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Understanding the response of a species or lineage to long-term environmental change is a critical aspect of evolutionary paleoecology. In order to do this, paleobiologists must have an excellent fossil record of a lineage and an independent source of environmental data in the same region. This situation occurs in the San Pedro area of southern California, where relatively new paleotemperature and paleoproductivity records enhance the well-known fossil gastropod record. We quantified shell morphology of late Pleistocene and Recent turritellid gastropods from this area and compared the timing of changes with temperature and productivity reconstructions for the region. Our results indicate that warm temperatures and moderate to high productivity are associated with larger shells and wider whorls. Cold temperatures and lower productivity are associated with smaller, narrower shells. We propose that warm temperatures and moderate productivity result in higher growth rates in turritellid gastropods. Our work also suggests that below a certain threshold temperature, productivity appears to have no influence on shell morphology. In other words, growth rate is unaffected by high productivity unless average temperatures are above a certain level. These results are consistent with models of shell deposition and with experimental results from living gastropods and bivalves reported in the literature.
Do mass extinctions grade continuously into the background extinctions occurring throughout the history of life, or are they a fundamentally distinct phenomenon that cannot be explained by processes responsible for background extinction? Various criteria have been proposed for addressing this question, including approaches based on physical mechanisms, ecological selectivity, and statistical characterizations of extinction intensities.
Here I propose a framework defining three types of continuity of mass and background extinctions—continuity of cause, continuity of effect, and continuity of magnitude. I test the third type of continuity with a statistical method based on kernel density estimation. Previous statistical approaches typically have examined quantitative characteristics of mass extinctions (such as metrics of extinction intensity) and compared them with the distribution of such characteristics associated with background extinctions. If mass extinctions are outliers, or are separated by a gap from background extinctions, the distinctness of mass extinctions is supported.
In this paper I apply Silverman's Critical Bandwidth Test to test for the continuity of mass extinctions by applying kernel density estimation and bootstrap modality testing. The method improves on existing work based on searching for gaps in histograms, in that it does not depend on arbitrary choices of parameters (such as bin widths for histograms), and provides a direct estimate of the significance of continuities or gaps in observed extinction intensities. I am thus able to test rigorously whether differences between mass extinctions and background extinctions are statistically significant.
I apply the methodology to Sepkoski's database of Phanerozoic marine genera. I conclude that mass and background extinctions appear to be continuous at this third level—continuity of magnitude—even though evidence suggests that they are discontinuous at the first and second levels.
Area cladograms produced by parsimony analysis of endemicity illustrate historically developed biogeographical associations among Caradocian, Ashgillian, Llandoverian, and Wenlockian bryozoans. Areas in North America, Siberia, and Baltica were organized into three provinces and 12 biomes over a time interval of 35 million years. Six of these biomes belonged to the North American-Siberian Province and became extinct during the Ashgillian. Three biomes represent a successional series of biogeographical associations in the Late Ordovician of Baltica, and the middle biome of this succession is most closely related to that of the Wenlockian platform in North America. All four Silurian biomes are represented in Late Ordovician local areas, indicating that the associations important in the recovery radiation were already in existence prior to the extinction events. Three of these four biomes expanded their geographic extent in the wake of the Late Ordovician extinctions. Several biome extinction and replacement events took place during lowstands of sea level, suggesting that biogeographic reorganizations took place as a consequence of habitat loss in epeiric seas. Biome development largely depended on the extent of major litho-topes and their intersections with deep ocean and climatic barriers. The loss of regional habitats, associated with marine regression, was a key factor in biome extinction and reorganization, and indicates that biogeography played a significant role in the Late Ordovician mass extinctions and Silurian recovery radiations. Vicariance hypotheses are needed to account for the development of barriers subdividing ancestral areas, whereas hypotheses of congruent dispersal are required to explain the appearance of biomes in geographically disjunct areas.
One can argue that the critical factors in evolution are competitive interaction, dispersal, and survival in that they determine the life and death of individuals and thus govern gene flow and the waxing and waning of populations; the byproduct of this cascade happens to be the origination and extinction of species and phylogenetic lineages. Perhaps evolutionary “truth” can be found only in petri dish experiments of lichens competing for space, through documentation of the wildly successful ecological invasion of purple loosestrife in ditches of northeastern North America, or by molecular phylogeographic analyses of the distribution of fruit fly lineages across the Hawaiian Islands. Nonetheless, there is something fundamentally intriguing in the fact that there are some 300,000 species of angiosperms currently distributed around the planet; an order of magnitude greater than other, far more ancient clades such as cycads, ferns, and horsetails and a discrepancy that has spurred centuries of evolutionary theorizing on the causes of their high relative diversity—Darwin's “abominable mystery.” These sorts of macroevolutionary questions necessitate research agendas that target global distributions of taxa over long intervals of geological time. Did mammals stay low to the ground until the large-bodied herbivore niches were opened up when an asteroid knocked off the last of the non-avian dinosaurids at the end of the Cretaceous? Was the transition from the dazzling variety of trilobites that dominated Cambrian and Ordovician marine ecosystems to the diverse ammonoid fauna of the later Paleozoic associated with differing intrinsic taxonomic turnover rates? Are species in tightly integrated communities such as reefs more vulnerable to extinction during intervals of climate change than species with lower levels of ecological connectivity?
Existing methods for point and interval estimation of the endpoints of the stratigraphic range of a fossil taxon under continuous sampling assume that the mean density of finds is constant over the stratigraphic range. These methods can perform badly when this mean density is not constant. Most seriously, if mean density declines toward the endpoint of interest, then the true coverage of the confidence interval for the true endpoint can be far below its nominal level, giving a false impression of estimation precision. Simple point and interval estimates that are designed to avoid this problem are presented. These methods are illustrated with the fossil record of two species of the Caribbean bryozoan Metrarabdotos.
A drawback to most existing methods of calculating confidence limits on fossil ranges is their assumption that the probability of collecting a taxon through a stratigraphic section is constant. Marshall (1997) described an approach that would circumvent this problem, but it requires knowing the probability of collection as a function of stratigraphic position. Multivariate paleoecological methods, such as detrended correspondence analysis (DCA), offer a means of estimating these probabilities. DCA axis 1 sample scores can be used to quantify facies change through a stratigraphic section, and to calculate the probability of collection of a taxon relative to DCA axis 1. From these two, the probability of collection of each taxon can be estimated for each horizon in the measured section. This approach is applied here to the Upper Ordovician Kope Formation of the Cincinnati, Ohio, area to distinguish between disappearances of taxa that are driven by facies change and taxon rarity and those that represent true regional extinction. This new approach to confidence limits could also be applied to test the synchroneity of extinction or origination at large-scale turnover events, such as mass extinctions and the turnover pulses that bound episodes of faunal stasis.
A major goal of paleobiological research since the early 1960s has been the reconstruction in quantitative terms of the history of biological diversity. Spearheaded by Valentine (1969), Raup (1972, 1976a, b), and Sepkoski (1979, 1981, 1984, 1990, 1993), this effort has yielded estimates of global diversity through time, as well as calculations of global rates and magnitudes of extinction and diversification. A consensus emerging in the early 1980s (Sepkoski et al. 1981) indicated that global marine invertebrate diversity rose through the Cambrian and Ordovician periods to a plateau, which with brief extinction-related interruptions was maintained from the mid-Paleozoic to the mid-Mesozoic. Beginning in the Cretaceous, diversity rose again, reaching a peak in the late Neogene. The five mass extinctions of the Phanerozoic, and more or less distinct episodes of diversification, were identified and distinguished from many lesser events (Raup and Sepkoski 1982). Comparable studies, with varying results, were conducted on land vertebrates (Benton 1985, 1989), land plants (Knoll et al. 1979; Niklas et al. 1980, 1983; Tiffney 1981; Knoll 1984), early protistans (Knoll 1994), insects (Labandeira and Sepkoski 1993), and life as a whole (Van Valen 1984, 1985; Van Valen and Maiorana 1985; Signor 1990; Valentine et al. 1991; Benton 1995; Courtillot and Gaudemer 1996; Miller and Foote 1996).
The principal conch parameters—whorl expansion rate, whorl overlap rate, umbilical width, and whorl thickness—of Early and Middle Devonian ammonoids have been extensively investigated. Stratophenetic analyses show long-term trends in the transformations of these characters over long periods of time, but sudden and rapid reversals can also be observed. On the basis of these four quantifiable conch parameters and supplementary qualitative characters, ten ammonoid morphs were distinguished. Reconstruction of the evolutionary history of these morphs reflects the existence of two major phylogenetic lineages, both already visible in Early Devonian faunas. The agoniatitid lineage is characterized by slow character development and leads to the Frasnian gephuroceratids; the anarcestid lineage displays rapid morphological evolution that leads to the late Givetian pharciceratids as well as the Middle and Late Devonian tornoceratids. Morphological evolution is interpreted as partly limited by geometrical and physical constraints.
Shell growth and morphogenesis were studied in nine species of Bivalvia from the viewpoint of theoretical morphology. The aperture map, or pattern of relative rate of shell accretion for each point around the aperture, received particular attention. Morphometric analyses indicate that the basic pattern of the aperture map is generally maintained throughout ontogeny, whereas both shell convexity and aperture shape commonly change with growth. Computer simulations show that posterior elongation of the aperture with growth cancels the effect of ontogenetic shell inflation to move the maximal growth point anteriorly. In the species examined, the coiling axis is generally inclined to the hinge axis toward the anterodorsal direction and is plunging to the dextral side of the valve. This condition allows ontogenetic shell inflation without modification of the basic pattern of the aperture map. The result indicates that ontogenetic change of shell form is architecturally constrained by a basic pattern of the aperture map, which is kept throughout ontogeny.
With a sample of 94 Pleistocene cranial capacities between the time period of 1.8 Ma and 50 Ka now known, we consider the evolution of cranial capacity in Homo, with the null hypothesis that the changes over time are a result of one process. We employ a new method that uses a resampling approach to address the limitations imposed on the methods of previous studies. To test the null hypothesis, we examine the distribution of changes in adjacent temporal samples and ask whether there are differences between earlier and later samples. Our analyses do not reject the hypothesis of a single process of brain size change, but they are incompatible with an interpretation of punctuated equilibrium during this period. The results of this paper are difficult to reconcile with the case for cladogenesis in the Homo lineage during the Pleistocene.
When ambitious large-scale projects are proposed, like the Paleobiology Database (Alroy et al. 2001) and Panama Paleontology Projects (Jackson and Johnson 2001), there is inevitably a negative reaction from various quarters, which is quickly forgotten when the projects start to yield results. Given that, it may be useful to sing the praises of “big picture” science more generally and I will do so after briefly addressing this discussion starter in the context of my own research.
Previous studies of overall arthropod disparity have compared Cambrian and Recent biotas, without considering taxa of intermediate age. This study explored morphological diversity among Carboniferous arthropods, primarily from the well-known Westphalian Mazon Creek Lagerstätte. Over 100 arthropod species, belonging to 48 orders, were examined. The data set is composed of nearly equal numbers of crustacean, arachnid, and insect species, with lower numbers of merostomes. Trilobites have not been found at Mazon Creek. However, some Late Carboniferous trilobite species were included in order to obtain a more representative picture of global Carboniferous arthropod disparity.
The absence, presence, or state of 66 shared characters was recorded for each species, as well as individual autapomorphies. Overall disparity was determined from the Euclidean distance analysis between taxa or variance along principal coordinates analyses (PCO) axes. Results indicate that arthropod disparity has not been greatly reduced throughout the Phanerozoic as was previously suggested. However, the regions of occupied morphospace have rotated over time.
One of the greatest challenges in using faunal assemblages to make ecological or paleoecological interpretations is determining the spatial scale over which such analyses are applicable. As a result, it has been difficult to use these assemblages to test hypotheses about spatial and temporal variability in populations. Here we show that it is possible to use strontium (Sr) isotopes from bones and vegetation to statistically constrain the area sampled in two Holocene predator accumulations in northeastern Yellowstone National Park, Wyoming. Previous studies have used these sites to elucidate local population responses to climatic change, by assuming that the specimens originated within ~5 km of the site. We used Sr analyses to construct a likelihood curve that describes the probability that our samples were collected within a given radius of each site. Our results indicate that the specimens in both sites were derived from non-overlapping populations and that the collection radius has not changed detectably over the past 3000 years. This work underscores the promise of this technique for ascribing source areas to paleontological, biological, and ecological specimens.
A theoretical morphologic model defining ligament formation in the Bivalvia is introduced. It is based on the spacing of a lamellar layer, the spacing of a fibrous layer, and the relative growth rate of the expanding ligament with respect to enlargement of the ligamental area. Most of the diverse patterns of bivalve ligaments are successfully modeled by computer simulations. Wide intraspecific variation of the ligamental pattern is observed in an arcid species, Tegillarca granosa. This appears to be a consequence of allometric change of morphogenetic parameters during growth, adjusted to maintain the relationship between ligament strength and shell weight. The distribution of actual ligaments, which does not fill the theoretical morphospace, shows potential evolutionary pathways of bivalve ligaments. Thus, it implies phylogenetic relationships between ligament types from the viewpoint of pattern formation.
The claim that measures of global biodiversity dynamics are meaningless is based upon several methodological problems, including underrepresentation of tropical regions in “global” Phanerozoic data sets, inaccuracies in taxonomic data, non-equivalence of higher taxa among groups of organisms, and uneven sampling intensity across groups, environments, and time intervals. Some of these problems are inherent in the fossil record, whereas others lie in documentation and interpretation of the subject. But the subject of global biodiversity is perfectly legitimate, even if problems persist in evaluating its full history. Moreover, recognition of the methodological problems has resulted in notable improvements in the Phanerozoic diversity database (e.g., Adrain and Westrop 2000; Alroy et al. 2001).
We examine patterns of intra-otolith variation in δ13C values of fossil Aplodinotus grunniens (freshwater drum) otoliths recovered from an archeological site in northeast Tennessee. We find three repeatable patterns: an initial increase early in ontogeny followed by relatively stable δ13C values as the fish ages, an initial strong covariation between seasonal δ18O and δ13C values, and a decrease with age in the magnitude of seasonal change in δ13C values. These last two observations are illustrated by seasonal least-squares linear regressions between δ13C and δ18O values that tend to progressively decrease in r2 value and slope with fish age. These patterns are evaluated by using a mass balance model in which otolith δ13C values are derived from dissolved inorganic carbon of ambient water mixing with carbon derived from metabolic processes. The proportion of metabolically derived carbon is found to be the dominant factor controlling intra-otolith variation in δ13C values.
Thus, the difference between maximum and minimum δ13C values from a single otolith (δ13Cmax-min) is postulated to reflect the total change in metabolic rate over the lifetime of a fish. δ13Cmax-min values significantly and negatively covary with average δ18O(CaCO3) values, suggesting either a higher total change in metabolic rate over the lifetime of a fish in cooler climates characterized by shorter growing seasons, or a decrease in summer/winter precipitation ratio. A proxy for metabolic rate preserved in otoliths would facilitate the understanding of evolutionary history in physiological traits of fishes and improve our understanding of bioenergetics.
All measurements and estimates have meaning, if consistent with peer-reviewed data and gifted with heuristic value. In pure science, we judge measures and estimation techniques by their importance to theory. We devise them for maximum relevance, and advance their reliability and precision by successive approximations. If better measures and estimation techniques come along, either by shifts in theoretical context or advances in technology, we discard the old and use the new.
The Desmostylia, an extinct order of mammals related to sirenians and proboscideans, are known from the late Oligocene to late Miocene of the North Pacific. Though often categorized as marine mammals on the basis of fossil occurrences in nearshore deposits, reconstructions of desmostylian habitat and dietary preferences have been somewhat speculative because morphological and sedimentological information is limited. We analyzed the carbon, oxygen, and strontium isotope compositions of enamel from Desmostylus and co-occurring terrestrial and marine taxa from middle Miocene sites in California to address the debate surrounding desmostylian ecology. The δ13C value of tooth enamel can be used as a proxy for diet. Desmostylus had much higher δ13C values than coeval terrestrial or marine mammals, suggesting a unique diet that most likely consisted of aquatic vegetation. Modern aquatic mammals tend to exhibit lower variability in δ18O values than terrestrial mammals. Both fossil marine mammals and Desmostylus exhibited low δ18O variability, suggesting that Desmostylus spent a large amount of time in water. Finally, the Sr isotope composition of marine organisms reflects that of the ocean and is relatively invariant when compared with values for animals from land. Sr isotope values for Desmostylus were similar to those for terrestrial, rather than marine, mammals, suggesting Desmostylus was spending time in estuarine or freshwater environments. Together, isotopic data suggest that Desmostylus was an aquatic herbivore that spent a considerable portion of its life foraging in estuarine and freshwater ecosystems.
Quantitative estimates of time-averaging in marine shell accumulations available to date are limited primarily to aragonitic mollusk shells. We assessed time-averaging in Holocene assemblages of calcitic brachiopod shells by direct dating of individual specimens of the terebratulid brachiopod Bouchardia rosea. The data were collected from exceptional (brachiopod-rich) shell assemblages, occurring surficially on a tropical mixed carbonate-siliciclastic shelf (the Southeast Brazilian Bight, SW Atlantic), a setting that provides a good climatic and environmental analog for many Paleozoic brachiopod shell beds of North America and Europe. A total of 82 individual brachiopod shells, collected from four shallow (5–25 m) nearshore (<2.5 km from the shore) localities, were dated by using amino acid racemization (D-alloisoleucine/L-isoleucine value) calibrated with five AMS-radiocarbon dates (r2 = 0.933). This is the first study to demonstrate that amino acid racemization methods can provide accurate and precise ages for individual shells of calcitic brachiopods.
The dated shells vary in age from modern to 3000 years, with a standard deviation of 690 years. The age distribution is strongly right-skewed: the young shells dominate the dated specimens and older shells are increasingly less common. However, the four localities display significant differences in the range of time-averaging and the form of the age distribution. The dated shells vary notably in the quality of preservation, but there is no significant correlation between taphonomic condition and age, either for individual shells or at assemblage level.
These results demonstrate that fossil brachiopods may show considerable time-averaging, but the scale and nature of that mixing may vary greatly among sites. Moreover, taphonomic condition is not a reliable indicator of pre-burial history of individual brachiopod shells or the scale of temporal mixing within the entire assemblage. The results obtained for brachiopods are strikingly similar to results previously documented for mollusks and suggest that differences in mineralogy and shell microstructure are unlikely to be the primary factors controlling the nature and scale of time-averaging. Environmental factors and local fluctuations in populations of shell-producing organisms are more likely to be the principal determinants of time-averaging in marine benthic shelly assemblages. The long-term survival of brachiopod shells is incongruent with the rapid shell destruction observed in taphonomic experiments. The results support the taphonomic model that shells remain protected below (but perhaps near) the surface through their early taphonomic history. They may be brought back up to the surface intermittently by bioturbation and physical reworking, but only for short periods of time. This model explains the striking similarities in time-averaging among different types of organisms and the lack of correlation between time-since-death and shell taphonomy.
Ecomorphological and biogeochemical (trace element, and carbon, nitrogen, and oxygen isotope ratios) analyses have been used for determining the dietary niches and habitat preferences of large mammals from lower Pleistocene deposits at Venta Micena (Guadix-Baza Basin, Spain). The combination of these two approaches takes advantage of the strengths and overcome the weakness of both approaches. The range of δ13Ccollagen values for ungulate species indicates that C3 plants were dominant in the diet of these mammals. δ13Ccollagen values vary among ungulates: perissodactyls have the lowest values and bovids the highest ones, with cervids showing intermediate values. The hypsodonty index measured in lower molar teeth and the relative length of the lower premolar tooth row indicate that the horse, Equus altidens, was a grazing species, whereas the rhino, Stephanorhinus etruscus, was a mixed feeder in open habitats. The similar δ13Ccollagen values shown in both perissodactyls does not reflect differences in feeding behavior with other ungulates, but rather a lower isotope enrichment factor in these monogastric herbivores than in ruminants, owing to their lower metabolic efficiency. The cervids Eucladoceros giulii and Dama sp. show low hypsodonty values, indicating that they were mixed feeders or browsers from forested habitats, an ecomorphologically based conclusion corroborated in the former by its low δ15Ncollagen content (canopy effect). Bovid species (Bovini aff. Leptobos, Soergelia minor, and Hemitragus albus) presumably inhabited open environments, according to their comparatively high hypsodonty and δ15Ncollagen values. Carnivore species (Homotherium latidens, Megantereon whitei, Pachycrocuta brevirostris, Canis falconeri, and Canis etruscus) exhibit higher δ15Ncollagen values than ungulates. These results record the isotopic enrichment expected with an increase in trophic level and are also supported by low bone Sr.Zn ratios. The elevated δ15Ncollagen value for a sample of Mammuthus meridionalis, which came from an individual with unfused epiphyses, confirms that it was a suckling animal. The δ15Ncollagen value of the scimitar-cat H. latidens is well above that obtained for the young individual of Mammuthus, which indicates that juvenile elephants were an important part of its diet. The hippo, Hippopotamus antiquus, yielded unexpectedly high δ15Ncollagen values, which suggest feeding on aquatic, non-N2-fixing plants. The high δ18Ohydroxyl values of bovids Hemitragus and Soergelia and of cervid Dama indicate that these ungulates obtained most of their water requirements from the vegetation. The megaherbivores and Eucladoceros exhibit the lowest δ18Ohydroxyl values, which suggest increased water dependence for them. Paleosynecological analysis was based on the relative abundance of species of large mammals from different ecological categories, determined by feeding behavior and locomotion types. The comparison of the frequencies of such categories in Venta Micena with those found in modern African communities indicates that the composition of the paleocommunity closely resembles those of savannas with tall grass and shrubs. The net above-ground primary productivity estimated for the on-crop biomass of the mammalian species preserved in the fossil assemblage also yields a figure congruent with that expected for an open environment.