Anthony J. Arnold

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.

Hydrodynamic strategies in the morphological evolution of spinose planktonic foraminifera

To counter gravitational settling, planktonic foraminifera adjust their buoyancies, in part by manufacturing low-density lipids or gases. The biochemical energy that a foraminifer expends in this way is a function of the speed at which it would otherwise settle if it did not expend this energy. In turn, the settling speed varies with foraminifer shape. We consider here foraminifera that have acicular spines, for example Orbulina universa and Globigerinoides sacculifer . Growing spines produces two counteractive effects: spines increase the weight of a foraminifer, and therefore tend to increase its settling speed; they also increase the fluid drag on the foraminifer, and therefore tend to decrease its settling speed. If growing spines is part of an evolutionary strategy to impede settling, then it is reasonable to expect that the advantage of increasing drag by growing spines outweighs the disadvantage of increasing weight. The complexity of foraminiferal shapes precludes directly solving the equations of fluid motion for drag and settling speed. We therefore appeal to the efficacy of dimensional analysis to define a coefficient of drag C D and a Reynolds number Re for spinose foraminifera. Experiments that involve settling scaled models of foraminifera (constructed from beeswax and pins) in viscous liquids are then used to confirm the forms of generalized dimensional formulae relating the settling speed W to test radius R , spine number n , spine length l , and spine radius r . Geometrically similar foraminifera whose spine arrangements possess quasispherical symmetry settle according to an inverse relation between C D and Re , homologous to Stokes9s law for spheres. Fluid drag systematically increases with both n and l . For given R , l , and r , a minimum settling speed occurs at an intermediate spine number n 0 . Similarly, for given R , n , and r , a maximum settling speed W 0 occurs at an intermediate spine length l 0 . Results suggest that insofar as there is disadvantage in settling rapidly, there is advantage in remaining small; or, if growth of tests occurs, there is advantage in manufacturing many long thin spines. Investments of mass and energy associated with this strategy must be weighed against those involved in achieving neutral (or positive) buoyancy by other mechanisms, and limitations on lengths of spines imposed by their finite strength. A comparison of the theory with modern foraminifera suggests that the geometries of adult Orbulina universa and Globigerinoides sacculifer , in the absence of external protoplasm, are well suited to impede settling. With external protoplasm, however, l is effectively decreased and the drag associated with spines is not sufficient to provide viscous settling unless the protoplasm possesses positive buoyancy. For an individual at or near a state of neutral buoyancy, drag associated with spines decreases the sensitivity with which its settling speed responds to unavoidable changes in the buoyancy of its protoplasm related to metabolic activity, and to changes in the density and viscosity of sea water related to external factors. The effect is to hydrodynamically dampen vertical motions that would otherwise occur if the individual did not possess spines. In contrast, the small drag associated with few short spines is advantageous to juveniles that must ascend from deep to shallow waters during their ontogenies. A partitioning of finite spine mass into many moderate to short spines is less effective in producing drag than one involving fewer long spines. Long spines, however, are more susceptible to mechanical breakage due to the torque that viscous forces apply to them. Foraminifera with approximately 10 (or fewer) spines that possess mechanical properties equivalent to those of spines of adult Orbulina universa and Globigerinoides sacculifer can withstand motions at speeds of only a few tenths of a centimeter per second (or more, depending on spine strength) without breakage. With increasing numbers of spines, the resulting hydrodynamic interaction among them has the effect of significantly reducing the chance of spine breakage related to momentarily rapid motions; this is attributable to a decrease in the proportion of the spine length l exposed to significant viscous forces, whereby the torque on individual spines is decreased. External protoplasm also reduces the chance of breakage by decreasing the length of spines exposed to surrounding fluid.

The species censorship problem: A general solution

The longevities of species constituting a statistical population have an underlying distribution whose form and parametric values reflect probabilities of origination and extinction through time. In the case that a part of the population is extant, the form of distribution and its parameters cannot be estimated directly from the longevity data without bias. Longevities of extant “censored” species are truncated and thus do not statistically represent the underlying distribution. The remaining “uncensored” species do not represent the true relative abundances of longevities. These biases can be defined from the probability densities for species longevitys and intervalr between successive originations of species. For realistic densities ofs andr, species with an intermediate longevity are preferentially censored. This simple, general result arises because the probability of censoring a species increases with its longevity, whereas the probability of censoring a given longevity varies with its relative abundance.

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.

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.

JOSEPH A. CUSHMAN AWARD

Dr. Barun K. Sen Gupta received the Joseph A. Cushman Award for Excellence in Foraminiferal Research on October 18, 2005, during the Cushman Foundation reception at the annual meeting of the Geological Society of America in Salt Lake City, Utah. The award is in recognition of his many contributions to our understanding of modern and Cenozoic benthic foraminifera. Barun has enjoyed a long and distinguished career as a researcher and educator that extends over more than four decades. As Barun takes his rightful place in a pantheon of Cushman Award recipients that begins with Fred B Phleger and Ruth Todd (1980) and extends to Martin A. Buzas and Richard K. Olsson (2004), it is appropriate to consider the role his work has played in the development of our discipline. The field of foraminiferology has now left behind its earliest phase dominated by taxonomic description, and gone on to see the establishment of stratigraphic and distributional frameworks and their integration into ecological, biological, evolutionary and geochemical studies, with the result that Foraminifera now play a vital role in our understanding of oceanographic and climatic change—fields that are critical to mankind’s stewardship of the Earth. Barun has played a leading part in the research thrust that has so positioned our discipline, and it is appropriate that we honor him for this contribution. Barun was born in Jamshedpur, India and educated at the Calcutta University, Presidency College, receiving B.Sc. with honors in 1951 and an M.Sc. First Class in 1954. In 1960, he went to …