Abstract: The modern structure of marine benthic ecosystems was largely established during the Jurassic and Early Cretaceous (200-100 Ma), a transition that has been termed the Mesozoic Marine Revolution (MMR). Although it has been suggested that the MMR marks an increase in the average energy consumption of marine animal ecosystems, this hypothesis has not been evaluated quantitatively. In this study, we integrate body size and abundance data from the fossil record with physiological data from living representatives to estimate mean per capita metabolic rates of tropical to subtropical assemblages of shallow-marine gastropods — a major component of marine ecosystems throughout the Meso-Cenozoic — both before and after the MMR. We find that mean per capita metabolic rate rose by ,150% between the Late Triassic and Late Cretaceous and remained relatively stable thereafter. The most important factor governing the increase in metabolic rate was an increase in mean body size. In principle, this size increase could result from secular changes in sampling and taphonomic biases, but these biases are suggested to yield decreases rather than increases in mean size. Considering that post-MMR gastropod diversity is dominated by predators, the net primary production required to supply the energetic needs of the average individual increased by substantially more than 150%. These data support the hypothesis that benthic energy budgets increased during the MMR, possibly in response to rising primary productivity.
Abstract: Phanerozoic trends in shell traits linked to post-mortem durability were evaluated for the most common fossil brachiopod, gastropod, and bivalve genera in order to test for changes in taphonomic bias. Using the Paleobiology Database, occurrence frequencies of genera were tabulated for 48 intervals of ~11 myr duration. The most frequently occurring genera, cumulatively representing 40% of occurrences in each time bin, were scored for intrinsic durability based on shell size, reinforcement (ribs, folds, and spines), life habit, and mineralogy.
Shell durability is positively correlated with the number of genera in a time bin, but durability traits exhibit different temporal patterns across higher taxa, with notable offsets in the timing of changes in these traits. We find no evidence for temporal decreases in durability that would indicate taphonomic bias at the Phanerozoic scale among commonly occurring genera. Also, all three groups show a remarkable stability in mean shell size through the Phanerozoic, an unlikely pattern if strong size-filtering taphonomic megabiases were affecting the fossil record of shelly faunas. Moreover, small shell sizes are attained in the early Paleozoic in brachiopods and in the latest Paleozoic in gastropods but are steady in bivalves, unreinforced shells are common to all groups across the entire Phanerozoic, organophosphatic and aragonitic shells dominate only the oldest and youngest time bins, and microstructures having high organic content are most common in the oldest time bins.
In most cases, the timing of changes in durability-related traits is inconsistent with a late Mesozoic Marine Revolution. The post-Paleozoic increase in mean gastropod reinforcement occurs in the early Triassic, suggesting either an earlier appearance and expansion of duraphagous predators or other drivers. Increases in shell durability hypothesized to be the result of increased predation in the late Mesozoic are not evident in the common genera examined here. Infaunality does increase in the late Mesozoic in all three groups, but it does not become more common than levels already attained during the Paleozoic, and only among bivalves does the elevated late Mesozoic level persist through the Holocene. These temporal patterns suggest control on the occurrence of durability-related shell traits by individual evolutionary histories rather than taphonomic megabiases. Our findings do not mean that taphonomic biases are absent from the fossil record, but rather that their effects apparently have had little net effect on the relative occurrence of shell traits that are generally thought to confer higher preservation potential over long time scales.
Abstract: Radiocarbon-calibrated amino acid racemization ages of 428 individually dated shells representing four molluscan taxa are used to quantify time-averaging and shell half-lives with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contains a distinct, essentially modern assemblage. Shells recovered at depths from 25 to 125 cm are age-homogeneous and significantly older than the surface layer. Taxon age distributions within sedimentary layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a sub-century scale. The age distributions and shell half-lives of four taxa (Ethalia, Natica, Tellina, and Turbo) are found to be largely distinct. Shell half-lives do not coincide with any single morphological characteristic thought to infer greater durability, but they are strongly related to a combined durability score based on shell density, thickness, and shape. These results illustrate the importance of bioturbation in tropical sedimentary environments, indicate that age estimates in this depositional setting are sensitive to taxon choice, and quantify a taxon-dependent bias in shell longevity and death assemblage formation.
Abstract: Amino acid racemization (AAR) is a cost-effective method for dating the large numbers of specimens required for time-averaging studies. Because the aim of time-averaging studies is to determine the structure of the age distribution, any data screening must be done cautiously and systematically. Methods to quantitatively assess the quality of AAR data and to identify aberrant specimens are underdeveloped. Here we examine a variety of screening criteria for identifying outliers and determining the suitability of specimens for numerical dating including: high serine concentrations (modern contamination), covariance of aspartic acid (Asp) and glutamic acid (Glu) concentrations (diagenetic influences), replication of measurements (specimen heterogeneity), and the relation between Asp and Ghl D/L values (internal consistency). This study is based on AAR analyses of 481 late Holocene shells of four molluscan taxa (Ethalia, Natica, Tellina, and Turbo) collected from shallow sediment cores from the central Great Barrier Reef. Different outliers are flagged by the different screening criteria, and 6% of specimens were found to be unsuitable for time-averaging analyses based on screening the raw AAR data. We recommend a hybrid approach for identifying outliers and specimens for numerical dating. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: 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.
Abstract: Numerical ages derived from amino acid racemization (AAR) geochronology are typically based on calibration curves that relate the extent of AAR to the age of independently dated specimens. Here, we compare options for developing calibration curves and quantifying age uncertainties using AAR data from 481 late Holocene shells, and AMS C-14 analyses of 36 shells of four molluscan taxa (Ethalia, Natica, Tellina, and Turbo) collected from shallow sediment cores from a back-reef lagoon of the central Great Barrier Reef. The four taxa differed substantially in the quality of their geochronogical results. Explicitly including data from specimens alive at the time of collection improves calibration curves, but weighting numerical ages based on their uncertainty has no effect. Calibration curve statistics do not adequately assess calibration uncertainty. The relation between ages inferred from different amino acids is recommended for identifying aberrant specimens and quantifying the uncertainty of inferred ages. For this study, the AAR ages based on two amino acids (aspartic acid and glutamic acid) exceed 200yr or 20% of their mean inferred age in similar to 15% of the specimens. Once these were removed, the mean age error (1 sigma) for individual specimens based on two amino acids analyzed in duplicate subsamples ranged from 5 3 to 142 yr for Tellina and Turbo, respectively, or about a 30% age error for these relatively young shells. This compares favorably with analytical errors estimated at 50 yr or 5 %. The presence of notable outliers undetectable using data from single amino acids emphasizes the importance of analyzing multiple amino acids. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: Radiocarbon-calibrated amino acid racemization ages of 250 individually dated Tellina shells from two sediment cores are used to quantify molluscan time averaging with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contain a distinct, essentially modern assemblage with a median age of only 5 yr. Sediment between 20 and 125 cm are age-homogeneous and significantly older than the surface sediment (median age 189 yr). Shell age distributions within layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a subcentennial scale. Shell size is an important correlate of shell half-life and an important determinant of the inferred age of sedimentary layers. These results illustrate the importance of bioturbation in these environments, indicate that age estimates in this depositional setting are sensitive to specimen choice, and document a size-dependent bias in death assemblage formation.
Abstract: How does the choice of size metric, specimen selection, and taxonomic level affect the results of macroevolutionary or ecological analyses? Four molluscan data sets are used to address this question as follows. First, the relationships among various size metrics are examined using a morphometric data set of Late Cretaceous-Oligocene veneroid bivalves. Second, the relationship between the size of bulk-sampled specimens and the size of species' type specimens is examined using bulk-sampled bivalves and gastropods from the Coffee Sand (Upper Cretaceous, Mississippi). Third, the same relationship is examined using mollusk-dominated field censuses from the type Cincinnatian (Upper Ordovician, Ohio, Indiana, and Kentucky). Fourth, the relationship between the size of the type species of a genus and median species size is examined using literature-derived measurements of bivalve type specimens from the recent eastern Pacific continental shelf. Together these data sets provide estimates of the biases imposed by measuring different kinds of specimens and using different methods of estimating body size. The geometric mean of length and height is highly correlated with more complex morphometrically based metrics and is our preferred bivalve size metric. Bulk or randomly sampled specimens are significantly smaller than species' type specimens for the Cretaceous dataset but significantly larger for the Ordovician dataset. Genus' type-species size is an unbiased estimate of median species size. These results suggest that large-scale studies can use the size of the type species of a genus as an unbiased proxy for a type-specimen size of a genus' median species, but that species' type-specimen size is a biased proxy for bulk or randomly sampled specimens. In addition, this study emphasizes the importance of using a single type of measurement within studies and suggests that combining multiple types of specimens (e.g., type specimens and bulk-sampled specimens) could lead to substantive errors.
Abstract: 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.
Abstract: Ecological interactions, such as predation and bioturbation, are thought to be fundamental determinants of macroevolutionary trends. A data set containing global occurrences of Phanerozoic fossils of benthic marine invertebrates shows escalatory trends in the relative frequency of ecological groups, such as carnivores and noncarnivorous infaunal or mobile organisms. Associations between these trends are either statistically insignificant or interpretable as preservational effects. Thus, there is no evidence that escalation drives macroecological trends at global and million-year time scales. We also find that taxonomic richness and occurrence data are cross-correlated, which justifies the traditional use of one as a proxy of the other.
Abstract: Question: Holding both the true number of taxa and their evenness constant, what is the effect of the shape of the abundance distribution on the number of sampled taxa? Method: We examine the effects of three types of abundance distribution (geometric, log-normal and Zipf) on the expected number of sampled taxa using Hurlbert's equation (1971, equation 14). First, we examine the differences in the number of sampled taxa for the three distributions given the same true number of taxa and true evenness. Second, we determine the sample sizes needed to find more taxa from a taxon-rich, low-evenness collection than found in a taxon-poor, high-evenness collection with the same model distribution. Conclusions: Independently of the true number of taxa and evenness, the shape of the abundance distribution affects the number of taxa expected in a sample. Given moderate to large sample sizes, a Zipf distribution will yield the most taxa, whereas a geometric distribution will yield the fewest. When comparing collections with the same model distributions, it takes the smallest sample sizes to recognize that a taxon-rich, low-evenness Zipf distribution has more taxa than does a taxon-poor, high-evenness Zipf distribution. It requires the largest sample sizes to do this when both are geometric distributions. A necessary implication of these results is that no simple evenness metric can predict the same number of sampled taxa given the same true number of taxa, true evenness and sample size but different model distributions.
Abstract: The Mesozoic-Cenozoic transition is generally seen as a pivotal time in the evolution of benthic marine assemblages but the details of the timing and drivers of these changes are poorly known. The Atlantic and Gulf Coastal Plains of the United States contain assemblages preserved as original aragonitic and calcitic material in unconsolidated sediments. This makes coastal plain assemblages ideally suited to paleoecological analyses. Data derived from bulk samples of the Coffee Formation (lower/middle Campanian: Mississippi) as well as published faunal lists from comparable samples of the Severn (Maastrichtian: Maryland), Providence (Maastrichtian: Georgia and Alabama), Stone City (Eocene: Texas), and Gosport (Eocene: Alabama) Formations are used to assess changes in taxonomic diversity and ecomorphological group (life habit and trophic group) composition through this time interval. These analyses find a significant decrease in rarefied-sample species richness from the Campanian through the Eocene, but no change in evenness. With the notable exception of the Stone City Formation, increases in carnivore (neogastropod) richness and abundance occur before the Campanian. Epifaunal suspension-feeding species are a smaller proportion of the sample richness in Eocene samples than in Cretaceous samples. Decreased relative epifaunal suspension-feeder biomass but unchanged relative numbers of epifaunal suspension-feeder individuals suggests a relative decrease in epifaunal suspension-feeder size. Infaunal suspension feeders increase in richness and abundance through the interval. The proportion of drilled bivalves and gastropods does not change through the interval. Changes found in the structure of local shallow-shelf benthic assemblages from the Campanian through the Eocene are generally small relative to the variability between samples. Formation-level variation between assemblages is high relative to the magnitude of the temporal signal, emphasizing the need for investigators to include multiple formations per interval in tests of temporal trends.
Abstract: This paper tests whether the most common fossil brachiopod, gastropod, and bivalve genera also have intrinsically more durable shells. Commonness was quantified using occurrence frequency of the 450 most frequently occurring genera of these groups in the Paleobiology Database (PBDB). Durability was scored for each taxon on the basis of shell size, thickness, reinforcement (ribs, folds, spines), mineralogy, and microstructural organic content. Contrary to taphonomic expectation, common genera in the PBDB are as likely to be small, thin-shelled, and unreinforced as large, thick-shelled, ribbed, folded, or spiny. In fact, only six of the 30 tests we performed showed a statistically significant relationship between durability and occurrence frequency, and these six tests were equally divided in supporting or contradicting the taphonomic expectation. Thus, for the most commonly occurring genera in these three important groups, taphonomic effects are either neutral with respect to durability or compensated for by other factors (e.g., less durable taxa were more common in the original communities). These results suggest that biological information is retained in the occurrence frequency patterns of our target groups.
Abstract: 1. Molecular approaches have increasingly revealed hidden genetic structure within ecologically important species, leading to the creation of sibling species whose ecological relevance is often unclear. A prime example is Daphnia galeata mendotae, which was split into D. dentifera and D. mendotae based on differences at two allozyme loci. 2. In a set of lake populations in Michigan USA, we test the geographical and temporal consistency of the genetic structure underlying this species split. We also test the morphological relevance of this molecular variation and its ecological significance in lakes. In essence, we ask: does recognition of these new species provide valuable information for plankton ecologists? 3. We found that D. dentifera and D. mendotae represent morphologically and ecologically distinct forms that are distributed among lakes in non-random fashion, which were remarkably stable over 6 years. Key differences between the species concern their body and head shape, vertical habitat use within lakes and distribution among lakes of different size. We hypothesise that these differences represent specialisation to habitats that differ in risk of invertebrate predation. 4. Reproductive barriers alone are insufficient to explain the pattern of genetic structure; in some lakes complete introgression is apparent. However, parent species and hybrids exhibit a stable co-existence in many lakes, which suggests that ecological specialisation reinforces divergence within this taxon.
Abstract: 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.
Abstract: This paper describes Byzantia obliqua, new genus and species. Byzantia is a small, distinctively ornamented, gastropod genus from the middle Permian (Leonardian-Guadalupian) of West Texas. This provides further evidence of the diversity of the Neritacea in the middle Permian and of the diverse endemic fauna of west Texas.
Abstract: A north central Texas Baptisia (Fabaceae, Papilionoideae, Thermopsideae) hybrid complex containing three parents (B. australis, B. sphaerocarpa, and B. bracteata) and the three F1 hybrids is investigated using morphological, spectrophotometric, and isozyme electrophoretic methods. Two of the three hybrids (B. X bushii and B. X bicolor) have been previously named, but the B. australis X B. sphaerocarpahybrid (B. X varicolor) is here described for the first time. This is the first report of sympatric occurrence of these three Baptisia species and all the possible F1 hybrids. Morphological evidence for introgressive hybridization involving B. australis and B. sphaerocarpa is presented.
