Does coordinated stasis yield taxonomic and ecologic stability?:
Middle Devonian Hamilton Group of central New York
Nicole Bonuso*
Cathryn R. Newton
James C. Brower
Linda C. Ivany
Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA
ABSTRACT
Statistical tests of coordinated stasis within the Middle Devonian Hamilton Group
demonstrate significant temporal changes in taxonomic composition and ecological structure of the macrofauna throughout a 5–6 m.y. time span. The analysis, based upon a
collection of .38,000 specimens obtained over a 20 yr period from the Hamilton Group
of central New York, used highly controlled sampling techniques, applied within a single,
well-defined lithofacies. Assemblages were tested for stability through time, as would be
predicted by the model of coordinated stasis. Our results reveal that within at least one
major Hamilton environment, taxonomic and ecological stability are not statistically significant and therefore do not support the hypothesis of coordinated stasis.
Keywords: coordinated stasis, paleoecology, Middle Devonian, Hamilton Group, statistics.
INTRODUCTION
The hypothesis of punctuated equilibrium (Eldredge and Gould,
1972) characterized evolution as essentially homeostatic for millions
of years (microevolution) until episodic disruption by abrupt speciation
events (macroevolution). Since punctuated equilibrium was first proposed, paleontologists have reported that not only single species, but
entire communities, underwent stasis followed by rapid community
turnover. Qualitative observations of recurring fossil assemblages
punctuated by abrupt faunal change led Brett and Baird (1995) to expand the idea of punctuated equilibrium to include entire communities,
a hypothesis they termed ‘‘coordinated stasis.’’ As proposed by Brett
and Baird (1995), community structure recurs within similar environments, and ;60% or more species persist with little morphologic
change. These periods of stasis persist for 3–7 m.y. and are followed
by drastic, turnover episodes lasting ;100,000 yr. Brett and Baird
(1995) suggested that this pattern is ubiquitous throughout the history
of marine life; if so, it has profound ramifications for the nature of
evolutionary processes. The purpose of this study therefore is to test
coordinated stasis statistically.
A logical place to test coordinated stasis is within the Middle
Devonian Hamilton Group (Fig. 1), described as one of the ‘‘best documented blocks of coordinated stasis’’ by Brett and Baird (1995, p.
296). The Hamilton Group exquisitely preserves invertebrate animals
that lived in the ocean-bottom muds of the Appalachian basin of Ontario, New York State, and Pennsylvania from ca. 440 Ma to 380 Ma.
Although the claim has been made that coordinated stasis is a general
phenomenon (e.g., Lieberman et al., 1995; DiMichele and Phillips,
1995; Morris, 1996), quantitative documentation of the original test
site has heretofore never been performed.
We test community stability throughout the Hamilton Group by
comparing taxonomic and ecologic structure within a relatively comparable environment. Our database of .38,000 specimens ranges
throughout the Hamilton Group and thus is comparable to that of Brett
*Present address: Department of Earth Sciences, University of Southern
California, Los Angeles, California 90089, USA; e-mail: [email protected].
Figure 1. Hamilton Group stratigraphy. Note position and number of sampled intervals. U.D.—Upper Devonian; Fras.—Frasnian; Eif.—Eifelian.
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; December 2002; v. 30; no. 12; p. 1055–1058; 4 figures.
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and Baird’s (1995) study. The statistical analyses presented here provide the first quantitative, testable documentation of evolutionary and
ecological patterns within the area first proposed to illustrate coordinated stasis.
DATA
To evaluate paleoecological patterns, we gathered high-resolution
relative-abundance data through the lower, middle, and upper parts of
the Hamilton Group of central New York State (Brower and Nye, 1991;
Newman et al., 1992; Newton et al., 2002; Bonuso et al., 2002).
Relative-abundance data are used because they are known to include
more information then qualitative, presence-absence data sets (Sneath
and Sokal, 1973; Gauch, 1982; Rahel, 1990). For this reason alone,
our analysis has the potential to reveal refined paleoecological patterns
not obtainable in earlier studies.
The test of coordinated stasis was constructed to maximize the
comparability of sampled data throughout the thickness of the Hamilton Group. Samples were obtained from a repeating, identical lithofacies, i.e., a medium-gray, noncalcareous shale interpreted by Bonuso
(2001) to represent an outer shelf environment. Moreover, the assemblages obtained from each sampling interval were taxonomically similar. Taxonomic consistency was maintained by a series of welldocumented protocols (Bonuso, 2001).
At each site, bulk samples were collected in 20–50 cm increments.
Bedding planes were uncovered in the field or in the laboratory, and
specimens were counted and identified to the species level. When attainable, a 300 specimen count was the cutoff per sample. However,
rarefaction curves (Brower and Nye, 1991; Bonuso et al., 2002) demonstrate that each sample size in our data set captures an adequate
number of specimens. Absolute counts were standardized to percentages before statistical analysis. Any species comprising ,1% of each
sample (i.e., five species total) was considered a rare taxon and was
removed from the database. We categorize taxa into three groups: abundant taxa ($10%), common taxa (9%–3%), and uncommon taxa
(#2%).
METHODS: TESTING STABILITY
To test for stability throughout the Hamilton Group, samples from
the five stratigraphic intervals were compared by using clustersignificance testing and the analysis of similarities (ANOSIM). The
most rigorous test—cluster-significance testing—is analogous to the
better-known discriminant analysis; however, cluster-significance testing is based upon the extent of overlap between two distinct clusters.
Rectangular distributions are used instead of Gaussian distributions because ecological data produce clusters that represent parts of a multidimensional swarm; thus, the distribution resembles a rectangular distribution rather than a normal Gaussian distribution (Sneath, 1977).
However, both rectangular and Gaussian distributions revealed similar
results.
To examine these data further, we compared between site differences against within-site differences by using ANOSIM. Through the
use of 5000 permutations and Bray-Curtis similarities, the one-way
ANOSIM analysis tests for spatial and temporal differences in community structure by combining permutation tests with the general
‘‘Monte Carlo’’ randomization approach. The test statistic calculated
(global R) reflects the differences between sites contrasted with differences within sites (Clarke, 1993). A predicted distribution of global R
is also calculated and compared to the actual global R to construct a
significance level (Clarke, 1993).
RESULTS
Resolving community patterns in this data set requires both nonparametric multidimensional scaling (NMDS) and agglomerative hi-
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Figure 2. Two-dimensional nonparametric multidimensional scaling
solution using Euclidean method on Bray-Curtis distance matrix between samples. Samples sites are superimposed on ordination plot.
Stratigraphic units are shown in Figure 1.
erarchical clustering because they are subject to different sources of
distortion. As seen in the NMDS plot, samples within each stratigraphic interval construct elliptical patterns with distinctive shapes that tend
to overlap (Fig. 2). The most prominent feature is that the Delphi
Station data are mostly separated from the Cardiff data. Similar results
occur within the cluster analysis; some but not all of the Delphi Station
and Cardiff samples cluster into separate groups, whereas the rest of
the samples cluster into one large group (Bonuso, 2001). In short, both
techniques conclude that there are numerous variations with an overall
similarity among temporal sites.
Although ordination and clustering techniques help the visualization of taxonomic and ecologic patterns, testing the stability of community patterns allows us to test our hypothesis. If coordinated stasis
holds true, cluster-significance testing should indicate that all temporal
sites have overlapping rectangular distributions (i.e., temporal samples
are similar through time). However, our results indicate that some temporal sites overlap, and others do not. According to this statistical test,
the Otisco samples are significantly different from all other temporal
sites except the Windom samples (Bonuso, 2001).
ANOSIM results suggest there is a 0 in 5000 chance that global
R comes from the predicted distribution (Bonuso, 2001). If coordinated
stasis were occurring, comparisons among temporal intervals should
yield similar distributions. Our results show clearly that between-site
variance and within-site variance do not derive from the same distribution. This finding strongly implies that significant taxonomic variations are occurring through time within this particular study.
PRESENCE-ABSENCE AND ABUNDANCE VARIATION
OVER TIME
There are marked presence-absence and relative-abundance variations over the temporal span of Hamilton deposition represented by
our data. The three most abundant taxonomic groups in our study are
Hamilton brachiopods: chonetids, Mucrospirifer, and Ambocoelia.
These three most frequently occurring taxa exhibit remarkable persistence in terms of presence-absence, but also they occur in consistently
high frequencies in each of the five data sets compared (i.e., ;40%,
12%, and 10% of all samples, respectively). That is, the three most
common contributors to lower Hamilton faunas persist throughout the
five stratigraphic intervals and contribute high percentages to each of
the upper Hamilton faunas (Bonuso, 2001).
By contrast, the common taxa, those ranked 4th through 20th,
vary markedly within our Hamilton sections. Some key taxa in the
lower Hamilton intervals—e.g., Leiorhynchus (brachiopod), Bembexia
(gastropod), and Emanuella (brachiopod)—do not occur at all in the
GEOLOGY, December 2002
Figure 3. Taxonomic patterns of 20 most abundant
taxonomic groups. Each square represents abundance pattern for particular taxonomic group. Within
squares, bars are in stratigraphic order and indicate
percentage found in each of five Hamilton Group formations (highlighted in Fig. 1). All abundances are
standardized to 100. Taxonomic groups are ordered
numerically from most abundant (i.e., number 1) to
least abundant (i.e., number 20).
upper Hamilton intervals (Fig. 3). Similarly, some frequent upper Hamilton Group taxa—such as Nuculoidea (bivalve), Tropidoleptus (brachiopod), and Palaeozygopleura (gastropod)—are entirely absent from
the lowermost Hamilton Group. Among taxa ranked 4th through 20th
in abundance, 53% fail to occur in either the uppermost Hamilton interval or the lowermost interval (Bonuso, 2001). In sum, within our
local study, more than half of the most common Hamilton taxa do not
span the entire Hamilton interval (Fig. 3).
Others of the 20 most common taxa exhibit differing patterns. For
example, Spinulicosta (brachiopod), Modiomorpha (bivalve), and Paracyclas (bivalve) extend through four or more of the temporal intervals,
but exhibit dramatically different frequencies among the intervals. Spinulicosta, for example, exists in relatively low proportions (i.e., ;2%)
in each of the lower four intervals but composes 11% of the last Hamilton interval. The frequency contrasts among the different intervals
reflect not only presence-absence distinctions among Hamilton faunas
of different ages, but also variable proportions of taxa within successive Hamilton communities.
Along with taxonomic variation, evidence suggests that dominant
ecological structure varies as well as the ‘‘players’’ within the dominant
ecological group (Fig. 4). For example, the majority of temporal intervals are dominated by suspension-feeding animals (i.e., those in the
Cardiff, Pecksport, Otisco, and Windom range from 78% to 89%). In
contrast, suspension feeders comprise 45% of Delphi Station fauna,
whereas infaunal deposit feeders and benthic crawlers comprise 24%
and 15% (Fig. 4). In addition, within the dominant ecological group
(i.e., suspension feeders), there is strong evidence suggesting that pedunculate suspension feeders replace reclining suspension feeders
through time (Fig. 4). Pedunculate suspension feeders represent 58%
GEOLOGY, December 2002
Figure 4. Ecological patterns within Hamilton
Group. Temporal sites are ordered stratigraphically.
and 33% of the Cardiff and Pecksport intervals and sharply decline in
numbers to 5% and 10% through time. Conversely, reclining suspension feeders range from 25% to 49% within the first three temporal
intervals and increase in number to constitute 68% and 84% within the
last two intervals (Fig. 4).
COMMUNITY PATTERNS AND ECOLOGICAL
MECHANISMS
If coordinated stasis obtains, faunal stability within a comparable
Hamilton Group environment should persist. This study demonstrates
that significant changes occur within taxonomic composition and eco-
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logical structure. Abundant taxa recur through time, whereas common
and uncommon taxa become locally extinct.
The persistence of abundant taxa has been explained by two mechanisms, the environmental-tracking model (Brett and Baird, 1995) and
the ecological-locking model (Morris et al., 1995). The environmentaltracking model suggests that taxa migrate with their preferred environment as it shifts in time and space, with no impetus for evolutionary
change. Morris et al. (1995) inferred that the preceding theoretical
model is unable to explain the coordination of stability and change
across unrelated taxa; therefore, they proposed the ecological-locking
model, which suggests that interspecific interactions introduce some
degree of stability to the community. Consequently, faunas can tolerate
modest fluctuations in the environment.
We consider that the abundant taxa (i.e., chonetids, Mucrospirifer,
and Ambocoelia) maintain their persistence by tracking a preferred environment. Implicated in the environmental-tracking model is the proposition that species behave independently. According to Valentine
(1973), independent species behavior results in an unstable ecosystem.
The instability can explain the frequent biotic replacement of the common and uncommon taxa. Low levels of correlation between the taxa
(r 5 ;0.12) further corroborate this conclusion.
The overall paleoecological pattern we observed consists of loosely structured paleocommunities in which the most abundant taxa are
tracking preferred environments and leaving the common and uncommon taxa free to come and go, depending on changes in ecological
parameters. It is interesting that other paleoecological studies have revealed similar paleoecological patterns within comparable time intervals. Patzkowsky and Holland (1997) reported turnover of Ordovician
taxa despite relatively consistent environments. These taxa were described as ‘‘independent of other taxa and not tightly integrated’’ (Patzkowsky and Holland, 1997, p. 420). Bennington and Bambach (1996)
concluded in their quantitative paleoecological study of Middle Pennsylvanian fauna that although some ‘‘paleocommunity types’’ recur,
statistically identical paleocommunities did not recur.
CONCLUSION
Through the use of presence-absence data, Brett and Baird (1995)
concluded that within a time span of 5–6 m.y., 80% of all species
persist in similar environments throughout the Hamilton Group. Brett
and Baird (1995) also predicted, as a causal mechanism for the observed stasis, that communities track stable and laterally shifting environments. These results suggest that independent taxa tracked preferred environments, therefore producing a loosely structured,
semistable ecology at the paleocommunity level. According to clustersignificance testing, four out of five intervals are significantly similar
and overlap; according to the ANOSIM technique, significant taxonomic variation is exhibited throughout the section. In addition, dominant
ecological patterns as well as key taxa within each ecological group
alternate through time. Although these results indicate that the most
abundant taxa recur and possibly account for superficial similarities
between temporal samples, taxonomic membership and ecological
structure change dramatically through time. Indeed, in this study, the
amount of variation surpassed the level of variation predicted by coordinated stasis.
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ACKNOWLEDGMENTS
Research was conducted as part of Bonuso’s M.S. thesis at Syracuse University and was supported by grants from the Petroleum Research Fund (administered by the American Chemical Society), the Paleontological Society, the
Syracuse University Department of Earth Sciences, and the John J. Prucha Field
Research Fund at Syracuse University. We also acknowledge C. Brett and G.
Baird for presenting the idea of coordinated stasis and thus forcing us to take
a closer look at the fossil record.
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Manuscript received May 24, 2002
Revised manuscript received August 20, 2002
Manuscript accepted August 23, 2002
Printed in USA
GEOLOGY, December 2002