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High-resolution estimates of temporal mixing within shell beds: the evils and virtues of time-averaging
- Michał Kowalewski, Glenn A. Goodfriend, Karl W. Flessa
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- Published online by Cambridge University Press:
- 20 May 2016, pp. 287-304
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This study quantifies the fine structure of time-averaging by using large samples of dated shells collected from within individual strata. Time-averaging results in both good and bad news for interpreting bioclastic deposits.
Nine samples of shells were collected from four Holocene cheniers on the Colorado Delta (Gulf of California) and 165 shells of the bivalve Chione fluctifraga were dated using 14C-calibrated amino acid racemization (D-alloisoleucine/L-isoleucine). The age range of shells within samples averages 661 years and, in seven out of nine samples, exceeds 500 years. The sample standard deviation ranges from 73 to 294 years and averages 203 years, far exceeding the dating errors (≪100 years) and potential variation in the life span of Chione (<10 years). Time-averaging is homogeneous among strata within cheniers but varies significantly among cheniers. Age-distributions of dated shells indicate that at 50-year resolution, the samples provide a continuous and uniform record for the entire interval. The actual sample completeness (63.6%) is very close to that predicted by simulations of sampling a 100% complete, uniform record (67.3%).
The bad news is that, no matter how carefully collected, data from shell beds may not be suitable for studying processes on timescales shorter than 102 to 103 years; explanations for faunal change that invoke reasoning or models derived from a strictly ecological point of view may rarely be justifiable. Also, notable differences in temporal resolution between the shell beds of seemingly identical origin imply that paleontological patterns (e.g., species diversity) may be affected by cryptic variation in time-averaging. The comparison of our data with time-averaging estimates obtained from other cheniers at coarser sampling resolutions indicates that pooling of samples (analytical time-averaging) can significantly reduce the temporal resolution of paleontological data.
The good news is that shell beds can record the optimal type of time-averaging: where paleobiological data are a time-weighted average of the faunal composition from the spectrum of environments that existed during the entire interval of time. Samples from single strata provide a long-term record that is representative of the predominating environments. Within the range of 14C dating, shell beds can provide a complete, high-resolution record, and thus may offer exceptional insights into the environmental and climatic changes of the last 40 thousand years.
Speciation and extinction asymmetries in paleontological phylogenies: evidence for evolutionary progress?
- Paul N. Pearson
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- Published online by Cambridge University Press:
- 20 May 2016, pp. 305-335
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This paper concerns paleontological phylogenies that have a “budding” configuration, wherein “ancestral” species persist through branching events to coexist with their “descendants.” Two principal tests are proposed for detecting patterns within such trees. The first test, called the “ancestor-descendant extinction test,” compares the number of cases in which, after a split, the ancestral species became extinct before its descendant with the number of cases in which the descendant became extinct before its ancestor. The second test, called the “ancestor-descendant speciation test,” compares the number of cases in which, after a split, the ancestral species gave rise to a further species with the number of cases in which the descendant species gave rise to a further species. The null hypothesis in each case is that the frequencies are equal, as predicted by a random Markovian branching model of evolution.
Five stratophenetic species-level phylogenies of three taxonomic groups, planktonic foraminifera, nannofossils, and graptoloids, are examined using these tests, including one (Paleogene planktonic foraminifera) that is presented for the first time. In all cases, the phylogenetic trees are found to be strongly nonrandom. The general pattern, although by no means expressed perfectly in every case, corresponds to a Simpsonian “step-series,” in which ancestor taxa are simultaneously more likely to become extinct and less likely to speciate than their coexisting descendants. It is shown that this pattern cannot simply be the result of simple age-dependent factors such as an increasing extinction risk in older taxa. Instead, the very fact that a species has given rise to another appears to increase its future extinction risk and decrease its likelihood of further speciation.
Many possible biases may affect the shape of paleontological phylogenies, which are as yet poorly understood and unquantified. One potentially important effect follows from the taxonomic subdivision of gradual chronoclines into artificial morphospecies, such as might conceivably induce a step-series pattern in the phylogeny. Even if this is the partial or entire reason for the observed patterns, it would appear to imply directional evolution in phyletic gradualism. Other possible artifacts are discussed, but they are regarded as probably too weak to produce the observed patterns.
If the pattern is not artificial, the fact that three of the best known fossil groups exhibit substantial asymmetries in speciation and extinction argues against the currently popular “nonprogressive” view of evolution. Instead, the evolutionary step-series pattern is consistent with the classical Darwinian concept of the general competitive superiority of newly evolved species over their ancestors and supports the idea of evolutionary progress.
A mathematical model for long-term patterns of evolution: effects of environmental stability and instability on macroevolutionary patterns and mass extinctions
- Satoshi Chiba
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- Published online by Cambridge University Press:
- 20 May 2016, pp. 336-348
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A simple mathematical model to examine the relationships between environmental instability and long-term macroevolutionary trends is presented. The model investigates the evolutionary changes that occur in certain population characters in an environment with physical disturbance. These quantitative genetic characters are related to intrinsic growth rates and mean carrying capacity. The model assumes that individual fitness is determined by these characters. I examine the likelihood of extinction under different degrees of environmental instability and for rapid change of environmental instability. The model suggests that characters that promote a high intrinsic growth rate and a low carrying capacity tend to evolve in the most unstable environments. This suggests that small body size, high fecundity, and simple forms evolve in unstable environments. The extinction probability of a population is the lowest for taxa possessing K-selected characters in the most stable environment. However, the extinction probability of a species (metapopulation) becomes lowest for r-selected species living in the most unstable environment and for the K-selected species living in the most stable environment, and it becomes the highest for taxa living in a moderately unstable environment. Increasing environmental instability changes the extinction probabilities of different taxa in different ways, due to differences in phenotypes and environments. The effect of environmental change is most serious for the K-selected taxa in the most stable environment. This also suggests that a continuously stable environment increases the extinction probability of taxa when environmental change occurs. Although catastrophic changes in environments are not presumed, these results are consistent with the existence of two “macroevolutionary regimes” in which a taxon's extinction rate and its characters differ for mass extinction and normal extinction. Mass extinction can occur as a result of long-term adaptation to a stable environment following a minor change of environment without catastrophes.
Fractal grain distribution in agglutinated foraminifera
- Kathryn Allen, Stephen Roberts, John W. Murray
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- Published online by Cambridge University Press:
- 20 May 2016, pp. 349-358
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A fractal geometry of clast size within the test wall in the Antarctic agglutinated foraminifera Hormosina mortenseni Cushman, 1910 and Cyclammina cancellata Brady, 1879 has been identified with the use of Scanning Electron Microscopic techniques. External surface and internal clast distributions in H. mortenseni display a self-similar distribution. C. cancellata has an internal self-similar grain arrangement, whereas the exterior surface shows an alternative grain distribution. Power law relationships between particle density and grain diameter enable values of fractal dimension (D) to be calculated; these “D-values” represent the absolute gradient of the power law relationship. The dimensions acquired from the foraminiferal study correspond well with those previously obtained from natural fractal geological structures and ideal fractals. The self-similar grain arrangement within walls of the foraminifera exists over three orders of magnitude, after which alternative methods of test wall construction are evident. This suggests that a limit exists where grain selection terminates. A self-similar grain distribution limits the amount of biologically produced adhesive material required by the foraminifera for constructing their tests.
Ontogeny of Trimerocephalus lelievrei (Trilobita, Phacopida), a representative of the Late Devonian phacopine paedomorphocline: a morphometric approach
- Catherine Crônier, Sabrina Renaud, Raimund Feist, Jean-Christophe Auffray
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- Published online by Cambridge University Press:
- 20 May 2016, pp. 359-370
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A detailed morphometric approach based on size and on outline analyses has been used on an exceptionally well-preserved assemblage of silicified trilobite exuvia, recovered from a Late Devonian limestone from southeastern Morocco. The material comprises a series of late larval to postlarval growth stages belonging to a single phacopine species, Trimerocephalus lelievrei Crônier and Feist, 1997.
Plurimodality of size distribution has allowed us to discriminate postlarval instars. Distinct dimensional classes of isolated parts are obtained using the intertooth distances on the posterior pygidial margin and the internotch distances in the cephalic vincular furrow, which are functionally linked during trilobite enrollment. Morphometric analysis of development permitted demonstration of progressive shape change in agreement with ontogenetic ordination and a comparison of the timing of size and shape changes. The main shape changes appear to occur early in development, and once the “adult” morphology is obtained, size increases significantly. The growth rate during ontogeny is estimated by analogy with extant deep-sea crustaceans. Exponential size increase resulting from constant duration of intermolt periods may be regarded as a life history strategy to compete in a nutrient-impoverished offshore environment. The particular phacopine mode of molting, which involves the opening of the neck joint after ankylosis of the facial sutures, occurred in Trimerocephalus lelievrei between the first two postlarval instars, later than in its ancestor. Trimerocephalus lelievrei occupies an intermediate position within the phacopine paedomorphocline as indicated by the delayed onset of ankylosis.
The masticatory apparatus of the armadillo Eutatus (Mammalia, Cingulata) and some allied genera: paleobiology and evolution
- Sergio F. Vizcaíno, María S. Bargo
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- Published online by Cambridge University Press:
- 20 May 2016, pp. 371-383
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The eutatines are a group of fossil armadillos traditionally regarded as herbivorous due to the particular morphology of the skull and teeth. Nevertheless, they have never been the subject of a detailed morpho-functional analysis. The masticatory apparatus of Eutatus (late Pliocene–early Holocene) is analyzed and compared with other eutatines (Proeutatus and Stenotatus from the Miocene, and Doellotatus and Ringueletia from the Pliocene) and with living armadillos (Euphractus and Dasypus). The masticatory muscles were reconstructed from origin and insertion scars; the occlusal pattern and mandibular movements were determined through the study of the craniomandibular joint, the shape and arrangement of the teeth and the symphysis, and the moment arms of the lines of action of the masseter and temporalis muscles were estimated. Skull and mandible shapes were compared using the Procrustean method Resistant-Fit Theta-Rho-Analysis (RFTRA). The analysis of the masticatory apparatus of the eutatines allows us to state that Eutatus and Proeutatus exhibit the most specialized morphology known for an herbivore with an armadillo-like skull pattern. Additionally, we can identify a morphological group from the Miocene Stenotatus to the Pleistocene Eutatus, including the Pliocene Doellotatus and Ringueletia as intermediate stages of a morphological line; Proeutatus deviates from this pattern. This morphological separation may reflect an early dichotomy within the well-defined clade of eutatines.
Using large mammal communities to examine ecological and taxonomic structure and predict vegetation in extant and extinct assemblages
- Kaye E. Reed
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- Published online by Cambridge University Press:
- 20 May 2016, pp. 384-408
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Evolutionary paleoecology is the study of paleoecological patterns of organization over time. However, identification of such patterns within modern communities must be made before any study over time can be attempted. This research analyzes mammalian ecological diversity of 31 African localities classified into eight vegetation types: forests, closed woodlands, closed woodland/bushland transition, bushlands, open woodlands, shrublands, grasslands, and deserts. Ecological diversity is measured as the relative proportions of large mammal trophic and locomotor behaviors within communities. Trophic and locomotor adaptations are assigned on the basis of published observations and stomach contents of 184 African mammal species. Communities are accordingly described on the basis of total percentages of mammalian trophic and locomotor adaptations. Since many paleoecology studies have been made using taxonomic uniformitarianism, this study also examines taxonomic community structure to compare with ecologically derived patterns.
Results indicate that particular types of vegetation have predictable percentages of arboreal, aquatic, frugivorous, grazing, etc. large mammals. Therefore, these adaptations, because they are predictable in extant assemblages, can be used to predict paleovegetation as well as to portray the community structure of fossil assemblages. Taxonomic groupings also can be used to predict vegetation in extant assemblages, and taxonomic patterns in communities are compared with ecological ones.
The mammalian communities of the Pliocene fossil locality Makapansgat, South Africa, are interpreted using these ecological and taxonomic methodologies. Trophic and locomotor adaptations are assigned for Makapansgat fossil mammals through morphological examination of each taxon. Vegetation type is predicted for these fossil localities, but ecological and taxonomic differences in the assemblages differ from extant communities.