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Tempo and mode of morphologic evolution near the origin of the radiolarian lineage Pterocanium prismatium
Abstract
Morphometric examination of cladogenesis and plyletic evolution in two late Neogene sister lineages of marine microfossils (Pterocanium prismatium and P. charybdeum, Radiolaria) from two equatorial Pacific sediment cores was undertaken to better understand the rate of cladogenesis and its relation to subsequent phyletic change. The origin of P. prismatium from P. charybdeum approx 4 Ma ago has been estimated to take place over an interval of approx 500 000 yr. Results show that the speciation event consists of two distinct phases. The first phase, cladogenesis, occurred relatively rapidly (on the order of 50 000 yr). A second phase of relatively rapid divergent phyletic evolution away from the commmon ancestral state followed in both descendant branches and continued for at least 50 000 yr after completion of the cladogenetic event. Net evolutionary rates over the next 2 ma appear to be much lower. Individual characters change by as much as two population standard deviations during cladogenesis, and by a total of approximately three standard deviations over 2.5 ma of phyletic evolution. Up to five population standard deviations of change during = or <50 000 yr of cladogenesis, and seven additional standard deviations of phyletic change over 500 000 yr are observed in multivariate (discriminant function) indices of morphologic difference. The measured pattern does not appear to be either strictly 'punctuated' or strictly 'gradual', but instead show features of both hypotheses.-Author
... The Family Pterocorythidae, described by Haeckel (1881) such as Stichopilium bicorne Haeckel (1887), show similarities to Natantesprifmata as their thorax and external shells are pyramidal when observed proximally due to their polyhedron nature. Similarly, Podobursa mersinensis sp.and Pterocanium prismatium (Lazarus, 1986) can appear prismatic in 2D cross-sections. However, these Radiolaria do not comprise chambers but consist of porate cephalis and commonly more than one postcephalic chamber is present (Haeckel, 1862). ...
Ovummuridae are calcareous, egg-shaped microfossils with an unknown taxonomic affinity. Their limited observation is due to their occurrence only within exceptionally preserved carbonate rocks that have undergone little to no diagenesis or aggrading neomorphism. The Much Wenlock Limestone Formation (Homerian) is famous for its exceptionally preserved and diverse fossil biota, but Ovummuridae have not been previously observed and reported from the formation. This paper introduces the population of Ovummuridae from the off-reef tract limestones of The Much Wenlock Limestone Formation, Shropshire, UK. The authors analysed 124 polished and etched thin sections using reflected light microscopy to detect the presence of Ovummuridae. A total of 6591 ovummurids were identified, including observations of several previously undescribed morphotypes of Ovummuridae. Three new genera, Munneckella, Natantesprifmata and Hartonella, and subsequently four new species, Minourella
wenlockiensis, Munneckella tribuscamera, Natantesprifmata rogersi and Hartonella oblonga, are introduced. The implications of this study suggest that reflected light microscopy is an effective and efficient method for observing calcareous microfossils. Ovummuridae may be more abundant than previously reported, their stratigraphic and
palaeogeographic range is further extended, and it is highlighted that the off-reef tract may have been the microfossils’ preferred environment.
... Pearson and Coxall (2014) observed transitions in the Hantkenina genus from a normal-spined to a tubulospined form within only 300 Kyr. In the case of the Pliocene radiolarian Pterocanium prismatium cladogenetic speciation from its ancestor P. charybdeum was reported to occur within 50 Kyr (Lazarus, 1986). ...
The mean test size of planktonic foraminifera (PF) is
known to have increased especially during the last 12 Myr, probably in terms
of an adaptive response to an intensification of the surface-water
stratification. On geologically short timescales, the test size in PF is
related to environmental conditions. In an optimal species-specific
environment, individuals exhibit a greater maximum and average test size,
while the size decreases the more unfavourable the environment becomes.
An interesting case was observed in the late Neogene and Quaternary size
evolution of Globorotalia menardii, which seems to be too extreme to be only explained by changes
in environmental conditions. In the western tropical Atlantic Ocean (WTAO)
and the Caribbean Sea, the test size more than doubles from 2.6 to 1.95
and 1.7 Ma, respectively, following an almost uninterrupted and
successive phase of test-size decrease from 4 Ma. Two hypotheses have been
suggested to explain the sudden occurrence of a giant G. menardii form: it was
triggered by either (1) a punctuated, regional evolutionary event or (2) the
immigration of specimens from the Indian Ocean via the Agulhas leakage.
Morphometric measurements of tests from sediment samples of the Ocean
Drilling Program (ODP) Leg 108 Hole 667A in the eastern tropical Atlantic
Ocean (ETAO) show that the giant type already appears 0.1 Myr earlier at this
location than in the WTAO, which indicates that the extreme size increase in
the early Pleistocene was a tropical-Atlantic-Ocean-wide event. A coinciding
change in the predominant coiling direction likely suggests that a new
morphotype occurred. If the giant size and the uniform change in the
predominant coiling direction are an indicator for this new type, the form
already occurred in the eastern tropical Pacific Ocean at the
Pliocene–Pleistocene boundary at 2.58 Ma. This finding supports the Agulhas leakage hypothesis. However, the hypothesis of a regional, punctuated
evolutionary event cannot be dismissed due to missing data from the Indian
Ocean.
This paper presents the Atlantic Meridional Overturning Circulation
(AMOC) and thermocline hypothesis in the ETAO, which possibly can be
extrapolated for explaining the test-size evolution of the whole tropical
Atlantic Ocean and the Caribbean Sea for the time interval between 2 and 8
Ma. The test-size evolution shows a similar trend with indicators for
changes in the AMOC strength. The mechanism behind this might be that
changes in the AMOC strength have a major influence on the thermal
stratification of the upper water column and hence the thermocline, which is
known to be the habitat of G. menardii.
Determination of the time sequence in the geologic record is a logical process that involves little “guesswork.” The geologic time scale is real, not a circular argument developed to show evolution. By applying Occam's Razor and assuming continuity, it is clear that biotic succession in apparent lineages demonstrates that species are capable of giving rise to others through modification with descent and have done so through geologic time. Although the fossil record is incomplete, biotic succession on a large scale provides a clear and unambiguous outline of the history of the change in life over time.
Science seeks explanations for observations and facts that begin as hypotheses. When hypotheses successfully survive rigorous testing, they mature into accepted theories. Evolution is the theory accepted as the explanation for the facts of biotic succession. The operational processes invoked in the theory of evolution (such as mutation, natural selection, isolation, and genetic drift) have been successfully tested many times. The general pattern of biotic succession follows the path expected or predicted for the course of the evolutionary development of the biosphere.
The biotic succession is the preserved result of the operation of the processes of organic evolution. Contributions from molecular biology and cladistics make it clear that the branching processes in evolution, operating over time, have produced all the diversity preserved in the fossil record. The current diversity of life is simply the product of its three and a half billion year history.
Microfossils are of prime importance in documenting patterns of evolution due to their great abundance (often tens of thousands to millions of specimens in a hand sample) and widespread distribution (in both time and space) in the fossil record. The term “microfossil” is often used for paleontological material that requires a microscope for its study, no matter what its biological affinities. For the purposes of this article we will be looking at the remains of protists (single-celled organisms). The several examples I discuss in this chapter are of three groups of planktonic (floating) protists, the calcareous nannoplankton (tiny plant-like protists whose single cell is covered in minute calcitic scales), the radiolaria (animal-like protists with siliceous shells) and the planktonic foraminifera (animal-like protists with calcitic shells). These organisms have been the subject of extensive study because the material from which they are often extracted, cores of deep-sea sediments, are usually comprised of a more complete sedimentological record ( i.e. , fewer breaks) than shallow shelf deposits. Hypotheses of evolutionary history have been constructed for many groups (lineages) of microfossils using specimens from deep-sea cores. Ancestor-descendent relationships have been recognized by tracking shape and form (morphologic) changes through time. This approach to reconstruction of evolutionary history provides an empirical record of morphologic evolution; that is, a record based on observations.
Microfossils are of prime importance in documenting patterns of evolution due to their great abundance (often tens of thousands to millions of specimens in a hand sample) and widespread distribution (in both time and space) in the fossil record. The term “microfossil” is often used for paleontological material that requires a microscope for its study, no matter what its biological affinities. For the purposes of this article we will be looking at the remains of protists (single-celled organisms). The several examples I discuss in this chapter are of three groups of planktonic (floating) protists: the calcareous nannoplankton (tiny plant-like protists whose single cell is covered in minute calcitic scales), the radiolaria (animal-like protists with siliceous shells), and the planktonic foraminifera (animal-like protists with calcitic shells). These organisms have been the subject of extensive study because the material from which they are often extracted, cores of deep-sea sediments, are usually comprised of a more complete sedimentological record (i.e., have fewer breaks) than shallow shelf deposits. Hypotheses of evolutionary history have been constructed for many groups (lineages) of microfossils using specimens from deep-sea cores. Ancestor-descendent relationships have been recognized by tracking shape and form (morphologic) changes through time. This approach to reconstruction of evolutionary history provides an empirical record of morphologic evolution; that is, a record based on observations.
Radiolarians are marine zooplankton possessing a tough, central capsular membrane that divides the cytoplasm into intracapsular (containing the nucleus, organelles, and food reserves) and extracapsular (with food-gathering rhizopodia and digestive vacuoles) portions (Figure 1). They bear two kinds of pseudopodia, the axopodia and filopodia. The axopodia extend radially through the ectoplasm and capsular membrane to the interior of the endoplasm. The axopodia are inserted into a special structure, the axoplast (Figure 1). The development of the axoplast and its complex is of fundamental importance in radiolarian taxonomy. For a detailed description of radiolarian cytology, biology, and reproduction, see Anderson (1983).
The morphological evolution was investigated in the tropical Neogene planktonic foraminiferal lineage Globorotalia menardii, G. limbata and G. multicamerata during the past 8 million years at ODP Hole 806C (Ontong-Java Plateau, western equatorial Pacific). This research is an extension of several similar studies since 2007 from the Caribbean Sea, the tropical Atlantic and the Eastern Equatorial Pacific. The goal is to find empirical and quantitative confirmation for morphological speciation – splitting or phyletic gradualism or accelerated evolution – in planktonic foraminifera with the above lineage as model objects. The present study from ODP Hole 806C serves as a test to discriminate between these evolutionary scenarios.
In the western equatorial Pacific warm and stable environments prevailed back to Pliocene times, and potential influences of Northern Hemisphere Glaciation are thought to bear less severely on shell size evolution than in the Atlantic Ocean. A slow and gradual pattern of shell size increase is therefore expected in the western tropical Pacific, in contrast to the intermittent rapid menardiform shell size increase during periods of intensified formation of warm water eddies in the southern to tropical Atlantic.
For this study a total of 11,101 specimens from 37 stratigraphic levels extending over the past 8 million years were morphometrically investigated thanks to the AMOR robot for imaging and microfossil orientation. Of those, 6080 specimens comprise the G. menardii–limbata-multicamerata plexus. Special attention was given to trends of spiral height (δX) versus axial length (δY) in keel view, for which bivariate contour- and volume-density diagrams were constructed to visualize speciation and evolutionary trends.
The investigation at Hole 806C showed, that G. menardii evolved in a more gradual manner than in the Atlantic. Contour plots of δX versus δY reveal modest bimodality between 3.18 Ma – 2.55 Ma with a dominant branch consisting of smaller G. menardii (δX<∼300 μm) persisting until the Late Quaternary, and a less dominant branch of larger G. menardii (δX>∼300 μm) until 2.63 Ma. There is evidence for cladogenesis – splitting with subsequent morphological divergence in the Late Pliocene G. menardii-limbata-multicamerata lineage, and which may be linked to changes in the thermocline. Due to the general scarcity of G. multicamerata at Site 806, the divergence was less clearly expressed than originally expected. Up-section, bimodality vanished but G. menardii populations shifted towards extra large shells between 2.19-1.95 Ma.
The morphological evolution of Pacific menardiform globorotalids contrasts the one observed in the Atlantic. This inter-oceanic asymmetry may indicate possible long-distance dispersal of G. menardii, at least during intermittent phases. For plankton biostratigraphy this implies, that correlating events represent more often than previously thought large scale environmental perturbations with local morphological ecophenotypic adaptations and nuances.
This paper presents, for the first time ever, a complete list of Cenozoic Polycystinea reported between 1834 and 2020. It records 6898 names of taxa originally described as new species or subspecies, assigned to the Class of Polycystinea, the most important group in the infraphylum Radiolaria. This list only attempts to provide an objective record of available and unavailable names, the latter including nomina oblita, nomina nuda, homonyms, invalid nomenclatorial acts, species wrongly described as Polycystinea and nomina dubia species with inexistent name-bearing specimens.
The mean test size of planktonic foraminifera (PF) is known to have increased especially during the last 12 Ma, probably in terms of an adaptive response to an intensification of the surface-water stratification. On geologically short timescales, the test size in PF is related to environmental conditions. In an optimal species-specific environment, individuals exhibit a greater maximum and average test size, while the size decreases the more unfavourable the environment becomes. An interesting case was observed in the late Neogene and Quaternary size evolution of Globorotalia menardii, which seems to be too extreme to be only explained by changes in environmental conditions. In the western tropical Atlantic Ocean (WTAO) and the Caribbean Sea, the test size more than doubles from 2.6 Ma to 1.95 Ma and 1.7 Ma, respectively, following an almost uninterrupted and successive phase of test size decrease from 4 Ma. Two hypotheses have been suggested to explain the sudden occurrence of a giant G. menardii form: it was triggered by either (1) a punctuated, regional evolutionary event or (2) the immigration of specimens from the Indian Ocean via the Agulhas Leakage. Morphometric measurements of tests from sediment samples of the Ocean Drilling Program (ODP) Leg 108 Hole 667A in the eastern tropical Atlantic Ocean (ETAO), show that the giant type already appears 0.1 Ma earlier at this location than in the WTAO, which indicates that the extreme size increase in the early Pleistocene was a tropical-Atlantic-Ocean-wide event. A coinciding change in the predominant coiling direction suggests that probably a new morphotype occurred. If the giant size and the uniform change in the predominant coiling direction are an indicator for this new type, the form already occurred in the eastern tropical Pacific Ocean at the Pliocene/Pleistocene boundary at 2.58 Ma. This finding supports the Agulhas Leakage hypothesis. However, the hypothesis of a regional, punctuated evolutionary event cannot be dismissed due to missing data from the Indian Ocean. This paper presents the AMOC/thermocline hypothesis, which not only suggests an alternative explanation for the sudden test-size increase in the early Pleistocene, but also for the test size evolution within the whole tropical Atlantic Ocean and the Caribbean Sea for the last 8 Ma. The test-size evolution shows a similar trend with indicators for changes in the Atlantic Meridional Overturning Circulation (AMOC) strength. The mechanism behind that might be that changes in the AMOC strength have a major influence on the thermal stratification of the upper water column, which is known to be the habitat of G. menardii.
The abrupt, widespread, and simultaneous extinction of radiolarian species makes them ideal stratigraphic markers. Five species became extinct in the North Pacific during the last 3 m.y. Four of these are used to define four stratigraphic zones. The boundary between the oldest two zones correlates with the Pliocene/Pleistocene boundary, as defined in southern Italy. These zones can be related through paleomagnetic stratigraphy to previously established radiolarian and foraminiferal zonations.
One species (Eucyrtidium matuyamai) evolved and became extinct during the last 2.5 m.y. Its evolution can be related to the invasion of and adaptation to a new habitat. The extinction of E. matuyamai shows a striking correlation with the magnetic reversal at the base of the Jaramillo event. The rapid evolution of this species probably reduced the genetic variability of the population, making it more vulnerable to extinction than other less rapidly evolving species. The environmental change that caused the extinction is unclear; however, there is suggestive evidence that it is in some way related to a reversal of the earth’s field. If reversals cause abrupt environmental changes, they may have played an important selective role through geologic time.
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.
The warm-water planktonic foraminiferal Globorotalia tumida lineage has been studied in a 10-Myr-long stratigraphic sequence (Late Miocene through Recent) from the Indian Ocean to determine long-term evolutionary patterns through the lineage's history, and particularly to study in great detail the evolutionary transition from G. plesiotumida to G. tumida across the Miocene/Pliocene boundary. Sampling resolution was very good, between 5 x 103 and IS x 103 yr across the Miocene/Pliocene boundary and about 2 x 105yr otherwise. The test shape was analyzed in edge view, permitting determinations of variation in inflation and elongation of the test. Shape was analyzed quantitatively using eigenshape analysis. This method represents the greatest proportion of variation observed among a collection of shapes by the least number of different shapes. The Late Miocene (10.4-5.6 Myr B.P.) populations exhibited only minor fluctuations in shape that did not result in any net phyletic change. This period of stasis was followed by an 0.6-Myr-long period (between 5.6 and 5.0 Myr B.P.) of gradual transformation of the Late Miocene morphotype {G. plesiotumida) into the Early Pliocene morphotype (G. tumida). The populations were again more or less in stasis in the Pliocene and Pleistocene (5.0 Myr to the present day), so that no major modifications of the newly evolved Early Pliocene morphotype occurred during these 5 Myr. Thus it would appear that the G. tumida lineage, while remaining in relative stasis over a considerable part of its total duration underwent periodic, relatively rapid, morphologic change that did not lead to lineage branching. This pattern does not conform to the gradualistic model of evolution, because that would assume gradual changes throughout the history of the lineage. It also does not conform to the punctuational model, because (1) there was no speciation (lineage branching) in this lineage and (2) the transition was not rapid enough (<1% of the descendant species' duration according to definition). For this evolutionary modality we propose the term “punctuated gradualism” and suggest that this may be a common norm for evolution—at least within the planktonic foraminifera.
In this paper, we present detailed quantitative studies of evolutionary changes over all or part of the stratigraphic ranges of five fossil radiolarian species from Pacific deep-sea sediment cores. Each of these species shows some variation of a distinctive evolutionary pattern: increase in size of measured morphologic characters, preceded and/or followed by an interval during which little or no significant change occurred.
One of the species studied (
Eucyrtidium matuyamai
) was allopatrically differentiated from another (
Eucyrtidium calvertense
). The others (
Calocycletta caepa, Pterocanium prismatium
and
Pseudocubus vema
) underwent phyletic change within a single lineage. Those species undergoing phyletic change towards larger size maintained almost constant variability of shell size over long periods of time, including periods of both rapid and extremely slow evolution. This constancy of variability suggests that diminution of selection against larger size may have acted as a stimulus to size increase. In contrast,
E. calvertense
decreased in variability during evolution towards smaller shell size. We believe this decrease may be interpreted as the result of two factors: (1) strong selection against larger size apparently exerted on this species by its direct descendant,
E. matuyamai
, during the neosympatric phase of speciation and (2) continuation of previous selection against very much smaller size.
One method of increasing the resolving power of paleontological inquiries is to assess the time distribution, or phenology, of perispeciational change. The rationale for this test is that, while concentration of morphologic change around the time of speciation is consistent with both known microevolutionary mechanisms and punctuated equilibria, punctuated equilibrium theory presupposes that most of the change occurs either before or during initial allopatry to produce morphologically static taxa. Considerable change in the initial phases of speciation could also result from known microevolutionary mechanisms, but if significant change occurs in the neosympatric phase, or at any time after the species have become differentiated, then that change is best explained by known microevolutionary mechanisms.
Amount and rate of morphologic change during each phase of the allopatric speciation process were determined for two cognate species of the radiolarian genus Eucyrtidium. In this case, morphologic change accruing to these species during the neosympatric phase was 2-3 times as great as that which occurred during the initial allopatric phase. Morphologic trends in both these species during the allopatric phase differed significantly from a random walk. The size differential that characterized this speciation event was not the product of a single large step early in the process but of a disproportionate number of small steps in one direction during neosympatry. Therefore, the hypothesis of no statistically significant change following the “first stage” of speciation, which is a major tenet of punctuated equilibrium theory, is falsified for this case.
A preliminary report is presented documenting the evolution of the Globorotalia crassaformis (Galloway and Wissler) lineage from the Early Pliocene G. cibaoensis (Bermudez) in Hydraulic Piston Core 72/516 on the Rio Grande Rise (western South Atlantic). A new technique, eigenshape analysis, is used to describe the gross changes in morphology, and the use of “Raupian” analysis of expansion rates, angular displacement, position relative to coiling axis, and translation rates of chambers is suggested as a useful vehicle for inferring the geometric causes of observed evolutionary changes. These changes include size increase, decrease in the number of chambers per whorl, inflation, reduction of relative height of aperture, and increase in the peripheral angle. The sequence is judged to be an example of phyletic gradualism.
We believe that punctuational change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous, even if tolerably continuous in ecological time). Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction. Phyletic gradualism is very rare and too slow, in any case, to produce the major events of evolution. Evolutionary trends are not the product of slow, directional transformation within lineages; they represent the differential success of certain species within a clade—speciation may be random with respect to the direction of a trend (Wright's rule).
As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. It could not be refuted by empirical catalogues constructed in its light because it excluded contrary information as the artificial result of an imperfect fossil record. With the model of punctuated equilibria, an unbiased distribution of evolutionary tempos can be established by treating stasis as data and by recording the pattern of change for all species in an assemblage. This distribution of tempos can lead to strong inferences about modes. If, as we predict, the punctuational tempo is prevalent, then speciation—not phyletic evolution—must be the dominant mode of evolution.
We argue that virtually none of the examples brought forward to refute our model can stand as support for phyletic gradualism; many are so weak and ambiguous that they only reflect the persistent bias for gradualism still deeply embedded in paleontological thought. Of the few stronger cases, we concentrate on Gingerich's data for
Hyopsodus
and argue that it provides an excellent example of species selection under our model. We then review the data of several studies that have supported our model since we published it five years ago. The record of human evolution seems to provide a particularly good example: no gradualism has been detected within any hominid taxon, and many are long-ranging; the trend to larger brains arises from differential success of essentially static taxa. The data of molecular genetics support our assumption that large genetic changes often accompany the process of speciation.
Phyletic gradualism was an a priori assertion from the start—it was never “seen” in the rocks; it expressed the cultural and political biases of 19th century liberalism. Huxley advised Darwin to eschew it as an “unnecessary difficulty.” We think that it has now become an empirical fallacy. A punctuational view of change may have wide validity at all levels of evolutionary processes. At the very least, it deserves consideration as an alternate way of interpreting the history of life.
A technique has been devised by which fine, hydrodynamically heterogenous grains may be mounted on a microscope slide in such a way that their distribution on the slide is random. The grains are settled from a well-mixed suspension onto the microscope slide where they are held in place by a thin film of gelatin. This technique allows the investigator to use any part of the slide as a representative subsample, and obviates the need for repeated splitting of samples and the counting of a very large number of grains.
The radiolarian subfamily Artiscinae in deep-sea sediments of the equatorial Pacific consists of both an ancestral lineage that has persisted from the Early Miocene to the Recent and a descendant lineage that originated, probably by allopatric speciation, in the Middle Miocene and became extinct at the end of the Miocene. Measurements of shell dimensions were made on samples of both lineages throughout their stratigraphic ranges. For almost all of the measured dimensions, morphologic rates of change increased in the ancestral lineage following the establishment of sympatry with the descendant lineage. Rates of change were consistently higher in the descendant lineage than in the ancestral lineage during sympatry. Differential rates of phyletic change exhibited by the 2 lineages resulted in significant divergence between them, which may be interpreted as possible character displacement at the lowest level of macroevolution.