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Examination of the neanic apparatuses of known populations of Nephrolepidina praemarginata, N. morgani, and N. tournoueri reveals that the equatorial chamberlets are arranged in spirals, along the direction of connection of the oblique stolons, giving the optical effect of intersecting curves. In N. praemarginata commonly 34 left- and right-oriented primary spirals occur from the first annulus to the periphery, 21 secondary spirals from the third to fifth annulus, 13 ternary spirals from the fifth to eighth annulus, following the Fibonacci sequence.
The number of the spirals increases in larger specimens and in more embracing morphotypes, and especially in trybliolepidine specimens; the secondary and ternary spirals from the investigated N. praemarginata to N. tournoueri populations tend to start from more distal annuli. An interpretative model of the spiral growth of Nephrolepidina is attempted.
The angle formed by the basal annular stolon and distal oblique stolon in equatorial chamberlets ranges from 122° in N. praemarginata to mean values close to the golden angle (137.5°) in N. tournoueri.
The increase in the Fibonacci number of spirals during the evolution of the lineage, along with the disposition of the stolons between contiguous equatorial chamberlets, provides new evidence of evolutionary selection for specimens with optimally packed chamberlets.
Natural selection favors individuals with the most regular growth, which fills the equatorial space more efficiently, thus allowing these individuals to reach the adult stage faster. We refer to this new type of selection as “golden selection.”
Model calculations predict that pathways of alpha- and beta-diversity in diversifying ecosystems notably differ depending on the relative role of competition, predation, positive effects of species' interactions, and environmental parameters. Four scenarios are discussed, in which alpha- and beta-diversity are modeled as a function of increasing gamma-diversity. The graphic illustration of this approach is herein called α-β-γ plot, in which the x-axis indicates increasing diversification rather than absolute time. In purely environmentally controlled systems, beta-diversity maintains near-maximum values throughout the diversification interval, whereas mean alpha-diversity increases linearly, with a slope being reciprocal to beta-diversity. A second scenario is based on the assumption that increasing richness will have predominantly positive effects on the addition of further species; here, alpha- and beta-diversity increase simultaneously (though not necessarily at the same rates) and without reaching a predictable upper limit. In ecosystems that are characterized by low competition between species, mean alpha-diversity asymptotically approaches a saturation level, whereas the increase in beta-diversity accelerates until alpha-diversity stagnates, and then continues to rise linearly. If competition is high, addition of species first increases beta-diversity until no further habitat contraction is possible, followed by a period in which alpha-diversity increase through adaptive divergence becomes the principal drive of diversification. Because there is a continuous transition between the late stage of the low-competition model and the early stage of the high-competition scenario, both can be combined in a single model of diversity partitioning under the premise of a diversity-dependent increase of competition. This summary model predicts three phases of diversity accumulation: (1) a niche overlap phase, (2) a habitat contraction phase, and (3) a niche differentiation phase. The models herein discussed provide a potential tool to assess the question which factors primary controlled the diversification of life over geological times.
Analysis of two independent data sets with increased taxonomic resolution (genera rather than families) using the revised 2012 timescale reveals that an extinction periodicity first detected by Raup and Sepkoski (1984) for only the post-Paleozoic actually runs through the entire Phanerozoic. Although there is not a local peak of extinction every 27 Myr, an excess of the fraction of genus extinction by interval follows a 27-Myr timing interval and differs from a random distribution at the p ∼ 0.02 level. A 27-Myr periodicity in the spectrum of interval lengths no longer appears, removing the question of a possible artifact arising from it. Using a method originally developed in Bambach (2006) we identify 19 intervals of marked extinction intensity, including mass extinctions, spanning the last 470 Myr (and with another six present in the Cambrian) and find that ten of the 19 lie within ±3 Myr of the maxima in the spacing of the 27-Myr periodicity, which differs from a random distribution at the p = 0.004 level. These 19 intervals of marked extinction intensity also preferentially occur during decreasing diversity phases of a well-known 62-Myr periodicity in diversity (16 of 19, p = 0.002). Both periodicities appear to enhance the likelihood of increased severity of extinction, but the cause of neither periodicity is known. Variation in the strength of the many suggested causes of extinction coupled to the variation in combined effect of the two different periodicities as they shift in and out of phase is surely one of the reasons that definitive comparative study of the causes of major extinction events is so elusive.
The mammalian fossil record of Spain is long and taxonomically well resolved, offering the most complete record of faunal change for the Neogene of Europe. We evaluated changes in diversification, composition, trophic structure, and size structure of large mammals over the middle and late Miocene with methods applied to this record for the first time, including ordination of fossil localities to improve temporal resolution and estimation of confidence intervals on taxa temporal ranges. By contrast, analysis within the traditional Mammal Neogene (MN) biochronology obscures important aspects of diversification. We used inferred temporal ranges of species and evaluated per capita rates of origination, extinction, diversification, and turnover over 0.5-Myr time intervals.
Three periods of significant faunal change occurred between 12.0 and 5.5 Ma: (1) From 12.0 to 10.5 Ma, elevated origination rates led to an increase in diversity without significant change in ecological structure. Immigrants and geographic-range shifts of species to lower latitudes during an interval of global cooling contributed to these faunal changes. (2) From 9.5 to 7.5 Ma, high extinction rates followed by high origination rates coincided with significant changes in taxonomic composition and ecological structure. These changes represent the Vallesian Crisis, with replacement of a fauna of forest affinities (with frugivores and browsers) by a fauna of open woodlands (with grazers and mixed feeders). (3) From 6.5 to 5.5 Ma, high extinction rates reduced diversity without substantial changes in ecological structure, and large mammal faunas became highly endemic across the northern Mediterranean region. This interval includes the Messinian Salinity Crisis, the desiccation of the Mediterranean basin. Extinction may have been caused by geographic isolation and aridification, with evolution of endemic lineages giving rise to new species in the early Pliocene. These distinct macroevolutionary patterns of faunal change correspond to different geographic scales of inferred climatic and tectonic drivers.
In marsupial mammals and their extinct relatives—collectively, metatherians—only the last premolar is replaced, but the timing of dental eruption is variable within the group. Our knowledge of fossils metatherians is limited, but is critical to understanding several aspects of the evolution and morphological diversification of this clade. We analyzed the sequence of eruption of 76 specimens of metatherians, including Sparassodonta, an extinct clade of specialized carnivores from South America. In Sparassodonta (1) the P3/p3 erupt simultaneously, in common with some didelphids (in other didelphids, p3 erupts before P3, whereas in the remaining didelphids, some peramelids, one caenolestid, and Pucadelphys this order is reversed); (2) the upper and lower molars at the same locus erupt more in synchrony than in other carnivorous metatherians in which the lower molars clearly precede the upper equivalents; (3) the upper canine in thylacosmilids and proborhyaenids is hypselodont; (4) species with similar molar morphologies have different morphologies of the deciduous premolars, suggesting diverse diets among the juveniles of different taxa; (5) deciduous teeth are functional for a long period of time, with thylacosmilids even retaining a functional DP3 in the permanent dentition. The retention of the DP3 and the hypertrophied and hypselodont upper canine of thylacosmilids represent clear heterochronic shifts. Specializations in the timing of dental eruption and in the deciduous tooth shape of sparassodonts are evolutionary mechanisms that circumvent constraints imposed by the metatherian replacement pattern and increase morphological disparity during ontogeny.
The record of the taxonomic evolution of North American ungulates is critical to our understanding of mammalian evolution and environmental change throughout the Cenozoic. The distribution of sampling in the ungulate fossil record over time and geographic space and the degree to which this biases the observed patterns of taxonomic evolution is poorly understood. To address these issues, I placed fossil collections and occurrences drawn from the Paleobiology Database into 2-Myr time intervals between 55 and 1 Ma. I determined the variation in numbers of fossil collections and occurrences, using three metrics to measure geographic variation: first, the area of the convex hull containing all collections in an interval, to determine the areal coverage of sampling; second, the mean pairwise geographic distance among collections as a measurement of the dispersion of collections within that area; and third, the interval-to-interval migration of the geographic centroid of all collections, to calculate changes in the geographic location of sampling. Each of these showed considerable variation over the Cenozoic, and both the area of the convex hull (ACH) encompassing all collections in an interval, and mean pairwise distance (MPWD) among them showed increasing trends over time.
To minimize the effect of variation in numbers of fossil samples over time, I used standard sample-standardization procedures. To minimize the effect of geographic variation in sampling over time, I standardized the area of sampling among intervals. I also employed both standardizations sequentially. Each standardization procedure had surprisingly little effect on observed patterns of taxonomic richness and rates. This indicates that, for North American ungulates, neither variation in number nor geographic distribution of fossil samples exerts an overwhelming influence on perceived macroevolutionary patterns. These results confirm the ungulate fossil record as a critical and faithful record for our understanding of Cenozoic environmental change and the mammalian evolutionary response.
We review and synthesize multiple biotic and abiotic proxies for marine nutrient and food availability, primary productivity, and food quality (stoichiometry) and propose what their relationships may have been to macroevolutionary processes, especially speciation. This review confirms earlier suggestions that there has been an overall increase in marine primary productivity over the Phanerozoic, but indicates that the increase has been irregular and that present levels may not be the peak. We integrate these indicators into a new estimate of relative primary productivity in the global ocean through the Phanerozoic. We then combine multiple, frequently conflicting ecological-evolutionary hypotheses into a general model for how primary production may affect speciation over geological time scales. This model, an elaboration and extension of the “speciation cycle” previously proposed by Grant and Grant, attempts to explain why an increase in food supply sometimes is associated with decreased diversity, and at other times with increased diversification. We propose some simple tests for the application of this model to the fossil record.
With a single complete mandible and 56 mandibular symphyseal fragments of various sizes, the Late Cretaceous Hungarian azhdarchid material has been considered one of the most extensive monospecific pterosaur assemblages in the world. Representing a broad size range, these elements have been thought to demonstrate a developmental series of Bakonydraco galaczi. As such, they were ideal to test whether absolute size and/or morphology reliably indicate relative ontogenetic stages in this pterosaur. Forty-five specimens were selected for multivariate morphometrics and classified into four size classes. After acquiring the morphometric data set, we thin-sectioned eight symphyses representing all size groups and classified them into relative ontogenetic stages based on qualitative microstructural inspection prior to quantitative histological analyses. Microstructural characters suggestive of developmental state were then quantified for intra- and interindividual uni- and multivariate analyses to test the correspondence among the results of qualitative and quantitative analyses. In contrast to our expectations, histological features identified the smallest specimen as an adult and not an early juvenile. The substantial size difference between this specimen and other adults, along with its distinct microanatomical and histological features, implies the presence of at least two pterosaur taxa in this symphysis assemblage. This hypothesis is further supported by multivariate morphometrics, which separate the smallest symphyses from all other specimens that form one continuous group. Although the latter group also shows considerable size variability in corresponding ontogenetic stages, this suggests developmental plasticity rather than the presence of even more taxa, and indicates that symphysis size and morphology are poor indicators of skeletal maturity in these animals. Hence, bone histology is an important independent test of the assessment of ontogenetic stage using size and morphology.