Proc. Natl. Acad. Sci. USA Vol. 92, pp. 11269-11273, November 1995 Evolution

The challenge of paleoecological stasis: Reassessing sources of evolutionary stability PAUL J. MORRIS*t, LINDA C. IVANYt, KENNETH M. SCHOPFi, AND CARLTON E. BRETT§ *Paleontological Research Institution, Ithaca, NY 14850-1398; tDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138; and §Department of Geological Sciences, University of Rochester, Rochester, NY 14627 Communicated by Stephen Jay Gould, Harvard University, Cambridge, MA, August 7, 1995

ABSTRACT The paleontological record of the lower and principle of evolutionary stability that results from constraints middle Paleozoic Appalachian foreland basin demonstrates imposed by . an unprecedented level of ecological and morphological sta- bility on geological time scales. Some 70-80%o of fossil mor- "Coordinated Stasis" phospecies within assemblages persist in similar relative abundances in coordinated packages lasting as long as -7 Data from the Hamilton Group of New York State provide million years despite evidence for environmental change and the best documented example of long-term faunal stability biotic disturbances. These intervals of stability are separated in the marine fossil record at the species level (Fig. 1). Within by much shorter periods of ecological and evolutionary the Hamilton Group, Brett and Baird (8, 9) recognize and change. This pattern appears widespread in the fossil record. describe some 20 recurring fossil assemblages, each charac- Existing concepts of the evolutionary process are unable to terized by a consistent, statistically recognizable taxonomic explain this uniquely paleontological observation of fauna- composition. These broadly defined biofacies are stable and wide coordinated stasis. A principle of evolutionary stability statistically recognizable, and many workers have identified the same sets of taxa independently (14-19). When examined that arises from the is explored here. We propose on both a spatial and temporal basis, biofacies are seen to be that hierarchical ecosystem theory, when extended to geolog- distributed across depth and turbidity gradients (3, 18, 20), and ical time scales, can explain long-term paleoecological stabil- each shows faunal, taphonomic, and sedimentological evi- ity as the result of ecosystem organization in response to dence for a narrow range of environmental conditions. Within high-frequency . The accompanying stability of a given biofacies, species persist stratigraphically from bottom fossil morphologies results from "ecological locking," in to top essentially unchanged morphologically, a characteristic which selection is seen as a high-rate response of populations noted as long ago as 1842 (14, 21) and verified more recently for that is hierarchically constrained by lower-rate ecological several species with morphological analyses (22, 23). Some 80% processes. When disturbance exceeds the capacity of the of the species found at the bottom of the unit are still present in system, ecological crashes remove these higher-level con- similar rank order at the top of the Hamilton Group, some 6 straints, and evolution is free to proceed at high rates of million years later (8, 9). However, very few (9-10% of 335 directional selection during the organization of a new stable Hamilton species; refs. 8 and 9) occur in units below or above ecological hierarchy. (Fig. 1). More extensive documentation of coordinated stasis in the Hamilton Group can be found in Brett and Baird (9). Paleontologists have long recognized that the fossil record seems to consist of packages of ecological and evolutionary stability Evidence for Disturbance bounded by episodes ofrelatively rapid change (e.g., refs. 1-5; see particularly refs. 6 and 7). Data recently presented from the Faunal stability in the Hamilton Group persisted in the face of Silurian and Devonian Periods of the Appalachian Basin, in both physical (abiotic) and biotic disturbances. Physical dis- particular the Middle Devonian Hamilton Group (8, 9), provide turbances are evident in the Hamilton Group on many time the first rigorous documentation of long-term faunal stability at scales, ranging from individual storm beds to episodes of the species level and show that it is possible to identify packages basin-wide mass mortality to widespread, lithologically distinct of rock, bounded in space and time, that contain coherent species horizons indicating changes in oxygenation or supply of clastic assemblages that persist for millions ofyears in the face ofphysical material on a time scale of 102 to 104 years. "Faunal tracking" and biotic These and (3) reflects biogeographic change associated with migration of disruptions. assemblages change rapidly various depth-parallel environments up to 200 km perpendic- synchronously to a new stable composition over comparatively ular to shoreline in response to relative sea-level fluctuations. shorter periods of time. Brett and Baird (8, 9) have called this Widespread biotic disturbances include ecological and incur- pattem oflinked stability and linked change "coordinated stasis." sion epiboles, stratigraphic horizons characterized by the Existing explanations for widespread evolutionary stasis have proliferation and transient of (respectively) rare tended to invoke either developmental constraints that limit taxa or taxa not otherwise present in the basin (Fig. 1). The change within individual lineages (10) or stabilizing selection reverse pattern, "outages" of otherwise characteristic mem- derived from the absence of environmental change (physical or bers of the fauna, is also observed (see closed and open circles biotic) that could induce speciation in multiple lineages (refs. 2 in Fig. 1). Epiboles and outages both occur within stratigraphic and 7; other explanations have been proposed for stasis in intervals on the order of decimeters and thus represent short particular cases-e.g., see refs. 11 and 12). A fossil record but ecologically significant periods of time (8, 9). After these characterized by coordinated ecological and evolutionary stabil- events, the original assemblage returns unaffected. ity, despite evidence for repeated disturbance, is incompatible Evidence for disturbances of different types on such a wide with these explanations. This pattern implies the need for a range of scales suggests that frequent opportunities for spe-

The publication costs of this article were defrayed in part by page charge tTo whom reprint requests should be addressed at the present address: payment. This article must therefore be hereby marked "advertisement" in Organismic and Evolutionary Biology Program, Morrill Science accordance with 18 U.S.C. §1734 solely to indicate this fact. Center, University of Massachusetts, Amherst, MA 01003. 11269 Downloaded by guest on September 29, 2021 11270 Evolution: Morris et al.11270 Evolution: Morris et al. ~~~~~Proc. Nati. Acad. Sci. USA 92 (1995)

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FIG. 1. Stratigraphic ranges of Middle to earliest Late Devonian fossil species of two broadly defined biofacies in the Hamilton Group of west-central New York (Cayuga to Owasco Lake outcrops). Symbols for range lines: thick line, species common to dominant; thin line, species present; arcuate curves to left/right, species migrated temporarily downslope (south)/upslope (north) from study area tracking favored environment; *, incursion epibole; 0, outage (see text). These data encompass a small subset of available Hamilton Group taxonomic ranges and are presented from a limited geographic perspective so as to illustrate faunal tracking. Note that most Hamilton-Tully species are first recorded at the base of subunit X, track their preferred environment in unison, maintain similar relative abundances throughout, and disappear in concert at the top of the interval. EE subunits are ecological-evolutionary "faunas" defined by Brett and Baird (8, 9); IX, Onondaga fauna; IXA, modified Onondaga assemblage plus unique elements of the Stony Hollow fauna; X, Hamilton-Tully fauna; XI, Genesee fauna. Adjoining portions of subunits IX and XI are included only for comparison. Relative sea-level curve is calibrated by fossil benthic assemblages (BA) as defined by Boucot (13); note that only the deeper end of this scale is shown; BA 3.5, diverse coral-rich offshore communities; BA 4, diverse brachiopod communities; BA 4.5, ambocoeliid communities; and BA 5, high-dominance leiorhynchid communities typical of dark-gray to black shale. Fossil species are subdivided roughly by biofacies into two groups: (i) those that characterize deeper water, dysaerobic areas typified by small ambocoeliid, chonetid, and leiorhynchid brachiopods (BA 4-5); (ii) those that typify diverse brachiopod biofacies of shallower, oxic, calcareous gray mudstones or argillaceous limestones (BA 3-4). Abbreviations for biostratigraphic zones, major shallowing pulses, and species are as in ref. 71. ciation occurred throughout the duration of the Hamilton cycle, limiting the duration of this transition to a maximum of Group (yet examples of in situ evolution are notably rare; Fig. between 100,000 and 400,000 years, at least an order of 1 is typical for the Hamilton Group, depicting a sole instance magnitude less than the duration of the stable fauna itself. The of inferred speciation in Heliophyllum-Hel h to Hel c). observation of coordinated stasis, therefore, cannot be dis- Biogeographic change indicated by faunal tracking in a com- missed as an artifact of incompleteness in the fossil record. plex basin implies opportunity for peripheral isolate formation The pattern observed in the Hamilton Group is not unique. and speciation. Basin-wide changes in clastic influx and oxygen The Silurian and Devonian Periods of the Appalachian Basin content imply physical pressures for adaptive change. Event preserve a record of 14 such intervals of coordinated stasis, each beds recording widespread mass deaths signify population interval spanning between 3 and 7 million years (refs. 8 and 9; and bottlenecks expected to produce evolutionary changes. Incur- see ref. 24). Between 65 and 80% of the species found at the base sion and ecological epiboles suggest both biotic challenges to of each interval are present in their appropriate biofacies up to the the ecosystem and biotic pressures for evolutionary change top of the interval in similar relative abundances. These stable (e.g., character displacement, competitive exclusion leading to periods are separated by geologically brief intervals during which changes in relative , or extinction of incumbents, long-ranging taxa become extinct, new species appear, and exotic etc.). Despite opportunities and impetus for evolutionary taxa successfully immigrate and become established. change, coordinated stasis persists throughout the duration of We argue that coordinated stasis is likely to be a much more the Hamilton Group, a period of 5 to 6 million years (based on general paleontological phenomenon than suggested solely by sequence and bio-stratigraphy; see refs. 3 and 9). The dramatic the above observations of Appalachian Basin faunas. The faunal change that marks the upper boundary of the Hamilton fundamental observation supporting the ubiquity of coordi- Group occurs within less than a single fifth-order sea-level nated stasis is the large paleoecological literature on biofacies. Downloaded by guest on September 29, 2021 Evolution: Morris et al. Proc. Natl. Acad. Sci. USA 92 (1995) 11271

Biofacies by definition represent stratigraphically persistent eco- Consideration of temporal scale is of central importance for the logical associations that are necessarily characterized by (i) mor- evaluation of ecological data (41, 42); while relative faunal phological consistency of the individual taxa and (ii) paleoeco- stability has been observed on a time scale of 106 years, studies at logical consistency in terms of relative abundance and higher and lower scales have yielded quite different results (e.g., membership (25). In addition, recent research provides several see refs. 43 and 44). striking examples of ecological stability lasting through periods of biogeographic and environmental change, including both marine Implications for Evolution invertebrate (26, 27) and terrestrial plant (28, 29) assemblages from Pennsylvanian cyclothems, Permian and Cenozoic mammal Coordinated stasis as a major theme in the fossil record has faunas (1, 30), and Pleistocene coral reef assemblages (31). More wide-ranging implications. Existing evolutionary frameworks, generally, the concentration of speciation and extinction events therefore, to remain vital, must be reconciled with these into discrete, simultaneous pulses (refs. 32-36; and presented as accumulated observations. Consider potential sources of sta- a general phenomenon in ref. 37) is another line of evidence bility and change recognized within current evolutionary the- suggesting the prevalence of coordinated stasis in the fossil ory (Fig. 2A). Punctuated equilibrium is readily able to explain record. Morphological stability ofcomponent species is suggested periods of stability and rapid change in a single lineage by by some case studies illustrating punctuated equilibrium (10). Few invoking developmental constraints upon the viability of vari- of these were designed to simultaneously examine stasis in ants, stabilizing selection, and the difficulty of reproductively multiple lineages or whole faunas; nevertheless, two of the most isolating variants (2, 10, 45, 46). Likewise, there are mecha- discussed cases also coincidentally show evidence for linked nisms within the classical Modern Synthesis that are capable of morphological stability and change across taxa (see ref. 38 for rift explaining low and high rates of evolutionary change in basin molluscs, and refs. 39 and 40 for bryozoans). Such studies, individual lineages (47, 48). Both of these theoretical frame- spanning taxonomic affinities and geologic time, support the works, however, are unable to explain: (i) the coordination of hypothesis that coordinated paleoecological and morphological both stability and change across multiple lineages of the sort stasis is a widespread phenomenon in the Phanerozoic Eon. observed in the Hamilton Group, and (ii) stability in the face

FIG. 2. Four frameworks for evolutionary theory (A), coupled with a conceptual model of the evolutionary process (B), showing levels at which explanations of evolutionary stability and instability operate. At right of B are two possible Darwinian entities, an individual and an isolated population, each of which has a clear origin, clear end, and some principle of heredity. Some aspect of the morphology is also shown as a frequency distribution; the location of the individual and isolated population within the distribution is shown relative to that of the species as a whole. Depicted at left of B from top to bottom are an ecosystem, the biogeographic range of a single species from that ecosystem, the ontogeny of an individual of that species, and the developmentally expressed genetics of that individual. All four evolutionary frameworks listed at top ofA consider sources of selective pressures that maintain or alter (stabilize or destabilize in boldface and italic type, respectively) the distribution of morphologies at several of these levels. In all cases, mutations produce the fundamental variability upon which selection can act, operating through the different survival of either individuals or isolated populations. Sources of stability/instability at the level of species and arise from the physical environment or ecological interactions. Only more traditional notions of paleoecological incumbency and ecological locking provide sources of evolutionary stability at the level of ecosystems. The stabilizing mechanism of paleoecological incumbency, a world in which all niches are full and incumbents have the advantage, is inconsistent with the fossil record of incursion epiboles and outages within blocks of faunal stability (Fig. 1). Only ecological locking provides a source of evolutionary stability, operating through natural selection, that can act in the presence of transient ecological and environmental change, and thus it alone can explain the observation of coordinated stasis. Downloaded by guest on September 29, 2021 11272 Evolution: Morris et al. Proc. Natl. Acad. Sci. USA 92 (1995)

of a changing environment [although this aspect has been strates for infaunal and epifaunal communities. Applying specifically addressed with Sheldon's (49) "Plus ca change, plus hierarchical ecology, the suite of biotic responses to these c'est la meme chose"]. Both the traditional Modern Synthesis perturbations (e.g., life history parameters, substrate prefer- and Punctuated Equilibrium are forced to rely upon the ences, species interactions) would have produced communities absence of environmental or ecological change to explain the that returned to one or a few stable states after perturbation. observation of coordinated stasis (2, 7). Stabilizing selection These ecosystems would therefore be capable of resisting some maintains unchanging morphologies only when the adaptive (but not all) longer-term biotic and physical perturbations. landscape is stable. As discussed above, we consider there to Thus, biofacies with similar species compositions tracked be strong arguments against such absence of change. Despite environments for millions of years until a rare event (very- this, coordinated stasis is maintained. low-frequency disruption) caused these systems to rapidly Coordinated stasis is therefore consistent only with a principle crash and reorganize (with high-frequency dynamics).§ Like of evolutionary stability arising from the level of the ecosystem, fire ecology on short time scales, it seems that to recasting ecology's role in macroevolution from one of only particular types of perturbation in hierarchically organized peripheral interest (but see refs. 7, 50, and 51) to that of an systems allows survival through more severe episodes of a important stabilizing force. It implies the existence of a mecha- similar type on geological time scales. nism by which ecological interactions prevent evolutionary change, resulting in long-lasting, stable systems capable of resist- "Ecological Locking" of Morphologies ing some types of disturbance. This stability has two aspects, one ecological, the other evolutionary. Ecosystems that exhibit coor- Our extension of hierarchical ecology may provide an expla- dinated stasis must be organized so as to produce a return to nation for paleoecological stasis, but the question of morpho- original species composition (or a limited set of possible compo- logical and evolutionary stability of the constituent taxa must sitions) after disruption. In addition, these ecosystems must be also be addressed. Observation in the wild suggests that organized so as to prevent both the morphologic change ofspecies directional selection is a common phenomenon (60) and is and the differentiation and/or persistence (52) of isolated popu- capable of driving evolutionary change at very rapid rates; yet, lations. Yet, these systems must also be capable of rapid ecolog- the rates of morphological evolution obtained from the fossil ical and evolutionary change when ecological associations are record are low relative not only to directional selection but also perturbed sufficiently that the system breaks down and con- to genetic drift (see references in refs. 61 and 62). An explanation straints are lifted. for this apparent discrepancy is the frequent reversal of morpho- logical trends observed within fossil species. At fine levels of Hierarchical Ecology and Paleoecological Stasis stratigraphic resolution, both morphological stability and gradual change appear to be ubiquitously underlain by high-frequency Hierarchical ecosystem theory (53-55) provides a general reversals in morphological trends (23, 63, 64). explanation for the production of long-term ecological stabil- We propose that the cause for these reversals, and hence the ity by short-term disruption. Its fundamental claim is that low rates of net morphological change, lies in the strength of biological systems are organized by disturbance into hierar- ecological interactions within a hierarchical framework. Spe- chically nested subunits (e.g., leaves, trees, forests) so as to act cifically, we suggest that directional selection is a high-rate as low-pass filters, insulating upper levels in the hierarchy from response of populations that is constrained by lower-rate the high-rate processes (or high-frequency events) that occur ecological processes. Lower-rate constraints upon directional at lower levels. Observation at limited temporal (or spatial) selection operate through the action of rules of limited com- scales will reveal rapid, high-frequency responses in lower- munity membership (65), in which interspecific level subunits (such as changes in leaf physiology in response (66) acting via character displacement and partition- to clouds, day, and night), while at broader scales of observa- ing is seen as the source for limitation (67-69). Such processes tion these high-frequency responses will be dampened, and not only prevent the successful introduction of new species but successively slower, lower-frequency (higher level) processes will also reduce the fitness of extreme variants within the commu- become apparent (such as seasonal growth rings, death nity. When these high-level constraints are removed by eco- and replacement of individual trees, and persistance of a see ref. logical crashes, temporarily removing interactions, evolutionary forest; 54). High-frequency perturbations cause eco- change is able to proceed at the high rates ofdirectional selection, systems to become organized hierarchically, ultimately making constrained only by the inherent developmental limitations of them resistant to lower-frequency perturbations of a similar and the of nature. Fire ecology demonstrates how a perturbation can be organisms vagaries allopatric speciation. on small This phenomenon, which we term ecological locking, must be disruptive temporal and spatial scales but stabilizing rooted in natural and (and even necessary) on larger scales (54). Even so, these selection be capable of preventing systems will still be susceptible to rare disruptions (exception- evolutionary change in both large populations (i.e., phyletic ally large or of an entirely different nature) that exceed the evolution) and small isolates (i.e., allopatric speciation) (Fig. capacity of the system (54, 55). During these times of ecosys- 2B). In large populations, we directly invoke low-frequency tem collapse, higher-level processes in the hierarchy will no ecological constraints (rules of limited membership) as the longer act to constrain the system; more rapid, lower-level mechanism that reverses selective trends and prevents sympa- processes will then determine the overall system response to tric speciation. In isolated populations, isolate persistence and perturbation until they are brought into check by the estab- allopatric speciation are inhibited through the operation of lishment of a new hierarchy. high-frequency ecological dynamics, such as responses to the We suggest that hierarchical ecology, considered over a removal of (70) or outstripping of resources. geological time scale, is capable of explaining the paleoeco- In essence, we suggest that ecological interactions maintain a logical stability and coordinated change observed at low static adaptive landscape and prevent both the long-term taxonomic levels in the fossil record [a notion first hinted at by O'Neill et al. (54)]. For example, in the Hamilton Group, §Interestingly, the rare, severe events suggested by Brett and Baird (8, high-frequency disruption of substrates by storm processes 9) to disrupt Appalachian Basin systems are episodes of widespread as of soft anoxia associated with transgressions. The single biofacies that persists (such winnowing substrates into shell pavements and through these turnovers is the Leiorhynchus fauna, an assemblage burial of epibenthic communities by mud blankets) has been characteristic of dysaerobic conditions (58). A similar pattern is also documented (3, 56, 57); this would have provided high- observed across blocks of stability in Midcontinent Pennsylvanian frequency spatial and temporal variability in habitable sub- cyclothems (59). Downloaded by guest on September 29, 2021 Evolution: Morris et al. Proc. Natl. Acad. Sci. USA 92 (1995) 11273

establishment of exotic taxa (as seen in incursion epiboles) and 23. Lieberman, B. S., Brett, C. E., & Eldredge, N. (1995) Paleobiology 21, evolutionary change of native species. However, when a dis- 15-27. 24. Boucot, A. J. (1990) in Paleocommunity Temporal Dynamics: The Long- ruption exceeds the homeostatic mechanisms of the ecosys- term Development of Multispecies Assemblies, ed. Miller, W., III (Univ. of tem, the stable state is not restored, and low-frequency eco- Tennessee, Knoxville), pp. 48-70. logical constraints are removed. This alteration opens the 25. Ivany, L. C., Schopf, K. M. & Brett, C. E. (1994) Geol. Soc. Am. Annu. Meet. ecosystem to immigration and presents species with a new Abstr. Programs 26, A453. 26. Bennington, J. B. (1993) Geol. Soc. Am. Annu. Meet. Abstr. Programs 25, adaptive landscape, simultaneously driving evolutionary change A459. in multiple lineages. 27. Bennington, B. J. (1994) Geol. Soc. Am. Annu. Meet. Abstr. Programs 26, A456. Concluding Remarks 28. DiMichele, W. A. & Phillips, T. L. (1992) Geol. Soc. Am. Annu. Meet. Abstr. Programs 24, A120. 29. DiMichele, W. A. (1993) Geol. Soc. Am. Annu. Meet. Abstr. Programs 25, The documentation of coordinated stasis in the fossil record A389. highlights the importance and utility of paleontology's unique 30. Vrba, E. S. (1992) J. Mammal. 73, 1-28. insight into the workings of evolution. These recent observa- 31. Jackson, J. B. C. (1992) Am. Zool. 32, 719-731. tions cast the widespread recurrence of biofacies in a new light, 32. Aubry, M.-P. (1992) in Eocene-Oligocene Climatic and Biotic Evolution, eds. Prothero, D. R. & Berggren, W. A. (Princeton Univ. Press, Princeton), pp. presenting researchers working within current frameworks of 272-309. evolutionary theory with a puzzling phenomenon that appears 33. Berggren, W. A. & Prothero, D. R. (1992) in Eocene-Oligocene Climatic to pervade the fossil record. As a starting point for futher and Biotic Evolution, eds. Prothero, D. R. & Berggren, W. A. (Princeton discussion, we propose a mechanism of stability arising from Univ. Press, Princeton), pp. 1-28. 34. McGowran, B., Moss, G. & Beecroft, A. (1992) in Eocene-Oligocene the ecosystem, derived from a hierarchical view of ecology, Climatic and Biotic Evolution, eds. Prothero, D. R. & Berggren, W. A. that operates through natural selection and is consistent with (Princeton Univ. Press, Princeton), pp. 178-201. both the ecological stability and morphological stability that 35. Lu, G. & Keller, G. (1993) Mar. Micropaleontol. 21, 101-142. define coordinated stasis. 36. Patzkowsky, M. E. & Holland, S. M. (1993) Geology 21, 619-622. 37. Stanley, S. M. (1979) Macroevolution, Pattern and Process (Freeman, San Francisco). We thank A. H. Knoll, T. K. Baumiller, S. J. Gould, J. B. C. Jackson, 38. Williamson, P. G. (1981) Nature (London) 293, 437-443. P. Colinvaux, and W. D. Allmon for helpful comments and discussion, 39. Cheetham, A. H. 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