Thomas D. Olszewski

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.

The preservational fidelity of evenness in molluscan death assemblages

Abstract The richness (number of species) and evenness (uniformity of species abundances) of death assemblages can differ from corresponding living communities due to processes such as between-habitat transport, environmental condensation, and differential taphonomic destruction. Analysis of 132 single-census live-dead comparisons of benthic molluscs from a variety of soft-bottom marine settings indicates that on average evenness does not differ greatly between live and dead assemblages, regardless of the particular depositional setting or grain size of associated sediment. However, individual death assemblages can deviate quite substantially from their corresponding living assemblages, especially if processed using a fine mesh. In addition, death assemblages collected using sieves with 2 mm mesh or coarser showed consistently and significantly greater evenness than corresponding living assemblages. These results are encouraging for broad-scale assessments of evenness in the fossil record based on the comparison of average values (rather than for individual assemblages) and where trends in evenness are the aim of the study. Our live-dead comparisons of richness sample-size corrected by rarefaction revealed that death assemblages were on average ∼1.45 times richer than the corresponding living assemblages regardless of rarefied size. In 63.6% of death assemblages both dead richness and dead evenness were greater than live, suggesting sufficient time-averaging to catch significant random or directional changes in the living community and/or introduction of individuals from outside the sampled habitat. In 12.9% of collections both dead richness and dead evenness were less than live, suggesting either rapid loss of dead shells so that dead diversity is depressed below the local living community or selective loss of taphonomically vulnerable taxa. In 18.2% of data sets dead richness was elevated but dead evenness was depressed relative to live: these are interpreted to reflect the addition of low-evenness allochthonous material. The remaining 4.5% of data sets had elevated dead evenness but depressed dead richness, suggesting that live and dead in this case may not be closely related. In seven available time series, temporal volatility in living communities over 6–24 months was considerable but could not account for observed (mostly higher) evenness values in corresponding death assemblages, whose evenness and composition were quite stable in the few examined studies. A densely sampled spatial transect shows that changes in living-assemblage evenness along an environmental gradient were preserved in the corresponding death assemblages, although dead evenness at any location on the gradient was substantially higher than living evenness.

Modeling the Influence of Taphonomic Destruction, Reworking, and Burial on Time-Averaging in Fossil Accumulations

Abstract A mathematical/computer model was constructed to explore the effects of changes in rates of taphonomic loss, sediment reworking, and burial on time-averaging in fossil deposits. Using a Monte Carlo algorithm, the main controlling variables were thickness of the taphonomically active zone (TAZ; the interval where shells are likely to be destroyed), depth of final burial (DFB; below which shells can no longer be reworked into the TAZ), and burial rate. Output included both the decay trajectory of shells above the DFB as well as the frequency distribution of post-mortem shell ages at final burial (entry into the fossil record). Output was justified ergodically as equivalent to time-averaging in a deposit. The model helps to illuminate several important concepts for time-averaging. For example, it clarifies that DFB and TAZ need not be coincident. In fact, the mechanism of shell sequestering (allowing shells to remain in a safe zone between the TAZ and DFB) as a means of shell survival for 100s or 1000s of years after death requires that DFB be below TAZ. In addition, the model demonstrates the use of a shells expected residence times above TAZ and DFB as a means of comparing time-averaging from different settings. Lastly, the model focuses attention on the potential variability in time-averaging among fossil deposits created under identical conditions due to stochastic variation. These results indicate the useful role of models in developing a process-based understanding of the nature and controls of time-averaging in fossil accumulations.

Dynamic response of Permian brachiopod communities to long-term environmental change

The fossil record preserves numerous natural experiments that can shed light on the response of ecological communities to environmental change. However, directly observing the community dynamics of extinct organisms is not possible. As an alternative, neutral ecological models suggest that species abundance distributions reflect dynamical processes like migration, competition, recruitment, and extinction. Live–dead comparisons suggest that such distributions can be faithfully preserved in the rock record. Here we use a maximum-likelihood approach to show that brachiopod (lamp shell) abundance distributions from four temporally distinct ecological landscapes from the Glass Mountains, Texas (of the Permian period), exhibit significant differences. Further, all four are better fitted by zero-sum multinomial distributions, characteristic of Hubbells neutral model, than by log-normal distributions, as predicted by the traditional ecological null hypothesis. Using the neutral model as a guide, we suggest that sea level fluctuations spanning about 10 Myr altered the degrees of isolation and exchange among local communities within these ecological landscapes. Neither these long-term environmental changes nor higher-frequency sea level fluctuations resulted in wholesale extinction or major innovation within evolutionary lineages.

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.

Change and stability in Permian brachiopod communities from western Texas

Abstract The silicified brachiopod faunas collected by G. Arthur Cooper and Richard E. Grant over decades of field work in the Permian Basin of western Texas provide a remarkable resource for studying change in the structure of fossilized ecological communities. Ordination analyses using 511 faunal lists that include 967 species (190 genera) from the Kungurian through the Capitanian Stages of the Guadalupe and Glass Mountains (∼15 myr) reveal four distinct, major stratigraphic clusters, each divisible into two to three subclusters. For the most part, these clusters coincide with third-order depositional sequences bounded by unconformities. Except for the youngest (Capitanian Stage), all the stratigraphic clusters show internal ordination patterns consistent with biogeographic and environmental (i.e., biotic gradients) control on the distribution of taxa. Despite this evidence for environmental sorting of taxa, Mantel tests indicate that patterns of generic co-occurrence break down from cluster to cluster despite sharing many of the same species and genera. This pattern cannot easily be explained by preservational or stratigraphic biases, suggesting that the processes that govern ecological communities may not lead to unique, strictly determined associations despite the presence of broadly similar species under similar environmental conditions. In contrast to the rest of the study interval, Capitanian clusters are less well structured (i.e., they do not show strong biotic gradients) than assemblages from any of the other clusters and do not show significant recurrence of generic associations. This may reflect evolution of carbonate margins in the basin from ramps to shelves with steep slopes over the duration of the study interval, a shift that may have changed the spatial distribution of (and covariation among) the environmental parameters that controlled brachiopod distributions.

TRACING BURIAL HISTORY AND SEDIMENT RECYCLING IN A SHALLOW ESTUARINE SETTING (COPANO BAY, TEXAS) USING POSTMORTEM AGES OF THE BIVALVE MULINIA LATERALIS

ABSTRACT Age spectra of Mulinia lateralis shells from the top 0–10 cm of the sediment column in Copano Bay, Texas, show three distinct populations: a young population with a highly skewed distribution ranging from 0 to 1 years, a middle-aged population that is more symmetrical ranging from 1 to 10 years with a peak ∼ 4 years, and a small, very old population ranging from 100 to > 10000 years. The young population is interpreted to record the rapid loss of shells from the taphonomically active zone at or near the sediment surface. The middle-aged population is interpreted to record a sequestered population of shells that has had time to accumulate below the taphonomically active zone. Although surface age spectra differ among depositional environments and at different locations, both of these populations are present in all sampled facies at multiple locations within the bay, indicating that the overarching controls on surface age spectra affect the entire bay. The very old surface population is present only in bay-margin sites and is interpreted to represent shells exhumed from eroding Holocene deposits. An 86-cm-long core taken at a bay-margin site near the mouth of the Aransas River contains shells spanning < 1 to 14,000 years but not preserved in stratigraphic order. The lack of stratigraphic order and the presence of distinct breaks in the postmortem age distribution suggest that 67% of shells in the core were recycled from older deposits, resulting in a 2–3 order of magnitude increase in the amount of time averaging in the sediment column relative to surface samples.

ENVIRONMENTAL DISRUPTIONS INFLUENCE TAXONOMIC COMPOSITION OF BRACHIOPOD PALEOCOMMUNITIES IN THE MIDDLE PERMIAN BELL CANYON FORMATION (DELAWARE BASIN, WEST TEXAS)

Abstract Paleontologists have identified patterns of stability in the diversity and composition of communities over millions to tens of millions of years within the fossil record. It is unclear, however, what processes are responsible for controlling the stability observed in these communities—for example, local species interactions within communities, local species-environment relationships, immigration of species from the regional biota. This study tests whether the taxonomic composition of local brachiopod paleocommunities occupying a slope environment across a ∼40 km2 region remains temporally uniform over a 5.4 myr interval of time recorded by the Pinery, Rader, Lamar, and Reef Trail Members of the middle Permian (Capitanian) Bell Canyon Formation (BCF). Two third-order sequence boundaries have been previously recognized in this formation, indicating episodes of environmental disruptions related to sea-level fall within the basin that can induce habitat tracking or immigration events from other basins. Presence-absence data analyzed by cluster analysis, ordination techniques, and rank-occurrence plots demonstrate that compositional turnover occurs within the BCF. Change in composition of the Lamar Member paleocommunities documents the replacement of established brachiopods with new taxa, including the immigration of two new brachiopod genera from outside North America, indicating the influence of regional-scale effects on community assembly. The results based on metacommunity models suggest that dispersal plays an important role in the assembly of brachiopod paleocommunities in the Delaware Basin. Results also indicate that fossil communities provide useful information for interpreting placement of sequence boundaries and the magnitude of disruption.

Geobiology: A Golden Opportunity and a Call to Action

Lately, the term “geobiology” has become very fashionable in the geosciences. It has appeared repeatedly in documents concerning future directions and funding of paleontological research, in job announcements for positions at a number of prominent universities, and will even serve as the title of a new journal scheduled to appear next year. My aim in this essay is to present a personal view of what geobiology is and its relationship to paleontology. Geobiology, which is more of a research agenda than a formal discipline, focuses on the processes and evolution of the coupled Earth-life system through time. It is characterized by contributions from disciplines as diverse as geochemistry, molecular biology, informatics, climate modeling, conservation biology, and, central to it all, paleontology. Because fossils offer the only direct record of ancient life on Earth, geobiology presents a golden opportunity to display the relevance of paleontology. During the late 1990s (way back in the 20th century), paleontologists were assessing the state of the field and trying to forecast future research directions; the term “geobiology” consistently appeared in these evaluations. In 1997, the Paleontological Society published a report to the National Science Foundation entitled “Geobiology of Critical Intervals” (Stanley, 1997; www.jhu.edu/∼eps/faculty/stanley/nsfgoci.html). This research initiative was “founded on the premise that modern paleontology, in combination with fields such as geochemistry, stratigraphy, plate tectonics, and ocean and atmospheric sciences, has great potential for exploring unique natural experiments on the structure and dynamics of the coupled earth-life system.” Geobiology also appeared as a heading in several places in the report of a 1997 Senckenberg World Conference entitled “Fossils and the Future: Paleontology in the 21st Century” (Lane et al., 2000; www.nhm.ac.uk/hosted_sites/paleonet/paleo21/rr/index.html). In 1999, the National Science Foundation sponsored a workshop on “Geobiology and the Earth Sciences in the Next Decade” (Flessa, 1999 …

The Sedimentary Record of Sea-Level Change

Angela L. Coe (Editor), 2003, Cambridge University Press, Cambridge, UK, 288 p. (Soft Cover, US