Adam Tomašových

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.

Catastrophic ocean acidification at the Triassic-Jurassic boundary

Palaeobotanical and geochemical evidence indicate a sudden rise in atmospheric carbon dioxide (CO2) across the Triassic-Jurassic boundary, probably reflecting the combined effect of extensive volcanic degassing and thermal dissociation of marine gas hydrates. Using carbon isotopes as a geochemical marker, we found that the onset of the CO2 emissions coincided with an inter- ruption of carbonate sedimentation in palaeogeographically distant regions, suggesting that hydro- lysis of CO2 led to a short but substantial decrease of seawater pH that slowed down or inhibited precipitation of calcium carbonate minerals. The cessation of carbonate sedimentation correlates with a major marine extinction event, which especially affected organisms with aragonitic or high-Mg calcitic skeletons and little physiological control of biocalcification. These findings strengthen current concerns that ocean acidification from industrial CO2 release threatens biotopes that are dominated by such organisms, in particular tropical reef systems.

Fidelity of variation in species composition and diversity partitioning by death assemblages: time-averaging transfers diversity from beta to alpha levels

Abstract Despite extensive paleoecological analyses of spatial and temporal turnover in species composition, the fidelity with which time-averaged death assemblages capture variation in species composition and diversity partitioning of living communities remains unexplored. Do death assemblages vary in composition between sites to a lesser degree than do living assemblages, as would be predicted from time-averaging? And is the higher number of species observed in death relative to living assemblages reduced with increasing spatial scale? We quantify the preservation of spatial and temporal variation in species composition using 11 regional data sets based on samples of living molluscan communities and their co-occurring time-averaged death assemblages. (1) Compositional dissimilarities among living assemblages (LA) within data sets are significantly positively rank-correlated to dissimilarities among counterpart pairs of death assemblages (DA), demonstrating that pairwise dissimilarity within a study area has a good preservation potential in the fossil record. Dissimilarity indices that downplay the abundance of dominant species return the highest live-dead agreement of variation in species composition. (2) The average variation in species composition (average dissimilarity) is consistently smaller in DAs than in LAs (9 of 11 data sets). This damping of variation might arise from DAs generally having a larger sample size, but the reduction by ∼10–20% mostly persists even in size-standardized analyses (4 to 7 of 11 data sets, depending on metric). Beta diversity expressed by the number of compositionally distinct communities is also significantly reduced in death assemblages in size-standardized analyses (by ∼25%). This damping of variation and reduction in beta diversity is in accord with the loss of temporal resolution expected from time-averaging, without invoking taphonomic bias (from differential preservation or postmortem transportation) or sample-size effects. The loss of temporal resolution should directly reduce temporal variation, and assuming time-for-space substitution owing to random walk within one habitat and/or temporal habitat shifting, it also decreases spatial variation in species composition. (3) DAs are more diverse than LAs at the alpha scale, but the difference is reduced at gamma scales because partitioning of alpha and beta components differs significantly between LAs and DAs. This indicates that the effects of time-averaging are reduced with increasing spatial scale. Thus, overall, time-averaged molluscan DAs do capture variation among samples of the living assemblage, but they tend to damp the magnitude of variation, making them a conservative means of inferring change over time or variation among regions in species composition and diversity. Rates of temporal and spatial species turnover documented in the fossil record are thus expected to be depressed relative to the turnover rates that are predicted by models of community dynamics, which assume higher temporal resolution. Finally, the capture by DAs of underlying variation in the LA implies little variation in the net preservation potential of death assemblages across environments, despite the different taphonomic pathways suggested by taphofacies studies.

Predicting the effects of increasing temporal scale on species composition, diversity, and rank-abundance distributions

Abstract Paleoecological analyses that test for spatial or temporal variation in diversity must consider not only sampling and preservation bias, but also the effects of temporal scale (i.e., time-averaging). The species-time relationship (STR) describes how species diversity increases with the elapsed time of observation, but its consequences for assessing the effects of time-averaging on diversity of fossil assemblages remain poorly explored. Here, we use a neutral, dispersal-limited model of metacommunity dynamics, with parameters estimated from living assemblages of 31 molluscan data sets, to model the effects of within-habitat time-averaging on the mean composition and multivariate dispersion of assemblages, on diversity at point (single station) and habitat scales (pooled multiple stations), and on beta diversity. We hold sample size constant in STRs to isolate the effects of time-averaging from sampling effects. With increasing within-habitat time-averaging, stochastic switching in the identity of species in living (dispersal-limited) assemblages (1) decreases the proportional abundance of abundant species, reducing the steepness of the rank-abundance distribution, and (2) increases the proportional richness of rare, temporally short-lived species that immigrate from the neutral metacommunity with many rare species. These two effects together (1) can shift the mean composition away from the non-averaged (dispersal-limited) assemblages toward averaged assemblages that are less limited by dispersal, resembling that of the metacommunity; (2) allow the point and habitat diversity to increase toward metacommunity diversity under a given sample size (i.e., the diversity in averaged assemblages is inflated relative to non-averaged assemblages); and (3) reduce beta diversity because species unique to individual stations become shared by other stations when limited by a larger but static species pool. Surprisingly, these scale-dependent changes occur at fixed sample sizes and can become significant after only a few decades or centuries of time-averaging, and are accomplished without invoking ecological succession, environmental changes, or selective postmortem preservation. Time-averaging results in less inflation of diversity at habitat than at point scales; paleoecological studies should thus analyze data at multiple spatial scales, including that of the habitat where multiple bulk samples have been pooled in order to minimize time-averaging effects. The diversity of assemblages that have accumulated over 1000 years at point and habitat scales is expected to be inflated by an average of 2.1 and 1.6, respectively. This degree of inflation is slightly higher than that observed in molluscan death assemblages at these same spatial scales (1.8 and 1.3). Thus, neutral metacommunity models provide useful quantitative constraints on directional but predictable effects of time-averaging. They provide minimal estimates for the rate of increase in diversity with time-averaging because they assume no change in environmental conditions and in the composition of the metacommunity within the window of averaging.

Modeling shelliness and alteration in shell beds: variation in hardpart input and burial rates leads to opposing predictions

Abstract Distinguishing the differential roles of hardpart-input rates and burial rates in the formation of shell beds is important in paleobiologic and sedimentologic studies, because high shelliness can reflect either high population density of shell producers or lack of sediment. The modeling in this paper shows that differences in the relative importance of burial rates and hardpart-input rates lead to distinct patterns with respect to the degree of shelliness and taphonomic alteration in shell beds. Our approach substantially complements other models because it allows computation of both shelliness and assemblage-level alteration. To estimate shelliness, we dissected hardpart-input rates into dead-shell production and shell destruction rates. To estimate assemblage-level alteration, we computed an alteration rate that describes how rapidly shells accrue postmortem damage. Under decreasing burial rates but constant hardpart-input rates, a positive correlation between alteration and shelliness is expected (Kidwells R-sediment model). In contrast, under decreased destruction rates and/or increased dead-shell production rates and constant burial rates (Kidwells R-hardpart model), a negative correlation between shelliness and alteration is expected. The contrasting predictions thus provide a theoretical basis for distinguishing whether high shell density in shell beds reflects passive shell accumulation due to a lack of sediment dilution or whether it instead reflects high shell input from a life assemblage. This approach should be applicable for any fossil assemblages that vary in shell density and assemblage-level alteration. An example from the Lower Jurassic of Morocco, which has shell-rich samples less altered than shell-poor samples, suggests that the higher shelliness correlates with higher community-level abundance and lower proportion of juveniles of the main shell producer, supporting the driving role of hardpart-input rates in the origin of the shell-rich samples in this case. This is of significance in paleoecologic analyses because variations in shelliness can directly reflect fluctuations in population density of shell producers.

The Effects of Temporal Resolution on Species Turnover and on Testing Metacommunity Models

Patterns of low temporal turnover in species composition found within paleoecological time series contrast with the high turnover predicted by neutral metacommunity models and thus have been used to support nonneutral models. However, these predictions assume temporal resolution on the scale of a season or year, whereas individual fossil assemblages are typically time averaged to decadal or centennial timescales. We simulate the effects of time averaging by building time‐averaged assemblages from local dispersal‐limited, nonaveraged assemblages and compare the predicted turnover with observed patterns in mollusk and ostracod fossil records. Time averaging substantially reduces temporal turnover such that neutral predictions converge with those of trade‐off and density‐dependent models, and it tends to decrease species dominance and increase the proportion of rare species. Observed turnover rates are comparable to an appropriately scaled neutral model: patterns of high community stability can be produced or reinforced by time averaging alone. The community attributes of local time‐averaged assemblages approach those of the metacommunity. Time‐averaged assemblages are thus unlikely to capture attributes arising from processes operating at small spatial scales, but they should do well at capturing the turnover and diversity of metacommunities and thus will be a valuable basis for analyzing the large‐scale processes that determine metacommunity evolution.

Accounting for the effects of biological variability and temporal autocorrelation in assessing the preservation of species abundance

Abstract Quantifying the effects of taphonomic processes on species abundances in time-averaged death assemblages (DAs) is pivotal for paleoecological inference. However, fidelity estimates based on conventional “live-dead” comparisons are fundamentally ambiguous: (1) data on living assemblages (LAs) are based on a very short period of sampling and thus do not account for biological variability in the LA, (2) LAs are sampled at the same time as the DA and thus do not necessarily reflect past LAs that contributed to the DA, (3) compositions of LAs and DAs can be autocorrelated owing to shared cohorts, and (4) fidelity estimates are cross-scale estimates because DAs are time-averaged and LAs are not. Some portion of raw (total) live-dead (LD) variation in species composition thus arises from incomplete sampling of LAs and from biological temporal variation among LAs (together  =  premortem component of LD variation), as contrasted with new variation created by interspecific variation in population turnover and preservation rates and by the time-averaging of skeletal input (together  =  postmortem component of LD variation). To tackle these problems, we introduce a modified test for homogeneity of multivariate dispersions (HMD) in order to (1) account for temporal autocorrelation in composition between LAs and DAs and (2) decompose total LD compositional variation into premortem and postmortem components, and we use simulations to evaluate the contribution of within-habitat time-averaging on the postmortem component. Applying this approach to 31 marine molluscan data sets, each consisting of spatial replicates of LAs and DAs in a single habitat, we find that total LD variation is driven largely by variation among LAs. However, genuinely postmortem processes have significant effects on composition in 25–65% of data sets (depending on the metric) when the effects of temporal autocorrelation are taken into account using HMD. Had we ignored the effects of autocorrelation, the effects of postmortem processes would have been negligible, inflating the similarity between LAs and DAs. Simulations show that within-habitat time-averaging does not increase total LD variation to a large degree—it increases total LD variation mainly via increasing species richness, and decreases total LD variation by reducing dispersion among DAs. The postmortem component of LD variation thus arises from differential turnover and preservation and multi-habitat time-averaging. Moreover, postmortem processes have less effect on the compositions of DAs in habitats characterized by high variability among LAs than they have on DAs in temporally stable habitats, a previously unrecognized first-order factor in estimating postmortem sources of compositional variation in DAs.

Effect of Extrinsic Factors on Biofabric and Brachiopod Alteration in a Shallow Intraplatform Carbonate Setting (Upper Triassic, West Carpathians)

Abstract Upper Triassic assemblages containing the terebratulid brachiopod Rhaetina gregaria from a shallow, intraplatform carbonate setting of the Fatra Formation are classified according to biofabric, geometry, and internal structure into 6 deposit types, which are interpreted as: (1) autochthonous primary biogenic, (2) autochthonous winnowed or sediment starved, (3) parautochthonous storm-wave, (4) parautochthonous storm wave/flow, (5) amalgamated storm-reworked, and (6) allochthonous (long-term current/wave) deposits. Their distribution on the bed scale correlates with depth-related environmental gradients in regard to the position of fair-weather wave base, average storm wave base, and maximum storm wave base. The biofabric, geometry, and internal structure of brachiopod deposits were predominantly influenced by: (1) storm activity, related to variations in sedimentation rates and water energy; and (2) original variations in composition and spatial distribution of life associations. Fossil assemblages preserved in brachiopod deposits have a wide range of temporal resolution, ranging from census to environmentally condensed types. Brachiopod assemblages in the storm-reworked deposits probably were affected by catastrophic mortality. The distinction of brachiopod deposit types based on deposit-level criteria does not wholly correspond to the classification of taphofacies types based on intensity of shell alteration. The biofabric and associated deposit-level properties reflect final depositional processes (i.e., the rate and permanence of burial), whereas shell alteration of brachiopods reflects mainly variation in the nature of pre-burial environmental conditions. The lowest degree of alteration (i.e., low levels of bioerosion, micritization, encrustation, and disarticulation) is associated with deposits that were affected by storm-induced sudden burial. In general, settings with high proportions of micritic mud (associated with mixed brachiopod-bivalve associations) are characterized by relatively low alteration of brachiopods. These settings are in sharp contrast to hard-bottom settings (associated with coral associations), in which bioerosion and micritization are high. This difference in shell alteration is the effect of extrinsic factors related to lower turbidity, higher proportion of hardparts and higher storm reworking in latter settings. Autochthonous/parautochthonous benthic associations dominated by the short-looped terebratulid Rhaetina gregaria are typical of settings below the fair-weather wave base, with background low-energy condition. This is in contrast to high-energy/hard-bottom occurrences of this association from other regions. The difference in preservation potential of brachiopods due to differential extrinsic factors (e.g., between hard- and soft-bottom settings) can substantially bias the understanding their ecology and temporal shifts in environmental preferences. Data about substantial bioerosion/micritization of brachiopods in some deposit types indicate their higher durability and inherently higher preservation potential in contrast to actualistic data about the poor resistance of modern brachiopods to destruction.

Long-term accumulation of carbonate shells reflects a 100-fold drop in loss rate

Shells in modern seabeds can be thousands of years old, far older than would be extrapolated from the rapid rates of shell loss detected in short-term experiments. An extensive shelldating program on the Southern California (USA) shelf permits rigorous modeling of the dynamics of shell loss in the mixed layer, discriminating the key rates of carbonate disintegration and sequestration for the first time. We find that bivalve shells experience an initially high disintegration rate l 1 (~ decadal half-lives) but shift abruptly, within the first ~500 yr postmortem, to a 100-fold lower disintegration rate l 2 (~ millennial half-lives) at sequestration rate t (burial and/or diagenetic stabilization). This drop permits accrual of a long tail of very old shells even when sequestration is very slow, and allows only a minority (<1%) of all shells to survive the first phase. These high rates of disintegration and low rates of sequestration are consistent with independent measures of high carbonate loss and slow sedimentation on this shelf. Our two-phase model thus reveals significant spatial and temporal partitioning of car bonate loss rates within the mixed layer, and shows how shell age-frequency distributions can yield rigorous and realistic estimates of carbonate recycling on geological time scales.

Preservation of Autochthonous Shell Beds by Positive Feedback between Increased Hardpart‐Input Rates and Increased Sedimentation Rates

The preservation of nonrapidly buried autochthonous shell concentrations with noncementing epifaunal animals in life position presents a taphonomic dilemma if in fact an increase in shelliness is driven by a decrease in sedimentation rate. A 150‐cm‐thick, densely packed shell bed with brachiopods from the Lower Jurassic of Morocco shows lower levels of postmortem alteration than shell‐poor beds, indicating that its formation is primarily governed by variations in hardpart‐input rates. Varying dominance and size structure of the main shell producer, brachiopod Zeilleria rehmanni, indicate that its increased population density was the main trigger in the shell bed formation. Thinner and less common microbial crusts in the shell bed than in shell‐poor beds indicate that higher shelliness is not due to lack of sediment. On the basis of actualistic data from modern mussel and oyster shell beds, the suspension feeding of a high‐density population leads to high biodeposition rates through production of feces and pseudofeces, which substantially exceed natural sedimentation rates. In addition, shell‐rich areas preferentially trap more suspended sediment than shell‐poor areas. Therefore, an increase in population density of shelly biodeposit producers should lead to higher biodeposition rates. This assumption is supported by a positive correlation between brachiopod shelliness and pellet abundance. Both active biodeposition and passive trapping of sediment would have increased sedimentation rate, thus leading to a decreased rate of shell destruction through suppression of predators or borers as well as stabilization and protection of the shell concentration. Under optimum ecologic conditions, these processes result in a positive feedback between an increased hardpart‐input rate and increased biogenic sedimentation rate. This scenario thus provides one alternative pathway for formation of well‐preserved shell concentrations formed by epifaunal suspension feeders. Identifying the importance of biodeposition is of environmental significance because it implies that carbonate sediment was produced largely in situ and was directly or indirectly related to the activity of shell producers. Recognizing the role of varying hardpart‐input rates in shell concentration genesis is of ecologic significance because shelliness can directly reflect abundance fluctuations of shell producers.