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Evidence that more than a third of Paleozoic articulate brachiopod genera (Strophomenata) lived infaunally
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 46 / Issue 3 / August 2020
- Published online by Cambridge University Press:
- 07 September 2020, pp. 405-433
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The Strophomenata, which includes two large orders, the Strophomenida and Productida, is the largest group of Paleozoic brachiopods. Nearly all uncemented strophomenatans possessed an unusual concave brachial valve. Most of these have been considered to have lived epifaunally, but had they rested on the seafloor, not only would they have faced intense predation, but their physical instability would have been fatal. I conclude that nearly all strophomenatans, like similar concavo-convex pectinid bivalves, lived infaunally by ejecting water to create a pit into which they descended, to be protected by sediment covering the concave valve. Strophomenatans have been discovered with this sediment preserved in place. If exhumed and turned upside down, a strophomenatan could have righted itself by squirting water. Many productides had anchoring spines, and some had hinge areas with stabilizing flanges. Small spines on the brachial valves of some productides served to trap disguising sediment. Evolutionary loss of hinge teeth within both the Strophomenida and Productida reduced the friction of valve clapping. Partly because of their slender shape, strophomenides were typically more vulnerable to exhumation than productides. Strophomenides also ejected water less effectively than productides and would have been less adept at righting themselves. The virtual disappearance of the strophomenides during the Devonian can be attributed to their vulnerability to intensifying benthic bulldozing and predation. The success of the productides during the late Paleozoic can be attributed to their relatively deep sequestration in the sediment and ability to right themselves and reburrow effectively when exhumed and overturned.
Competitive exclusion in evolutionary time: the case of the acorn barnacles
- Steven M. Stanley, William A. Newman
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- Journal:
- Paleobiology / Volume 6 / Issue 2 / Spring 1980
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- 17 July 2017, pp. 173-183
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Barnacles living along rocky shores provide the classic example of competitive dominance in the marine ecosystem: by means of firm attachment and rapid growth, balanoid barnacles commonly undercut and overgrow the genus Chthamalus, which is thereby restricted to the upper fringe of the intertidal zone, where balanoids are physiologically incapable of living. Today, after perhaps less than 50 Myr of evolution, balanoid barnacles are in the midst of rampant adaptive radiation, being represented by about 273 species, of which about half are free-living species of intertidal or shallow subtidal habitats. Chthamaloid barnacles, in contrast, are on the decline, having originated at least 70 Myr ago but today comprising only about 53 living species, approximately 40 of which occupy the uppermost intertidal. The remainder persist as localized, relict, and often disjunct populations. Through competitive exclusion, balanoid barnacles have apparently caused the ecological restriction and decline of the chthamaloids. The balanoids have an advanced feeding mechanism, but the most important adaptive breakthrough leading to their competitive success was probably the origin of a tubiferous wall structure, which affords rapid skeletal growth for the efficient monopolization of free space and for the destruction of chthamaloids.
Target Markets for Retail Outlets of Landscape Plants
- Steven C. Turner, Jeffrey H. Dorfman, Stanley M. Fletcher
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- Journal:
- Journal of Agricultural and Applied Economics / Volume 22 / Issue 1 / July 1990
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- 09 September 2016, pp. 177-184
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Merchandisers of landscape plants can increase the effectiveness of their marketing strategies by identifying target markets. Using a full information maximum likelihood tobit procedure on a system of three equations, target markets for different types of retail outlets in Georgia were identified. The results lend support and empirical evidence to the premise that different retail outlet types have different target markets and thus should develop different market strategies. The estimated target markets are identified and possible marketing strategies suitable for each type of retail outlet are suggested.
Memoir 4: An Analysis of the History of Marine Animal Diversity
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 33 / Issue S4 / Fall 2007
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- 09 September 2016, pp. 1-55
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According to when they attained high diversity, major taxa of marine animals have been clustered into three groups, the Cambrian, Paleozoic, and Modern Faunas. Because the Cambrian Fauna was a relatively minor component of the total fauna after mid-Ordovician time, the Phanerozoic history of marine animal diversity is largely a matter of the fates of the Paleozoic and Modern Faunas. The fact that most late Cenozoic genera belong to taxa that have been radiating for tens of millions of years indicates that the post-Paleozoic increase in diversity indicated by fossil data is real, rather than an artifact of improvement of the fossil record toward the present.
Assuming that ecological crowding produced the so-called Paleozoic plateau for family diversity, various workers have used the logistic equation of ecology to model marine animal diversification as damped exponential increase. Several lines of evidence indicate that this procedure is inappropriate. A plot of the diversity of marine animal genera through time provides better resolution than the plot for families and has a more jagged appearance. Generic diversity generally increased rapidly during the Paleozoic, except when set back by pulses of mass extinction. In fact, an analysis of the history of the Paleozoic Fauna during the Paleozoic Era reveals no general correlation between rate of increase for this fauna and total marine animal diversity. Furthermore, realistically scaled logistic simulations do not mimic the empirical pattern. In addition, it is difficult to imagine how some fixed limit for diversity could have persisted throughout the Paleozoic Era, when the ecological structure of the marine ecosystem was constantly changing. More fundamentally, the basic idea that competition can set a limit for marine animal diversity is incompatible with basic tenets of marine ecology: predation, disturbance, and vagaries of recruitment determine local population sizes for most marine species. Sparseness of predators probably played a larger role than weak competition in elevating rates of diversification during the initial (Ordovician) radiation of marine animals and during recoveries from mass extinctions. A plot of diversification against total diversity for these intervals yields a band of points above the one representing background intervals, and yet this band also displays no significant trend (if the two earliest intervals of the initial Ordovician are excluded as times of exceptional evolutionary innovation). Thus, a distinctive structure characterized the marine ecosystem during intervals of evolutionary radiation—one in which rates of diversification were exceptionally high and yet increases in diversity did not depress rates of diversification.
Particular marine taxa exhibit background rates of origination and extinction that rank similarly when compared with those of other taxa. Rates are correlated in this way because certain heritable traits influence probability of speciation and probability of extinction in similar ways. Background rates of origination and extinction were depressed during the late Paleozoic ice age for all major marine invertebrate taxa, but remained correlated. Also, taxa with relatively high background rates of extinction experienced exceptionally heavy losses during biotic crises because background rates of extinction were intensified in a multiplicative manner; decimation of a large group of taxa of this kind in the two Permian mass extinctions established their collective identity as the Paleozoic Fauna.
Characteristic rates of origination and extinction for major taxa persisted from Paleozoic into post-Paleozoic time. Because of the causal linkage between rates of origination and extinction, pulses of extinction tended to drag down overall rates of origination as well as overall rates of extinction by preferentially eliminating higher taxa having relatively high background rates of extinction. This extinction/origination ratchet depressed turnover rates for the residual Paleozoic Fauna during the Mesozoic Era. A decline of this fauna's extinction rate to approximately that of the Modern Fauna accounts for the nearly equal fractional losses experienced by the two faunas in the terminal Cretaceous mass extinction.
Viewed arithmetically, the fossil record indicates slow diversification for the Modern Fauna during Paleozoic time, followed by much more rapid expansion during Mesozoic and Cenozoic time. When viewed more appropriately as depicting geometric—or exponential—increase, however, the empirical pattern exhibits no fundamental secular change: the background rate of increase for the Modern Fauna—the fauna that dominated post-Paleozoic marine diversity—simply persisted, reflecting the intrinsic origination and extinction rates of constituent taxa. Persistence of this overall background rate supports other evidence that the empirical record of diversification for marine animal life since Paleozoic time represents actual exponential increase. This enduring rate makes it unnecessary to invoke environmental change to explain the post-Paleozoic increase of marine diversity.
Because of the resilience of intrinsic rates, an empirically based simulation that entails intervals of exponential increase for the Paleozoic and Modern Faunas, punctuated by mass extinctions, yields a pattern that is remarkably similar to the empirical pattern. It follows that marine animal genera and species will continue to diversify exponentially long into the future, barring disruption of the marine ecosystem by human-induced or natural environmental changes.
Presentation of the Paleontological Society Medal to Alfred G. Fischer
- Steven M. Stanley
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- Journal:
- Journal of Paleontology / Volume 70 / Issue 6 / November 1996
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- 20 May 2016, p. 1098
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Presentation of the Charles Schuchert Award of The Paleontological Society to Donald R. Prothero
- Steven M. Stanley
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- Journal of Paleontology / Volume 66 / Issue 4 / July 1992
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- 20 May 2016, p. 712
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Ideas on the timing of metazoan diversification
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 2 / Issue 3 / Summer 1976
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- 08 April 2016, pp. 209-219
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Fossil data suggest that the great majority of metazoan classes that existed in the Early Cambrian arose after about 700 my ago. The rectangular model of evolution, which views most evolutionary change as being concentrated in speciation events, can easily accommodate the implied rate of divergence.
If, as some authors believe, the eukaryotic cell arose long before the start of the Cambrian, an explanation is required for the delay of large-scale metazoan divergence. The advent of sexuality may have triggered diversification, not by accelerating phyletic evolution, as traditionally believed, but by making possible speciation and, hence, adaptive radiation. Another important delaying factor may have been the near-saturation of Precambrian algal systems in the absence of cropping by herbivores. Uncropped Precambrian systems should have been self-limiting in terms of diversification. When advanced heterotrophy finally arose, self-propagating feedback systems of diversification should have been set in motion. Even if the eukaryotic cell arose later than commonly envisioned and triggered the radiation of the Metazoa about 700 my ago, the earlier absence of sexuality and cropping may have delayed the transition from the prokaryotic condition to the eukaryotic condition.
Predation defeats competition on the seafloor
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 34 / Issue 1 / Winter 2008
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- 08 April 2016, pp. 1-21
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… the snail, whose tender horns being hit Shrinks backward in his shelly cave with pain, And there, all smothered up, in shade doth sit, Long after fearing to creep forth again….
— William Shakespeare, Venus and Adonis (1593)
For many decades, ecology textbooks presented classical competition theory without reservation. The central principle here is that two species sharing an essential resource that is in limited supply cannot coexist for long because the competitively superior species will eliminate the other one. The implication is that ecological communities should be characterized by division of resources among species, or niche partitioning. Thus, it is understandable that many paleontologists have continued to invoke concepts of competitive exclusion and niche partitioning in their studies of ancient guilds and communities. By now, however, there is a large body of neontological literature demonstrating that interspecific competition and resource partitioning play only a minor role in many ecological communities—especially benthic marine communities, which are the primary focus of the following discussion. Predation and physical disturbance inflict so much damage on biotas of the seafloor that populations of one species seldom monopolize a potentially limiting resource, except sporadically and locally. As a result, it is uncommon for any species to drive another to extinction through competitive exclusion—or even to force another species to drastically change its exploitation of any environmental resource throughout its geographic range. Furthermore, what particular species or group of species occupies a particular microhabitat is often simply a matter of time of arrival.
Rates of evolution
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 11 / Issue 1 / Winter 1985
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- 08 April 2016, pp. 13-26
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For some higher taxa, species can be identified in the fossil record with a high degree of reliability. The great geological durations of species indicate that phyletic evolution is normally so slow that little change occurs within a lineage during 105–107 generations. Failure to recognize sibling species in the fossil record has no bearing on this conclusion because they embody virtually no morphological change. Although slowness is the rule, we have no more precise assessment of morphological rates of phyletic evolution for any major taxon. Morphological data that have been assembled to assess rates of phyletic evolution have been meager, unrepresentative, and predominantly reflective of nothing more than body size. Net selection pressures within long segments of phylogeny—even ones that embrace large amounts of evolution—are infinitesimal and seemingly unsustainable against random fluctuations. This suggests that natural selection operates in a highly episodic fashion.
Rates of adaptive radiation and extinction at the species level can be estimated for many taxa and, from them, rates of speciation in adaptive radiation. Species selection should universally tend to increase rate of speciation and decrease rate of extinction, yet these rates are positively correlated in the animal world, apparently because they are linked by common controls: both rate of speciation and rate of extinction seem to increase with level of stereotypical behavior and to decrease with dispersal ability. Only a few “supertaxa” have been able to combine high rates of speciation with moderate rates of extinction.
Chronospecies' longevities, the origin of genera, and the punctuational model of evolution
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 4 / Issue 1 / Winter 1978
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- 08 April 2016, pp. 26-40
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Taxonomists working with late Cenozoic faunas tend to view living species as starting points for the evaluation of chronospecies (i.e., segments of evolutionary lineages subjectively designated as species) that extend backward in time from the Recent. This practice makes it possible to construct a survivorship curve for late Cenozoic chronospecies by evaluating all fossil lineages believed to have survived to the present day. A survivorship curve is produced by plotting the fraction of these lineages existing at any time that have not undergone enough phyletic evolution that their extant representatives are assigned to new species. This kind of surviviorship curve has been plotted for chronospecies of mammals using the beginning of the Würm, rather than the Recent, as an endpoint in order to avoid the effects of the Würm and post-Würm mass extinction. The survivorship curve reveals that all but a small fraction of established chronospecies have long durations relative to intervals of time during which distinctive higher taxa have arisen. Phyletic turnover of species has been remarkably slow. Most net evolutionary change must have been associated with saltational speciation. Even the large majority of genera must have arisen rapidly by one or more divergent speciation events. Estimates of rates of extinction suggest that the bottleneck effect, in which constriction of a lineage is followed by re-expansion as a distinct species, cannot be a major source of evolutionary change. These conclusions, based on the evaluation of mammalian phylogeny, seem also to apply to other taxa of animals, supporting the punctuational model of evolution. The long durations of hominid species imply that the evolution of humans, like that of other mammals, conforms to this model.
Approximate evolutionary stasis for bivalve morphology over millions of years: a multivariate, multilineage study
- Steven M. Stanley, Xiangning Yang
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- Journal:
- Paleobiology / Volume 13 / Issue 2 / Spring 1987
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- 08 April 2016, pp. 113-139
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We have calculated rates of evolution for 19 lineages of Neogene bivalves over time intervals ranging from 1 ma to 17 ma. Our morphometric comparisons are based on 24 variables, for which we have made more than 43,000 individual measurements normalized for shell size. We initially assessed evolutionary changes in shape for 19 early Pliocene (4 ma old) species of bivalves by comparing these forms to their closest living relatives, which in 12 cases have traditonally been assigned to the same species. To make our study unbiased and comprehensive, we included all species that met certain predetermined guidelines and that offered suitable fossil material for measurement. We compared early Pliocene and Recent populations using (1) all 24 variables treated separately, (2) multivariate distance (Mahalanobis' D), based on the full set of variables, and (3) eigenshapes for shell outlines. For these comparisons, we used as a yardstick the same measures of morphologic distance applied to pairs of geographically separated Recent populations that belong to eight of the living species to which the fossil populations were compared. As it turns out, with minor exceptions, the distribution of morphologic distances between 4 ma old and Recent populations resembled the distribution of distances between conspecific Recent populations.
We calculated net rates of evolution separating pairs of populations that belong to single lineages. For all intervals of time, the distribution of differences between population means for individual variables is remarkably similar to a comparable distribution representing the comparison of pairs of conspecific Recent populations from separate geographic regions. Because morphologic differences between populations do not vary greatly with evolutionary time, measured “rates” of evolution, on the average, decrease with interval of measurement. Because these differences resemble intraspecific variability, however, the rates do not represent significant evolution. Evolution has followed a weak zigzag course, yielding only trivial net trends.
The weak and reversible “trends” that we measured yield net rates averaging less than 10 millidarwins, which is much lower than most rates previously reported for marine invertebrates (average ~200 millidarwins for a 1 ma interval and ~60 millidarwins for a 10 ma interval). We attribute this disparity (1) to the fact that most previously published rates have been calculated when a significant amount of evolution was recognized in advance (often for a poorly documented lineage) and (2) to the fact that most measured variables have represented nothing more than some measure of body size. We conclude that shape, as opposed to size, has been highly stable in bivalve evolution over millions of years and 106–107 generations. We conclude that to characterize rates or evolution for any group of organisms, one must employ a large, unbiased sample of measurements for numerous well-documented lineages, and one must segregate data depicting shape from data depicting size.
Why clams have the shape they have: an experimental analysis of burrowing
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 1 / Issue 1 / Winter 1975
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- 08 April 2016, pp. 48-58
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The prosogyrous shape and flattened lunule of a typical clam shell form a blunt anterior, the function of which is related to the forward-and-back rocking motion of the shell in burrowing. Analysis of movies has revealed that each rocking motion of a morphologically typical clam, Mercenaria mercenaria (Linné), involves purely rotational movement, with no translational component. The clam is able to burrow by “walking” its way downward only because the axis of backward rotation lies to the anterior of the axis of forward rotation. Experiments with burrowing robots show that the blunt anterior serves to shift the axis of backward rotation anteriorly, thus aiding in downward progress. The prosogyrous condition and the rotational mechanism of burrowing are fundamental adaptations of burrowing clams and were apparently present in the ancestral bivalves of the Cambrian.
Delayed recovery and the spacing of major extinctions
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 16 / Issue 4 / Fall 1990
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- 08 April 2016, pp. 401-414
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Approximate periodicity for peak rates of global extinction during the past 250 m.y. may have resulted from delayed recovery following major extinction events. Two components can be envisioned for such delays: persistence of inimical environmental conditions for some time after the onset of the crisis, and slow restoration of vulnerable taxa. This general hypothesis is consistent with statistical evidence of linkage between measured rates of extinction of marine invertebrate genera for contiguous stages and substages of the geologic column. The nine broad valleys between the “periodic” peak rates for the past 250 m.y. exhibit only three trivial secondary peaks, indicating that, if the pattern is not artifactual, trends in global rates of extinction have not readily been abruptly reversed. Moreover, the smooth observed trends reflect the fact that regional crises tend to remove many species but few genera. To some degree, high rates of extinction that precede peak rates must represent bias of the incomplete fossil record (the Signor-Lipps effect). High rates that immediately follow peak rates also may, to a degree, reflect biological legacy: (1) final extinction of weakened genera or (2) extinction of new genera that contain few species or represent failed evolutionary “experiments.” Nonetheless, there is evidence that protracted intervals of stressful environmental conditions contributed to high rates of extinction preceding or following certain peak intervals, including the Scythian, Cenomanian, Early Paleocene, and Early Oligocene. The reef-building rudists, for example, suffered heavy extinction during both Cenomanian and Turonian time and then failed to recover quickly.
The late Neogene record of bivalve molluscs in the Western Atlantic offers a more detailed picture of delayed recovery. Here early intervals of glacial expansion caused heavy extinction, leaving an impoverished, eurythermal fauna that was virtually unaffected by late Pleistocene glacial episodes. The episode of heavy extinction in Late Eocene time exhibits a similar phenomenon on a worldwide scale. Among the planktonic foraminifera, warm-adapted stenothermal species died out, and eurythermal forms predominated throughout Oligocene time; restoration of vulnerable, stenothermal species proceeded gradually during the Miocene Epoch. This example of delayed recovery and others like it following earlier global crises may have prevented such crises from following one another in rapid succession, yielding an appearance of periodicity.
Population size, extinction, and speciation: the fission effect in Neogene Bivalvia
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 12 / Issue 1 / Winter 1986
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- 08 April 2016, pp. 89-110
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The extinction of a species represents reduction of both geographic range and population size to zero. Most workers have focused on geographic range as a variable strongly affecting the vulnerability of established species to extinction, but Lyellian percentages for Neogene bivalve faunas of California and Japan suggest that population size is a more important variable along continental shelves. The data employed to reach this conclusion are Lyellian percentages for latest Pliocene (∼2 ma old) bivalve faunas of California and Japan (N = 245 species). These regions did not suffer heavy extinction during the recent Ice Age, and for each region the Lyellian percentage is 70%–71%.
Discrepancies in population size appear to explain the following differences in survivorship to the Recent (Lyellian percentage) for three pairs of subgroups: (1) burrowing nonsiphonate species (42%) versus burrowing siphonate species (84%), which suffer less heavy predation; (2) burrowing nonsiphonate species of small size (73%) versus burrowing nonsiphonate species of large body size (96%); (3) Pectinacea (30%) versus other epifauna (71%), which suffer less heavy predation. During the Mesozoic Era, when predation was less effective in benthic settings, mean species duration for the Pectinacea was much greater (∼20 ma).
Along the west coast of North and Central America, mean geographic range is greater for siphonate species of large body size than for siphonate species of small body size and greater still for pectinacean species. These ranges are inversely related to mean species longevity for the three groups, which indicates that geographic range is not of first-order importance in influencing species longevity. Species with nonplanktotrophic development neither exhibit narrow geographic ranges along the west coast of North and Central America nor have experienced high rates of extinction in California and Japan.
Rates of extinction are so high for Neogene pectinaceans and nonsiphonate burrowers that without enjoying high rates of speciation these groups could not exist at the diversities they have maintained during the Neogene Period. They are apparently speciating rapidly because of the fission effect: the relatively frequent generation of new species from populations that are fragmented by heavy predation. Thus, ironically, there may be a tendency for high rates of speciation to be approximately offset by high rates of extinction. Only if mean population size for species in a particular group becomes extremely small is it likely to result in a high rate of extinction and a low rate of speciation—and hence a dramatic decline of the group. The fission effect may contribute to the general correlation in the animal world between rate of speciation and rate of extinction.
Competition wins out overall: Reply to Paine
- William A. Newman, Steven M. Stanley
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- Journal:
- Paleobiology / Volume 7 / Issue 4 / Fall 1981
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- 08 February 2016, pp. 561-569
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A number of facts indicate that the diversity of chthamaloid barnacles is much lower now than it has been in the past. Among these are the relict, often disjunct geographic distributions of distinctive chthamaloid higher taxa and their representation by only a few living species. The living chthamaloid barnacles also have a disjunct bathymetric distribution: as we will document here in detail, they are concentrated in the intertidal zone and in deep water. In his rejoinder to our paper, Paine makes no cogent argument against our interpretations of this powerful evidence.
Our case that the balanoids have competitively displaced the more primitive chthamaloids is based on several facts: (1) the Chthamaloidea have dwindled during the Cenozoic Era, so that most have relict or refugial distributions, whereas the Balanoidea have undergone a remarkable adaptive radiation; (2) balanoid species are known to defeat chthamaloid species in competition for space; (3) the competitive success of the balanoids can be attributed to an inherent biological feature—a tubiferous skeleton, which fosters rapid growth; (4) the peak diversity of living balanoids coincides precisely with the bathymetric gap in the chthamaloids’ distribution.
We argue that the radiation of balanoids during the past 40 Myr or so has caused a decline in chthamaloid diversity by reducing and destabilizing low intertidal and shallow subtidal populations, thus elevating average rate of extinction and depressing average rate of speciation. This model allows for exceptions to the rule of balanoid competitive dominance, but contrary to Paine's claim, the exceptions that exist are either trivial or support our view.
Paine offers no alternative to our competition hypothesis. He suggests that an increase in predation has caused a decline in chthamaloid body size, but neither a Cenozoic trend in body size nor one in predation pressure has empirical support; furthermore, the body-size hypothesis has no direct bearing on the case we have presented.
Natural clades differ from “random” clades: simulations and analyses
- Steven M. Stanley, Philip W. Signor III, Scott Lidgard, Alan F. Karr
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- Journal:
- Paleobiology / Volume 7 / Issue 1 / Winter 1981
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- 08 February 2016, pp. 115-127
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Using computer simulations and analytic calculations, we have evaluated whether conspicuous expansions and contractions of natural clades may have represented chance fluctuations that occurred while probabilities of speciation and extinction remained equal and constant. Our results differ from those of previous workers, who have not scaled generating parameters empirically at the species level. We have found that the waxing and waning of many real clades have almost certainly resulted from changes in probabilities of speciation and extinction. For some of these changes, likely explanations are evident. The emplacement of adaptive innovations, for example, has at times elevated probability of speciation. We conclude that chance factors have not played a dominant role in producing dramatic changes in standing diversity within speciose higher taxa.
Infaunal survival: alternative functions of shell ornamentation in the Bivalvia (Mollusca)
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 7 / Issue 3 / Summer 1981
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- 08 February 2016, pp. 384-393
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Two kinds of adaptation aid infaunal bivalve mollusks in sustaining their life positions against potentially disruptive water movements: (1) ability to retard scour of surrounding sediment and (2) ability to reburrow rapidly if exhumed.
The shell of the venerid Anomalocardia brasiliana is ornamented with concentric ridges that are asymmetrical in cross-section; experiments show that these ridges aid the animal in burrowing by gripping the sediment during backward rotation of the shell. The shell of the less deeply burrowing venerid Chione cancellata also bears concentric ridges, but these are symmetrical and experiments show that they hinder burrowing; other experiments demonstrate that these ridges reduce scour of sand from around a partly exposed animal. The single cardiid species Trachycardium egmontianum possesses adaptations comparable to those of the two venerid species, in the form of spines of two varieties; experiments show that the flared anterior spines facilitate burrowing and the cupped posterior spines reduce scour.
Conspicuous ornamentation of the kinds considered here was rare throughout the Paleozoic. Its evolutionary deployment occurred primarily during the post-Paleozoic adaptive diversification of infaunal bivalves, which was triggered by the evolution of efficient burrowing mechanisms. The general premium on maintaining infaunal life positions was accentuated for bivalves after the Paleozoic by the origins of important modern predatory taxa.
An ecological theory for the origin of Homo
- Steven M. Stanley
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- Journal:
- Paleobiology / Volume 18 / Issue 3 / Summer 1992
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- 08 February 2016, pp. 237-257
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The genus Homo evolved its pronounced encephalization through postnatal extension of the high rate of brain growth that characterizes all primates in utero. Linked to this extension was delayed development, which represented an enormous ecological sacrifice because it produced the longest postnatal interval of physical helplessness in the Mammalia and forced mothers to carry infants.
Graphs relating brain growth to body growth indicate a pongid pattern of development for gracile australopithecines, implying that infants could cling to mothers whose forelimbs were occupied with climbing. Also present were several postcranial traits that would have made the adults more adept climbers than modern humans. Habitual use of these inherited traits is suggested by the fact that evolution failed to eliminate certain ones, such as short legs and long pedal phalanges, that impeded terrestrial locomotion. Moreover, the intensity of predation by large, swift, social carnivores must have compelled australopithecines to use trees as refuges, in the manner of chimpanzees and baboons; australopithecines probably also gathered some of their food in trees. Gracile australopithecines failed to expand their brain size, experiencing general evolutionary stasis for more than 1.5 m.y. I propose that this stability resulted from these animals' semiarboreal mode of life: First, their postcranial morphology remained compromised by selection pressures to maintain both terrestrial and arboreal adaptations. Second, by requiring that neonates be mature enough to cling to mothers, obligate arboreal activity precluded encephalization of the kind that characterizes Homo; this evolutionary constraint has previously been overlooked.
In contrast to australopithecines, early Homo approached H. erectus in pelvic configuration and brain size. A new brain-body growth curve for early Homo indicates extension of the fetal pattern well into the postnatal interval, implying that neonates were highly immature so that adults had to be fully terrestrial. Homo evolved shortly after the onset of the modern ice age about 2.5 Ma. Fossil pollen and carbon isotopes in paleosols record a contraction of forests in Africa at this time. I propose that this represented a crisis that led to the evolution of Homo by compelling some australopithecine populations to adopt a fully terrestrial existence. Although ecologically difficult, this behavioral restriction finally made possible encephalization through the evolution of delayed development. During the ecological crisis, a large brain evolved in at least one population of gracile australopithecines. An advanced tool industry and cunning behavior were of such great adaptive value for avoiding predators and expanding food resources on the ground that selection for encephalization soon overrode the problems imposed by helpless infants.
The fossil record of antelopes and micromammals provides a test of the idea that environmental forcing opened the way for the evolution of Homo: both of these groups experienced heavy extinction of forest-adapted species about 2.5–2.4 Ma and a rapid proliferation of species adapted to unforested habitats. The transformation of the hominid clade during Plio-Pleistocene time did not follow a simple pattern. Homo may have arisen either by anagenetic transformation of a “bottlenecked” species or by speciation, and it may not have evolved immediately with the onset of climatic change. Furthermore, just as a few forest-adapted antelope species survived the biotic crisis, a small-brained gracile taxon with arboreal adaptations may have persisted to the start of the Pleistocene. Robust australopithecines survived into the Pleistocene, perhaps because a broad vegetarian diet reduced their need to migrate frequently between home bases. With their extinction in mid-Pleistocene time, about the time that savannahs became widespread, only Homo remained.
Macroevolutionary differences between the two major clades of Neogene planktonic foraminifera
- Steven M. Stanley, Karen L. Wetmore, James P. Kennett
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- Journal:
- Paleobiology / Volume 14 / Issue 3 / Summer 1988
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- 08 February 2016, pp. 235-249
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Being of especially high quality, the Neogene fossil record of planktonic foraminifera offers special opportunities for assessing patterns of extinction and speciation. A variety of metrics indicates that within this group the mean duration of lineages has been much shorter (rate of extinction has been higher) for the globorotaliid clade than for the globigerinid clade. Furthermore, in the globorotaliid clade rates of extinction and speciation have not been closely linked to changes in diversity, but rather have been relatively high even at times when diversity has undergone little change. Thus, the globorotaliid clade has undergone more rapid evolutionary turnover than the globigerinid clade. Data for living species reveal that neither geographic range nor temperature tolerance is the primary factor controlling lineage duration. On the other hand, there is evidence that lineages marked by low abundance (small population size) are relatively short-lived. The reason that globorotaliid lineages have generally survived for shorter intervals, on the average, may be that their populations have been less abundant and less stable. Usually they live deeper in the water column, where food is often sparse, and many flourish only in areas of upwelling. Furthermore, the globorotaliids lack symbiotic algae for nutritional support. The same ecological factors may have accelerated speciation in the globorotaliid clade, by causing species to be patchily distributed. Thus, population size and structure have been more important than geographic range in determining rates of extinction and speciation. This parallels the situation for Neogene marine bivalves.
For planktonic foraminifera, as for Western Atlantic Bivalvia, the normal pattern of extinction was reversed in late Pliocene time, apparently in response to climatic cooling. The globigerinids suffered a sudden pulse of extinction. The deeper dwelling globorotaliids fared better; probably many of their species benefited from elevation of the seasonal thermocline into the photic zone. At the same time, rate of speciation declined in the globorotaliid clade, which supports the idea, inferred from the evolutionary history of marine bivalves, that an increase in the size and stability of populations should depress both rate of extinction and rate of speciation.
Biogeographic patterns and Plio-Pleistocene extinction of Bivalvia in the Mediterranean and southern North Sea
- Sergio Raffi, Steven M. Stanley, Raffaella Marasti
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- Journal:
- Paleobiology / Volume 11 / Issue 4 / Fall 1985
- Published online by Cambridge University Press:
- 08 February 2016, pp. 368-388
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An evaluation of the history of polysyringian species (filibranchs and eulamellibranchs) reveals that the huge early Pliocene bivalve fauna of the Mediterranean Basin (MB) and North Sea Basin (NSB) suffered heavy extinction during late Pliocene and early Pleistocene time. This is evidenced by low survivorship of early Pliocene species in the Recent and by a decline in species richness of the two basins from 323 known early Pliocene species to 198 living species.
Several kinds of evidence indicate that cooling rather than the areal effect of eustatic sea-level lowering was the primary cause of the excessive extinction: (1) heavy Plio-Pleistocene extinction of mollusks was not global but concentrated around the margins of the northern Atlantic—an ocean fringed by polar ice caps; (2) taxa of tropical affinities were most severely affected; (3) heavy extinction occurred in the MB in areas not marked by facies change; (4) in the MB the onset of extinctions coincided with the onset elsewhere, but because of tectonic activity, water depths in the MB were not under tight eustatic control; (5) 14 species present in both the MB and NSB during early Pliocene time are now restricted to waters south of the NSB; and (6) the majority of species common to the two basins during the early Pliocene (eurythermal species) have survived to the present.
Molluscan data support palynological evidence that the climate in the MB was warmer and less seasonal in early Pliocene time than today, when latitudinal temperature gradients are steeper. Molluscan evidence indicates that the North Sea is exceptional in being less seasonal (though cooler) today than in early Pliocene time, and we attribute this anomaly to the local effects of the Gulf Stream, which was strengthened in mid-Pliocene time by the uplift of the Isthmus of Panama.
The heavy extinction in the MB and NSB about 3.2–3.0 ma ago approximately coincided with the earliest deposition of glacial tills in Iceland and with isotopic shifts in the tests of planktonic foraminifers preserved in deep-sea cores. Additional heavy extinction probably coincided with a pulse of severe cooling in late Pliocene time, 2.5–2.4 ma ago. Heavy extinction of mollusks in the MB and NSB continued into the early Pleistocene but not into the middle and late Pleistocene, apparently because by this time it was primarily only eurythermal species that survived. Today the molluscan faunas of the MB and NSB are unusually eurythermal; few species are restricted to a single biogeographic province.