Evolutionary Faunal Proportions
The transition between the trilobite-dominated Cambrian EF and the brachiopod-dominated Paleozoic EF appears at the temporal
scale of this study to occur gradually and uniformly in North American fossil assemblages
from mixed carbonate-shale and shale environments located below normal wave base
(Fig. 3). It is of course possible that a more rapid transition actually occurs in the upper Arenigian (Whiterockian [Li and Droser 1999;
Droser and Finnegan 2003]) or in the Llanvirnian and Llandeilian, which are all unrepresented in this study. There is no significant
change in relative genus richness in the three
faunas between the Caradocian and the Ashgillian, implying that most of the transition
between the Cambrian and Paleozoic EFs, at
least in terms of relative genus richness in offshore environments, was completed by the
Caradocian. The Modern EF, which is represented by approximately three times as many
gastropods as bivalves in these assemblage
data, exhibits little increase in relative genus
richness during the Ordovician and remained
only a small fraction of the total community
in the environments studied here.
In the Cambrian, genera from the Cambrian
EF dominate benthic assemblages almost to
the exclusion of all other elements. In the Late
Ordovician, Paleozoic genera are dominant,
but to a lesser degree (Fig. 3). The Early Ordovician is an important transitional interval
with communities of intermediate composition that are unlike those of the Cambrian and
later Ordovician. The average intermediate
composition of assemblages in the Early Ordovician is not the result of pooling disparate
samples that are individually Cambrian-dominated (Cambrian-like) or Paleozoic-dominated (Ordovician-like); instead, most of the Early Ordovician assemblages in the data set are
composed of 40–60% Cambrian EF genera
(gray points in Fig. 3) and are therefore generally distinct from older and younger assemblages.
It is important to realize that in Figure 3,
and in all other figures, the mean assemblage
SEPKOSKI’S EVOLUTIONARY FAUNAS
FIGURE 3. Relative genus richness of each evolutionary
fauna in local assemblages. Gray points are values for
single samples. Stippled datapoint is for literature-derived estimate in Tremadocian. Spacing of points along
abscissa is proportional to geologic time. Mean 6 one
standard deviation shown.
proportion is obtained by taking the unweighted average of proportions observed in
each sample, not by pooling all of the samples
from a time interval and then calculating a
single proportion for each of the three faunas.
Thus, an average value of 0.58 for the Cambrian fauna in the lower Arenigian (Fig. 3) in-
549
dicates that a given sample is, on average,
composed of 58% Cambrian EF genera.
The intermediate composition of lower-Arenigian assemblages documented here is at
least broadly consistent with the results of Li
and Droser (1999: Figs. 6, 9), who also reported approximately equal abundance of shellbeds dominated by Cambrian and Paleozoic
fauna elements in the lower Arenigian of the
North American Basin and Range province. Li
and Droser (1997) also reported data for Cambrian shell beds that are consistent with these
assemblage-level data. This consistency
emerges despite the fact that assemblage data
presented herein were collected primarily
from a very different taphofacies. Most of the
lower Arenigian samples in this study were
collected from the muds and thin carbonates
that are in many cases directly interbedded
with the skeletal tempestites and other shell
accumulations tabulated by Li and Droser
(1997, 1999). This suggests that the relative
abundance of taxa in shell accumulations
agrees broadly with presumably less time-averaged and less taphonomically altered intervening assemblages (Kidwell 1986), at least at
the taxonomic level relevant to the identification of the evolutionary faunas. Additional
section-by-section data from skeletal concentrations and intervening sediments are required in order to test this hypothesis, but the
agreement between these two estimates of the
relative richness of the evolutionary faunas is
noteworthy. This is particularly true because
the reliability of shellbed data has been justifiably questioned (e.g., Westrop and Adrain
1998).
Another ecologically important way of tabulating faunal proportions in local assemblages is to count the total number of individuals
that belong to each fauna rather than the total
number of genera, as in Figure 3. Because
specimens need only be identified to class level to determine their faunal affinity, this approach has the additional advantage of circumventing any potential problems, inconsistencies, or errors in genus-level taxonomy that
may exist between the evolutionary faunas in
this tabulation. Figure 4 presents the same assemblage data shown in Figure 3, but with
evolutionary faunal proportions calculated on
550
SHANAN E. PETERS
FIGURE 4. Mean relative abundance of each evolutionary fauna in local assemblages. Gray points are values
for single samples. Spacing of points along abscissa is
proportional to geologic time. Mean 6 one standard deviation shown.
the basis of number of individuals. Although
the overall temporal pattern in mean assemblage relative abundance is similar to that for
mean relative genus richness, there are some
notable differences between the genus- and individual-based tabulations.
First, the variance in many time intervals is
greater for proportions calculated on the basis
of individuals (Fig. 4) than it is for relative genus richness (Fig. 3). The most notable example of this occurs in the lower Arenigian,
where variance in the proportion of individuals in the Cambrian and Paleozoic EFs far exceeds variance in the proportion of genera in
each fauna. In general, high variance in the
lower Arenigian is consistent with the intermediate average composition of these assemblages because the standard error of sample
proportions is greatest when true proportions
are near 0.5 (Hayek and Buzas 1997: Fig. 8.3).
However, the difference in variance between
sample genus proportions and individual proportions cannot be explained by this fact
alone.
To evaluate potential sources of variance in
the individual-based tabulation for the lower
Arenigian, the influences of lithology and
depth zone (which are not entirely independent in these data) were examined. Figure 5
shows the effects of these environmental parameters on the proportion of Cambrian faunal elements calculated on the basis of genera
and individuals, as in Figures 3 and 4. The
proportion of individuals belonging to the
Cambrian fauna in the lower Arenigian is significantly different between lithologies (p 5
0.05) and depth zones (p 5 0.01) according to
a nonparametric Kruskal-Wallis test. However, there is no significant variation in relative
genus richness across these same environmental variables (Fig. 5; p . 0.83; results similar
for ANOVA). Thus, the relative genus richness
of the Cambrian EF in lower Arenigian samples tends to be consistent, regardless of depth
zone or lithology, but relative abundance
tends to be more variable across depth zones
and lithologies. The lower Arenigian Paleozoic EF exhibits a similar pattern, but individuals tend to be more abundant in shallower
water and carbonate-dominated environments. The effect in the Paleozoic EF is also
not as strong (p 5 0.03 for depth effect in individuals; p 5 0.14 for lithology effect in individuals; p . 0.74 for both effects in proportion of genera). The lower Arenigian Modern
EF, which is dominated by gastropods in the
assemblages studied here, does tend to have a
SEPKOSKI’S EVOLUTIONARY FAUNAS
551
FIGURE 5. Effects of lithology and depth on relative taxonomic richness (top) and relative abundance (bottom) of
the Cambrian EF in the lower Arenigian. Gray points are values for single samples; dark lines represent mean of
samples 6 one standard error of the mean. There is no significant difference in relative genus richness between
depths and lithologies (p . 0.10; Kruskal-Wallis test; ANOVA), but relative abundance differs significantly (p #
0.05); wb refers to wave base; N, individuals; S, genera. See text for discussion.
greater proportion of individuals in shallower-water settings and in carbonate-dominated
samples, but the effect is not significant (p .
0.1). However, the possibility that gastropods
(which dominate the Modern EF in these assemblage data) are more prevalent in carbonate-dominated settings than in siliciclastic settings is consistent with the lithofacies preferences reported by Novack-Gottshall and Miller (2003).
Unfortunately, the sample sizes used to test
for significant environmental variation in lower Arenigian faunal proportions are quite
small. Nevertheless, greater between-sample
variance in the proportion of individuals is
the expected consequence of sampling an environmental gradient occupied by taxa that
tolerate a range of environmental conditions,
but that achieve peak abundance in specific
settings. Provided that taphonomic differences between the evolutionary faunas have not
caused the selective destruction of individuals
from different faunas in different depth zones
and lithologies (which is a distinct possibility), this result suggests that there are significant differences in the preferred environment
of the evolutionary faunas even within the relatively restricted lithofacies and depth zone
covered here. Broader-scale patterns in the environmental distribution of the evolutionary
faunas are discussed below.
Another difference between the genusbased and individual-based tabulations
shown in Figures 3 and 4 is the strong domi-
552
SHANAN E. PETERS
FIGURE 6. Relative genus richness plotted against relative abundance (proportion of specimens) for each evolutionary fauna in each sample. Lines are least-squares
linear regression lines calculated after excluding assemblages with zero values for a fauna. Including zero values increases all slopes, but relative relationships remain qualitatively similar. Regressions are significant (p
, 0.001); r2 values are 0.73 (Cambrian), 0.72 (Paleozoic),
0.67 (Modern). Slopes for the Cambrian and Paleozoic
fauna are 0.61. Slope for Modern fauna is 0.93. See text
for discussion.
nance of the Paleozoic EF in the Late Ordovician when assemblage proportions are calculated on the basis of specimens. Most Late
Ordovician assemblages are dominated by the
Paleozoic EF in terms of specimens, but with
respect to the number of genera, the Cambrian
and Modern EFs tend to be better represented.
The comparatively large proportion of individuals belonging to the Paleozoic EF for a
given proportion of genera is a general feature
of the assemblage data.
Figure 6 shows local relative genus richness
plotted as a function of local relative abundance for each of the three faunas. Each point
in Figure 6 represents a single sample and
each sample has three points (one for each of
the three faunas). Least-squares linear regression lines are shown for each fauna. To prevent
undue weighting of assemblages that are
missing a fauna altogether, as commonly occurs in the Cambrian for example, regression
lines are calculated with zero values omitted,
but including zero values does not change the
qualitative relationships.
The slopes of the Cambrian and Paleozoic
EFs shown in Figure 6 are indistinguishable,
but the intercept of the Paleozoic EF is sub-
stantially less than the intercept of the Cambrian EF. This indicates that an increase in the
proportion of individuals tends to correspond
to an equivalent increase in the proportion of
genera in both the Cambrian and Paleozoic
EFs, but that for a given proportion of genera,
the Paleozoic EF tends to have more individuals. The slope of the Modern EF is substantially larger than the slope for the Cambrian
and Paleozoic EFs, indicating that an increase
in the proportion of individuals in the Modern
EF results in a larger increase in relative genus
richness. However, the range of values for the
Modern EF is not large in these data so the nature of the richness versus abundance relationship is difficult to evaluate.
The difference between the intercepts of the
lines for the Cambrian and Paleozoic EFs
shown in Figure 6 could be the result of variation in the way individuals are counted (see
‘‘Data and Methods’’) or systematic variation
in the reliability and consistency of genus-level taxonomy between faunas. Alternatively, if
the Paleozoic EF really does tend to have a
greater proportion of individuals for a given
proportion of genera, then this result may
have many interesting biological implications
relating to, for example, the ability of the Paleozoic EF to accommodate and recruit a
greater number of individuals per genus per
unit area of seafloor. This may be possible because of the overall low metabolic requirements (Bambach 1993) and larvae settlement
preferences of brachiopods and other Paleozoic EF taxa relative to those of the Cambrian
and Modern EFs. The pattern may also reflect
variation in the degree to which the evolutionary faunas are differentiated morphologically.
If the Cambrian EF tends to have a larger suite
of distinguishing characters and a more dispersed distribution of taxa within morphospace, then many genera may be recognized for
a given number of individuals. Likewise, if the
Paleozoic EF tends to be comparatively depauperate in morphological characters and to
have a rather densely occupied morphospace,
then relatively few genera may be recognized
for a given sample size. However, adequately
testing of these and other hypotheses and establishing the generality of the patterns
SEPKOSKI’S EVOLUTIONARY FAUNAS
553
shown in Figure 6 require additional field
data.
Local versus Global Evolutionary
Faunal Proportions
The relationship between local, assemblagelevel relative genus richness of the three faunas and their relative genus richness in Sepkoski’s global compilation is shown in Figure
7. There is remarkably good agreement between the two data sets, despite the very different scales at which they were compiled. Although the overall correlation is quite high, it
is important to note that not all of the points
fall perfectly on the 1:1 line. For example, the
lower Arenigian (Ibexian) assemblages studied here have, on average, more genera belonging to the Cambrian fauna than would be
predicted on the basis of Sepkoski’s global
compilation. Although residual variation
around the 1:1 line is present, evaluating the
meaning of this rather small amount of variation is difficult. Subtle changes in the average
sampled environment from stage to stage, for
example, could easily drive residual variation
(see following discussion on onshore-offshore
patterns), but quantifying this effect is impossible without high-resolution environmental
data. It is also possible that the residuals reflect the fact that only North American assemblages have been sampled in this study. In
some cases, biogeographic differences in the
apparent timing of clade diversification, such
as the rather late arrival of bivalves to Laurentia (Babin 2000; Sánchez and Babin 2003; Novack-Gottshall and Miller 2003), could explain
residual variation. Interestingly, local relative
genus richness of the Modern EF in the lower
Arenigian is far less than predicted by Sepkoski’s global compilation, which is precisely
what is expected on the basis of a late arrival
of bivalves to North America (Novack-Gottshall and Miller 2003: Fig. 3). If bivalves are excluded from Sepkoski’s compilation in the
Early Ordovician, then the Arenigian point
shown in Figure 7 falls near the one-to-one
line. Another plausible explanation for residual variation is imperfect stage assignment in
Sepkoski’s data. For example, many of the Arenigian genera in Sepkoski’s compilation are
not resolved to substage. In the analyses pre-
FIGURE 7. Mean relative genus richness in local assemblages plotted against relative genus richness in Sepkoski’s global compilation. Unassigned genera are excluded from the calculation but results are similar if
they are included (Table 2). Stippled datapoint shows
literature-derived Tremadocian estimate. The y-error
bars are 6 one standard error of the mean; x-error bars
are 95% binomial confidence limits for global proportion. Dashed lines are 1:1 lines. Points are labeled according to Sepkoski’s stages: MidC (Middle Cambrian),
Dres (Dresbachian), Trep (Trempealeauan), Aren (Arenigian), Cara (Caradocian), Ashg (Ashgillian).
554
SHANAN E. PETERS
sented here, all unresolved first occurrences
were omitted because many of these may in
fact represent upper Arenigian occurrences.
Regardless of the reasons for the departures
from the 1:1 line, the magnitude of the residual variation is small; the largest deviation in
Figure 7 is 12%.
Figure 8 shows the relationship between local assemblages and global relative genus
richness when assemblage proportions are
calculated on the basis of number of specimens. In general, results are consistent with
the assemblage-level relative genus richness
tabulation. As discussed above, one of the
main differences between the genus-based
and specimen-based tabulation is that Late
Ordovician assemblages are more strongly
dominated by the Paleozoic EF when proportions are calculated from the number of individuals. This is shown by positive residuals
on the 1:1 line for the Ashgillian and Caradocian. Another difference is the low slope of
the Modern EF in Figure 8. This indicates that
the proportion of individuals representing the
Modern fauna in local assemblages tends to be
low in comparison to relative genus richness
in the global compilation. This is perhaps not
surprising given the known environmental
preferences of bivalves (Miller 1989; NovackGottshall and Miller 2003), which are greatly
outnumbered by gastropods in these assemblage data. Nevertheless, the strength of the
correlation, which measures the association
between global richness and local abundance,
remains high for the Modern EF.
Figures 7 and 8 compare faunal proportions
for genera from the three faunas only. In these
cases, unassigned genera (Fig. 1) were omitted
from the calculations. However, including unassigned genera does not substantively affect
the results. Table 2 shows the linear productmoment correlation coefficients between global and assemblage faunal proportions calculated by using all genera, including those that
are unassigned to an evolutionary fauna (all
taxa). Also shown in Table 2 are correlation coefficients for comparisons in which only genera belonging to an evolutionary fauna are included (faunas only), as in Figures 7 and 8. Because random time-series are often spuriously
correlated, the significance of the correlations
FIGURE 8. Mean relative abundance in local assemblages plotted against relative genus richness in Sepkoski’s
global compilation. Unassigned genera are excluded
from the calculation but results are similar if they are
included (Table 2). The y-error bars are 6 one standard
error of the mean; x-error bars are 95% confidence limits
for global proportion. Dashed lines are 1:1 lines. Points
are labeled by stage as in Figure 7.
SEPKOSKI’S EVOLUTIONARY FAUNAS
TABLE 2. Linear product-moment correlation coefficients between the mean proportion of an evolutionary
fauna in local assemblages and the equivalent proportion in Sepkoski’s global genus compilation for six stratigraphic intervals in the Cambrian and Ordovician as in
Figure 7. ‘‘Faunas only’’ refers to proportions calculated
with unassigned genera omitted. ‘‘All taxa’’ refers to
proportions calculated with unassigned genera included. ‘‘Genera’’ refers to assemblage proportions calculated by using the number of genera in each fauna. ‘‘Individuals’’ refers to assemblage proportions calculated
by using the number of individuals.
Faunas only
All taxa
Genera
Cambrian
Paleozoic
Modern
0.986
0.997
0.764
0.976
0.996
0.828
Individuals
Cambrian
Paleozoic
Modern
0.997
0.996
0.937
0.984
0.989
0.960
in Table 2 was not tested. Although correlations are consistent between tabulations, the
slopes of the relationships shown in Figures 7
and 8 increase when all taxa are counted (total
fauna; plots not shown). This is because Sepkoski’s global compilation includes taxa, such
as radiolarians, that do not belong to an evolutionary fauna and that were not counted in
local assemblages. The effect of including
these taxa is to decrease uniformly the global
proportions of the evolutionary faunas relative
to the assemblage data. However, the
strengths of the assemblage-global correlations are not markedly influenced by unassigned genera (Table 2).
Environmental Distribution of the
Evolutionary Faunas
The large-scale patterns of faunal change
during the Ordovician, including onshore-offshore expansion of the evolutionary faunas,
have been well documented (e.g., Sepkoski
and Sheehan 1983; Sepkoski and Miller 1985;
Miller 1989; Patzkowsky 1995; but see Westrop
et al. 1995; Westrop and Adrain 1998). Trilobite-dominated assemblages of the Cambrian
EF occupied all environments in the Cambrian, but these were superseded during the Ordovician radiation by brachiopod-dominated
assemblages of the Paleozoic EF. Sepkoski and
Sheehan (1983) and Sepkoski and Miller
555
(1985) found that the Paleozoic EF first became prevalent in shallow-water environments and then rapidly expanded to become
dominant over most parts of the shelf during
the Ordovician radiation. They also found that
the mollusk-dominated Modern EF exhibits a
similar pattern, in that it first achieved dominance in nearshore environments and then
slowly expanded offshore, albeit in an irregular fashion (Sepkoski 1991). Although there
is widespread consensus regarding the overall
pattern, the underlying evolutionary and ecological processes responsible for the shifting
environmental distribution of the evolutionary faunas are still under discussion (Droser
et al. 1996; Westrop and Adrain 1998). For example, Adrain et al. (1998) found that some
trilobite groups depart from the stereotypical
Cambrian EF syndrome and were active participants in the Ordovician radiation, suggesting that the apparent replacement of the Cambrian EF by the Paleozoic EF was more a matter of dilution than of displacement (Westrop
et al. 1995; Westrop and Adrain 1998).
The results of this study (Table 2, Figs. 7, 8)
may at first seem to contradict an onshore-offshore gradient in the composition of communities because the gradient hypothesis predicts that faunal proportions in a given environmental zone should not reflect global relative genus richness. Instead, under the
onshore-offshore hypothesis, local assemblages should be disproportionately dominated by
genera from the evolutionary fauna preferentially occupying that depth zone. At the very
least, environmental patterns in the distribution of the evolutionary faunas predict environmental heterogeneity in their relative richness.
To evaluate the onshore-offshore signal in
these data, Early Ordovician (Tremadocian
and Arenigian) and Late Ordovician (Caradocian and Ashgillian) assemblages were separately assigned to three depth zones relative
to maximum storm wave base (as in Fig. 5).
This was possible because although the total
environmental gradient encompassed in this
study is relatively narrow when compared
with the complete spectrum of shelf environments, sufficient variability exists to make approximate depth divisions (see discussion of
556
SHANAN E. PETERS
FIGURE 9. Depth effects on the relative genus richness
of Sepkoski’s evolutionary faunas in Early and Late Ordovician assemblages. Data on left represent assemblages located below normal wave base (deep subtidal).
Within deep subtidal environments, assemblages were
distributed according to their approximate position relative to maximum storm wave base (swb); below, near,
and above swb from left to right in the deep subtidal
zone. Data on right are for published faunal lists from
intertidal and shallow nearshore environments. Means
in each depth bin 6 one standard error of the mean are
shown. Symbols for the faunas as in Figure 1.
criteria used in ‘‘Data and Methods’’). A limited number of additional intertidal and shallow-water nearshore data were derived from
the literature to supplement the onshore offshore analysis by providing a much wider
range of depth zones and environments (see
Appendix). Lithologically, the nearshore literature data range from algal-laminated dolomitic carbonates to shaly flat-pebble conglomerates.
Figure 9 shows the effect of depth on the local relative genus richness of each evolutionary fauna for the Early and Late Ordovician.
In Early Ordovician deep subtidal environments, there is rather little difference in mean
relative genus richness within each fauna (Fig.
9). However, in Early Ordovician intertidal
and nearshore environments, not only are all
three faunas more equally represented in
terms of relative genus richness, but their
rank-order relative richness is completely different (Fig. 9). In the nearshore environments
sampled here, the rarest of the three faunas in
deep subtidal environments, the Modern EF, is
dominant. The other two faunas have each
shifted down by one rank in nearshore settings. A similar pattern is observed in the Late
Ordovician, although each fauna tends to be
more variable within Late Ordovician deep
subtidal environments.
Considerably more data on the relative and
absolute abundance of the three evolutionary
faunas across high-resolution environmental
transects (including both absolute water
depth and substrate type) are required in order to fully resolve and test the onshore-offshore pattern of faunal succession and to distinguish this pattern from substrate-driven
environmental preferences (e.g., Miller 1989;
Miller and Connolly 2001; Novack-Gottshall
and Miller 2003). Nevertheless, these data
support a broad onshore-offshore distribution
of evolutionary faunas that is consistent with
previous assemblage-based studies.
Agreement between the proportions of evolutionary faunas in the assemblages studied
here and in Sepkoski’s global genus compilation (Table 2) may be at least partly the result
of choosing a depth zone that harbors most of
the known Cambrian and Ordovician genera.
Other environments may contribute such a
small proportion of total global richness in the
lower Paleozoic that their unique signals do
not contribute significantly to the global compilation. Clearly, as shown in Figure 9, had
this field study been conducted in intertidal
and shallow-water environments, there would
have been marked disagreement between the
global compilation and local assemblages.
Perhaps Sepkoski’s global compilation provides a good estimate of the average assemblage-level faunal composition in a range of
marine environments, but not in more restrict-
SEPKOSKI’S EVOLUTIONARY FAUNAS
ed or more taphonomically biased settings,
such as the intertidal zone.
Discussion
Sepkoski’s global compilation of marine animals has recently come under sharp criticism,
both for its taxonomic and stratigraphic inaccuracies (e.g., Adrain and Westrop 2000; Jeffery 2001; although the former found that errors were randomly distributed and therefore
of little consequence to the overall pattern)
and for its naivety with respect to large-scale
bias by differential fossil preservation in time
and space (e.g., Raup 1976; Allison and Briggs
1993; Cherns and Wright 2000; Peters and
Foote 2001, 2002; Smith 2001; Westrop and
Adrain 2001; Wright et al. 2003). Nevertheless,
some of the most interesting and fundamental
features of Sepkoski’s global compilation, and
other similar global taxonomic databases, are
based on relative proportions and are therefore immune to many of the first-order sample
size biases that afflict global fossil data.
The patterns of relative genus richness
among the evolutionary faunas in Sepkoski’s
(2002) global genus compilation appear to be
very accurately borne out within individual
marine assemblages from deep subtidal environments in North America. This is remarkable and unexpected given the dramatically
different scales of analysis and fundamentally
disparate nature of the data. It is also surprising given that regional factors can influence
the structure and composition of local communities. Miller (1997) pointed out that disparate regional effects, summed over the global record, produce the patterns that emerge in
global compilations. For example, Patzkowsky
and Holland (1993) credited a regional decline
in the importance of brachiopods across a
stratigraphic boundary in the Middle Ordovician of eastern North America to shifting
water mass characteristics and sediment input
instigated by the Taconic orogeny. Bivalves
and gastropods also appear to have had different histories on different paleocontinents in
the Ordovician (Babin 2000; Sánchez and Babin 2003; Novack-Gottshall and Miller 2003).
Even if there is not geographic variation in biological patterns, unequal sampling of marine
habitats in time and space can influence large-
557
scale paleobiological patterns (e.g., Westrop
and Adrain 2001). It is likely that some of the
rather small amount of residual variation in
the local-global relationships results from disparate regional histories and unequal environmental sampling in Sepkoski’s data, but
the results presented here suggest that Sepkoski’s global data can serve as a reasonable
proxy for local assemblage composition in
some environments and vice versa.
The agreement between faunal proportions
in local assemblages and in global compilations may also have biological implications for
how communities are assembled. For example,
the correspondence may indicate that the
communities studied here are composed of
taxa that are drawn randomly from the available global pool. Alternatively, and perhaps
more likely, the agreement may indicate that
most of the known Cambrian and Ordovician
genera occupy this depth zone so that their
signal overwhelmingly dominates the global
pattern. This interpretation is supported by
several studies that have identified deep subtidal environments as among the most taxonomically rich environments in the Paleozoic
fossil record (e.g., Lockley 1983; Sepkoski
1988; Patzkowsky 1995; Adrain et al. 2000). Of
course it is also possible that the agreement
between local and global proportions reflects
disproportionate representation of North
America in the published paleontological literature and therefore geographic bias in Sepkoski’s global genus database. Testing the environmental and geographic sampling in Sepkoski’s data requires additional information
about the environmental and geographic distributions of the constituent taxa and is therefore difficult at this time. However, at least
some of the correspondence between local and
global proportions may indicate that the environments studied here harbor much of the
world’s known marine biodiversity and that
North America is an important component of
Sepkoski’s compilation. Even so, this would
not necessitate a strong positive correlation
between relative richness estimated on the basis of literature-compiled genus first and last
occurrences and local community composition. Thus, the possibility that Sepkoski’s compilation is North American biased does not
558
SHANAN E. PETERS
detract from the fact that it appears to have
captured ecologically relevant parameters in
some Cambrian and Ordovician deep subtidal
communities.
Sepkoski’s evolutionary faunas have been
discussed extensively, and this study may be
interesting for its simultaneous treatment of
global and local patterns, but a perennial
question goes something like this: ‘‘What are
the evolutionary faunas anyway?’’ When analyzed at the global scale, the answer is quite
simple, but somewhat dissatisfying. Globally,
the evolutionary faunas are no more than
groups of taxa with covarying richness patterns and characteristic rates of turnover that
sequentially replace one another as the dominant faunal elements during the Phanerozoic.
But why they should exhibit this pattern in the
first place is far from obvious. The evolutionary faunas are certainly not phylogenetically
coherent, nor are there clear ecological niches
or strategies that unify them. Even in Sepkoski’s original factor analysis, the evolutionary
faunas are not fully distinct but exhibit fuzzy
boundaries that are often overlooked. For example, the Gastropoda score rather well on
two factors, the ‘‘Modern’’ and the ‘‘Paleozoic’’ (Sepkoski 1981), making them, statistically
speaking, part of both evolutionary faunas.
Thus, it is interesting but perhaps not surprising that within an evolutionary fauna,
some clades might behave in a way that is distinctly at odds with the expected pattern (Adrain et al. 1998). Nonetheless, the results of
this study seem to strengthen the ecological
significance of the evolutionary faunas, as defined on the basis of global first and last occurrence data. This is especially true because
the taxonomic richness of a group does not
necessarily translate into local ecological
dominance (e.g., Wing et al. 1993).
Sepkoski’s (1979, 1981) specific threephased kinetic model that sought to explain
the macroevolutionary history of the evolutionary faunas has recently been questioned
by Alroy (2004), and it seems clear that at the
genus level, turnover rates do not neatly divide marine animal classes into three distinct
groups. Nevertheless, the results of this study
indicate that the relative genus richness of
Sepkoski’s evolutionary faunas is not just a
global phenomenon but is locally manifested
in at least some marine assemblages in very
ecologically relevant ways. These results do
not support Sepkoski’s specific macroevolutionary model for the three faunas, but they do
suggest the possibility that some of the ecological interactions that might be inferred on
the basis of global relative genus richness
could be relevant in local communities.
Although this study in no way answers the
question of how the evolutionary faunas maintain coherence at such different scales of analysis (measured here in terms of the agreement
between global relative richness and local relative richness and abundance), these data do
suggest some possibilities that can be tested
further, many of which have already been
suggested in some form or another by Bambach (1993) and others. For example, the relationship between local relative abundance
and local relative genus richness (Fig. 6) as
well as the onshore-offshore patterns of faunal
dominance, may reflect fundamental differences between the faunas in such parameters
as biomass and metabolic requirements. It is
not a stretch to imagine phylogenetically diverse and ecologically disparate taxa having
similar characteristics in this regard. This was
a major theme of Bambach’s (1993) ‘‘seafood
through time’’ paper, and it would be interesting if such low-level differences among unrelated taxa could have such far reaching evolutionary consequences. Of course this does
not explain mechanistically how these specific
characteristics could result in differential
global rates of evolution and local relative
richness, but other life-history traits have been
linked to rates of evolution and large-scale
patterns of richness (e.g., Jablonski 1986) so it
is not inconceivable that this may be the case.
Whatever the ultimate explanation for the coherence of the evolutionary faunas, playing off
local and global patterns of relative abundance and richness in conjunction with a comparative ecological approach will undoubtedly help to better understand Sepkoski’s
three great evolutionary faunas.
Acknowledgments
Fieldwork was supported by grants from
the Paleontological Society, the Environmental
SEPKOSKI’S EVOLUTIONARY FAUNAS
Protection Agency STAR Fellowship program,
the Society of Sigma Xi, and the University of
Chicago Hinds Fund. B. Gaines, S. Finnegan,
M. Foote, K. Karns, S. Kidwell, A. McGowan,
and A. Ziegler provided helpful discussion. P.
Wagner provided a picture of his favorite
poorly preserved Ordovician snail. S. Finnegan, M. Foote, H. De Simone, and T. Rothfus
provided invaluable field assistance at various
stages of this work. K. Karns and M. Behrendt
offered excellent specimen preparation skills.
M. Foote, D. Jablonski, S. Kidwell, and A. I.
Miller helped to improve early drafts of this
manuscript. I also thank J. Adrain, P. Sheehan,
and M. Patzkowsky for very helpful reviews.
Literature Cited
Adrain, J. M., and S. R. Westrop. 2000. An empirical assessment
of taxic paleobiology. Science 289:110–112.
Adrain, J. M., R. A. Fortey, and S. R. Westrop. 1998. Post-Cambrian trilobite diversity and evolutionary faunas. Science 280:
1922–1925.
Adrain, J. M., S. R. Westrop, B. D. E. Chatterton, and L. Ramsköld. 2000. Silurian trilobite alpha diversity and the end-Ordovician mass extinction. Paleobiology 26:625–646.
Allison, P. A., and D. E. G. Briggs. 1993. Paleolatitudinal sampling bias, Phanerozoic species-diversity, and the End-Permian extinction. Geology 21:65–68.
Alroy, J. 2004. Are Sepkoski’s evolutionary faunas dynamically
coherent? Evolutionary Ecology Research 6:1–32.
Ausich, W. I., and D. J. Bottjer. 1982. Tiering in suspension-feeding communities on soft substrata during the Phanerozoic.
Science 216:173–174.
Babin, C. 2000. Ordovician to Devonian diversification of the
Bivalvia. American Malacological Bulletin 15:167–178.
Bambach, R. K. 1985. Classes and adaptive variety: the ecology
of diversification in marine faunas through the Phanerozoic.
Pp. 191–253 in J. W. Valentine, ed. Phanerozoic diversity patterns: profiles in macroevolution. Princeton University Press,
Princeton, NJ.
———. 1993. Seafood through time: changes in biomass, energetics, and productivity in the marine ecosystem. Paleobiology 19:372–397.
Best, M. M. R., and S. M. Kidwell. 2000. Bivalve taphonomy in
tropical mixed siliciclastic-carbonate settings. I. Environmental variation in shell condition. Paleobiology 26:80–102.
Brett, C. E., and G. C. Baird. 1986. Comparative taphonomy: a
key to paleoenvironmental interpretation based on fossil
preservation. Palaios 1:207–227.
Brett, C. E., S. E. Speyer, and G. C. Baird. 1986. Storm-generated
sedimentary units: tempestite proximality and event stratification in the Middle Devonian Hamilton Group of New York.
New York State Museum Bulletin 457:129–156.
Cherns, L., and V. P. Wright. 2000. Missing mollusks as evidence
of large-scale, early skeletal aragonite dissolution in a Silurian
sea. Geology 28:791–794.
Droser, M. L., and S. Finnegan. 2003. The Ordovician radiation:
follow-up to the Cambrian explosion. Integrative and Comparative Biology 43:178–184.
Droser, M. L., R. A. Fortey, and X. Li. 1996. The Ordovician radiation. American Scientist 84:122–131.
Ellingsen, K. E. 2002. Soft-sediment benthic biodiversity on the
559
continental shelf in relation to environmental variability. Marine Ecology Progress Series 232:15–27.
Erwin, D. H., and S. L. Wing, eds. 2000. Deep time: Paleobiology’s
perspective. Paleobiology 26 (Suppl. To No. 4).
Foote, M. 2000. Origination and extinction components of taxonomic diversity: general problems. Pp. 74–102 in Erwin and
Wing 2000.
Gould, S. J. 2000. Beyond competition. Paleobiology 26:1–6.
Gould, S. J., and C. B. Calloway. 1980. Clams and brachiopods
—ships that pass in the night. Paleobiology 6:383–396.
Hayek, L., and M. A. Buzas. 1997. Surveying natural populations. Columbia University Press, New York.
Hill, G. W., K. A. Roberts, J. L. Kindinger, and G. D. Wiley. 1982.
Geobiologic study of the south Texas outer continental shelf.
U.S. Geological Survey Professional Paper P1238.
Jablonski, D. 1986. Larval ecology and macroevolution in marine-invertebrates. Bulletin of Marine Science 39:565–587.
Jeffery, C. H. 2001. Heart urchins at the Cretaceous/Tertiary
boundary: a tale of two clades. Paleobiology 27:140–158.
Kidwell, S. M. 1986. Models for fossil concentrations: paleobiologic implications. Paleobiology 12:6–24.
Li, X., and M. L. Droser. 1997. Nature and distribution of Cambrian shell concentrations: evidence from the Basin and
Range Province of the western United States (California, Nevada, and Utah). Palaios 12:111–126.
———. 1999. Lower and Middle Ordovician shell beds from the
Basin and Range Province of the western United States (California, Nevada, and Utah). Palaios 14:215–233.
Lidgard, S., F. K. McKinney, and P. D. Taylor. 1993. Competition,
clade replacement, and a history of cyclostome and cheilostome bryozoan diversity. Paleobiology 19:352–371.
Lockley, M. G. 1983. Brachiopod dominated palaeocommunities
from the type Ordovician. Palaeontology 26:111–145.
Miller, A. I. 1989. Spatio-temporal transitions in Paleozoic Bivalvia: a field comparison of Late Ordovician and upper Paleozoic bivalve-dominated fossil assemblages. Historical Biology 2:227–260.
———. 1997. Dissecting global diversity patterns: examples
from the Ordovician radiation. Annual Review of Ecology
and Systematics 28:85–104.
Miller, A. I., and S. Connolly. 2001. Substrate affinities of higher
taxa and the Ordovician Radiation. Paleobiology 27:768–778.
Novack-Gottshall, P. M., and A. I. Miller. 2003. Comparative
geographic and environmental diversity dynamics of gastropods and bivalves during the Ordovician Radiation. Paleobiology 29:576–604.
Palmer, A. R. 1998. A proposed nomenclature for stages and series for the Cambrian of Laurentia. Canadian Journal of Earth
Sciences 35:323–328.
Patzkowsky, M. E. 1995. Gradient analysis of Middle Ordovician
brachiopod biofacies: biostratigraphic, biogeographic, and
macroevolutionary implications. Palaios 10:154–179.
Patzkowsky, M. E., and S. M. Holland. 1993. Biotic response to
a Middle Ordovician paleoceanographic event in eastern
North America. Geology 21:619–622.
Peters, S. E. 2004. Evenness in Cambrian–Ordovician benthic
marine communities in North America. Paleobiology 30:325–
346.
Peters, S. E., and M. Foote. 2001. Biodiversity in the Phanerozoic: a reinterpretation. Paleobiology 27:583–601.
———. 2002. Determinants of extinction in the fossil record. Nature 416:420–424.
Powell, E. N., H. K. M. Parsons, C. W. Russell, G. M. Staff, G. T.
Gilbert, C. E. Brett, S. E. Walker, A. Raymond, D. D. Carlson,
S. White, and E. A. Heise. 2002. Taphonomy on the continental
shelf and slope: two-year trends—Gulf of Mexico and Bahamas. Palaeogeography, Palaeoclimatology, Palaeoecology
184:1–35.
560
SHANAN E. PETERS
Raup, D. M. 1976. Species diversity in the Phanerozoic: an interpretation. Paleobiology 2:289–297.
Sánchez, T. M., and C. Babin. 2003. Distribution paléogéographique des mollusques bivalves durant l’Ordovicien. Geodiversitas 25:243–259.
Sepkoski, J. J., Jr. 1979. A kinetic model of Phanerozoic taxonomic diversity. II. Early Phanerozoic families and multiple equilibria. Paleobiology 5:222–251.
———. 1981. A factor analytic description of the Phanerozoic
marine fossil record. Paleobiology 7:36–53.
———. 1984. A kinetic model of Phanerozoic taxonomic diversity. III. Post-Paleozoic families and mass extinctions. Paleobiology 10:246–267.
———. 1988. Alpha, beta, gamma: where does all the diversity
go? Paleobiology 14:221–234.
———. 1991. A model of onshore-offshore change in faunal diversity. Paleobiology 17:68–77.
———. 1996. Competition in macroevolution: the double wedge
revisited. Pp. 211–255 in D. Jablonski, D. H. Erwin, and J. H.
Lipps, eds. Evolutionary paleobiology. University of Chicago
Press, Chicago.
———. 2002. A compendium of fossil marine animal genera.
Bulletins of American Paleontology 363, 560 p.
Sepkoski, J. J., Jr., and A. I. Miller. 1985. Evolutionary faunas and
the distribution of Paleozoic benthic communities in space
and time. Pp. 393–396 in J. W. Valentine, ed. Phanerozoic diversity patterns. Princeton University Press, Princeton, NJ.
Sepkoski, J. J., Jr., and P. M. Sheehan. 1983. Diversification, faunal change, and community replacement during the Ordovician radiations. Pp. 673–718 in M. J. S. Tevesz and P. L. McCall,
eds. Biotic interactions in Recent and fossil benthic communities. Plenum, New York.
Sepkoski, J. J., Jr., F. K. McKinney, and S. Lidgard. 2000. Competitive displacement among post-Paleozoic cyclostome and
cheilostome bryozoans. Paleobiology 26:7–18.
Smith, A. B. 2001. Large-scale heterogeneity of the fossil record:
implications for Phanerozoic biodiversity studies. Philosophical Transactions of the Royal Society of London B 356:351–
367.
Vermeij, G. 1977. The Mesozoic marine revolution: the evidence
from snails, predators, and grazers. Paleobiology 3:245–258.
Webby, B. D. 1998. Steps towards a global standard for Ordovician stratigraphy. Newsletters on Stratigraphy 36:1–33.
Westrop, S. R., and J. M. Adrain. 1998. Trilobite alpha diversity
and the reorganization of Ordovician benthic marine communities. Paleobiology 24:1–16.
———. 2001. Sampling at the species level: impacts of spatial
biases on diversity gradients. Geology 29:903–906.
Westrop, S. R., J. V. Tremblay, and E. Landing. 1995. Declining
importance of trilobites in Ordovician nearshore paleocommunities: dilution or displacement? Palaios 10:75–79.
Wing, S. L., L. J. Hickey, and C. C. Swisher. 1993. Implications
of an exceptional fossil flora for Late Cretaceous vegetation.
Nature 363:342–344.
Wright, P., L. Cherns, and P. Hodges. 2003. Missing mollusks:
field testing taphonomic loss in the Mesozoic through early
large-scale aragonite dissolution. Geology 31:211–214.
Appendix
Literature Sources
Literature-derived data used in this paper are referenced below. A list of formations used in the study follows each citation.
If the formations cited preserve shallow-nearshore and intertid-
al environments, then the formations are followed by a parenthetical statement indicating that these data represent shallow
habitats that were used to generate Figure 9. If a citation does
not indicate a shallow-water environment, then the data represent the deeper-water settings described in ‘‘Data and Methods.’’ See http://dx.doi.org/10.1666/02055.s1 for deep subtidal data from both field and literature sources.
Steptoean:
Shaw, A. B. 1956. A Cambrian Aphelaspis fauna from Steele
Butte, near Boulder, Wyoming. Journal of Paleontology 30:48–
52. Dry Creek Shale.
Early Ordovician (taxonomic lists only):
Cloud, P. E., and V. E. Barnes. 1957. Early Ordovician sea in central Texas. In H. S. Ladd, ed. Treatise on marine ecology and
paleoecology. Geological Society of America Memoir 67:163–
214. Tanyard Formation, Honeycut Formation, Gorman Formation
(shallow).
Hintze, L. F., L. F. Braithwaite, D. L. Clark, R. L. Ethington, and
R. F. Flower. 1972. A fossiliferous Early Ordovician reference
section from the western United States. Proceedings of the International Paleontological Union 5:385–399. House Limestone.
Lochman, C. B., and J. L. Wilson. Stratigraphy of Upper Cambrian-Early Ordovician subsurface sequence in Williston Basin. American Association of Petroleum Geologists Bulletin
51:883–917. Deadwood Formation (shallow).
Mazzullo, S. J., and G. M. Friedman. 1977. Competitive algal colonization of peritidal flats in a schizohaline environment: the
Early Ordovician of New York. Journal of Sedimentary Petrology 47:398–410. Middle member, Great Meadows Formation,
Fort Ann Formation (shallow).
Sando, W. J. 1957. Beekmantown Group (Early Ordovician) of
Maryland. Geological Society of America Memoir 68. Stonehedge Limestone, Rockdale Run Formation (shallow).
Shaw, A. B. 1958. Stratigraphy and structure of the St. Albans
Area, northwestern Vermont. Geological Society of America
Bulletin 69:519–568. Lower Highgate Formation.
Wilson, J. L. 1954. Late Cambrian and early Ordovician trilobites from the marathon Uplift, Texas. Journal of Paleontology
28:249–285. Lower Marathon Formation and Woods Hollow Formation.
Late Ordovician:
Bayer, T. N. 1967. Repetitive benthonic community in the Maquoketa Formation (Ordovician) of Minnesota. Journal of Paleontology 41:417–422. Maquoketa Formation.
Bretsky, P., and J. J. Bermingham. 1970. Ecology of the Paleozoic
scaphopod genus Plagioglypta with special reference to the
Ordovician of eastern Iowa. Journal of Paleontology 44:908–
924. Maquoketa Formation (shallow).
Frey, R. C. 1987. The occurrence of pelecypods in early Paleozoic epeiric-sea environments, Late Ordovician of the Cincinnati, Ohio area. Palaios 2:3–23. Sample A; Kope Formation.
Patzkowsky, M., and S. M. Holland. 1999. Biofacies replacement
in a sequence stratigraphic framework: Middle and Late Ordovician of the Nashville Dome, Tennessee, USA. PALAIOS
14:301–323. Lebanon Formation, Hermitage Formation, Arnheim
Formation, Sequatchie Formation.
Springer, D. A. 1982. Community gradients in the Martinsburg
Formation (Ordovician), southwestern Virginia. Ph.D. dissertation, Virginia Polytechnic Institute, Blacksburg, Va. Martinsburg Formation (shallow).
Titus, R., and B. Cameron. 1976. Fossil communities of the lower
Trenton Group (Middle Ordovician) of central and northwestern New York State. Journal of Paleontology 50:1209–
1225. Lower Trenton Group (shallow).