John Alroy

Phanerozoic trends in the global diversity of marine invertebrates.

It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

Effects of sampling standardization on estimates of Phanerozoic marine diversification.

Global diversity curves reflect more than just the number of taxa that have existed through time: they also mirror variation in the nature of the fossil record and the way the record is reported. These sampling effects are best quantified by assembling and analyzing large numbers of locality-specific biotic inventories. Here, we introduce a new database of this kind for the Phanerozoic fossil record of marine invertebrates. We apply four substantially distinct analytical methods that estimate taxonomic diversity by quantifying and correcting for variation through time in the number and nature of inventories. Variation introduced by the use of two dramatically different counting protocols also is explored. We present sampling-standardized diversity estimates for two long intervals that sum to 300 Myr (Middle Ordovician-Carboniferous; Late Jurassic-Paleogene). Our new curves differ considerably from traditional, synoptic curves. For example, some of them imply unexpectedly low late Cretaceous and early Tertiary diversity levels. However, such factors as the current emphasis in the database on North America and Europe still obscure our view of the global history of marine biodiversity. These limitations will be addressed as the database and methods are refined.

Dynamics of origination and extinction in the marine fossil record

The discipline-wide effort to database the fossil record at the occurrence level has made it possible to estimate marine invertebrate extinction and origination rates with much greater accuracy. The new data show that two biotic mechanisms have hastened recoveries from mass extinctions and confined diversity to a relatively narrow range over the past 500 million years (Myr). First, a drop in diversity of any size correlates with low extinction rates immediately afterward, so much so that extinction would almost come to a halt if diversity dropped by 90%. Second, very high extinction rates are followed by equally high origination rates. The two relationships predict that the rebound from the current mass extinction will take at least 10 Myr, and perhaps 40 Myr if it rivals the Permo-Triassic catastrophe. Regardless, any large event will result in a dramatic ecological and taxonomic restructuring of the biosphere. The data also confirm that extinction and origination rates both declined through the Phanerozoic and that several extinctions in addition to the Permo-Triassic event were particularly severe. However, the trend may be driven by taxonomic biases and the rates vary in accord with a simple log normal distribution, so there is no sharp distinction between background and mass extinctions. Furthermore, the lack of any significant autocorrelation in the data is inconsistent with macroevolutionary theories of periodicity or self-organized criticality.

The shifting balance of diversity among major marine animal groups.

No Guide to the Future Although fossils can provide glimpses of evolution, the accuracy for predictions made on the basis of commonality among geographically and hierarchically distinct taxa is unknown. Alroy (p. 1191; see the Perspective by Marshall) looked at the global marine fossil record and found that major taxonomic groups of animals have distinctive patterns of diversification and unpredictable responses to mass extinctions. Extrapolating from the current ongoing extinction event predicted a geologically rapid rebound of a number of species, although the future taxonomic composition of the marine biosphere could not be forecast. Future assemblies of animals following mass extinction cannot be predicted by analyses of Phanerozoic fossils. The fossil record demonstrates that each major taxonomic group has a consistent net rate of diversification and a limit to its species richness. It has been thought that long-term changes in the dominance of major taxonomic groups can be predicted from these characteristics. However, new analyses show that diversity limits may rise or fall in response to adaptive radiations or extinctions. These changes are idiosyncratic and occur at different times in each taxa. For example, the end-Permian mass extinction permanently reduced the diversity of important, previously dominant groups such as brachiopods and crinoids. The current global crisis may therefore permanently alter the biosphere’s taxonomic composition by changing the rules of evolution.

New methods for quantifying macroevolutionary patterns and processes

Abstract This paper documents a series of methodological innovations that are relevant to macroevolutionary studies. The new methods are applied to updated faunal and body mass data sets for North American fossil mammals, documenting several key trends across the late Cretaceous and Cenozoic. The methods are (1) A maximum likelihood formulation of appearance event ordination. The reformulated criterion involves generating a maximally likely hypothesized relative ordering of first and last appearances (i.e., an age range chart). The criterion takes faunal occurrences, stratigraphic relationships, and the sampling probability of individual genera and species into account. (2) A nonparametric temporal interpolation method called “shrink-wrapping” that makes it possible to employ the greatest possible number of tie points without violating monotonicity or allowing abrupt changes in slopes. The new calibration method is used in computing provisional definitions of boundaries among North American land mammal ages. (3) Additional methods for randomized subsampling of faunal lists, one weighting the number of lists that have been drawn by the sum of the square of the number of occurrences in each list, and one further modifying this approach to account for long-term changes in average local species richness. (4) Footes new equations for instantaneous speciation and extinction rates. The equations are rederived and used to generate time series, confirm that logistic dynamics result from the diversity dependence of speciation but not extinction, and define the median duration of species (i.e., 2.6 m.y. for Eocene–Pleistocene mammals). (5) A method employing the G likelihood ratio statistic that is used to quantify the volatility of changes in the relative proportion of species falling in each of several major taxonomic groups. (6) Univariate measures of body mass distributions based on ordinary moment statistics (mean, standard deviation, skewness, kurtosis). These measures are favored over the method of cenogram analysis. Data are presented showing that even diverse individual fossil collections merely yield a noisy version of the same pattern seen in the overall continental data set. Peaks in speciation rates, extinction rates, proportional volatility, and shifts in body mass distributions occur at different times, suggesting that environmental perturbations do not have simple effects on the biota.

Constant extinction, constrained diversification, and uncoordinated stasis in North American mammals

Abstract The coordinated stasis model has far-reaching implications. Among them are three important predictions concerning diversity dynamics that I test here against the Cenozoic fossil record of terrestrial North American mammals. First, origination and extinction rates should be correlated; second, turnover should be a composite function of very low background rates and occasional, dramatic turnover pulses; and finally, stasis should result from ecological (niche) incumbency, with the domains of incumbent species being defined by ecological similarity, which in the case of mammals corresponds closely with taxonomic affinity. The data used to test these hypotheses are standing diversity levels and counts of originations and extinctions for 1193 genera and 3161 species. Instead of relying on a traditional time scale comprised of “ages” having uneven and unpredictable durations, the diversity curve is computed directly from a multivariate ordination of 3870 faunal lists, and then sectioned into 1.0 m.y. intervals. The lists span the late Cretaceous through late Pleistocene interval, exclusive of the Wisconsinan, and are taxonomically standardized to remove junior synonyms, out-dated combinations, and nomina dubia. Because Cretaceous and Paleocene diversity dynamics are idiosyncratic, only the last 55 intervals (Eocene-Pleistocene: 55-0.01 Ma) are analyzed. The test of origination and extinction rates shows that an apparent correlation between them is a statistical artifact related to the necessary coincidence of first and last appearances for taxa known from just one interval. The test of variation in turnover shows that most of the observed extinction rates could be generated by a single, invariant underlying rate, wheras origination rates show many well-defined pulses. Furthermore, origination pulses within particular orders are not fully coincident. The very largest pulses of origination therefore seem to be mediated by key adaptations within particular groups, not by the general opportunity to fill niches opened up by extinction. Both of these tests argue against the idea of sweeping “reorganization” intervals bounding placid “stasis” intervals, and against Vrbas turnover pulse hypothesis. Finally, tests for niche incumbency, based on plots of per-taxon turnover rates against standing diversity, show that incumbency is widespread and mediated by the suppression of origination at high diversity levels in all groups. Extinction is a far less important controlling factor. Because orders are ecologically distinct, but random subsamples of the entire data set actually show stronger controls than groupings based on ordinal affinity, it appears that niche space has little or no important ecological substructuring. Therefore, mammalian diversity seems to be integrated at the highest possible taxonomic level, in opposition to the coordinated stasis concept of static guilds. On balance, the results indicate that although the data are robust and provide strong support for the niche incumbency model and the idea of diversity equilibrium, they generally disconfirm the unique predictions of coordinated stasis.

THE FOSSIL RECORD OF NORTH AMERICAN MAMMALS : EVIDENCE FOR A PALEOCENE EVOLUTIONARY RADIATION

Paleontologists long have argued that the most important evolutionary radiation of mammals occurred during the early Cenozoic, if not that all eutherians originated from a single common post-Cretaceous ancestor. Nonetheless, several recent molecular analyses claim to show that because several interordinal splits occurred during the Cretaceous, a major therian radiation was then underway. This claim conflicts with statistical evidence from the well-sampled latest Cretaceous and Cenozoic North American fossil record. Paleofaunal data confirm that there were fewer mammalian species during the latest Cretaceous than during any interval of the Cenozoic, and that a massive diversification took place during the early Paleocene, immediately after a mass extinction. Measurement data show that Cretaceous mammals were on average small and occupied a narrow range of body sizes; after the Cretaceous-Tertiary mass extinction, there was a rapid and permanent shift in the mean. The fact that there was an early Cenozoic mammalian radiation is entirely compatible with the existence of a few Cretaceous splits among modern mammal lineages.

Global climate change and North American mammalian evolution

Abstract We compare refined data sets for Atlantic benthic foraminiferal oxygen isotope ratios and for North American mammalian diversity, faunal turnover, and body mass distributions. Each data set spans the late Paleocene through Pleistocene and has temporal resolution of 1.0 m.y.; the mammal data are restricted to western North America. We use the isotope data to compute five separate time series: oxygen isotope ratios at the midpoint of each 1.0-m.y. bin; changes in these ratios across bins; absolute values of these changes (= isotopic volatility); standard deviations of multiple isotope measurements within each bin; and standard deviations that have been detrended and corrected for serial correlation. For the mammals, we compute 12 different variables: standing diversity at the start of each bin; per-lineage origination and extinction rates; total turnover; net diversification; the absolute value of net diversification (= diversification volatility); change in proportional representation of major orders, as measured by a simple index and by a G-statistic; and the mean, standard deviation, skewness, and kurtosis of body mass. Simple and liberal statistical analyses fail to show any consistent relationship between any two isotope and mammalian time series, other than some unavoidable correlations between a few untransformed, highly autocorrelated time series like the raw isotope and mean body mass curves. Standard methods of detrending and differencing remove these correlations. Some of the major climate shifts indicated by oxygen isotope records do correspond to major ecological and evolutionary transitions in the mammalian biota, but the nature of these correspondences is unpredictable, and several other such transitions occur at times of relatively little global climate change. We conclude that given currently available climate records, we cannot show that the impact of climate change on the broad patterns of mammalian evolution involves linear forcings; instead, we see only the relatively unpredictable effects of a few major events. Over the scale of the whole Cenozoic, intrinsic, biotic factors like logistic diversity dynamics and within-lineage evolutionary trends seem to be far more important.

Similarity of Mammalian Body Size across the Taxonomic Hierarchy and across Space and Time

Although it is commonly assumed that closely related animals are similar in body size, the degree of similarity has not been examined across the taxonomic hierarchy. Moreover, little is known about the variation or consistency of body size patterns across geographic space or evolutionary time. Here, we draw from a data set of terrestrial, nonvolant mammals to quantify and compare patterns across the body size spectrum, the taxonomic hierarchy, continental space, and evolutionary time. We employ a variety of statistical techniques including “sib‐sib” regression, phylogenetic autocorrelation, and nested ANOVA. We find an extremely high resemblance (heritability) of size among congeneric species for mammals over ∼18 g; the result is consistent across the size spectrum. However, there is no significant relationship among the body sizes of congeneric species for mammals under ∼18 g. We suspect that life‐history and ecological parameters are so tightly constrained by allometry at diminutive size that animals can only adapt to novel ecological conditions by modifying body size. The overall distributions of size for each continental fauna and for the most diverse orders are quantitatively similar for North America, South America, and Africa, despite virtually no overlap in species composition. Differences in ordinal composition appear to account for quantitative differences between continents. For most mammalian orders, body size is highly conserved, although there is extensive overlap at all levels of the taxonomic hierarchy. The body size distribution for terrestrial mammals apparently was established early in the Tertiary, and it has remained remarkably constant over the past 50 Ma and across the major continents. Lineages have diversified in size to exploit environmental opportunities but only within limits set by allometric, ecological, and evolutionary constraints.

Plant and mammal diversity in the Paleocene to early Eocene of the Bighorn Basin

Abstract Abundant plant and vertebrate fossils have been recovered from fluvial sediments deposited in the Bighorn Basin, Wyoming, during the first 13 m.y. of the Tertiary. Here we outline and discuss changes in the composition and diversity of floras and faunas during this period, which includes the recovery of terrestrial ecosystems from the K/T boundary extinctions, and later, during the Paleocene-Eocene transition, the greatest global warming of the Cenozoic. Floral diversity has been studied at three levels of spatial resolution: sub-local (at individual collecting sites), local (along a single bed or stratigraphic horizon), and basin-wide (regional). Sub-local diversity shows a moderate increase from the early to late Paleocene, followed by a decrease across the Paleocene/Eocene boundary, then an increase into the later early Eocene. Local heterogeneity was lower in Paleocene backswamp floras, although distinct groups of species dominated in different local fluvial settings such as backswamps and alluvial ridges. Heterogeneity of backswamp forests increased by about 65% from the early to late Wasatchian (early Eocene). The number of plant species inferred from the Bighorn Basin dataset rose gradually from the Puercan to an early Clarkforkian peak of about 40 species, declined sharply to about 25 species by the Clarkforkian/Wasatchian boundary, then rose through the Wasatchian to about 50 species. A regional analysis of mammalian genera shows high turnover and a rapidly increasing number of genera within a million years of the K/T boundary (10–50 genera), a slight decline to 40 genera by the early Clarkforkian, then an increase from 40 to 75 genera by the late Wasatchian. Our analyses found no major extinctions in mammals during the Paleocene and early Eocene in the Bighorn Basin, but a one-third decrease in the number of plant species at about the Paleocene/Eocene boundary. Rates of taxonomic turnover were much higher for mammals than plants. The diversity trends for plants and mammals show little congruence, implying that the two groups responded in a very different manner to post K/T extinction opportunities. There is also little congruence between plant diversity levels and change in mean annual temperature (MAT) as inferred from foliar physiognomy.