The comments by Miller et al. boil down to the questions
of the scope of our data set and its relevance to community reorganization
during the Ordovician. Although we were constrained by our desire
to include only high-quality, quantitative data in our analysis, we feel
that we have produced a comprehensive survey of continental scale.
It is true that our Upper Ordovician data are from exclusively Canadian
sites, but in discussing our work, Miller et al. treat Canada as if it
were the size of Rhode Island or Delaware. Canada is actually larger
than the continental United States and our Upper Ordovician study sites
are spread across the country: southern Ontario, Manitoba, the Arctic
Archipelago,
and the Mackenzie Mountains of the northern Cordillera. Lines connecting
these regions circumscribe an area of more than 3,000,000 km2. As
even a casual glance at our appendix will show, our data are not confined
to shallow-water carbonates, but include a number of collections from subtidal
shales and storm deposits (including cool-water facies from the Trenton
Group [Brookfield 1988]) as well as from a variety of nearshore, buildup,
and deep subtidal settings. It is also worth pointing out that our
Middle Ordovician data are more extensive than those from the Upper Ordovician
and yet still show levels of alpha diversity that are comparable to the
Late Cambrian, a result that is not predicted by alternative models of
trilobite history during the Ordovician Radiation (Sepkoski and Sheehan
1983; Sepkoski and Miller 1985).
Miller et al.'s comments are all the more perplexing
when one compares the scale of our observations with those that are marshaled
to counter them. The Cincinnati Arch and Nashville Dome areas are
separated by a N-S distance of only about 250 km and a published estimate
of the total area of the basin is only 250,000 km2 (Patzkowsky and Holland
1997). In Droser's work (Droser et al. 1996), the Ordovician Radiation
is apparently captured adequately in the Ibex region of Utah, an area that
is orders of magnitude smaller than our geographic coverage.
We make no apologies for having restricted our analysis
to Laurentia. Laurentian data have been the foundation of contemporary
efforts to characterize the Ordovician radiations (Sepkoski and Sheehan
1983; Sepkoski and Miller 1985; Droser et al. 1996) and it is a perfectly
reasonable place to start. We have expanded our work geographically
to other parts of the world and temporally into the Silurian and, while
the results will be presented fully elsewhere (Adrain, Westrop, and B.
D. E. Chatterton unpublished data), some of our conclusions are relevant
to this discussion.
Table 1 presents data for Middle and Upper Ordovician
shallow and deep subtidal facies of Laurentia (data set expanded from that
used by Westrop and Adrain [1998]), Baltica, and Avalonia, with smaller
amounts of information for Australia, Armorica, and various Chinese terranes
united under "other provinces." Much of the published information
for extra-Laurentian trilobite faunas lacks the detailed abundance data
that we assembled for Laurentia, but a subset can be rarefied and we have
tabulated expected species number [E(Sn)] at a standard sample size of
90 individuals. In both raw species counts and rarefied data, mean
values for Laurentian alpha diversity are in line with values obtained
from other biogeographic regions; indeed, highest mean values for shallow
subtidal species richness occur outside Laurentia.
In this context, species richness data tabulated
by Miller et al. (Tables 1 and 2) for the Cincinnati region and the Nashville
Dome of central Tennessee are decidedly low and are certainly not representative
of either Laurentian or global patterns. The depauperate nature of
the data for shallow subtidal facies of the Nashville Dome is particularly
striking. Indeed, one would be hard pressed to find another Middle
Ordovician shallow subtidal sequence anywhere else in North America with
as few as three or four trilobite species. For example, Middle Ordovician
shallow subtidal collections from eastern Tennessee in our data set (Benbolt
Formation at Cedar Springs and Evans Ferry [erroneously listed as from
Virginia in our appendix]) contain more than double the number of species.
Similarly, slabs from a single horizon in the Middle Ordovician Bromide
Formation of Oklahoma (Westrop unpublished data) yield ten species [n =
95; E(Sn) = 9.84) in a surface area of only 0.5m2. Other Middle Ordovician
shallow subtidal collections in our data set from Ontario (Trenton Group,
Verulam Formation), the Upper Mississippi Valley (Platteville Group), the
Mackenzie Mountains (Sunblood and Esbataottine Formations), and the Laurentian
terrane of the Girvan area, Scotland (Stinchar Limestone), tell the same
story. Moreover, in the Silurian, mean values of E(Sn) for Llandovery
(9.73) and Wenlock (8.6) and Ludlow (8.83) shallow subtidal trilobite
collections
from Laurentia (Adrain, Westrop, and B. D. E. Chatterton unpublished data)
are also substantially higher than those reported from the Nashville Dome.
The compilation for the Nashville Dome is said to
be based on "fieldwork and published literature." And yet, modern
systematic studies of Ordovician trilobites do not exist for this area.
Indeed, one has to go back as far as works such as Bassler (1932)
just to find faunal lists. There has been a long history of
collecting in the Cincinnati region but, even here, a revision of the trilobite
fauna is long overdue. Thus, lack of modern systematic treatment alone
may explain some of the disparity between Miller et al.'s data and the
global picture. Until trained systematists undertake a comprehensive
field-based revision of the faunas in both areas, their relevance to questions
of Ordovician trilobite diversity remains uncertain.
Our data indicate that trilobite alpha diversity
was maintained at levels comparable to the Cambrian in Middle to
Upper Ordovician shelf environments of Laurentia and that these values
are paralleled in other biogeographic regions. In our experience,
the only Laurentian shelf facies in which trilobites assemblages are of
low diversity (typically fewer than four species) are those of nearshore
settings and, especially, in the Taconic clastic wedges associated with
infilling of the Appalachian basin (flysch of Lehmann et al. 1995).
The fact that trilobites are locally abundant in shales in the Cincinnati
Arch region does not falsify the latter observation. These occurrences
of trilobites in shales are limited to a few stratigraphic intervals, each
of which is no more than a meter in thickness (Frey 1987; Schumacher and
Shrake 1997), that can be traced over parts of southern Ohio and eastern
Indiana. Although two trilobite genera (Isotelus and Flexicalymene)
may represent up to about 25% of specimens in these mollusc-dominated
paleocommunities
(Frey 1987), we think that the complete absence of a variety of other trilobite
taxa that account for most of the species diversity in typical Middle and
Upper Ordovician shallow subtidal biofacies (e.g., illaenids, cheirurids,
encrinurids, pterygometopids, and lichids [see, for example, Ludvigsen
1978]) is far more significant and supports our conclusions. The
"trilobite shales" certainly attest to the fact that a small subset of
species could be locally abundant in Laurentian clastic facies, but they
tell us little about Ordovician trilobite paleoecology in general.
The shell bed study by Droser and colleagues (Li
and Droser, in Droser and Sheehan 1995: p. 88; Droser et al. 1996) is not
directly relevant to the question of trilobite diversity because their
data document only relative abundances of classes or phyla. This
provides a record of community change at the broadest scale, but it is
hardly an unbiased archive. We do not contest a decline in relative
abundances of trilobites but we hesitate to infer the drop in absolute
abundances implied by Droser et al. As we argued in our paper, the
same pattern of apparent decline will result simply from the great expansion
of community membership during the Ordovician Radiation. Moreover,
because of the effects of taphonomic factors, a literal interpretation
of the shell bed record is at best hazardous. Clearly, we did not
mean to imply that trilobites could not form shell beds-our own work indicates
otherwise-but Miller et al. miss the point. The paper that they cite
(Westrop 1986) demonstrated that size- and/or shape-sorting in Cambrian
shell beds can produce dramatically different relative abundance profiles
for trilobite assemblages drawn from the same biofacies. Similar
taphonomic obstacles to interpretation have been documented in Cretaceous
non-marine microvertebrate assemblages (Blob and Forillo 1996) and we find
it hard to believe that Ordovician shell beds would be immune from these
types of problems. The use of shell bed types is certainly a novel
approach to documenting the changing composition of Ordovician faunas,
but the very nature of the data demands utmost caution when drawing conclusions.
Taphonomic overprint may make direct estimation
of the absolute abundances of trilobites an intractable problem.
However, there may be indirect approaches. If we are correct in our
assessment of alpha diversity, then any significant real decline in trilobite
abundances in local habitats would have occurred in the face of essentially
constant species richness. This would translate into a drop
in mean population sizes which, in turn, should be expressed by an increase
in species turnover rates and a reduction in species longevity. An
accurate compilation of species ranges of even Laurentian trilobites would
be a major undertaking and is clearly beyond the scope of this note.
However, Foote's (1988) analysis of generic survivorship indicates that
no change in turnover rates took place during the Middle and Upper
Ordovician.
Rather, the pivotal point in the survivorship patterns lies between the
Cambrian and Ordovician, with the lower Tremadoc being somewhat
transitional.
As recognized by Foote (1988: p. 268), the increase in genus survivorship
and longevity between the Cambrian and Ordovician has a variety of potential
explanations, including differences in traditional approaches to
systematics.
However, genus level turnover in the Middle and Upper Ordovician
is not likely to be influenced by taxonomic bias and appears to be as stable
as our estimates of alpha diversity.
In a final attempt to dismiss our conclusions, Miller
et al. suggest that we may simply have failed to sample those localities
in which the Ordovician Radiation was "going full tilt." Here,
we believe that they are being disingenuous. They contrast our work
with the earlier study by Westrop et al. (1995) on Cambrian-Ordovician
nearshore paleocommunities, without noticing that collections and localities
used in that study are incorporated into our present analysis. Moreover,
Miller (Sepkoski and Miller 1985) used data from many of the same regions
in his characterization of the Ordovician reorganization of communities.
For example, we both included collections from the Middle Ordovician of
the Mackenzie Mountains (communities 61-65 in Sepkoski and Miller: p. 183)
and yet Sepkoski and Miller had no doubt that the Ordovician Radiation
was "going full tilt" in that area. The Ordovician Radiation was
also alive and well in Middle Ordovician of western New York, a major source
of information in Sepkoski and Miller's study (1985: p. 183, communities
67-70, 75, 79, 80-81, 83-84), but Miller et al. are apparently unwilling
to accept the relevance of data from the same facies and faunas some 200
km to the west in south-central Ontario. In Manitoba, the diverse
trilobite fauna of the Red River Formation is part of a paleocommunity
dominated by tabulate and rugose corals, stromatoporoids, receptaculitids,
and cephalopods (Westrop and Ludvigsen 1983), and the same paleocommunity
type (the "Arctic Ordovician Fauna") is associated with the equally rich
trilobite assemblages of the Arctic Archipelago (Thorsteinssen 1958; Bolton
et al. 1977). If the "Arctic Ordovician Fauna" is an unequivocal
product of the Ordovician Radiation in Nevada (Sheehan, in Sheehan and
Droser 1995: p. 83), then it is surely of the same origin in Canada. There
is no reason to believe that our data are anything but an accurate reflection
of the diversity history of trilobites in habitats that record the Ordovician
Radiation in Laurentia.
There is a broad consensus that trilobites became
reduced in relative importance during the Ordovician Radiation, but we
are still far from understanding how this was accomplished. Our
conclusions
about the stability of Laurentian trilobite alpha diversity are novel and
run counter to conventional wisdom. The available data indicate that
within-habitat species richness was equally stable in other biogeographic
regions. Rather than struggling to dismiss these results, we suggest
that a more fruitful approach is to use them to constrain hypotheses about
the dynamics of the radiation. They indicate that community expansion,
rather than displacement of one evolutionary fauna by another, is a dominant
theme. They also suggest that controls on taxonomic diversity of
trilobites during the Ordovician are lodged at scales beyond those regulating
species richness in local habitats. In a new compilation of global
taxonomic richness (Adrain et al. 1998), we have corroborated a general
Ordovician decline in the number of trilobite genera. Even here,
the pattern is more complex than previously appreciated, with a subset
of taxa, the Whiterock Fauna, undergoing a dramatic Middle Ordovician radiation
alongside members of the Paleozoic Evolutionary Fauna. The apparent
decoupling of local species diversity from global taxonomic diversity suggests
that factors such as declining provinciality during the Ordovician (Whittington
and Hughes 1972) may be important in the history of the group. By
taking a fresh look at trilobite diversity at a variety of scales, it may
be possible to gain new perspectives on the history of community reorganization
during the Ordovician.
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Stephen R. Westrop. Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019. E-mail:swestrop@ou.edu
Jonathan M. Adrain. Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom. E-mail: j.adrain@nhm.ac.uk