An extensive body of recent research on the Ordovician
Radiation has shown that the continuous, period-long transition among
evolutionary
faunas exhibited at the global level (Sepkoski 1981) was anything but continuous
when evaluated at more local levels (see Droser et al. 1996, Miller 1997a).
Local and regional transitions were characterized by abrupt changes in
the diversities and abundances of faunal elements, often in association
with equally dramatic changes to paleoenvironmental conditions induced
by local, regional, or global phenomena (Patzkowsky and Holland 1993, 1996;
Droser and Sheehan 1995; Droser et al. 1995, 1996; Miller and Mao 1995;
Miller 1997b).
While the sum of these transitions produced a synoptic
decrease in the relative contribution of trilobites to global diversity
and concomitant increases among elements mainly of the Paleozoic Evolutionary
Fauna, regional and local shifts in trilobite diversity and abundance typically
did not mirror this global average. Indeed, where environmental conditions
were favorable, it was not uncommon for trilobites, bivalve molluscs, and
other elements of the Cambrian and Modern Evolutionary Faunas to dominate
the biota (e.g., Bretsky 1969, 1970a,b; Babin et al. 1982; Babin and
Gutirrez-Marco
1991; Babin 1993, 1995; Frey 1987a,b; Miller 1989; Sßnchez
and Waisfeld 1995; Cope 1996; Waisfeld and Sßnchez 1996).
In this context, Westrop and Adrain (1998)
have demonstrated convincingly that trilobites were major, stable
contributors to alpha diversity, throughout the Ordovician, among
Ordovician biotic assemblages located mainly in tropical, carbonate environments
in western Canada, the Canadian Arctic, and Newfoundland. However,
we are concerned that their characterization of these biotas in the context
of the Ordovician Radiation and "onshore-offshore" patterns of faunal
diversification is misleading for several reasons:
1. While they acknowledge
that biotic patterns exhibited by the paleocontinent of Laurentia might
not be indicative of broader, global patterns, Westrop and Adrain downplay
this concern and imply that their documented patterns reflect trilobite
diversities worldwide. However, throughout the Ordovician, biotic
compositions appear to have varied markedly from
paleocontinent-to-paleocontinent
(Miller 1997b, Miller and Mao in press), and no single paleocontinent reflects
a global microcosm. To be fair, it must be acknowledged that the
"onshore-offshore" interpretations that Westrop and Adrain call directly
into question (Sepkoski and Sheehan 1983; Sepkoski and Miller 1985) may
be compromised similarly because they are based almost entirely on data
from Laurentia (Mao and Miller 1994; Mao 1995; Miller and Mao in press).
2. Given their geographic
limitations within North America, Westrop and Adrain's data do not reflect
regional variations throughout Laurentia. Westrop and Adrain allude
to this concern by suggesting, for example, that trilobites were affected
negatively by environmental transitions in central and eastern Laurentia
associated with the Taconic Orogeny. The limited diversity
of trilobites in these settings can perhaps best be illustrated with data
compiled for two regions in the midcontinent affected by the onset of orogenic
activity: the Cincinnati Arch and Nashville Dome. Data from Upper
Ordovician strata of the Cincinnati Arch are summarized in Table 1, compiled
at the scale of Holland and Patzkowsky's (1996) six Late Ordovician
sequences (C1 through C6). These trilobite diversities are based on known
reports of trilobites from the region and represent over 100 years of intensive
collection by professionals and amateurs. In addition, this region
has been studied by workers with well-known reputations for taxonomic splitting,
so it is unlikely that these diversities significantly underestimate true
diversity. The first number in each column reports this total diversity
and is based on fieldwork, museum collections, and published literature.
Based on our field experience, many of these trilobite species are either
so rare or stratigraphically restricted that they would not be encountered
in most samples. Thus, the second number (in parentheses) reflects
the species diversity that would likely be encountered from each of these
units in a sample of 90 trilobites, equivalent to the rarified sample
sizes of Westrop and Adrain. A perusal of these data indicates that Cincinnati
Arch trilobites are uniformly less diverse than nearly any of the Upper
Ordovician samples reported by Westrop and Adrain.
The situation is as extreme or even more so on the
Nashville Dome. The trilobite diversity data shown in Table 2 are
based on field work (Patzkowsky and Holland unpublished data) and published
literature and represent all reported species from the Middle Ordovician
(M1 through M6) and Late Ordovician (C1 through C5) of the Nashville Dome.
Most of these species are sufficiently common to be encountered in large
samples, so they are comparable to the rarified samples of Westrop and
Adrain. Again, these diversities are consistently at the low end
of the values reported by Westrop and Adrain.
While Westrop and Adrain would probably not quarrel
with these numbers, they would argue that these patterns were endemic to
portions of Laurentia overprinted by the Taconic Orogeny. However, this
begs the following question: how large an area must be affected by the
by-products of tectonic activity before such an overprint is viewed as
more than just an exception to a larger pattern? Taken together, the Appalachian
Basin, Cincinnati Arch, Nashville Dome, and other regions affected by the
Taconic Orogeny and volcanism constituted a significant areal percentage
of Laurentia in the Middle and Late Ordovician. Further, the effects
of orogeny and volcanism were by no means limited to eastern and central
Laurentia; tectonic activity was on the rise in several venues around the
world, and these areas may have been hotbeds of diversification (Miller
and Mao 1995; see below).
Moreover, it should not be assumed that the physical
effects of orogeny and other tectonic activity were uniformly detrimental
to trilobites. In the Cincinnati region, for example, the primary
environmental effect of the Taconic Orogeny was the contribution of fine-grained
siliciclastic sediment, ubiquitous in Cincinnatian strata but manifested
most dramatically in intermittent claystones known locally as "butter
shales."
The two most common Cincinnatian trilobites, Flexicalymene and Isotelus,
exhibit their greatest abundances in these claystones (e.g., Frey
1987a,b).
Coupled with observations that the global diversity histories of orders
constituting the class were rather disparate from one another (cf. Adrain
1996), this serves as a reminder that any attempt to characterize broadly
the paleontology of an entire class is bound to overlook markedly different
attributes exhibited by many of its constituents.
In the Cincinnati and Nashville regions, orogeny-induced
Middle Ordovician changes in paleoceanographic conditions, including upwelling
and associated cooling of surface waters, abated during the Late
Ordovician.
This was reflected in the return to the area of certain articulate brachiopods,
corals, and molluscs that favored tropical, carbonate settings (Patzkowsky
and Holland 1993, 1996; Holland 1997). Although several trilobite
genera first appeared on the Cincinnati Arch during this C4-C5 invasion,
no net change in trilobite diversity occurred during this interval on either
the Cincinnati Arch or the Nashville Dome (Tables 1 and 2). Furthermore,
no change in trilobite diversity was apparent on the Nashville Dome at
the onset of the Taconic Orogeny in the M4-M5 interval (Table 2); trilobite
diversity was uniformly low in the entire region throughout the Middle
and Late Ordovician. Thus, in the Nashville area, Westrop and Adrain's
suggested linkage of trilobite decline to these paleoceanographic effects
is questionable.
3. While they ascribe the
precipitous Lower-to-Middle Ordovician decline in trilobite abundance in
the Great Basin as manifested in shell beds (Droser et al. 1996) to taphonomic
effects, Westrop and Adrain overlook the fact that many trilobite-rich
biotas are contained in storm-deposited strata in the Great Basin and
elsewhere.
Indeed, trilobite shell beds are common components of Middle Ordovician
strata in the Great Basin. However, at the base of the Whiterock,
the absolute number of trilobite-dominated shell beds decreases dramatically
and the number of polytaxic shell beds that include trilobites remains
constant, even though the total number of shell beds increases, augmented
by the addition of shell beds dominated by articulate brachiopods.
Moreover, there is no evidence of a taphonomic transition associated with
passage from Lower to Middle Ordovician strata (Li and Droser unpublished
data). In fact, Westrop (1986) demonstrated that trilobite material
contained in Upper Cambrian storm-deposits of southern Alberta was sufficiently
robust to be preserved in great abundance, despite being physically sorted
and transported after death. Thus, it is difficult to accept the
suggestion that the limited abundances of trilobites in Middle Ordovician
strata of Great Basin localities resulted mainly from taphonomic effects,
especially given that older, storm-deposited strata in these venues frequently
contain abundant trilobites.
Droser et al. (1996) pointed out that trilobite
diversity was far from depleted in Middle Ordovician strata of the Great
Basin and that the loss of Lower Ordovician trilobite taxa was offset by
a substantial Middle Ordovician radiation of trilobites, including
members of higher taxa not seen in older strata anywhere in Laurentia (Adrain
1996; Fortey and Droser 1996). Thus, the decline in trilobite abundance
may have been associated with a decline in the previously incumbent trilobite
fauna. In the Great Basin, the base of the Middle Ordovician marks
a significant shift among trilobite clades and corresponds with the initial
diversification of a new trilobite fauna.
4. Because their analyses
were limited to trilobites, it is difficult to know whether the study localities
used by Westrop and Adrain were venues in which the Ordovician Radiation
was ongoing actively. If the authors wish to show that trilobite
diversity and abundance persisted in the face of the radiation of other
faunal elements, it is not sufficient to show that trilobite diversity
was maintained in the Middle and Upper Ordovician settings that they
investigated.
They must also demonstrate directly the dilution effect that they claim:
that in these same strata, the diversity of other faunal elements, including
members of the Paleozoic Fauna, was increasing. In an earlier paper
that focused on nearshore environments, Westrop et al. (1995) provided
evidence that other taxa (primarily gastropods, rostroconchs, and bryozoans)
diversified through an interval in the Cambro-Ordovician during which trilobite
diversity remained relatively constant. This kind of analysis was not extended
by Westrop and Adrain to the suite of samples and environments included
in the present study. Thus, can they be confident that they
have not simply isolated a number of venues in which trilobites continued
to persist, while declining elsewhere in locations where the Radiation
was going full tilt? Most of Westrop and Adrain's data come from
tropical, carbonate-dominated paleoenvironments in a limited geographic
portion of Laurentia. However, as suggested by Miller and Mao (1995,
in press) and Miller (1997a,b), several major Ordovician taxa diversified
most appreciably in settings in which terrigenous sedimentation occurred
with at least some regularity.
In most respects, the approach used by Westrop and
Adrain is laudable: they provided a synthetic treatment of highly resolved
data emanating largely from their own fieldwork and systematic
investigations.
If their efforts were repeated for other taxa and in other major venues
around the world, the result would almost certainly be a more highly resolved
sense of the spatio-temporal texture of Ordovician biodiversity.
Moreover, we wish to emphasize that we are not claiming a role for competitive
interaction in the spatio-temporal history of trilobites; if anything,
our collective experience points to the importance of physical mechanisms
in producing these patterns. It is simply our view that Westrop
and Adrain overstepped their data in making claims about the nature of
the Ordovician Radiation that were unwarranted, given the limited geographic
and taxonomic coverage of their investigation.
Acknowledgements
We thank the National Aeronautic and Space Administration (Exobiology
Grant NAGW-3307 to A. I. M.), the National ScienceFoundation (Grants EAR-9204916
to A. I. M.; EAR-9204445 and EAR-9705732 to S. M. H.; EAR-9219731 and
EAR-9706021to
M. L. D.; EAR-97-05829 to M. E. P.), the National Geographic Society (M.
L. D.), and the Petroleum Research Fund of the American Chemical Society
(S. M. H. and A. I. M.) for supporting our research on Ordovician
paleobiology.
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Arnold I. Miller. Department of Geology, Post Office Box 210013, University of Cincinnati, Cincinnati, Ohio 45221-0013
Steven M. Holland. Department of Geology, University of Georgia, Athens, Georgia 30602-2501
Mary L. Droser. Department of Earth Sciences, University of California, Riverside, California 92521
Mark E. Patzkowsky. Department of Geosciences, Pennsylvania State
University, University Park, Pennsylvania 16802-2714