Final Whorl Research Articles

Phyletic gradualism and punctuated equilibrium in the late Neogene planktonic foraminiferal clade Globoconella

Substantial geographic coverage in paleontological study is essential in testing evolutionary models of phyletic gradualism and punctuated equilibrium. We present a multivariate morphometric study of the late Neogene planktonic foraminiferal clade Globoconella using specimens from four Deep Sea Drilling Project sites (DSDP 284, 207A, 208, and 588) along a latitudinal traverse in the southwest Pacific.During the Late Miocene (7 Ma to 5 Ma), populations of the ancestral species Globorotalia (Globoconella) conomiozea formed a geographic cline showing continuous morphological variation from the temperate sites (DSDP 284 and 207A) to the warm subtropical sites (DSDP 208 and 588). Populations living to the south had higher conical angle and fewer chambers in the final whorl compared to the northern populations. Nevertheless, populations across the entire cline exhibited a coherent, directional trend towards having larger conical angle and fewer chambers through time. At the Miocene/Pliocene boundary, the intensification of the Tasman Front (Subtropical Divergence) possibly isolated the peripheral populations in the warm subtropics from the central stocks of the temperate water masses. The evolutionary trends became decoupled: the central populations gradually lost their keel and transformed into G. (G.) sphericomiozea, while the peripheral populations in the warm subtropical areas retained their keel and evolved into a flattened species, G. (G.) pliozea.The gradual transformation of G. (G.) conomiozea terminalis (a form retaining a keel) into G. (G.) sphericomiozea (a form lacking a keel) occurred during an interval of about 0.2 m.y., with all measured morphologic variables showing continuous and steady changes. The evolution of the central populations follows the model of phyletic gradualism. In peripheral populations, the origin of the descendant species G. (G.) pliozea from the ancestor G. (G.) conomiozea terminalis occurred very rapidly within an interval of less than 0.01 m.y. The population size of G. (G.) pliozea was small at the incipient stage at about 5.05 Ma, but increased rapidly to become dominant during the next 0.2 m.y. when the ancestral species G. (G.) conomiozea terminalis became locally extinct. Following speciation, G. (G.) pliozea exhibited morphological stasis for about 0.6 m.y., until the central stock form G. (G.) puncticulata migrated back to the warm subtropics; during the next 0.5 m.y. of their sympatry, there is no sign of hybridization between these two sister species. The evolution of G. (G.) pliozea follows the model of punctuated equilibrium.The evolution of the Globoconella clade shows both phyletic gradualism and punctuated equilibrium. These two “alternative” evolutionary models complement each other rather than being mutually exclusive. Both models are indispensable towards providing a complete picture of the evolution of Globoconella.

Evolutionary changes in some Late Neogene planktonic foraminiferal lineages and their relationships to paleoceanographic changes

The timing of evolutionary changes in some late Neogene lineages of planktonic foraminifers and major paleoceanographic events are evaluated to assess possible correspondences between paleoclimatic change (in a broad sense) and evolutionary innovations. Previously published data on evolutionary changes in the Globorotalia inflata lineage, the Globorotalia tumida lineage, and the Globorotalia truncatulinoides lineage, and new data on the Sphaeroidinella dehiscens lineage, are used in the study. Most evolutionary rate changes in these lineages are shown to occur near times of major paleoceanographic events. For example, rapid increase in conicality and weakening of the keel in the G. inflata lineage in the latest Miocene coincided with the 6.2‐Ma event (global cooling, sea level regression, and shift in carbon isotope ratios in foraminiferal tests). Disappearance of keeled morphotypes (Globorotalia conomiozea) in the same lineage, and appearance of nonkeeled forms with rounded periphery (Globorotalia puncticulata) coincided with the return of warmer, more stable climatic conditions, and transgressions at the Miocene/Pliocene boundary (5.3 Ma). Phyletic transformation from Globorotalia plesiotumida into G. tumida also took place across this boundary, and supplementary apertures first developed in the S. dehiscens lineage. Supplementary apertures in the S. dehiscens lineage were rare through the early Pliocene but became rapidly dominant in the populations in the interval between 3.4 and 3.3 Ma, where major paleoceanographic changes occurred (the 3.4‐to 3.2‐Ma event marked by cooling of surface waters and increased production of bottom waters at high latitudes, resulting in strengthening of ocean vertical and horizontal gradients). Forms without a supplementary aperture became extinct only 0.3–0.4 m.y. later (3.0 Ma), and at about the same time, G. inflata appeared in the fossil record. Although evolutionary change exhibits striking temporal correlation with climatic change, some evolutionary time series do not show any obvious relationship to paleoclimate: chamber number in the final whorl in the G. inflata lineage and conicality in G. truncatulinoides.

The taxonomy and stratigraphic record of Globorotalia mayeri Cushman and Ellisor in New Zealand

Scanning electron microscope photographs of Globorotalia mayeri Cushman and Ellisor and G. peripheroronda Blow and Banner from both Trinidad and New Zealand illustrate differences in wall structure that suggest the two species are unlikely to be related. The wall of G. peripheroronda shows an uneven pore size throughout the test, with a major size difference between the ultimate and previous chambers, a smooth, flat area between the pores, and a pustulose umbilical area. In contrast, the pores of G. mayeri are more even in size and density throughout the test, with raised or ridged areas between the pores, and an umbilical area that lacks pustules. Stratigraphic variation in Globorotalia mayeri of changes in chamber separation, number of chambers in the final whorl, size of chambers in the final whorl, height of aperture, size of apertural lip and peripheral inflation are considered to be insufficient to warrant seperate specific designation. However, these characters may reflect environmental parameters as yet poorly understood and the retention of names, either as subspecies or formae, for specimens with such characters may have merit. Thus the synonymy of G. pseudocontinuosa Jenkins, G. semivera (Hornibrook), G. continuosa Blow and G. mayeri nympha Jenkins with G. mayeri extends the range of New Zealand G. mayeri back to the Oligocene, as occurs in tropical-equatorial areas. The ancestor of Globorotali (Globorotalia) mayeri is probably G. nana Bolli, and the ancestor of Globorotalia (Fohsella) peripheroronda is possibly G. kugleri Bolli.

The potential of morphometrically based phylo-zonation: Application of a Late Cenozoic planktonic foraminiferal lineage

Phylo-zonations (or lineage-zonations) are based upon morphological changes within individual evolutionary lineages. These zonations, although potentially of use for stratigraphic subdivision and correlation, often suffer from a lack of quantitative exactness in the definitions of chronospecies. Thus exact reproducibility is hindered for stratigraphic determinations. The potential of morphometrically defined phylo-zonations is demonstrated on a temperate South Pacific Late Cenozoic lineage of planktonic foraminifera ( Globorotalia conoidea through intermediate forms to Globorotalia inflata in DSDP Site 284) exhibiting phyletic gradualism. Our sampling interval is about 0.1 m.y. during the last 8 m.y. Changes in the number of chambers in the final whorl, test conicalness, percentage of keeled forms, and test roundness or inflatedness, are used to quantitatively define the following five chronospecies: G. conoidea (Late Miocene; 6.1–>8.3 m.y.), G. conomiozea (latest Miocene; 5.3–6.1 m.y.), G. puncticulata sphericomiozea (earliest Pliocene; 4.5–5.3 m.y.), G. puncticulata puncticulata (Early-Middle Pliocene; 2.9–4.5 m.y.), and G. inflata (Late Pliocene-Quaternary; 0–2.9 m.y.). This phylo-zonation is directly applicable to temperate cool subtropical Southern Hemisphere areas where the evolution took place (Kennett, 1967, 1973; Scott, 1979). It is still not known if the lineage occurs elsewhere; thus the applicability of the phylo-zonation over broader areas is still uncertain. Trends in general size and aperture shape seem to be climatically controlled, and thus may be only of local stratigraphic utility. The practical applications of morphometric phylo-zonation for stratigraphy is to a large extent dependent upon the amount of time and effort required to statistically define the trends. Experiments with large numbers of subsamples from this lineage demonstrate that accurate stratigraphic determinations are possible from measurements on only 15 specimens per sample, except for those very close to chronospecies boundaries.

Phyletic gradualism in a Late Cenozoic planktonic foraminiferal lineage; DSDP Site 284, southwest Pacific

Shape measurements have been made on planktonic foraminifera from a South Pacific Late Miocene to Recent temperate evolutionary lineage (Globorotalia conoidea through intermediate forms to G. inflata in DSDP Site 284). The sampling interval is about 0.1 Myr over nearly 8 Myr. Gradual evolution (phyletic gradualism) clearly occurs in all but one measured parameter. No clear evidence exists for abrupt evolutionary steps (punctuated equilibria) within the bioseries. If they occur, they are the exception rather than the rule. The number of chambers in the final whorl decreases almost linearly, despite known paleoceanographic oscillations within the temperate water mass. Mean size and apertural shape variations seem to correlate with paleoceanographic change. It is speculated that certain major morphological changes that took place within this evolutionary bioseries (i.e. loss of keel, rounding of periphery) developed in response to a major latest Miocene cooling, associated with instability in the water column and resulting adjustments of the test structure to water density changes. Changes exhibited in shape measurements may offer a precise method of stratigraphic correlation between temperate South Pacific Late Cenozoic sequences. Four species and two subspecies, long recognized to form the basis of this lineage, are redefined biometrically.

BIOMETRIC DIFFERENTIATION BETWEEN RECENT GLOBIGERINA BULLOIDES AND GLOBIGERINA FALCONENSIS IN THE SOUTHERN INDIAN OCEAN

Morphometric analysis has been carried out on the morphologically similar Recent planktic foraminiferal species Globigerina bulloides d’Orbigny and Globigerina falconensis Blow in southern Indian Ocean surface sediments from subtropical to northern subantarctic waters (30°S to 46°S). Previous non-biometric studies have suggested that these two forms grade into each other at such latitudes and that they may represent subspecies or phenotypic variants of a single species. Initial separation of the two forms is most readily based on the presence or absence of a distinct apertural lip in G. falconensis and G. bulloides respectively. A small percentage of specimens which have intermediate apertural characteristics and which do not allow clear taxonomic differentiation are allocated to either species by multivariate morphometric analysis. The studies have shown that the two forms exhibit distinct differences in patterns of variation in relative abundance, kummerform relative abundance, coiling direction and test size and shape. Essential morphologic differences are related to thickness (depth) of test and final chamber, and apertural dimensions with G. falconensis being more compressed and having smaller apertures than G. bulloides. Kummerform chambers in G. falconensis are more elongated than in G. bulloides, and this difference is amplified with cooler temperatures. Optimum environmental conditions for G. bulloides (relative abundances between 25 and 43%) coincide with the subantarctic water mass (yearly average surface-water temperatures between about 6 and 13°C) and for G. falconensis (relative abundances between 8 and 27%) with subtropical waters (temperatures above about 13°C). Kummerform relative abundances in G. falconensis samples (44 to 78%) are consistently higher than in G. bulloides samples (23 to 48%) but are not related to species relative abundances or surface­water temperature as in G. bulloides. Coiling in G. bulloides samples is normally dominantly sinistral and shows trends related to surface-water temperature. Globigerina falconensis is normally randomly coiled and coiling ratios lack relations with temperature. Globigerina bulloides is larger in size than G. falconensis at average annual surface-water temperatures less than about 18°C while G. falconensis is larger at warmer temperatures. Variations in test size of G. bulloides are related to changes in surface-water temperature whereas test size in G. falconensis varies randomly with temperature. Test size in normalform G. falconensis is positively correlated with relative abundances of G. falconensis. Likewise, size in both normalform and kummerform G. bulloides is positively correlated with its relative abundances. This is strong support for the hypothesis that optimum growth occurs in areas of optimum adaptation. Globigerina bulloides and G. falconensis do not show latitudinally correlated patterns with respect to: gross shape of test, relative size and shape of aperture and final chamber, rate of chamber expansion, or number of chambers in final whorl Size of aperture in kummerform G. falconensis increases with decreasing winter surface-water temperature. Relative apertural size in the same form increases with increasing latitude. Kummerform chambers in G. falconensis become gradually more elongated with decreasing temperature. Oxygen isotopic analysis suggests that normalform G. falconensis dwell at a shallower average depth in the water column than normalform G. bulloides. The biometric analyses thus show that G. bulloides and G. falconensis tend to react as different species in all essential foraminiferal parameters and therefore the taxonomic distinction between these forms should be retained.

Origin and Development of Turborotalia pachyderma (Ehrenberg): ABSTRACT

End_Page 1690------------------------------Studies of the morphology and ultrastructure of Turborotalia pachyderma (Ehrenberg) support the concept that this species arose from T. continuosa (Blow) in the upper part of Neogene Zone 12; both species occur together up to the upper Miocene zones N16 or N17, which is the upper limit of T. continuosa. Early chambers in T. pachyderma have the highly arched, laterally directed aperture of T. continuosa. This is a lineage quite distinct from that leading from T. globorotaloidea (Colom) to the Neogloboquadrina dutertrei (d'Orbigny) group in the upper Miocene (N17-N18). The pachyderma group invariably has a well-developed thickened imperforate lip and the umbilicus is small or closed; the dutertrei group has an open umbilicus, and the apertural edge has only a smooth band (temperate form such as the subspecies subcretacea) or toothlike flanges (tropical forms such as the typical dutertrei). Preferential coiling ratios occur almost at the base of the range for T. pachyderma, derived from its ancestor T. continuosa. Sinistral populations adjusted to cold polar waters whereas dextral populations adapted to temperate waters, providing one of the better planktonic indices of paleoceanography for the late Neogene. Upper Miocene and Pliocene forms have mostly 4 to 4½ chambers in the final whorl; the aperture varies from being almost a closed slit to more of an open arch. In the glacial Pleistocene, additional variants developed sometimes with 5 chambers in the final whorl; other variants show the progressive restriction of the aperture, especially the development of a highly thickened wall in polar waters. In wall thickening, the early stage is that with little thickening (incompta of Cifelli), a second stage is that in which some thickening is added (pseudopachyderma of Cita, Silva, and Rossi), and finally a third stage is the typical thick-walled form. Wall thickening progressively reduces the depressions between the chambers until it is very difficult to recognize the individual chambers in extreme cases. This transition occurs as the populations settle progressively deeper in the water column. End_of_Article - Last_Page 1691------------

Geographical Variation of Nonionellina flemingi

Nonionellina flemingi (Vella) reaches reproductive maturity at earlier stages in lower latitudes. Adults from the continental shelf off northern New Zealand are small and trochospiral, while those from off the subantarctic islands are commonly twice the size and planispiral. The mean length and number of chambers in the final whorl of a local population is an inverse function of the temperature at which the animals are living. The relationship is used in a palaeotemperature study of the New Zealand Upper Miocene to Recent.

Stratigraphic Utility of Sphaeroidinella Cushman in Louisiana: ABSTRACT

The evolutionary line which leads to the living Sphaeroidinella dehiscens (Parker and Jones) has been divided into two genera, at least six species, and several subspecies. The sequence of forms is discussed here as a temporal cline. Irrespective of nomenclature, the evolutionary forms of Sphaeroidinella s. l. have proved stratigraphically useful in the upper Miocene and younger sediments of Louisiana. Development progressed from a trochospiral form with discrete, inflated chambers and a single aperture, to the living form in which the coiling pattern is obscure, the chambers are merged, and multiple apertures are present. Single aperture forms (Sphaeroidinellopis of Banner and Blow) disappeared at the end of the Pliocene. Late Miocene and earliest Pliocene specimens have a thickened apertural lip at maturity. Pleistocene and Recent specimens resemble irregularly ruptured spheres. The lips of the several apertures tend to be recurved and are never thickened. The coiling spiral becomes lower with time. Whereas ther is a trend in the direction of fewer chambers in the final whorl of stratigraphically younger forms, this feature must be used with great caution as juveniles always have fewer chambers than adults. The safest single index to stratigraphic age is the degree of chamber separation. All stages of gradation between species can be observed. End_Page 2165------------------------------ If one uses the Globorotalia fohsi forms to mark time datums, then the stratigraphic ranges of Sphaeroidinella forms in Louisiana are not contemporaneous with their ranges in the Caribbean region. The evolution of Sphaeroidinella is apparently similar in both areas. There is some evidence that Sphaeroidinella dehiscens is a deep-water variant of Globigerinoides sacculifer. Benthonic associates of fossil specimens suggest that Sphaeroidinella did not require great water depths in the past. End_of_Article - Last_Page 2166------------