PETER J. HARRIES,1 ERLE G. KAUFFMAN2 & THOR A. HANSEN3
1 Department of Geology, University of South Florida, 4202 E. Fowler Ave, SCA 203,
Tampa, FL 33620-5200, USA 2Department of Geological Sciences, CB-250, University of Colorado, Boulder,
CO 80309-0250, USA
3Department of Geology, Western Washington University, Bellingham, WA 98225, USA
Abstract: Mass extinction intervals are characterized by three dynamic processes: extinction, survival, and recovery. It has been assumed that the taxa surviving a mass extinction are composed predominantly of eurytopic groups and opportunistic/disaster species. However, high-resolution stratigraphic and palaeontological analyses of several mass extinction intervals show that the repopulation of the global ecosystem takes place among ecologically and genetically diverse and complex taxa and occurs far too rapidly to be solely attributed to rapid radiation from a few ecological generalists. We suggest a number of potential survival mechanisms or strategies (sensu Fryxell 1983) which have evolved in diverse taxa and which could have allowed them to survive mass extinction intervals. These mechanisms consist of: rapid evolution, preadaptation, neoteny/progenesis, protected and/or unperturbed habitat, refugia species, disaster species, opportunism, broad adaptive ranges, persistent trophic resources, widespread and rapid dispersion, dormancy, bacterial-chemosymbioses, skeletonization requirements, reproductive mechanisms, larval characteristics and chance. Because of the wide variety of potential survival mechanisms, the range of survivors may be far higher than previously hypothesized. This would account, in part, for the diversity and evolutionary state of Lazarus taxa and for the rapid re-establishment of some complex ecosystems following many mass extinction intervals, without calling on "explosive" radiation from generalist/opportunist stocks following a mass extinction interval.
The Phanerozoic record of marine invertebrates is punctuated by numerous, geologically short-term intervals (generally < 3 Ma) during which biotic diversity and abundance declined significantly ( < 40% at the familial level and < 63% at the generic level; Raup & Sepkoski 1986). The evolutionary and ecological patterns of extinction during these crises may be significantly different from normal background patterns (Jablonski 1986a), and they are followed abruptly by major evolutionary radiations (or revolutions sensu Wiedmann 1973) which change the character of the global biota. These intervals are termed mass extinctions and have been recognized by palaeontologists for over a century (Cuvier 1812; Newell 1967).
Yet mass extinction is only one part of a three-stage, geologically dynamic biological process and is closely linked to subsequent survival and recovery intervals or, collectively, repopulation (see Harries & Kauffman 1990). Jablonski (1986a) pointed out that mass extinction events may be one of the most important factors in evolution - reducing diversity significantly and opening ecospace for the rapid evolution of new forms. Episodes of repopulation may record some of the most rapid, large-scale biotic changes in life history. Yet, the mechanisms of survival and the nature of repopulation during and following mass extinctions have been poorly studied.
This paper is focused on adaptive traits that may aid in survival through the exceptional stresses of a mass extinction interval. Although most survival mechanisms discussed herein are common to living taxa and have some geological expression, their extensive testing across well-studied mass-extinction intervals needs to be performed in order to validate or discard them. This will also have implications as to their effectiveness relative to magnitude, duration, and causation among Phanerozoic mass extinctions. We expect to find differences in the relative effectiveness of various survival strategies associated with different mass extinction events.
It has been proposed that marine taxa which survive mass extinction events have one or more of the following characteristics: (1) broad adaptive range or eurytopy; many of these taxa are trophic generalists as well (e.g. Sheehan &
From Hart, M. B. (ed.), 1996, Biotic Recovery from Mass Extinction Events, Geological Society Special Publication No. 102, pp. 41-60
Hansen 1986); (2) opportunistic taxa adapted to highly stressed environments (see Levinton 1970) which characterize intervals of mass extinction but are normally found in low population numbers within most robust ecosystems (e.g. Palmer 1988); and (3) taxa living primarily or commonly in protected niches or refuges (Vermeij 1986) including the deep ocean. These models predict that survivors of mass extinction should normally have long species ranges, low diversity during the survival interval, usually large, widely dispersed populations prior to the extinction interval(s) (although the opposite may be true for opportunists and refugia species), broad adaptive ranges and thus broad environmental and biogeographic distributions prior to the mass extinction event where not excluded through competition. Their domination of newly available ecospace immediately following mass extinction events should occur rapidly and should give rise through punctuated evolution to diversifying successor species with progressively more restricted niches. In addition, Lazarus taxa (Flessa & Jablonski 1983) - species that survive the extinction event(s) but disappear from the record for an interval spanning a portion or the entire mass extinction event -should reappear abruptly during and following the flood of ecological generalists. In this scenario of survival and recovery, a short interval with little or no obvious biota would be followed by the rapid colonization of the ecospace with a few, usually small-sized (indicative of stress?) eurytopic and opportunistic taxa. These would, in turn, be succeeded by the reappearance of Lazarus taxa and the rapid radiation of new, more specialized taxa derived from these more generalized stocks (e.g. Surlyk & Johansen 1984).
However, a number of observations of recovery intervals following Cretaceous and Tertiary mass extinctions suggest a more complex picture (Kauffman 1984, 1988a; Keller 1986, 1988a, b, 1989; Hansen et al. 1987; Hansen 1988; Hansen & Upshaw 1990; Harries & Kauffman 1990; Harries 1993a). In many well-studied recovery intervals, especially in temperate settings (1) Lazarus taxa which return after mass extinction events are ecologically and evolutionarily more diverse and specialized than previously hypothesized; (2) the first marine recovery biotas are far more diverse, and arise much too rapidly (100500 ka) to be accounted for by radiation from a few generalized, eurytopic stocks; (3) deeper water and/or poleward, diverse marine assemblages are much less affected than their Tropical counterparts by mass extinction (Kauffman 1979, 1984, 1988; Wilde & Berry 1986, unless oxygen depletion or other chemical 'poisoning' events are operative. This is due to their broad adaptive ranges and colder temperature tolerance and suggests that a robust Temperate gene pool may remain following mass extinction to provide genetically and ecologically advanced stocks for rapid recolonization of those environ-ments/ecospace (i.e. shallow water and more equatorward) which were most affected by the perturbations associated with mass extinction events (but see discussion Raup & Jablonski 1993 and in Kauffman & Harries, this volume).
If these factors hold true, we would expect diverse survival mechanisms, many of which evolved to cope with normal background conditions operated during the perturbations of mass extinction. This resulted in high levels of ecological/genetic specialization among a certain portion of the survivors. This would explain not only the rapid diversification and apparent rapid emplacement of new taxa shortly after some mass extinction events, and especially individual extinction steps, but also the apparent survival of complex ecosystems across mass extinction boundaries (e.g. the highly diverse bryozoan-brachiopod mound associations found in the Maastrichtian and Danian intervals of the Danish Chalk; Birkelund & Hakansson 1982; Surlyk & Johansen 1984, and Danian deep-water reefs). Although some of the taxa comprising such communities/ecosystems pass through the boundaries unaffected, even newly appearing taxa perform very similar trophic and ecological functions as their extinct predecessors or ancestors (Hansen, 1988). It is quite possible that these new taxa evolved from extinct forms rapidly enough to prevent destruction of complex ecosystems. Thus, we believe that the mechanisms of survival are diverse, and these ideas are at least partially corroborated by the initial data collected at highly refined levels of resolution from the Cenomanian-Turonian (C-T), Cretaceous-Tertiary (K-T) and Eocene-Oligocene (E-O) mass extinction boundaries and their survival-recovery intervals.
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