Scott Lidgard

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.

Angiosperm Diversification and Paleolatitudinal Gradients in Cretaceous Floristic Diversity

The latitudinally diachronous appearance of angiosperm pollen during the Cretaceous is well documented, but the subsequent diversification and accompanying significant changes in floristic dominance have not been assessed quantitatively for a wide range of paleolatitudes. Trend surfaces fitted to within-palynoflora diversity data from 1125 pollen and spore assemblages show that angiosperms first become floristically prominent in low paleolatitude areas(∼20�N to 20�S). Non-magnoliid dicotyledons show a similar but slightly delayed pattern of increase and are the principal component of angiosperm diversity from all areas sampled. Monocotyledons and magnoliid dicotyledons are significant primarily in low to middle paleolatitude palynofloras(∼50�N to 20�S) during the latest Cretaceous. As angiosperms become increasingly prevalent the importance of most non-angiosperm taxa either decreases or remains unchanged. The only apparent exception is a striking increase in gnetalean diversity concurrent with the initial angiosperm diversification at low paleolatitudes.

Abundance distributions imply elevated complexity of post-Paleozoic marine ecosystems

Likelihood analyses of 1176 fossil assemblages of marine organisms from Phanerozoic (i.e., Cambrian to Recent) assemblages indicate a shift in typical relative-abundance distributions after the Paleozoic. Ecological theory associated with these abundance distributions implies that complex ecosystems are far more common among Meso-Cenozoic assemblages than among the Paleozoic assemblages that preceded them. This transition coincides not with any major change in the way fossils are preserved or collected but with a shift from communities dominated by sessile epifaunal suspension feeders to communities with elevated diversities of mobile and infaunal taxa. This suggests that the end-Permian extinction permanently altered prevailing marine ecosystem structure and precipitated high levels of ecological complexity and alpha diversity in the Meso-Cenozoic.

Comparing palynological abundance and diversity: implications for biotic replacement during the Cretaceous angiosperm radiation

The Cretaceous radiation of angiosperms initiated a major reorganization of terrestrial plant communities as dominance by pteridophytic and gymnospermic groups eventually gave way to dominance by angiosperms. Previously, patterns of biotic replacement have been assessed using measures based on taxonomic diversity data. However, using measures of both abundance and diversity to investigate replacement patterns provides more information about macroecological change in the fossil record than either can provide alone. Analyses of an updated and expanded database of North American palynological samples from Cretaceous sediments document a rapid increase in angiosperm diversity and abundance within individual fossil palynofloras (represent- ing local/subregional vegetation). New analyses of floristic diversity patterns support previous results and indicate that the decline of free-sporing plants is more pronounced than the decline of gymnosperms. In contrast, analyses of abundance data appear to show that the decline of gym- nosperms is far more pronounced than the decline of free-sporing plants. Detailed examination of both data sets segregated by paleolatitude shows that this apparent contradiction reflects biogeo- graphical differences in the patterns of vegetational change (e.g., free-sporing plants declined in abundance at lower latitudes) as well as sampling bias (e.g., greater sampling in the northern region in the Late Cretaceous). Analyses accounting for these biases support the conclusion that as an- giosperms radiated, free-sporing plants rather than gymnosperms (in this case, mainly conifers) experienced the most pronounced decline. A thorough understanding of the Cretaceous radiation of angiosperms will require both abundance and diversity data. It also will require expanding the analyses presented here into other geographic regions as well as sampling more completely at all spatial scales.

Spatial patterns of diversity in the sea: bryozoan species richness in the North Atlantic

1. We have examined large-scale geographical patterns in species richness for continental shelf bryozoan assemblages in the North Atlantic. Bryozoans are common and often abundant benthic organisms, but they have not previously been examined at this scale of resolution. 2. Assemblage species richness was estimated by sample species richness. This was highest at intermediate depths (10-75 m) at all latitudes where there were sufficient data, but there was no statistically significant variation with depth for the overall data set (all latitudes pooled). Mean assemblage species richness showed no significant variation with latitude, although the highest individual values were generally from lower latitudes. There is thus as yet no convincing evidence for a latitudinal cline in the alpha diversity of North Atlantic bryozoans. 3. Pooling of data into bins of 10 degrees of latitude, or into biogeographic provinces, to estimate regional species richness revealed significant undersampling. Two independent techniques to correct for this undersampling revealed a latitudinal cline in the regional species richness of North Atlantic bryozoans, with a peak around 10-30°N, and a steady decrease in richness north to 80°N. 4. Two measures of beta diversity (Whittaker and Jaccard) revealed relatively high turnover, presumably related to habitat heterogeneity within regional bins or to significant environmental variation across bins. There was a tendency for beta diversity to be higher at lower latitudes, as would be expected from a combination of a latitudinal cline in regional diversity with a mean assemblage species richness invariant with latitude. Null models were used to clarify the expected relationship between the two measures of beta diversity, and these indicated a strong influence of species-abundance structure in the North Atlantic bryozoan data.

Natural clades differ from "random" clades; simulations and analyses

Using computer simulations and analytic calculations, we have evaluated whether conspic- uous expansions and contractions of natural clades may have represented chance fluctuations that oc- curred 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.

Competition, clade replacement, and a history of cyclostome and cheilostome bryozoan diversity

One of the striking yet scarcely documented episodes of clade replacement in the post- Paleozoic fossil record is the decline of cyclostome Bryozoa and the corresponding, rapid diversi- fication of cheilostome Bryozoa. These clades are closely associated morphologically and phylo- genetically, and their ecological similarities have previously led to the inference that competition was a primary cause of the overt pattern of replacement. Alternatively, previous compilations of bryozoan families and genera have implied that extinctions at the Cretaceous/Tertiary boundary differentially affected cyclostomes, and thus were also an important factor in the transition. We first evaluated the ecological context for competition between the two clades, then updated and reexamined the history of absolute family diversity for bryozoans in consecutive geologic stages from Jurassic to Recent. The resulting trends echo the patterns shown in earlier family level com- pilations, but indicate a slight shift in the frequency of cheilostome family originations from Late Cretaceous to early Paleogene. The relative fall in cyclostome family diversity at the Cretaceous/ Tertiary boundary is significantly less than shown in earlier genus level compilations. We then assessed these various compilations of absolute diversity by analyzing species counts and percentages in 728 fossil assemblages, primarily from North America and Europe, over the same time interval. Cyclostome species overwhelmingly dominate assemblages from Jurassic through Cenomanian, then decline significantly in average percentage dominance through the Campanian. Cheilostomes are predominant in Campanian and later assemblages. Cyclostome species percentages do decrease overall through the Tertiary, but this decrease is small and non-uniform, varying around 30%, with a sharp drop in the Late Neogene. Our within-assemblage results indicate that as cheilostomes radiate, their mean species diversity, maximum diversity, and variance all increase, thereby ac- counting for much of the decline in average percentage of cyclostomes within assemblages. While this result does not exclude a role for competition, an hypothesis of relative decline in cyclostome species richness based on competitive extinction alone seems unlikely. Further, despite decreases in absolute species counts following end-Cretaceous extinctions, within-assemblage percentages of cheilostome or cyclostome species show only slight change relative to one another. Comparison of these and earlier diversity compilations indicates that the dynamics of bryozoan clade replacement may be perceived differently at different ecologic scales or taxonomic ranks.

Competitive displacement among post-Paleozoic cyclostome and cheilostome bryozoans.

Abstract Encrusting bryozoans provide one of the few systems in the fossil record in which ecological competition can be observed directly at local scales. The macroevolutionary history of diversity of cyclostome and cheilostome bryozoans is consistent with a coupled-logistic model of clade displacement predicated on species within clades interacting competitively. The model matches observed diversity history if the model is perturbed by a mass extinction with a position and magnitude analogous to the Cretaceous/Tertiary boundary event. Although it is difficult to measure all parameters in the model from fossil data, critical factors are intrinsic rates of extinction, which can be measured. Cyclostomes maintained a rather low rate of extinction, and the model solutions predict that they would lose diversity only slowly as competitively superior species of cheilostomes diversified into their environment. Thus, the microecological record of preserved competitive interactions between cyclostome and cheilostome bryozoans and the macroevolutionary record of global diversity are consistent in regard to competition as a significant influence on diversity histories of post-Paleozoic bryozoans.

Comparative ecology of bryozoan radiations; origin of novelties in cyclostomes and cheilostomes

Cyclostome and cheilostome bryozoans diversified at different times and consequently in different ecological contexts. Cyclostomes began their rebound from a Perm-Triassic bottleneck in the early Jurassic, prior to increases in bioturbation, durophagous predation, and other ecological changes of the Mesozoic Marine Revolution. Cheilostomes did not appear until the latest Jurassic and rapid diversification began only in the mid-Cretaceous, when the Mesozoic Revolution was well under way. We compare the radiations of these two groups to test for similarities in the within-group patterns of origin of biologically significant novelties, and for between-group differences that might be due to ecological context or group attributes. As seen for other invertebrate t a u , within-group novelties were not concentrated in onshore settings, in contrast to origination patterns at the ordinal level. Differences in environment of first occurrence and rapidity of novelty acquisition were not obviously related to the Mesozoic Revolution, or to the distinction between zooidand colony-level characters. The contrast in novelty acquisition rates may partly reflect group-specific constraints. In cyclostomes, novelties appeared rather evenly over 100 Ma, whereas in cheilostomes many of the novelties appeared in the Late Albian-Early Cenomanian during a period of rapid diversification. Despite a slow start (Late Jurassic-mid Cretaceous), the cheilostome radiation entered an explosive phase that may characterize successful establishment of groups founded late in the Phanerozoic.

Simple versus complex models of trait evolution and stasis as a response to environmental change

Significance Paleontologists have long argued about what the fossil record call tell us about how species evolve over long periods. Simple models such as stasis and random walks have been used to explore evolutionary patterns, but these have not always adequately captured the ways in which traits change over time in fossil species. Here we find that models with complex evolutionary dynamics are often favored, especially for fossil series that sample many populations, suggesting that the underlying evolutionary reality is likely more complex than represented by simplified—though still useful—models of trait change. Previous analyses of evolutionary patterns, or modes, in fossil lineages have focused overwhelmingly on three simple models: stasis, random walks, and directional evolution. Here we use likelihood methods to fit an expanded set of evolutionary models to a large compilation of ancestor–descendant series of populations from the fossil record. In addition to the standard three models, we assess more complex models with punctuations and shifts from one evolutionary mode to another. As in previous studies, we find that stasis is common in the fossil record, as is a strict version of stasis that entails no real evolutionary changes. Incidence of directional evolution is relatively low (13%), but higher than in previous studies because our analytical approach can more sensitively detect noisy trends. Complex evolutionary models are often favored, overwhelmingly so for sequences comprising many samples. This finding is consistent with evolutionary dynamics that are, in reality, more complex than any of the models we consider. Furthermore, the timing of shifts in evolutionary dynamics varies among traits measured from the same series. Finally, we use our empirical collection of evolutionary sequences and a long and highly resolved proxy for global climate to inform simulations in which traits adaptively track temperature changes over time. When realistically calibrated, we find that this simple model can reproduce important aspects of our paleontological results. We conclude that observed paleontological patterns, including the prevalence of stasis, need not be inconsistent with adaptive evolution, even in the face of unstable physical environments.