the potential of morphometrically based phylo-zonation: application of a late cenozoic planktonic...

Marine Micropaleontology, 7 (1982) 285--296 285 Elsevier Scientific Publishing Company, Amsterdam --Printed in The Netherlands

THE P O T E N T I A L OF M O R P H O M E T R I C A L L Y BASED P H Y L O - Z O N A T I O N : A P P L I C A T I O N OF A L A T E C E N O Z O I C P L A N K T O N I C F O R A M I N I F E R A L L I N E A G E

BJORN A. MALMGREN and JAMES P. KENNETT

Department of Geology, Stockholm University, Box 6801, S-113 86 Stockholm (Sweden) Graduate School of Oceanography, University of Rhode Island, Kingston, R.I. 02881 (U.S.A.)

(Revised version accepted March 24, 1982)

Abst rac t

Malmgren, B.A. and Kennett, J.P., 1982. The potential of morphometricaily based phylo-zonation: application of a Late Cenozoic planktonic foraminiferal lineage. Mar. Micropaleontol., 7 : 285--296.

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 Globo- rotalia 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.

I n t r o d u c t i o n

Biostrat igraphic zona t ions have been based on various manifes ta t ions o f the faunal and floral world, such as appearances and ex- t inc t ions o f taxa, jo in t occur rences o f taxa, and peak abundances . Phylo-zones (or lineage- zones) , based on successive t a x o n o m i c units in a phyle t ic lineage, represent one o f the mos t useful me thods for biostrat igraphic

subdivision and correlat ion. The practical usefulness o f phy lo -zona t ions is m u c h depen- den t on the exactness o f the def ini t ions o f the taxa in this con t i nuum. If the taxa are no t dis t inct ly def ined, ambigui ty will result abou t the exact limits o f the taxa, and this m a y seriously impair the reproducibi l i ty o f phylo-zona t ions .

Morphome t r i c analyses of phyle t ic evolu- t ion can represent bo th a c o m p l e m e n t a r y

0377-8398/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

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and alternative approach to traditional usage of phylo-zonations. It allows the transitions between the established taxa to be exactly and objectively defined on the basis of morphometric differences in size, shape, or meris- tic characters. It also allows reproducibility of the individual species concept among different workers, and provides stability of taxonomic usage.

Morphometric analyses may also aid in detecting subtle morphologic changes within any of the chronospecies, which otherwise may appear unchanging. Such trends may in turn be used in the establishment of more refined subdivisions.

A morphometr ic approach to the study of phyletic evolution may be particularly valu- able in areas of low fossil diversities, where traditional biostratigraphy is not applicable. Here, morphometrically defined phylo-zonations may provide a basis for detailed subdivisions and correlations.

The definition of chronospecies within individual lineages in continuous sedimentary sequences can thus be difficult in the absence of morphometr ic work. This can be a particular problem when the lineages are studied by the inexperienced worker. Incorrect species designations, in turn, can lead to incorrect biostratigraphy and paleoenvironmen- tal interpretations. Morphometric definitions within continuous lineages should be more widely applied by micropaleontologists.

Two principal models exist to explain evolutionary processes in a lineage: phyletic gradualism and punctuated equilibrium (Eldredge and Gould, 1972; Gould and Eldredge, 1977). Phyletic gradualism holds that new species arise from the slow and steady transformation of entire populations, providing unbroken gradational fossil series linking separate chronospecies. Punctuated equilibrium explains the appearance of new species by rapid speciation in a small, periph- erally isolated subpopulation. In other areas than that in which the evolution is occurring, the lineage would be expected to show a series of sharp morphological breaks in a

step-like fashion. The evolutionary trends predicted by each model have equal potential for the establishment of morphometric phylo-zonations. Zonal boundaries at break points (thus at chronospecies boundaries) would be appropriate in lineages exhibiting punctualism. The location of chronospecies boundaries in gradualistic lineages is more arbitrary, but still definable.

The establishment of morphometr ic phylo- zones requires large series of measurements on the entire morphology and morphometr ic evaluation of "average characters" in com- bination with timeseries analysis. Micro- fossils offer almost unprecedented oppor- tunities for morphometr ic analyses of chrono- clinal variations in various lineages, because many species occur in extremely large numbers in rather continuous sequences. Numer- ous deep-sea sequences are available from different oceans and water masses through the Deep Sea Drilling Project (DSDP). The hydraulic piston coring technique (HPC) now used by the DSDP is very promising for studies of chronoclines, because it enables coring of more continuous, undisturbed, and more easily dated sequences than with rotary drilling.

We show the potential of morphometric phylo-zonation by applying this method to a lineage of planktonic foraminifera, Globo- rotalia conoidea via intermediate forms to Globorotalia inflata from Late Miocene through Recent sediments in the temperate South Pacific. We believe that the same approach is applicable to most lineages of planktonic foraminifera and other micro- fossils. The evolutionary changes taking place in this lineage have been described morphometrical ly by Malmgren and Kennett (1981), and partly by Kennett (1966) and Scott (1979, 1980). The species within this lineage have formed the foundation for temperate biostratigraphic schemes in the South- ern Hemisphere (Jenkins, 1967; Kennett , 1973; Kennett and Vella, 1975; Berggren, 1977) and the Northern Hemisphere (Poore and Berggren, 1975; Keller, 1978), where

287

they are widespread and persistent in Late Cenozoic marine sections on land and in the oceans.

Microscopic measuring techniques are lab- orious in the collection of the data to establish morphometr ic phylo-zonations. Future routine measurements using microprocessor- controlled video image analyzers {see, for example, Lohmann, 1982) should significant- ly reduce the time and effort involved in col- lecting the data. For stratigraphic analysis of long sequences using morphometr ic phylo- zonal schemes, it is desirable that measurements are kept to a minimum. Thus we have conducted experiments to determine minimum sample sizes for mean morphometr ic values.

The Globorotalia conoidea--G, inflata lineage

The lineage begins with G. conoidea (Waiters) in the Late Miocene (Tongaporutuan Stage of New Zealand). This form is keeled, moderately conical, and has about 4.5--5 chambers in the final whorl. It evolved suc- cessively into keeled, highly conical, about 4.25 chambered forms called G. conomiozea Kennett in the latest Miocene (Kapitean). At the Miocene--Pliocene boundary, the keel rapidly disappeared, the periphery became rounded, and the number of chambers was reduced to about 4 (G. puncticulata sphericomiozea (Waiters)). During the Pliocene and Pleistocene, further peripheral rounding and decrease in number of chambers gave rise to G. puncticulata puncticulata (Des- hayes) (Early Pliocene) and later (Late Plio- cene) to G. inflata (d'Orbigny) (about 3.25-- 3.5 chambers).

We strongly believe that the taxa measured represent morphologically grading chronospecies within a single lineage. Berggren (1977) instead believed that G. puncticulata is derived from G. cibaoensis Bermudez. Berggren did not observe the transition from G. conomiozea to G. puncticulata, but his study was based on sedimentary sequences from middle latitudes in the South Atlantic,

where this evolution may not have taken place. Despite this alternate view on evolu- t ionary relationships, our data represent ob- jective criteria for biostratigraphic subdivision and correlation.

Material and methods

The morphologic analysis of this lineage was based on material from DSDP Site 284 to the east of central New Zealand (lat. 40°30'S, long. 167°41'E; water depth 1068 m). We examined the entire 208 m of the carbonate section (foraminiferal-nannofossil ooze), which ranged in age from the Late Miocene (Tongaporutuan Stage) to the present day. Stratigraphic correlations with dated New Zealand sequences (Louti t and Kennett , 1979) indicate that the base of the section is 8.3 m.y.

Measurements were made in 72 samples in the section at an average sampling interval of about 0.1 m.y. Stratigraphic resolution is slightly bet ter in the Late Miocene and Early Pliocene. In each sample, we randomly picked between 40 and 60 specimens of the G. conoidea to G. inflata lineage from the greater than 150-t~m fraction. We included only relatively thin-walled specimens without a calcite crust, because the measurements required clear definitions of the sutures. Kum- merform specimens (specimens with reduced final chamber size) were also not included, because small final chambers may greatly affect several morphologic measurements. Measurements were carried out without knowledge of depth in the sequence to guarantee that they were unbiased. In our approach, the measurements were made on all members of the lineage, and thus no prior identification as to species was made.

Six variables were measured (Plate I): (I) Number o f chambers in the final whorl,

estimated at quarter chamber increments (Nch). Accurate estimates were normally possible, but where doubt existed, the number of chambers was determined by measuring, using dimensions a and b as defined

288

A B

Nw ~ I N W I

C

I

PLATE I

Nw t

I:)

F

[:)r

1

G H I

289

in Plate I. (2) Conical angle (A). This is defined in edge

view as the angle (in degrees) between the ventral surface of the final chamber and a reference line extending between the angular junctions (keel) of the ultimate and antepenultimate chambers.

(3) Presence or absence of a keel. (4) Roundness of the periphery (Pr). Pr is

defined in edge view as the minimum distance from the point of maximum inflation of the final chamber to a reference line determined from two points on the spiral side of the test (the suture between the final chamber and first chamber of the last whorl, respectively, and the previous whorls).

(5) Apertural shape as determined by the ratio between mean apertural height (Ah) and mean apertural width (Aw). This measure better reflects observable variations in apertural shape than allometry coefficients.

(6) General size as measured by the width of the final chamber (Nw). Redundancy analysis showed this variable to be the math- ematically most stable size measure (Malm- gren and Kennett , 1981).

Sample mean values and coefficients of variation are given in Malmgren and Kennett (1981).

R e s u l t s and d i s c u s s i o n

Evolution versus paleoclimatic trends

Phyletic changes in the analyzed variables

(Fig. 1) are discussed in detail in Malmgren and Kennet t (1981). The exhibited changes are interpreted by them to represent phyletic gradualism.

Two of the variables (general size and apertural shape) are believed to be related to climatic fluctuations during the Late Ceno- zoic. General sizes were largest and apertures most slit-like during periods of climatic cool- ings and/or glacial development (Kapitean and Late Pliocene--Pleistocene, Kennett , 1978). This tendency is slightly less pro- nounced for apertural shapes in the Late Pliocene and Pleistocene. General sizes were smaller and apertures more highly arched during periods of relative paleoceanographic stability and warmth, for example, the Early--Middle Pliocene (Keigwin et al., 1979).

Similar trends have been observed in the cool-water planktonic foraminifer Globigerina bulloides d'Orbigny in Quaternary sediments from the Southern Ocean (Malmgren and Kennett , 1978). This species was also larger during cool (glacial) episodes and smaller during warm (interglacial episodes). In Recent surface sediments from the Southern Ocean (Antarctic through temperate), fossil tests of G. bulloides range from large in cool waters to small in warm waters (Malmgren and Kennett , 1976). The variations in the Quaternary sequences reflect episodic lati- tudinal expansions and contractions of the cool water mass carrying the larger specimens across the core sites. The size variations in

PLATE I

Scanning electron micrographs o f A and D: G. conomiozea; B, E, H, and I: G. puncticulata; C and F: G. inflata; and G : G. conoidea. Nw is the maximum width of the final chamber, which is here a measure o f general size; A is the conical angle; Pr is the roundness o f the periphery; and Aw and A h are the max imum width and height, respectively, o f the aperture. /k is defined in edge view as the angle (in degrees) between the ventral surface of the final chamber and a reference line extending between the angular junctions (keel) o f the ultimate and antepenultimate chambers. Pr is defined in edge view as the min imum distance from the point o f max imum inflation of the final chamber to a reference line determined from two points on the spiral side o f the test (the suture between the final chamber and first chamber of the last whorl, respectively, and the previous whorls). The number of chambers in the final whorl was determined at quarter chamber increments. In doubtful cases, it was determined as the whole number of chambers plus a/(a+b). Specimen G has 4.5 chambers and specimen H has 3.7 5 chambers.

290

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d

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• ~ . = ¢~ c . .~ ~

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~ I I ~ ~

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03 co'

:C '),

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the G. conoidea-G, inflata lineage probably also reflect similar movements of water masses. These movements involved the temperate water mass across the core site, since assemblages indicative of other water masses (e.g. the Subantarctic water mass) are not present at this location in the Late Cenozoic (Kennett and Vella, 1975).

Although our investigations have so far been limited to one section, it is possible to discriminate between morphologic changes due to reversible paleoclimatic change and those that have resulted from evolution. Since general size and apertural shape seem to be related to paleoclimatic changes, these variables would have changed geographically, and thus shall be of less value for stratigraphic correlation. The trends exhibited by the other variables {number of chambers, conical angle, presence or absence of a keel, and roundness of the periphery} were not directly climatically controlled, and did not result from migration of populations. The con- spicuously linear evolutionary decrease in number of chambers is remarkable, because it took place at times of known paleoclimatic oscillations (Malmgren and Kennett , 1981). The upward decrease in chamber number and increase in conicalness and/or peripheral rounding are, therefore, considered to represent irreversible evolutionary trends. The same trends should, therefore, be recorded elsewhere within the range of the lineage populations. The range of these populations is known to extend throughout the temperate water mass in the Southern Hemi- sphere. The evolutionary trends have been observed in their entirety only in this water mass -- specifically in DSDP Sites 284 and 207A (Kennett, 1973) and in many New Zealand marine sections (Kennett, 1966; Scott, 1979, 1980). The appearance of members of the lineage in sediments from warmer waters to the north is considered to be due to migration rather than evolution (Kennett , 1973). The first appearance of G. conomiozea in the warm subtropics (DSDP Site 208) slightly later in the latest

291

EPOCH

PLEIST.

PLIOCENE

M I O C E N E

N.Z, ~Cem~te h STAGE

0

HAUTAWAN - 2 0 -

M p 4 0 - -wp.__

6 0 -

8 0 - OPOITIAN

I 0 0 -

120-

140 - KAPITEAN

1 6 0 -

T O N G A - 180-

P O R U T U A N - 2 0 0 -

VEAN No. of CHAMBERS MEAN ROUNDNESS, PERIPHERY 3-2 3 . 6 4 . 0 4 . 4 4 . 8 2 0 4 0 , 6 , 0 , 8 0 I 0 0 . 1 2 0 / . L I T ' . ` I ~ . I i I I I I 1 I I L I I i t I I J i I I j ~ 1 ~ *

~ I ° ' ~ -I

Fig. 2. Stratigraphic ranges of the component chronospecies of the G. conoidea--G, inflata lineage.

Miocene is a result of migration following its evolution in temperate waters. No evolutionary gradation is apparent between G. conoidea and G. conomiozea at this latitude (Kennett, 1973).

Morphometric phylo-zonation

Samples within the lineage were assigned to particular species using the traditional semiquantitative approach (Kennett and Vella, 1975). The boundaries between the chronospecies were then accurately defined using certain values for the morphometric variables. As might be expected, the boundaries of species defined only by general observations coincide with distinct stages in individual chronoclines (Fig. 1). The boundary between G. conoidea and G. conomiozea occurs at a distinct upward increase in the rate of evolution of the conical angle; the boundary between G. conorniozea and G. puncticulata sphericomiozea coincides with the beginning of the loss of keel and of the peripheral rounding; and the boundary between G. puncticulata sphericomiozea and

G. puncticulata puncticulata coincides with a distinct increase in the evolutionary rate in roundness of the periphery. The boundary between G. puncticulata puncticulata and G. inflata is difficult to define on simple semiquantitative observations because of the gradations. The chronoclines reflect this. Since there is no distinct shift in any of the variables, we have based the boundary between these two species on mean number of chambers (less than 3.60 chambers in the final whorl) and in part on peripheral rounding (greater than 71 t~m, although G. puncticulata exhibits mean peripheral rounding values of up to 77 pm) (Table I).

The morphometric redefinitions of the component species of the G. conoidea-- G. inflata lineage are summarized in Table I (their stratigraphic ranges are shown in Fig. 2). The mean number of chambers is the most significant variable in the establishment of this zonation. Only two samples out of 72 analyzed samples are incorrectly allocated using this variable (Table II). This zonation optimizes the distinctions among the chronospecies.

292

TABLE I

Morphometric redefinition of the component chronospecies of the G. conoidea-G, inflata lineage. Depth ranges in DSDP Site 284 and biochronology of phylo-zones based on these chronospecies. Ages were interpolated from dated New Zealand sections (Loutit and Kennett, 1979), assuming constant sedimentation rates between age estimates: Miocene--Pliocene boundary, 5.3 m.y.; Tongaporutuan--Kapitean boundary, 6.1 m.y.; and the base of the sequence, 8.3 m.y.

Chronospecies Depth Age Mean Mean Percentage Mean roundness range (m.y.) number of conical of keeled of the periphery (m) chambers angle forms (~m)

(degrees)

G. inflata 0--68 0--2.9 3.25--3.60 -- 0 71--125 G. puncticulata puncticulata 68--120 2.9--4.5 3.60--4.00 -- 0 49--77 G. puncticulata sphericomiozea 120--142 4.5--5.3 3.90---4.10 -- 0--100 30--49 G. conomiozea 142--159 5.3--6.1 4.10--4.35 60--69 100 -- G. conoidea 159-->206 6.1-->8.3 4.35--<4.75 <57--63 100 --

A La te Neogene sample o f u n k n o w n age f rom the t e m p e r a t e w a t e r mass m a y be re fe r red to any o f these p h y l o - z o n e s by using the s t r a t egy shown in Fig. 3. F i r s t ly , the sample m a y be d i c h o t o m i z e d by de te r - min ing the pe r cen t age o f kee led forms and the mean n u m b e r o f chamber s . F o r accura te numer ica l e s t ima tes o f these var iables , it is i m p o r t a n t t ha t d a t a are o b t a i n e d f rom speci-

mens lack ing th ick ca lc i te crus t and in which the spiral su tures are visible. I f all spec imens have a keel and the mean n u m b e r o f chambers is g rea te r than (or equal to) 4 .10, the sample is re fe rab le to the G. cono idea - -G .

c o n o m i o z e a pa r t o f the l ineage (La te Mio- cene) . When spec imens lack a d i s t i nc t keel and the mean n u m b e r of c h a m b e r s is less t han 4.10, the sample is a l loca ted to the

TABLE II

"Classification matrices" showing numbers of samples from Site 284 correctly and incorrectly allocated as to species using evolutionary trends in mean number of chambers and mean roundness of the periphery. Mean number of chambers is used to discriminate among G. conoidea, G. conomiozea, G. puncticulata, and G. inflata (Fig. 3). Mean roundness of the periphery is used as a criterion for distinguishing G. puncticulata sphericomiozea and G. puncticulata puncticulata.

Number of chambers (72 samples) predicted species

G. conoidea G. conomiozea G. puncticulata G. inflata

Actual species

Actual subspecies

G. conoidea 23 0 0 0 G. conomiozea 1 9 0 0 G. puncticulata 0 0 28 1 G. inflata 0 0 0 10

Roundness of the periphery (29 samples) predicted subspecies

G. p. sphericomiozea G. p. puncticulata

G. p. sphericomiozea 13 0 G. p. puncticulata 0 16

293

' ~ k ~ inf,roto /,

r-T°ng a 2 ~ e ~ pLpor utuan~ [ Phocene |

mean Pr

i

~72~ ........ . . . . . . . . . . . ? ~ eorhest L ~ , ~~'1 Pi . . . . . . j

n~an Nch £~I~J i

Phocene - Pleistocene]

Fig. 3. Application of morphometric phylo-zonation based on the G. conoidea--G, inflata lineage. Strategy for taxonomic and biostratigraphic determinations of a Late Cenozoic sample from temperate waters in the Southern Hemisphere.

G. puncticulata--G, inflata part of the lineage (Pliocene-Pleistocene).

More detailed stratigraphic determinations of a Late Miocene sample may be accom- plished through further use of the number of chambers. A mean greater than 4.35 indicates G. conoidea (Tongaporutuan Stage) and a smaller mean indicates G. conomiozea (Kapitean Stage). The two species cannot be uniquely separated using the mean conical angle, since there is overlap of angles (Table I). However, in non-overlapping intervals, the conical angle can assist in stratigraphic determinations (Fig. 3).

Similarly, the number of chambers forms the basis for distinguishing G. puncticulata (Early-Middle Pliocene) from G. inflata (Late Pliocene--Pleistocene). If the mean is greater than 3.60, the sample is referable to G. puncticulata, and if it is smaller, the sample is G. inflata. A range of overlap exists in the mean roundness of the periphery between the two species (between values of 71 pm and 77 um; Table I). Values outside this range assist in the discrimination between the two forms.

The two subspecies of G. puncticulata can be distinctly separated on the basis of the roundness of the periphery (see "classification matr ix" in Table II). The cut-off point is at a mean roundness of 49 pm. Smaller values mark G. puncticulata sphericomiozea (earliest Pliocene) and larger values mark G. puncticulata puncticulata (Early- Middle Pliocene). A confirmation of this distinction may be obtained from the number of chambers (Fig. 3). Scott (1980) established a criterion for distinguishing these subspecies based on the frequency of keeled forms in a sample. When at least 5% of the specimens do not have a keel, they are assigned to G. puncticulata sphericorniozea, whereas all specimens of G. puncticulata puncticulata lack a keel. If this criterion were applied to the sequence at Site 284, the transition would have occurred at about 130 m {Fig. 1), which is about 10 m (0.4 m.y.) below the boundary defined using roundness of the periphery and number of chambers (Table I).

294

M i n i m u m sample size

E x p e r i m e n t s were m a d e to d e t e r m i n e the m i n i m u m n u m b e r o f spec imens needed for cor rec t ch ronospec ies iden t i f ica t ion based on the mean n u m b e r o f c h a m b e r s {Fig. 4). Six samples were d rawn at r a n d o m f r o m each of the intervals > 0 . 2 0 , 0 .16- -0 .20 , 0 . 1 1 - 0 . 1 5 , 0 . 0 6 - 0 . 1 0 , and 0 .00- -0 .05 mean n u m b e r o f chambe r s away f rom the value at the ch ronospec ies bounda r i e s {4.35, 4 .10, and 3.60, respec t ive ly ; Tab le I). In each sample the m e a n n u m b e r o f cham be r s were d e t e r m i n e d for six r a n d o m subsamples consis t ing of 5, 10, 15, . . . N spec imens (N is the to ta l sample size). These represen t on ly a small f rac t ion o f po t en t i a l subsampl ing categories . The subsample means were t hen c o m p a r e d with the range o f means def in ing the respect ive chronospec ies . The m i n i m u m

n u m b e r o f spec imens is de f ined in each subsample as the lowes t n u m b e r (e.g. 5, 10, 15, • . . ) for which co r rec t a l loca t ions were ob- ta ined for all subsequen t ly larger sample sizes. F o r e x a m p l e , if a mean based on 5 spec imens yields the cor rec t chronospec ies , and this is t rue also for larger subsample sizes, the required m i n i m u m is 5 specimens . Near ch ronospec ies boundar ies , a l though cor rec t a l loca t ions m a y be p rov ided by 5 and 10 spec imens , this m a y no t be so for 15 and 20 specimens . In the case o f all larger subsamples giving cor rec t values, the m i n i m u m n u m b e r is de f ined as 25. In m o s t samples , 15 spec imens are necessary in m o s t e x p e r i m e n t s for cor rec t ch ronospec ies alloca- t ion {Fig. 4). Cor rec t a l loca t ion is possible even on on ly 5 spec imens at some levels, bu t on app roach ing ch ronospec ie s boundar ies , the required n u m b e r d is t inc t ly increases.

llll1. 1 !i Z 5 15 25 5 15 25 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5.15 25 W

:11 1 l L_ 5 15 25 5 15 25 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25

LU

i! .L L_L k h,,

5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 35 5 15 25 35 5 15 25 >&O 5 15 25 >40

NUMBER OF SPECIMENS

Fig. 4. Experiments to determine minimum number of specimens required for allocation of a sample from known stratigraphic level to correct chronospecies using mean number of chambers. This variable is used to discriminate among G. conoidea, G. conomiozea, G. puncticulata, and G. inflata (Fig. 3). Six samples were randomly drawn from each of five different intervals of sample means away from values at chronospecies boundaries (>0.20, 0.16-0.20, 0.11-0.15, 0.06-0.10, and <0.05 mean number of chambers). These samples were taken from those used to establish the trends. In each sample, six random subsamples (experiments) of 5, 10, 15 . . . specimens were analyzed for mean number of chambers. Depending on how these means compared with the actual mean, minimum sample sizes were assessed.

295

1 LILI X 5 !5 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 2~1 5 15 25 W

0 6 1 6 5

4

3

2

1

5 15 25 >~0 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 >40 5 15 25 ~40

NUI '4BER OF S P E C I M E N S

Fig. 5. Experiments to determine minimum sample sizes using mean roundness of the periphery. This variable is used for differentiating samples of G. puncticulata sphericomiozea and G. puncticulata puncticulata (Fig. 3). Four random subsamples were taken from four intervals of sample means (>15 urn, 11--15 urn, 6--10 urn, and < 5 ~m) from the separating value (49 urn). For further details, see caption of Fig. 4.

Similar experiments were carried out for the roundness of the periphery, which is required to distinguish between G. puncticulata sphericorniozea and G. puncticulata puncticulata (Fig. 5). Four samples were randomly drawn from each of the following intervals: ~15 pm, 11--15 ~m, 6--10 pm, and <5 ~m. Six subsamples were tested from each interval. Also for this variable, few measurements are needed for correct deci- sions: 15 specimens is a suitable number for all samples, except very close to a boundary.

C o n c l u s i o n s

(1) Five morphometrically defined chronospecies are recognized within the Late Mio- cene -- Recent planktonic foraminiferal lineage from Globorotalia conoidea via intermediate forms to Globorotalia inflata in temperate regions of the South Pacific. The following five chronospecies were quantitatively defined: 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 .)

(2) The phyio-zones are readily quan-

titatively defined using only one or two measured variables (number of chambers in the final whorl and peripheral rounding) and at most stratigraphic levels by measurements of only 15 specimens.

(3) The small sample sizes determined here to be sufficient for quite precise taxonomic determinations greatly enhance the practical use of morphometric phylo- zonations. Applications of this technique should not generally require much more effort than for standard biostratigraphic approaches.

(4) Our phylo-zonation conforms with the previously described component species of the lineage. Evolutionary trends within the ranges of most of the chronospecies should in the future provide much higher biostratigraphic and correlation precision.

(5) Once the lineage is accurately tied to a paleomagnetic stratigraphy and integrated chronology, it will be possible to assign accurate geologic ages to any measured samples within the lineage.

A c k n o w l e d g e m e n t s

Parts of this research were carried out while B.A.M. was a Visiting Research Scientist at the University of Rhode Island in the

296

s u m m e r s o f 1 9 7 8 a n d 1 9 7 9 . We t h a n k B S r j e S / / w e n s t e n , S t o c k h o l m U n i v e r s i t y , a n d N a n c y

P e n r o s e , U n i v e r s i t y o f R h o d e I s l a n d , f o r t e c h n i c a l a s s i s t a n c e .

Th i s r e s e a r c h was s u p p o r t e d b y g r a n t s f r o m U.S . N a t i o n a l S c i e n c e F o u n d a t i o n t o J . P . K . ( D P P 7 8 - 0 8 5 1 2 , D i v i s i o n o f P o l a r P r o g r a m s , a n d O C E 7 9 - 1 4 5 9 4 , C E N O P ) a n d a g r a n t f r o m t h e S w e d i s h N a t u r a l S c i e n c e R e s e a r c h C o u n c i l to B .A .M. ( G 4 0 7 6 - 1 0 1 ) .

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the potential of morphometrically based phylo-zonation: application of a late cenozoic planktonic...

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