William C. Parker

Biogeography of planktonic Foraminifera

There are several important unresolved issues in the area of modern planktonic foraminiferal biogeography. The large-scale latitudinally symmetrical faunal provinces do not appear to show a consistent relationship to comparably-scaled hydrographic features. The origins of these provinces are therefore likely to be understood by reference to other causal factors. The first of these is the degree to which smaller-scale hydrographic features such as current, gyre, and frontal systems combine to play a role in determining larger-scale latitudinal distribution patterns. The second is the role historical processes — particularly, latitude-dependent rates of speciation and extinction, and the development of tectonic barriers — have played in establishing faunal provinces and the global diversity gradient. A number of additional factors, including depth habitats, bipolarity, pore size, coiling direction, and iterative evolution, are indirectly related to physicochemical characteristics of the foraminiferal habitat.

Paedomorphosis and the origin of the Paleogene planktonic foraminiferal genus Morozovella

The evolutionary origin of Morozovella angulata from its immediate ancestor, Praemurica uncinata, is preserved in Paleocene sediments from the Gulf of Mexico. This event represents the beginning of the morozovellid radiation and marks the first appearance of keeled planktonic fo- raminifera after the Cretaceous/Tertiary extinction. Parallel biometric and isotopic analyses were performed on size-segregated specimens from a succession of stratigraphic horizons. The biometric data reveal a temporal pattern of variation consistent with paedomorphosis. The appearance of angulose juvenile chambers in the otherwise rounded ancestral form (Praemurica uncinata) results in an allometry that becomes more pronounced upsection. At the origin of M. angulata, the juvenile morphology of the ancestor is retained throughout the entire ontogeny. Isotopic analysis of this sequence reveals the gradual acquisition of an increasingly heavy adult 813C signal relative to that of the juvenile, while the 8180 data display no temporal or size-related trends. The temporal in- crease seen in the slope of the 813C/size relationship may reflect the evolution of an increased de- pendency on photosymbionts.

IS EXTINCTION AGE DEPENDENT

Abstract Age-dependent extinction is an observation with important biological implications. Van Valens Red Queen hypothesis triggered three decades of research testing its primary implication: that age is independent of extinction. In contrast to this, later studies with species-level data have indicated the possible presence of age dependence. Since the formulation of the Red Queen hypothesis, more powerful tests of survivorship models have been developed. This is the first report of the application of the Cox Proportional Hazards model to paleontological data. Planktonic foraminiferal morphospecies allow the taxonomic and precise stratigraphic resolution necessary for the Cox model. As a whole, planktonic foraminiferal morphospecies clearly show age-dependent extinction. In particular, the effect is attributable to the presence of shorter-ranged species (range < 4 myr) following extinction events. These shorter-ranged species also possess tests with unique morphological architecture. The morphological differences are probably epiphenomena of underlying developmental and heterochronic processes of shorter-ranged species that survived various extinction events. Extinction survivors carry developmental and morphological characteristics into postextinction recovery times, and this sets them apart from species populations established independently of extinction events.

Species survivorship in the Cenozoic planktonic foraminifera; a test of exponential and Weibull models

The species survivorship curve for Cenozoic planktonic foraminifera was fitted to exponential (age-independent) and Weibull (age-dependent) models. Model fits that account for censorship bias have substantially lower slopes (extinction probabilities) than suggested by censored data. The best-fit Weibull model has a shape parameter significantly greater than one (,B = 1.19), which normally indicates an age-dependent increasing probability of extinction. Deviation of Weibull /3 from 1.0 (exponential case) seems closely related to temporal variation in records of very short-lived taxa (?1 my). The shape of the curves are not sensitive to Corrected Species Survivorship adjustment of longevities, but may reflect taxonomic procedure or temporal variability in extinction probability.

Use of principal components analysis in petrology: an example from the Martinsville igneous complex, Virginia, U.S.A.

SummaryThe Martinsville igneous complex is located in the Smith River allochthon, within the Piedmont of southwestern Virginia, U.S.A. This Ordovician complex consists of two main plutonic units: the mafic Rich Acres suite and the Leatherwood Granite. Four lithologic phases can be recognized in the Rich Acres and two are present in the Leatherwood. Major- and trace-element analyses from these six phases have been examined by principal components analyses (PCA); the first two principal components account for 86.9 percent of the total variance in the database, as opposed to about 35 percent for the first two original variables. Examination of variable loadings and sample scores for these two principal components has led to a number of observations about which original chemical variables best characterize the database. Mixing lines, control lines, and the “lever rule” can be used on bivariate PC plots as they can on bivariate plots of original chemical variables.Results of the PCA coupled with field and petrographic relationships allow for some hypotheses to be posed concerning petrogenetic relationships among the lithologic units. Among these hypotheses are 1) some type of mixing process occurred between the Leatherwood and Rich Acres; 2) the lithologic phases within the Rich Acres form one cogenetic suite, and 3) the Rich Acres and Leatherwood apparently are not comagmatic, in contrast to earlier suggestions. PCA can also be used to place constraints on different crystal-fractionation models. Results for PCA are compared with those for discriminant function analysis (DFA); PCA indicates a compositional continuum between most groups, whereas DFA shows large compositional gaps. The results for PCA seem to be closer to the true situation.ZusammenfassungDer magmatische Komplex von Martinsville liegt im Smith River Allochton innerhalb des Piedmont des südstlichen Virginia, USA. Der Komplex besteht aus zwei tektonischen Haupteinheiten: die matische Rich Acres Abfolge und der Leatherwood Granit. Radiometrische Daten für den letzteren zeigen Ordovicisches Alter. Auf der Basis von unterschiedlicher Mineralogie und Textur existieren in der Rich Acres Abfolge vier lithologische Phasen, und im Leatherwood Granit zwei.Die Spurenelemente dieser sechs Phasen wurden durch Hauptkomponenten Analyse (PCA) untersucht; die ersten beiden Hauptkomponenten machen 86,9% der totalen Varianz in der Datenbasis aus, im Gegensatz zu ungefähr 35% für die ersten zwei Originalvariablen. Untersuchung der variablen Gewichtungen und der Proben „Scores” für diese zwei Hauptkomponenten zeigen, welche ursprünglichen chemischen Variablen die Datenbasis am besten charakterisieren. Beispielsweise bestätigt PCA, daß MgO eine der wichtigen Variablen ist, die die Datenbasis charakterisieren, SiO2 jedoch nicht. Außerdem können Mischlinien, Kontrollinien und die Hebelregel auf Bivarianten von Hauptkomponenten benützt werden, ebenso wie auf bivarianten Plots die ursprüngliche chemische Variable darstellen:Hauptkomponentenanalyse, zusammen mit den Gelände- und petrographischen Beziehungen ermöglicht es, Hypothesen bezüglich der petrogenetischen Beziehungen zwischen den lithologischen Einheiten aufzustellen. Diese umfassen:1.einen Misch-Vorgang, sehr wahrscheinlich Magma-Mischung, zwischen dem Leatherwood und dem Rich Acres Komplex2.die vier lithologischen Phasen innerhalb des Rich Acres Komplex bilden eine cogenetische Suite und3.die Rich Acres und Leatherwood Komplexe sind offensichtlich nicht comagmatisch im Gegensatz zu früher vorgelegten Anregungen. Die PCA-Diagramme können auch dazu benützt werden, um verschiedene Kristall-Fraktionierungsmodelle einzuschränken. Die Ergebnisse von PCA werden mit denen der diskriminanten Funktionsanalyse (DFA) verglichen; PCA weist auf ein Kontinuum der Zusammen setzungen zwischen den meisten Gruppen hin, während DFA große Lücken im Spektrum der Zusammensetzungen erkennen läßt. Die Ergebnisse für PCA scheinen der tatsächlichen Situation näher zu kommen.

Deviation from Red Queen behaviour at stratigraphic boundaries: evidence for directional recovery

Abstract Boundary-defining events influence the evolutionary behaviour of post-extinction survivors. The Cox proportional hazards model takes into account the varying background extinction rates characteristic of boundaries and enables survivorship analysis of post-boundary behaviour. Survivorship results from the Middle Cretaceous to recent planktonic foraminifera reveal two intriguing observations. First, they indicate significantly age-dependent extinction probabilities in populations of species following two boundaries: Cenomanian-Turonian (C-T) and Cretaceous-Tertiary (K-T); the survivors are short lived and show rapid turnover. Characteristics that might mediate this macroevolutionary behaviour are clearly distinct from those that precede the extinction. We hypothesize that the rapid taxonomic turnover during post-extinction macroevolutionary recovery is driven by the lingering expression of ‘passport’ characteristics, where the primary adaptive value was during the preceding extinction. Second, age-dependency of extinction oscillates through time. Many survivorship curves averaging long-term data have exponential or near-exponential form: suggesting a lack of age-dependence consistent with the Red Queen hypothesis. The boundary events discussed here, analysed in higher-resolution 15 Ma subsets, demonstrate perturbation of some post-extinction populations toward positive age-dependence, and are followed by long intervals suggestive of recovery. Red Queen behaviour, when measured over very long time-spans, appears to be the time-averaged result of these boundary-generated oscillations between short-term positive age-dependence and longer-term return to nearly age-independent Red Queen behaviour.

Aspects of the post-Cretaceous recovery of the Cenozoic planktic foraminifera

Abstract Survivorship analysis was used to extract unbiased (censorship-corrected) estimates of the species longevity distribution of the Cenozoic planktic Foraminifera. These estimated distributions provide the basis for monte carlo simulations of the post-Cretaceous recovery in the group. Using the achievement of a stable longevity distribution as a criterion for recovery yields a predicted recovery time of approximately 15 Myr. This prediction agrees well with the observed recovery time, but significant patterns of deviation from simulations suggest that extinction and origination rates covary and that rapid turnover during the Paleocene recovery period may have significantly reduced mean longevity during that time. A significant fit to a Weibull survivorship model suggests that there is an longevity-dependent increase in extinction probability which may be explained by temporal inhomogeneity in the origination rates of short-lived species. The scale of the inhomogeneity suggests it is unlikely to be an artifact of taxonomic practice or sampling error. Censorship-corrected mean longevity is reported to be 9.17 Myr.

A general class of test statistics for Van Valen's Red Queen hypothesis

Van Valens Red Queen hypothesis states that within a homogeneous taxonomic group the age is statistically independent of the rate of extinction. The case of the Red Queen hypothesis being addressed here is when the homogeneous taxonomic group is a group of similar species. Since Van Valens work, various statistical approaches have been used to address the relationship between taxon age and the rate of extinction. We propose a general class of test statistics that can be used to test for the effect of age on the rate of extinction. These test statistics allow for a varying background rate of extinction and attempt to remove the effects of other covariates when assessing the effect of age on extinction. No model is assumed for the covariate effects. Instead we control for covariate effects by pairing or grouping together similar species. Simulations are used to compare the power of the statistics. We apply the test statistics to data on Foram extinctions and find that age has a positive effect on the rate of extinction. A derivation of the null distribution of one of the test statistics is provided in the supplementary material.

Corks, Buoyancy, and Wave-Particle Orbits

The motion of open ocean waves and associated water-particle orbits constitute a focused topic that occupies a lasting niche in our arsenal of simple, pedagogically compelling things to offer introductory students in the geosciences. The topic provides a vehicle for introducing the general subject of wave behavior, a universally important phenomenon in the geosciences; it challenges students to understand counterintuitive ideas regarding the difference between wave and water-particle motions, serves as a base for advanced topics (for example, oscillatory ripple formation), and is readily “accessible” to students in that wave motions and particle orbits can be easily demonstrated or “tested” in either lab or field conditions. But unless students are exposed to this topic in a manner that goes well beyond introductory-text explanations, possibly in advanced courses, they are not apt to gain an understanding of ocean-wave behavior beyond that provided by purely kinematic explanations. Because of the pedagogical importance of gravity waves – of which ocean waves are an example – and because ambiguities exist in current introductory-text descriptions of waves, we assemble and summarize a dynamical explanation of their behavior. A notable bonus of this explanation is that it provides a simple, concise introduction to the ideas of buoyancy and gravitational forces – also universally important subjects in the geosciences – and how these forces interact during wave motion. Specifically, the orbit of a water particle beneath a train of ocean waves involves a clockwise motion when viewed from a perspective where the waves are moving from left to right. The vertical component of this motion is associated dynamically with pressure fluctuations about the average pressure state such that the buoyancy force exceeds the gravitational force beneath wave troughs, and the gravitational force exceeds the buoyancy force beneath wave crests. The horizontal component of the motion is associated with these pressure fluctuations wherein the pressure decreases horizontally in the direction of wave motion between wave crests and their leading troughs, and the pressure increases horizontally in the direction of wave motion between wave troughs and their leading crests. These forces alternately accelerate and decelerate the water with simultaneous vertical and horizontal components leading to approximately circular orbits. Peak water (orbit) speeds decrease with increasing wave speed. The attenuation of orbits with increasing depth is related to downward attenuation of the fluctuating pressure field, rather than being attributable to frictional damping. This dynamical explanation can be concisely presented at several levels. The first involves a qualitative description that appeals to graphical representations of the pressure and velocity fields beneath waves. The second appeals to a simple trigonometric representation of waves and the simplest possible form of Eulers equations combined with the approximation of hydrostatic conditions. The third involves solving linearized forms of Eulers equations.