2. Taxonomic changes within a clade.
is nonquantitative, but it is based on well-documented features of the fossil record.
In this chapter, we review our method of paleoecological levels and discuss some possible ecological underpinnings that have caused the criteria we use to identify these various levels to be empirically observable features of the fossil record. Similarly, we demonstrate the utility of this approach through comparative analyses of a number of the "Big 5" mass extinctions (Raup and Sepkoski 1982) and their associated recoveries, as well as the Ordovician radiation.
This method is not meant as a replacement of previous paleoecological or taxonomic approaches for the understanding of major events in the history of life, but rather as an additional means of analysis to be used in conjunction with these other approaches. In this way we can further identify a variety of paleo-ecological shifts and in particular the paleoecological significance of an event.
Major events in life's Phanerozoic history, such as mass extinctions and radiations, are typically identified by examining changes in taxonomic diversity (e.g., Sepkoski 1979, 1981). Preliminary paleoecological studies indicate, however, that the relative magnitudes of changes measured by taxonomic diversity were not the same as the relative magnitudes of associated ecological changes (e.g., Brenchley 1989). Thus, taxonomic and ecological changes may have been decoupled.
Changes in paleoecological systems are expressed so that there are scales of change, and some changes are far more important than others. This structuring provides a means to scale or rank paleoecological changes. We categorize types of paleoecological changes into four ranks that we term "paleoecological levels" (table 4.1). These paleoecological levels are not hierarchical nor additive, but they are ordered.
Changes at the first level are of the greatest magnitude and represent the advent of a new ecosystem. These types of changes include the beginning of life on planets such as Earth and Mars, and on Earth the advent of metazoan life on land, the sea floor, the deep sea, and the pelagic realm (e.g.,Rigby 1997). On Earth these types of changes only happened once, and once they happened they have not been reversed (as far as we know). The best candidate for such a reversal, Seilacher's (1992) Vendobionta (Ediacaran fauna), now seems to have persisted into the Cambrian (e.g., Knoll 1996; Jensen, Gehling, and Droser 1998). In many respects, these types of ecological breakthroughs represent functional thresholds. Because they are at such a large scale and are unidirectional, first level changes will seldom play a part in an analysis of trends through time.
Changes at the second level occur within an established ecosystem and represent major structural changes at the largest ecological scale. Structural changes include the first appearance of, or changes in, ecological dominants of higher taxa within an ecosystem, such as the shift from trilobite- to brachiopod-dominated shallow soft-substrate paleocommunities in the Ordovician. Large-scale shifts in the nature of ecospace utilization are also included. Bambach (1983) introduced adaptive strategies as a means of evaluating paleoecological changes through time (e.g., figures 4.1 and 4.2). These include, for example, categories such as epifaunal mobile suspension feeders and pelagic carnivores. Whereas many workers have utilized the term guild for these categories, Bam-bach (1983) referred to guilds as smaller subgroups within these adaptive strategy categories. Thus, we have proposed the term Bambachian megaguilds for the adaptive strategies of Bambach (Droser, Bottjer, and Sheehan 1997). Addition or reduction in the number of Bambachian megaguilds serves as a useful signal of second level changes.
The development or collapse of metazoan carbonate buildups (e.g., Copper 1994; Stanley and Beauvais 1994) represents a major shift in the ecological structure of the marine ecosystem. The presence or absence of such buildups is dependent on climate and paleogeography and therefore is not as much an ecological signal as an environmental one. Reefs tend to disappear early in major extinction phases and thus are advance indicators of mass extinctions and global environmental crises (Copper 1994). The collapse of a reef system during a mass extinction and subsequent redevelopment of a reef system based on new higher level taxa can be considered a second level structural change within an ecosystem. The reappearance of reefs with essentially the same components is not considered a major structural shift. When metazoan reefs are lost, they are commonly replaced by a resurgence of stromatolite formation (e.g., Schubert and Bottjer 1992; Lehrmann, Wei, and Enos 1998), and so a significant increase of normal marine stromatolites also serves as a signal of structural changes. The presence of small local metazoan buildups within a largely silici-clastic or nonreefal setting would not be considered a structural shift.
Changes at the third level include community-scale shifts within an established ecological structure, in particular the appearance or disappearance of community-types. A community-type is "the aggregate of local communities and communities that have similar, but not identical, taxonomic membership and occur in similar, but not necessarily the same environments" (Bam-bach and Bennington 1995). Within a community-type, the filling-up of Bambachian megaguilds and an increase in tiering complexity (e.g., Bottjer and Ausich 1986) would also constitute third level changes.
Changes at the fourth level involve the appearance or disappearance of paleocommunities such as a succession of similar brachiopod communities (e.g., Boucot 1983; Harris and Sheehan 1996). These fourth level changes are common throughout the Phanerozoic and are similar in magnitude to most minor ecological changes.
Although paleoecological levels are not hierarchical or additive, there is a cascade effect, usually from the top to the bottom. If there is a second level change, it will be accompanied by third and fourth level changes. This is a "trickle-down" paleoecological effect rather than the building effect that occurs in the taxonomic system, where higher taxonomic levels are a means of grouping species according to their relatedness.
These four paleoecological levels provide a means to compare and rank ecological shifts associated with taxonomic events. How do we determine paleoecological levels for an event? Obviously the first step is the recognition of these events from taxonomic data. Some signals of paleoecological-level changes can be recognized through taxonomic shifts (e.g., the early Mesozoic transition from brachiopods to bivalves). However, in order to recognize other signals, such as an increase in tiering complexity or the addition of new paleo-communities, original paleoecological data must be collected in the field.
Similarly, in order to address the question of whether a specific ecological shift corresponds with a taxonomic shift, new paleoecological data must also be obtained.
As has already been noted, much discussion of the paleoecology of major events in the history of life, in particular, mass extinctions, has focused on bio-geography (e.g., Sheehan 1979; Jablonski 1986,1987; McGhee 1996; Erwin et al. 1996). Furthermore, much attention has also been paid to understanding the variations in extinctions of benthic versus pelagic organisms (e.g., Paul and Mitchell 1994; Levinton 1996). Although these measures have provided important insight into the paleoecology of mass extinctions, as discussed previously, these approaches cannot be used to recognize the extent that an extinction degraded the structure of an ecological system. In addition, the comparative paleoecological signature of radiations and recoveries has been little studied. In the following sections we use our system of paleoecological levels to analyze the Ordovician radiation and various components of the Big 5 mass extinctions and their associated recoveries.
The Precambrian-Cambrian radiation was the most significant event in the history of marine metazoans. Changes at all paleoecological levels occurred through this time interval as metazoans became established in Earth's seas. Clearly, a series of changes at several levels occurred as communities proceeded from Ediacaran assemblages to the Tommotian fauna ("small shellies") to typical members of the Cambrian Fauna. This radiation, regardless of its potential triggers or timing (e.g., Wray, Levinton, and Shapiro 1996), was a metazoan ecological event in which organisms were evolving into ecospace that had never before been occupied.
However, in many ways, the Ordovician radiation provides a simpler opportunity to examine paleoecological changes through the course of a radiation because there is a record of skeletal metazoans long before and after the Ordovician radiation, and there is a continuous marine record. In particular, it is not complicated by such phenomena as the advent of skeletalization or taph-onomic biases associated with soft-bodied faunas. Paleoecological changes associated with the Ordovician radiation of marine invertebrates include second, third, and fourth level changes. However, evidence from both spores (Gray 1985) and trace fossils (Retallack and Feakes 1987) suggests that the initial radiation of complex life onto land occurred in the Ordovician; this constitutes a first level change.
In the marine realm, second level changes included a shift in ecological dominants from trilobite- to brachiopod-dominated shallow shelf paleocom-munities (Droser and Sheehan 1995). We also observe a shift in ecological dominants in hardgrounds from echinoderm- to bryozoan-dominated paleo-communities (Wilson et al. 1992). Both of these changes resulted in the establishment of marine systems that were to last most of the Paleozoic. There were also new Bambachian megaguilds (figure 4.1), including deep mobile burrow-ers (Droser and Sheehan 1995). Similarly, the Ordovician witnessed the advent of stromotoporoid reefs, which dominated the reef ecosystem through the Devonian.
A major part of the Ordovician story is at the third level, where essentially Bambachian megaguilds were "filled" up to their Paleozoic levels (figure 4.1). In the Cambrian many of the Bambachian megaguilds had one or two clades, whereas by the end of the Ordovician, several megaguilds had up to eight different clades (figure 4.1; Droser and Sheehan 1995).
Additional third level changes included increases in tiering complexity from two to four levels in epifaunal suspension feeders (Bottjer and Ausich 1986), and in the shallow marine infaunal realm there were up to three tiers (as opposed to Skolithos piperock) (Droser, Hughes, and Jell 1994). There also was the appearance of new community types. These include a Receptaculites-macluritid high-energy nearshore community-type, new orthid community-types, and a bivalve-trilobite community-type in offshore muds (Droser and Sheehan 1995). The nature of the development of these new community-types still needs further study with additional field work.
Abundant fourth level changes, in the form of new paleocommunities, accompanied these second and third level changes. This demonstrates the "trickle down" effect that results from the existence of second and third level changes. The nature of these new paleocommunities is also currently under investigation.
Late Ordovician Mass Extinction and Silurian Recovery
The Late Ordovician mass extinction was the second-largest extinction in the history of metazoan life (Sepkoski 1981; Sheehan 1989). As much as 50% of all marine species became extinct (Brenchley 1989). However, ecologically, only third and fourth level changes occurred (Droser et al. 2000). Although reef communities were strongly affected by cool temperatures, the Silurian reefs that appeared soon after the extinction were mostly composed of the subfam-
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