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Chapter 44. Oligocene to Quaternary ostracods of the central equatorial Pacific (Leg 85, DSDP-IPOD)

Chapter 44. Oligocene to Quaternary ostracods of the central equatorial Pacific (Leg 85, DSDP-IPOD)

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1-Location map, sites 571 through 575, Leg 85 DSDP. Depth contours in kilometres. From Text-fig.

1 of Theyer et al., (1985). Location and present depth of sites as follows: 575, 5"51'N, 13So02.16'W, 4536111;

674, 4"12.52'N, 133"18.57'W, 4555m; 573, Oo29.92'N, 133"18.57'W, 4309m; 572, 1"26.09", 113"50.52'W,



General Comments

Analysis of over 55,025 cc aliquots from the core inventory of Leg 85 has revealed the presence of a biocoenose containing at least 47 species of ostracods (Table 1). The assemblages are

sparse (0 to 35 specimens/sample)and low in diversity (maximum number of species/sample = 11).

Approximately one-third of the samples processed were devoid of ostracods. Because of the limited

recovery, our results most likely underestimate the diversity of past faunas in the central equatorial

Pacific as well as the stratigraphical range of rare species. Two ecostratigraphic units have been

defined on the basis of the vertical distribution of ostracod species. A Lower Ecozone (Oligocenelower Miocene) is characterised by an internally uniform fauna. The Upper Ecozone (middle

Miocene-Quaternary) contains a lower interval marked by a reduction in diversity, evolutionary

changes in lineages of Poseidonamicus. Bradleya and Abyssocythere and by a transformation of

the overall morphological aspect of the fauna.

Lower Ecozone (Oligocene to lower Miocene)

Faunas in this ecozone are relatively diverse (30-35 species per biostratigraphic zone; Text-fig.

2) and contain abundant representatives of Poseidonamicus and Bradleya. Taxa such as Xestoleberis

chamela, Eocytheropteron trinidadensis and Krithe kollmani which originated in the Eocene of the

Tethys and Caribbean, are present. Twenty-three species have been recorded in the lower Oligocene; an additional nihe make their appearance in the upper Oligocene. Within the Oligocene

to lower Miocene interval, there are no stratigraphical range terminations without replacement

except for species with isolated occurrences (e.g. Bythocypris sp.). Agrenocythere antiquata evolved

into A. hazelae in the late Oligocene as previously suggested by Benson (1972). Stability in the

composition of the faunas and relative abundance of species in the Oligocene to lower Miocene

of the central equatorial Pacific is one of the most noteworthy results of our investigation.

Upper Ecozone (middle Miocene-Quaternary)

A series of local extinctions, initial appearances and apparent speciation events which took









Abyssocythere australis Benson, 1971

Abyssocytherejaponica Benson, 1971

Abyssocythere sp. 1

Abyssocythere trinidadensis

(van den Bold, 1957)

Abyssocythereis sulcatoperferata

(Brady, 1880)

Agrenocythere antiquata Benson, 1972

Agrenocythere hazelae

(van den Bold, 1946)

Ambocythere c.f. A. caudata

van den Bold, 1965

Ambocythere challengeri Benson, 1983

Argilloecia sp.

Bairdia oarion van den Bold, 1972

Bairdoppilata sp.

Brachycythere mucronalatum

(Brady, 1880)

Bradleya dictyon (Brady, 1880)

Bradleya johnsoni Benson, 1983

Bradleya pygmaea Whatley, Harlow,

Downing and Kesler, 1985

Bythocypris sp.

Eocytheropteron trinidadensis

(van den Bold, 1960)

Eucythere sp.

Henryhowella sp.

“Hyphhlocythere” sp.

Krithe kollmani Pokorny, 1980

Krithe morkoveni van den Bold, 1960

Krithe reversa van den Bold, 1958

Range in Pacific

Total Range

Upper Miocene (N16)

Upper Miocene-Recent

-Upper Pliocene (N19)


Upper Miocene (N17)


-Quaternary (N23)

Upper Oligocene (P21)

-Upper Miocene (N17)

Late Cretaceous

Lower Oligocene (P19)

-Upper Miocene (N17) -Upper Miocene


Lower Oligocene (P19)

-Quaternary (N23)

Middle Eocene

Lower Oligocene (P19)

-Upper Oligocene (P21) -Upper Oligocene

Upper Oligocene

Upper Oligocene (P21)

-Upper Miocene (N17) -Recent

Lower Oligocene (P20)

-Upper Miocene (N17)


Lower Miocene (N8)

-Quaternary (N23)

Lower Oligocene (P16)

-Quaternary (N23)

Eocene-Upper Miocene

Lower Oligocene (P16)

-Upper Miocene (N17)

Lower Oligocene (P20)

-Upper Miocene (N17)


Lower Oligocene (P18)

-Quaternary (N23)

Upper Miocene-Recent

Upper Miocene (M17)

-Quaternary (N23)

Lower Oligocene-Upper

Lower Oligocene (P19)

Miocene (Recent?)

-Upper Miocene (N17)


Pliocene (N19, N21)

Lower Oligocene (P16)

Upper Oligocene (P21)

-Upper Pliocene (N19)

Lower Oligocene (P20)

-Upper Miocene (N17)

Lower Oligocene (P19)

-Quaternary (N23)

Lower Oligocene (P19)

-Lower Miocene (N6)

Lower Oligocene

(P18, P19)

Lower Miocene (N6)

-Quaternary (N23)

Middle Miocene (N9)

-Quaternary (N23)

Middle Eocene

-Upper Pliocene

Upper Eocene

-Lower Oligocene









Krithe sp. 1

Krithe sp. 2

Krithe sp. 3

Krithe sp. 4

Krithe sp. 5

Krithe sp. 7

Krithe vandenboldi Steineck, 1981

Messinella guanajayemis

van den Bold, 1969

“Oxycythereis” sp.

Parakrithe sp. 1

Parakrithe vermunti

van den Bold, 1946

Phacorhabdotus sp.

Poseidonamicus ex. gr. major

Poseidonamicus ex. gr. punctatus

Proabyssocypris sp.

Rockallia vscripta Whatley, Uffenorde,

Harlow, Downing and Kesler, 1982

“Suhmicythere” suhmi (Brady, 1880)

Range in Pacific

Lower Oligocene (P19)

-Quaternary (N23)

Lower Miocene (N5)

-Quaternary (N22)

Lower Oligocene (P19)

-Quaternary (N23)

Lower Oligocene (P18)

-Quaternary (N23)

Lower Miocene (N4)

-Upper Pliocene (N21)

Pliocene (N18-N21)

Lower Miocene ( N 5 )

-Upper Pliocene (N19)

Lower Oligocene (P19)

Upper Oligocene (P21)

-Quaternary (N23)

Lower Oligocene (P19)

-Middle Miocene (N9)

Lower Oligocene (P19)

-Upper Miocene (N17)

Lower Oligocene (P19)

-Quaternary (N22)

Lower Oligocene (P19)

-Quaternary (N23)

Upper Oligocene (P21)

-Upper Miocene (N17)

Upper Oligocene (P21)

-Middle Miocene (N10)

Lower Miocene (N5)

Total Range


Middle Eocene

-Lower Miocene



Upper OligoceneRecent

Lower Miocene



Upper Oligocene (P21)

-Upper Pliocene (N19)



Upper Miocene (N17)

(Brady, 1880)

-Quaternary (N23)

Upper Eocene

“Thallasocythere” bermudezi

Lower Miocene (N5)

(van den Bold, 1960)

-Middle Miocene (N10) -Middle Miocene

Lower Oligocene (P20)

Trachyleberis sp.

-Quaternary (N22)

Middle Eocene

Xestoleberis chamela

Lower Oligocene (P20)

van den Bold, 1960

-Middle Miocene (N12) -Middle Miocene

Sources for total ranges-van d& Bold (1977,1981),Benson (1971,1972),Benson and Peypouquet (1983). Pokornf

(1980), Steineck (1981), Whatley et. al. (1982,1983). The N and P zones given in parentheses are the standard

planktonic foraminifera] zones as defined and used by Leg 85 scientists.Their age assignments are followed thoughout this paper.

place in the middle to late Miocene dramaticaly transformed the nature of the ostracod faunas in

Leg 85 cores. Fifteen species become locally extinct at this time; these terminations occur over

an extended period of time (Text-fig. 2) but there is a concentration in upper Miocene sediments

(N17).Coincident with the latter extinctions are two species-replacementsthat are parts of evolutionary lineages : Bradleyayohnsoni -> B. dictyon, Abyssocythere sp. 1 -> A. japonica. Limited

Oligocene to Quaternary Ostracods of Central Equatorial Pacific 601

froabyssocypris sp.

Xestoleberis chamela

“Thallasocythere” acanthoderma













2-Stratigraphical range of selected species and simple species diversity per zone in the middle and

upper Miocene. Planktonic foraminifera1 zones taken from Mayer, Theyer and others (1985). Simple species

diversity is the number of species summed across all samples in a zone.

data.does not allow us to establish whether these speciation events took place locally or whether the

replacements were related to the immigration of the descendant species. An important characteristic

of Upper Ecozone assemblages is the rarity of Poseidonamicus relative to the occurrence of Bradleya (Text-fig. 3). Only 8 species survived the middle and upper-Miocene faunal event as recorded

in the Leg 85 cores to be represented in Pliocene and younger sediments whose faunas are characterised by low species diversity (Text-fig. 2) and by the dominance of Krithe spp., Bradleya

dictyon, Brachycythere mucronalatum and by Thallasocythere acanthoderma.

In addition to their distinctive taxonomic composition and population structure, faunas of the

Upper Ecozone are recognizable by a shared morphological style. By the Pliocene, most of the

quantitatively significant species possess a carapace morphology that conforms to the model elucidated by Benson (1977b, 1979) as diagnostic of ostracods inhabiting the deep-sea. The krithinids

and Brachycythere mucronalatum increase in size; in some examples the length of the adult carapace exceeds 1400 pm. Thallasocythere acanthoderma and Henryhowella sp. bristle with a dense

network of ominous-looking spines (Pl. 1, figs. 1-3, 10, P1. 3, fig. 8). The reticularpatternof

Bradleya dictyon is subdued and simplied compared to its progenitor B. johnsoni (compare P1.

2, fig. 1 with figs. 24). These morphologies have the effect of reducing carapace mass relative to

size and may confer a selective advantage, when cold, CaCO, undersaturated bottom-water is

presentq by minimizing the metabolic energy needed to secrete the shell following moulting.








Bradkya spp.---





















TEXT-HG.3-Relative abundance of Bradleyu and Poseidonumicusversus age. Vertical scale is the number of specimens summed across all samples in a given zone.

Taxonomic Notes

Abyssocyt here

A. trinidudensis ranges from the lower Oligocene to the upper Miocene (N17). This upper limit

may represent a true extinction as other authors (van den Bold, 1977, fig. 7; Benson, 1971) noted

the absence of this species from Pliocene sediments. This species dates back at least to the late



1-3 Henryhowellasp. 1. 1) spinoseform, adult left valve, 574A, 14-3, Middle Miocene (N2), x%.

2) reticulate form, adult left valve, 572C, 3-1, Quaternary (N22), x 77. 3) long form, adult right valve, 572C.

4-6,Pliocene (N21), x 72. Fig. 4. Thuflusocytherebermudezi (van den Bold). Subadult left valve, 574, 19-7,

Middle Miocene (NlO-Nl l), x 72. Figs. 5 , 7. Agrenocyfhere huzelae (van den Bold). 5 ) adult right valve, 575,

7-1, Middle Miocene (N12), x 67. 7) detail of castrum, same specimen as 5, X 170. Fig. 6. Phucorhubdotussp.

Adult left valve, 575A, 19cc, Early Miocene (NS), x 112. Fig. 8. Abyssocythereis sulcutoperforatu (Brady).

Subadult left valve, 575, 8-2, Middle Miocene (NlO), x 100. Fig. 9. Bruchycythere mucrondatum (Brady).

Adult left valve, 572c, 16-2, Upper Miocene (N17), x 64. Fig. 10. “Thallasocythere” ucunthodermu (Brady).

Adult right valve, 572c, %5, Pliocene (N19),x 87. Fig. 11. Abyssocythere trinidadensis (van den Bold). Adult

left valve, 575,9-3, Middle Miocene (N8), x 73. Fig. 12. Ambocythere chuffengeriBenson. Adult left valve,

572A, 9-6, Pliocene (N19)p x 110.

Oligocene to Quaternary Ostracods of Central Equatorial Pacific 605

Cretaceous (van den Bold, 1957) and had entered the abyss by the middle Eocene (Steineck et ul.,

1984). The morphological stability displayed by this species over a period of 55 my is remarkable

even for deep-sea ostracods (compare P1. 1, fig. 1 1 with van den Bold, 1957, P1. 3, fig. 1).

Abyssocythere sp. 1 first appeared in the upper Oligocene and persists upcore to the upper Miocene (N17) when it may have given rise to A. juponicu. Both species are recognized by the elevated

vertically-aligned muri which dominate the sculptural relief. A. sp. 1 is distinguished by a domeshaped muscle scar node (absent in A. juponicu) and by the extension of the anterior muri from the

dorsum to the ventro-lateral complex (Pl. 3, fig. 5). In the descendant species, presumably homologous elements close ventrally to form U-shaped structures (Benson, 1971, fig. 11, P1. 1. fig. 8).


Brudleyu johnsoni was first described from the Lower Miocene of the South Atlantic (Benson

and Peypouquet, 1983, p. 81 l), but it had been previously recorded (as B. dictyon) in Oligo-Miocene faunas from the Caribbean (Steineck, 1981, PI. 2, fig. 14; Steineck et ul., 1984, figs. 6i-k; van

den Bold, 1981, P1. 4, fig. 17). In the present study it occurs in Oligocene to upper Miocene sediments. Brudleyujohnsoni is identified by a massive, primary reticulation composed of excavate muri,

an elevated and robust bridge structure and subdued secondary muri which subdivide the primary

fossae in the anterior, dorso-median and posterior regions (Pl. 2, fig. 1). It appears to have

evolved into B. dictyon (sensu Benson and Peypouquet, 1983, P1. 3, fig. 4) within zone N17 by

development of a more open and irregular reticulum with less relief, (PI. 11, figs. 2-4). Brudleya

dictyon of present usage may be conspecificwith B. lordhowensis described by Whatley et. al. (1984)

from the Pliocene of the S.W. Pacific. Benson (1972, P1. 9, figs. 1-12) figured a recent or subfossil

specimen that is close to B. johnsoni raising the possibility that this species is extant. Contrary to the

assertions of Whatley et. ul. (1984), the B. dicyon lineage has been present at abyssal depths in

the Pacific since the earliest Oligocene.


Two species complexes, differing in the pattern of anterior ornamentation, occur in Leg 85

material. The P . major complex (Oligocene-Quaternary), is identified by an anterior reticular

field containing 1) a mural system which obliquely traverses the region between the muscle-scar

node and the anterior margin and 2) large, oval to semi-circular fossae (Pl. 2, figs. 8-11, P1.

3, figs. 1-3). Some specimens (Pl. 3, fig. 1) of Poseidonamicus ex. gr. major in the Lower Ecozone are similar to the form figured by Steineck el. ul. (1984, figs. 7c-e), others may belong to

P . riogrundensis Benson, (Pl. 11, figs. 8, 9) and a third group of individuals may belong to an

undescribed species. Few Middle Miocene specimens were discovered. These are difficult to categorize because of the lack of well-preserved adults. Individuals found in the Upper Ecozone are

considered to fall within the range of variation of P . miocenicus Benson (Pl. 11, figs. 10, 1 1 ; see

Benson and Peypouquet, 1983, P1. 3, figs. 2, 3; Benson, 1983, fig. 1).

PLATE2-Pig. 1. BradleyajohnsoniBenson. Adult left valve, 572D, 33-5, Middle Miocene (N9), x 81. Figs. 2-4. Bradfeya dicryon (Brady). 2) adult left valve, 572A, 2-1, Quaternary (N22), x 62. 3) adult right valve, 572A, 1-3,

Quaternary (N23), x 70. 4) closeup of the central mural loop and posterior end of bridge, same specimen as

2, x 190. Fig. 5. Poseidonamicus ex. gr. P. punctafus Whatley, Harlow, Downing and Kesler. Adult right

valve, 574A, 21-3, Middle Miocene (N9), x 103. Fig. 6. Amboryrhere cf. A. cnudara van den Bold. Subadult

right valve, 573, 6-1, Pliocene (NZl), x 101. Fig. 7. "Hyphalocyrhere" sp. Adult right valve, 574C, 6-2,

Lower Miocene (h6), x 99. Figs. 8,9. Poseidonamicus ex. gr. P . nrajor (Brady). Form similar to P. riograndensis Benson. 8 ) adult left valve, 574A, 21-1, Lower Miocene (N6). 9) adult right valve. same sample as 8. Both

x 88. Figs. 10, 11. Poseidonanmicus ex. gr. P . major Brady. Form similar 10 P. rniocenicus Benson. 10) adult

right valve, 572C, 13-2, Pliocene (N18). 11) adult left valve, same siiniple as 10. Both \ 82. Fig. 12. Brad/eya

pygmaea Whatley, Harlow, Downing and Kesler. Adult right valve. 572C. S-4, Pliocene (N19), \ 80.

Oligocene to Quaternary Ostraeods of Central Equatorial Pacifc 607

The Poseidonamicus punctatus complex, including P . anteropunctatus, P . punctatus, and P .

praenudus (Whatley et. al., 1983), is distinguished by an array of fine to coarse anterior punctae.

In many specimens, the punctae are set into a raised surface similar to the levatum of Abyssocythere (Pl. 2, fig. 5). Several subadult specimens from Miocene samples resemble P. praenudus

(see Whatley et al., 1983, fig. 2) in the near-total suppression of the reticulation but the chance

of confusing this species with the moults of related forms is great. Most adult specimens (Pl. 2,

fig. 5 ; P1. 3, fig. 2) possess coarse anterior punctae and an orthogonal, posterior reticulum with

broad non-excavate muri suggesting referral to P . punctatus. If lower Miocene specimens from the

central equatorial Pacific are indeed correctly placed in P. punctatus (Pl. 3, fig. 2), then this

species would predate P. anteropunctatus, its presumed ancestor in the phylogeny of Whatley et

al. (1983, fig. 2).


A significant bimodal variation in the size and shape of the carapace is present in assemblages

of this genus in the Leg 85 cores. Both morphotypes are present at all stratigraphical levels and

commonly co-occur in the same sample (see P1. 1, figs. 1-3, P1. 3, figs. 6, 7). Consistent morphological differences other than proportion are not evident. Perhaps, this is an example of extreme sexual dimorphism?

Henryhowellids in the Oligocene and lower Miocene typically possess a prominent muscle-scar

node, posterior latitudinal ridges (or their remnants), and a hispid-reticular ornamentation (Pl.

3, figs. 6, 7). Those in the Upper Ecozone are larger, have longer and more delicate spines and

lack the muscle-scar node and posterior ridges. Individuals with dominantly hispid or dominantly

reticulate ornamentation can often be found in the same sample (Pl. 1, figs. 1-3). LeRoy and

Levinson (1974, Pls. 10-12) illustrated similar variation in Recent populations of Henryhowella

from the Gulf of Mexico. In Upper Miocene to Quaternary specimens, the reticular pattern is

similar to that in deep-sea species referred to Echinocythereis; elongate morphs of this age may

be conspecific with E. circumdentata (Brady) (Brady, 1880, p. 180, P1. 26 figs 2a-c; Puri and Hulings, 1976, p. 269, P1.17, figs. 3-6). However, in those specimens where the adductor scars are visible,

the intact, U-shaped frontal-scar is diagnostic for HenryhowelIa (M. Kontrovitz, pers. comm.,

1985). Until the morphological lability of Pacific populations of Henryhowella can be partitioned

into evolutionary or ecophenotypic components, the material at hand is retained in a single taxon

left in open nomenclature.


Brachycythere mucronalatum (Pl. 1, fig. 9) is a long-ranging and morphologically-stable species.

It was first described by Brady (1880, p. 140-141, P1. 33, Figs. 8a-d) from a number of locations

in the North and South Atlantic and eastern Pacific. Puri and Hulings (1976, p. 307, PI. 22, figs.

PLATE3-Figs. 1,3. Poseidonamicus ex. gr. P. major (Brady). 1) adult left valve, 575A, 12-2, Lower Miocene (NS),

x 84. 3) adult left valve, 572D, 32-2, Middle Miocene (N9), ~ 9 0 .Fig. 2. Poseidonamicusex. gr.P.punctatus

Whatldy, Harlow, Downing and Kesler. Adult right valve, 575A, 12-2, Lower Miocene(NS), X 101. Fig. 4.

Messinella guanujayensis van den Bold. Adult right valve, 574C, 33-1, Lower Oligocene (P19), X 160. Fig. 5 .

Abyssocythere sp. 1. Adult right valve, 573-12cc, Upper Miocene (N17), x 103. Figs. 6-8. Henryhowella

sp. 6 ) short form with node, adult left valve. 575A, 33-1, Lower Miocene (N4), x 84.7) long form, adult left

valve, 573B, 19-1, Lower Miocene (N4), x 101. 8) detail of spine terminations, dorsal margin of adult right

valve, 574A, 14-3, Middle Miocene (N12), X 1190. Figs. 9,lO. Phacorhabdotus sp. 9) adult right valve, 575A,

19cc, Lower Miocene (NS), ~ 1 1 0 10)

. interior of subadult (?) right valve, 573B, SCC, Middle Miocene

(N13/N14), x 110. Fig. 11. Eocytheropteron trinidadensis (van den Bold). Interior view and hinge, adult right

valve, 572D, 15-1, Middle Miocene (N13), X 117. Fig. 12. Abyssocythereis sulcatoperforata (Brady). Adult

right valve, 572C, 6-5, Upper Pliocene (N19), X70. Fig. 13. Oxycythereis sp. Subadult left valve, 573B,

18-2,,Lower Miocene (N4), x 94.





TEXT-FIG.&Suggested ontogenetic development in Brachycythere mucronalatum. A, early instar (573B. 30-2,

Upper Oligocene), 900 pm long x 600 pm high. B, late instar (575A, 12-1, Lower Miocene), 1100 x 750 pm.

C, adult (573, 2-2, Quaternary), 1250 x 1000 pm. Note the proportional decrease in length and development

of smoothlyarched dorsal margin with maturation.

14-1 8) commented that their lectoholotype lacks the broadly-arched dorsum discussed and

illustrated by Brady. Text-figure 4 suggests that they selected a subadult.


Two taxonomic philosophies have been applied to deep-sea representatives of this genus. The

first (van den Bold, 1977, 1981; Pokornq, 1980) defines narrowly circumscribed species on the

basis of traditional internal and external carapace features. The other, (Kaesler and Lohmann,

1972; Peypouquet, 1979; Benson and Peypoquet, 1983) emphasizes the environmental plasticity

of the krithinid carapace under the influence of environmental parameters. In Peypouquet's (1979)

taxonomy, a few, highly-variable species are differentiated by the number and length of pore

canals in the dorsal third of the anterior vestibule; ecomorphotypes of these species vary in length

(up to 250%), shape, vestibular size and shape and valve overlap. Different species can approach

each other in overall appearance so completely as to be virtually indistinguishable (Benson and

Peypouquet, 1983, P1. 5, figs. 5, 7). Such extreme variation has not been demonstrated in other

euhaline, podocopid ostracods. Although often criticized by others (Pokornq, 1980; Whatley,

1983; Steineck et. al., 1984), this system has given rise to palaeoenvironmental intepretations

consistent with sedimentological and geochemical data (Ducasse and Peypouquet, 1978, 1979;

Benson and Peypoquet, 1983).

Attempts to develop a taxonomy of Leg 85 krithinids began with the establishment of two labile

species: Krithe C (= K. vandenboldi and related forms) and K. D (= K. rnorkhoveni) of Peypouquet (1979) (Text-fig. 5), However, a consistent ecomorphotypic stratigraphy related to interpretable environmental change could not be demonstrated. Subsequently, a more conservative

taxonomy based on limited intraspecific variation was adopted (Table 1).

Discrimination of species of Krithe remains a formidable undertaking regardless of the taxonomic approach used. In Leg 85 material, a middle Miocene to Quaternary population with reverse

overlap closely resembles K. reversa (van den Bold, 1958; P1. fig. 5; Text-fig. 5B). In the Caribbean where it is widespread, K. reversa has same stratigraphical range as in the central equatorial

Pacific (van den Bold, 1977, fig. 5). In the Pacific, K. 3 is identical to K. reversa except for normal

valve overlap (Text-fig. 5C) and slightly smaller size. Krifhe sp. 3 first appeared in the lower

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Chapter 44. Oligocene to Quaternary ostracods of the central equatorial Pacific (Leg 85, DSDP-IPOD)

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