The Future of the Fossil Record David Jablonski
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E VOLUTION VIEWPOINT The Future of the Fossil Record David Jablonski The fossil record provides a powerful basis for analyzing the controlling questions. The answers will give us a much factors and impact of biological evolution over a wide range of temporal fuller view of the interplay between the two and spatial scales and in the context of an evolving Earth. An increasingly great themes of evolutionary biology: diversity interdisciplinary paleontology has begun to formulate the next generation and adaptation. of questions, drawing on a wealth of new data, and on methodological 2) Why are major evolutionary innova- advances ranging from high-resolution geochronology to simulation of tions unevenly distributed in time and space? morphological evolution. Key issues related to evolutionary biology in- One of the most striking patterns to emerge clude the biotic and physical factors that govern biodiversity dynamics, from the fossil record is that biological inno- the developmental and ecological basis for the nonrandom introduction of vations—the breakthroughs that open new evolutionary innovations in time and space, rules of biotic response to ecological opportunities and evolutionary environmental perturbations, and the dynamic feedbacks between life and pathways—do not arise randomly. Regard- the Earth’s surface processes. The sensitivity of evolutionary processes to less of exactly when the major lineages actu- rates, magnitudes, and spatial scales of change in the physical and biotic ally split, the Cambrian explosion represents environment will be important in all these areas. a uniquely rich and temporally discrete epi- sode of morphological invention for the It is hard to resist the fossil record as a measured in terms of taxa, range of body metazoan phyla (9). Smaller pulses follow source of spectacular evolutionary tri- forms, or modes of life—has been one of net mass extinctions, not least being the exuber- umphs, grotesqueries, and catastrophes. increase through geologic time. However, ant Cenozoic radiation of mammals, after 100 But of course it can be much more than this general trend has been anything but million years of fairly monotonous morphol- that—a laboratory of evolutionary experi- smooth. Diversification has occurred episod- ogies in the shadow of the dinosaurs. ments, sometimes richly replicated, as in ically, has been interrupted by extinction Understanding both the onset and the termi- regional extinction pulses, and sometimes events, and has had (in the oceans anyway) at nation of such bursts is a major challenge. confoundingly unique, as in the Cambrian least one prolonged episode of little net Critical tests for the trigger or damper of the explosion of metazoan life 530 million change. Cambrian explosion have been difficult. Poten- years ago. The next wave of research will The complex trajectory of taxonomic diver- tial mechanisms are plentiful and fall roughly build on an array of empirical and method- sity through time has proven robust to contin- into an extrinsic set of ecological or physical ological advances that, judging by the out- ued sampling (3) (Fig. 1) and, as shown by triggers and brakes (10) and an intrinsic set of come of several recent workshops on new simulations, to very different phylogenetic ap- thresholds in the increasing complexity and lat- directions for the coming decades (1), will proaches to grouping species into higher taxa er stabilization of developmental systems. foster a paleontology that is more interdis- (4). But diversity time series become increas- However, without a time machine to per- ciplinary than ever. ingly jagged and disparate at lower taxonomic form reciprocal transplant experiments be- Paleontology has recently accelerated on levels and on regional scales, both because tween Cambrian and modern seas, the rival several fronts. Combined with advances in al- sampling is less complete and because lower- hypotheses so far have resisted falsification; lied fields in the earth and life sciences, from diversity lineages really are almost inevitably clearly, broadly multidisciplinary work is es- high-resolution geochronology to molecular de- more volatile. Just how do diversity dynamics sential to crack this problem. velopmental biology, paleontology is beginning at these finer scales sum to produce the global, The origin of evolutionary novelty, like to take a renewed approach to a wide array of family-level pattern? At least some biotic shifts biodiversity dynamics, appears to have a new and old scientific issues. A battery of new that appear to be protracted when analyzed strong spatial component. For post-Paleozoic ways to generate and test hypotheses by using globally, such as the massive Ordovician diver- marine invertebrates, and for at least some the ever-growing breadth, volume, and quality sity increase in the sea and the Mesozoic mod- plant and vertebrate groups, major novelties of empirical data is being abetted by the devel- ernization of marine predators and of prey de- first occur mainly in disturbed habitats—for opment of synoptic global and regional data- fenses, are more abrupt but occur at slightly example, onshore marine settings—and in the bases (Fig. 1) and by the influx of techniques different times when examined regionally (5). tropics. Where phylogenetic relationships are for simulation and mathematical modeling. When diversity dynamics are dissected into known in detail (and more work here would Analyses with methods derived from probabil- constituent clades (that is, monophyletic evolu- be especially valuable), these first appearanc- ity theory, for example, can now directly con- tionary lineages—for example, crabs or their es tend to be primitive species basal to their front sampling biases once considered crippling scallop prey), how well can clade-specific turn- groups. Yet lower level novelties, such as (2). Among the opportunities opened by these over rates be predicted by biotic factors such as those defining species and genera, are evi- developments, four key, interrelated research body size and feeding habits (6), and how often dently freer to originate according to the en- questions have emerged at paleontology’s inter- do clade dynamics interact to damp or enhance vironmental and latitudinal gradients of their section with evolutionary biology (1). those rates (7)? When are “taxon-free” dynam- individual clades. Here is where Van Valen’s 1) What are the rules that govern biodi- ics, based strictly on the acquisition and persis- maxim that “evolution is the control of de- versity dynamics, and do they apply at all tence of morphological traits or the occupation velopment by ecology” (11) comes into full temporal and spatial scales? The overall trend and emptying of regions within morphometri- play. What is it about certain environments or of plant and animal biodiversity—whether cally defined multivariate spaces (“morpho- regions that elicits, or conversely dampens, spaces”), most concordant or most out of phase major evolutionary novelty? Do these factors with taxonomic trends (8)? Paleontologists, change through time, and could there be un- Department of Geophysical Sciences, University of Chicago 5734 South Ellis Avenue Chicago, IL 60637, now taking advantage of advances in phyloge- suspected patterns in the types of novelties USA. E-mail: [email protected] netic methods, have just begun to explore these that conform to or break the spatial rules—for 2114 25 JUNE 1999 VOL 284 SCIENCE www.sciencemag.org E VOLUTION example, novelties generated in particular The initial state of the system must be a resolution geochemical methods, researchers ways, such as through changes in develop- crucial variable for understanding responses are beginning to focus on time intervals when mental timing? to perturbation, but this has been little ex- significant changes occurred in biological The long-simmering partnership of pale- plored. To take one possibility, do biotic materials and biogeochemical cycling, on ontology and developmental biology is heat- systems respond differently during global how steady states are maintained, and on the ing up nicely. Rapidly expanding knowledge greenhouse versus ice-age intervals (for ex- roles of biological innovations in perturbing of the molecular mechanisms underlying the ample, because greenhouse sea levels are and stabilizing those cycles. Biotic changes development of complex forms, and the re- higher and thus damp temperature variation) probably altered the nature of the fossil rec- markable conservation of developmental path- or according to the recency of similar pertur- ord itself, not only through increases in the ways among groups, can be combined with bations (for example, do taxa or ecosystems robustness of skeletal materials and the activ- the fossil record to provide a multilevel por- fatigue or harden with repeated stresses)? ities of predators, but also biogeochemically trait of the sequence of events leading to the General, predictive rules for the nonlinear because of burrowers that acidify pore waters origin and elaboration of complex features response of biological systems to perturba- by sediment irrigation (18). Such secular and such as arthropod limbs or the tetrapod skull tion will be of more than academic interest broadly fluctuating changes need to be quan- (12). given the stresses being imposed on today’s tified and factored into the next generation of Such interactions with developmental biosphere by human activities. large-scale paleontological analyses.