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Chapter 61. Geographical isolation in marine species: Evolution and speciation in Ostracoda, I

Chapter 61. Geographical isolation in marine species: Evolution and speciation in Ostracoda, I

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812 T.M. CRONIN



A PROSPECTUS FOR EVOLUTIONARY

STUDIES IN OSTRACODA

This paper presents the background and initial results of a series of studies specificallydesigned

to investigate speciation and morphologic stasis in marine Ostracoda. My ultimate objectives are

1) to assess the geographical classification of speciation by determining the relative frequency of

types of allopatric, sympatric and parapatric speciation, 2) to estimate the relative frequency and

circumstances of rapid and gradual morphological change, and 3) to assess the role of climate and

tectonics in causing geographical isolation and speciation. The decision to conduct these studies

stems from several trends emerging in evolutionary palaeontology. First, there are few case studies

using fossil sequences that adequately address the question of speciation (Gingerich, 1985, Table 2).

Of greater concern are questions about paleontologists’ ability to identify species in fossil material

(Schopf, 1980, 1982). It may be that Schopf’s concern about fossil species reflects the approaches

adopted by paleontologists, which may not be appropriate for answering current evolutionary

questions, and not the fossil record itself. With few exceptions, paleontological studies of speciation have unnecessarily overemphasized stratigraphic completeness and high sampling resolution

(Gingerich, 1985). Clearly, no stratigraphical sequence is complete, an idea known long before the

comprehensive studies of Schindel (1980, 1982) and Dingus and Sadler (1982). Despite assertions

by theorists that geographical coverage is of paramount importance (Eldredge and Gould, 1972;

Schopf, 1982; Valentine and Jablonski, 1983), the apparent reluctance by paleontologists to delineate biogeographical patterns of speciation using fossils has been a major flaw in methodology.

A stratophenetical approach is important, but in order to critically assess phylogenetic hypotheses,

the geographical scope of a study should also be stated in the context of the paleogeography of the

taxon (see Young, 1984). Climatically-induced shifts in species’ biogeography,which form the

empirical data for many paleoclimatologists, suggests that migration rather than evolutionary origination, accounts for the first stratigraphical appearances of most species in rock sections. It should

therefore be stressed at the outset that comprehensive geographical coverage deserves as much

priority as stratigraphical sampling interval.

Another topic is the question of abiotic. extrinsic factors and their effect on evolution. Widely

divergent schools of evolutionary thinking hold that geographical isolation caused by climatic and

tectonic events plays an important role in speciation. Some models of genetic changes during

speciation postulate extrinsic events to initiate speciation (Carson, 1982). In advocating a cladistic

approach, Eldredge and Cracraft (1980, p. 121) pointed out that isolation by geographical barriers

is a mechanism for the disruption of “within species patterns of parental ancestry” but that isolation is not prima facie evidence for two species. Valentine and Jablonski (1983) discussed modes

of speciation by small founder populations, large vicariant populations. and in clines and offered

hypotheses about which mode might be more common in groups with various reproductive strategies and biogeographies.

Mayr (1982) has maintained that peripatric speciation is the most ccmmon type, but that speciation does not necessarily result from all founder events. However, we still cannot answer the question of which environmental events are more likely to result in speciation, which in extinction, and

which in stasis. Is it true that a smaller population, when isolated, is more likely to “pass through

the bottleneck of deleterious heterozygosity and reorganize itself genetically” (Mayr, 1982, p. 6)?

If so, how can we identify such events in fossils and associate them with documented environmental

changes? If sympatric and parapatric speciation are found to be common, do they support genetic

models of speciation (Templeton, 1980) and relegate geography to a secondary role?

Climatic change represents an obvious candidate to explain evolutionary trends and has frequen-



Geographical Isolation in Marine Ostracoda 873



tly been invoked as such (Cracraft 1982; Stanley, 1984; Valentine, 1984; Vrba, 1985). Indeed, it is

almost always possible for a paleontologist to find a climatic event in the geological record to

“associate” with a faunal or floral diversification or extinction. Yet simple age equivalence of a

faunal and a climatic event does not imply a causal relationship. More rigorous testing of suspected

abiotic influences on evolution is possible and necessary (Cronin, 1985 and below).

A third reason behind these studies is the unrealized potential of Ostracoda for understanding

evolutionary processes. Ostracods have ideal characteristics for evolutionary research - sexual

reproduction, abundantly fossilized carapaces with easily identified homologous features for

morphometric study, and growth through moulting yielding a clear ontogeny (Cronin, 1985; Reyment, 1985). Many problems associated with assumptions of the “paleontologic approach” (see

Eldredge and Novacek, 1985, p. 69,70) are minimized by using ostracods. In addition to distinguishing ontogenetic and sexual dimorphism from other morphological variation, the ostracod carapace affords an opportunity to study the exoskeleton of the whole organism, not just a single part,

such as the vertebrate tooth. Mention should also be given to important new studies, among them

papers on ostracod histology and morphology (Okada, 1982), the relation between ecology, reproduction and morphology (Kamiya, this volume), and various aspects of polymorphism (Reyment, 1985; Abe, 1983; Ikeya and Ueda, this volume), that together boost our confidence that

biological species can be recognized from carapace morphology. In an important paper, Kamiya

(this volume) has demonstrated how distinct carapace shapes in two related extant species of Loxoconcha directly reflect different copulatory positions in different microhabitats. One species

is phytal in habitat, and requires a rounded posterior shape and circular-oval lateral shape to

copulate on the plants; the other species is flattened posteriorly and rectangular laterally to copulate

on the sand bottom. Therefore, not only does carapace shape distinguish two species but it is functionally related to the primary means of distinguishing two species-the criterion of reproductive

isolation.



TESTING

EVOLUTIONARY

HYPOTHESES

USINGFOSSILS

While some authors have emphasized species diversity and overall faunal patterns to test evolutionary laws (Hoffman and Kitchell, 1984), my approach focuses on species and monophyletic

groups for which there is strong evidence for a common ancestor. I agree with Eldredge and Novacek (1985) that cladistic and paleontological methodologies of phylogentic analysis differ from

one another more in style than in substance, and that both are confronted with similar subjective

decisions. In my ostracod studies, relationships among species are known with varying degrees of

confidence that will be stated in each case. While subjectivity and judgement will always play a role,

testing genealogical hypotheses is treated as an iterative process, in which securing new modern

and fossil material confirms or refutes suspected phylogenies.

Three points about the approach should be stressed: [l] As stated explicitly by more and more

evolutionists (Stenseth and Smith, 1984; Vrba, 1984), the fossil record provides a unique, albeit

imperfect, opportunity to test hypotheses about the influence of environment on organisms. Each

study in this series of papers represents an effort to understand the evolutionary history of a

species or monophyletic group of species, selected on the basis of their ecology, biogeography

and exposure to known climatic or tectonic changes. One school of vicariance biogeography has

criticized the role of fossils in biogeography and systematics, stating “recent distributions . . .

provide the only unequivocal data of biogeography” (Patterson, 1981, p. 464). To test vicariance

versus dispersal explanations of why a taxon lives in an area, some propose seeking concordant

biogeographical trends in separate lineages subjected to the same vicariant events (Rosen, 1978).



However, different taxa will not necessarily respond in similar ways to the same extrinsic event

(see Endler, 1982; Young, 1984). Although methodologically sound in trying to falsify hypothesized

genealogies, this vicariant-cladistic approach tends to ignore valuable geologic evidence for

environmental change.

The fossil record itself can be used both to test phylogenetic hypotheses (Young, 1984), and,

more importantly, hypotheses about the causal relationships between organisms and their environments. In one example, I documented a period of speciation in the ostracod Puriunu, which I hypothesized was caused by gradual climatic and oceanographical change along the eastern United

States about 4 to 3 million years ago (mya) (Cronin, 1985). This idea was tested in three ways: 1)

by examining Puriuna’s response to cyclic climatic change, considered by palaeoclimatologists to be

qualitatively distinct from gradual climatic change (no speciation events were found, Cronin, 1985);

2) by examining the entire endemic ostracod fauna off the eastern United States for evolutionary

first appearances during the last 5 million years to see if other genera diversified when Puriana

did (about 50 % out of 127 ostracod species originated during this time, Cronin, in press a); 3) studies

have been started on ostracods from the western north Pacific near Japan where a distinct ostracod

fauna was subjected to similar climatic changes in temperate and subtropical climatic zones to those

in the western North Atlantic. In summary, for some extant marine groups having a fossil record,

paleobiogeography is known as well as modern distributions so that both hypotheses about genealogical relationships and hypotheses about the causal relationship between speciation (vicariant

and dispersal) and extrinsic events can be tested and falsified.

[2] I do not share Schopf’s (1982) pessimism about identifying biological species from fossilized

hard parts. With the exception of sibling species, distinguishing among species of ostracods is

possible as demonstrated by a growing literature (e.g., Cronin, 1985, and below; Reyment,

1982, 1985; Abe, this volume; Ikeya and Tsukagoshi, this volume) provided one has adequate

sample size and geographical coverage. [3] I place emphasis on intraspecific variability, particularly

by comparisons among completely or partially isolated populations. Unless morphological

variability within a species is understood over its geographical range, phylogenetic relationships

among related species cannot be determined.

A wide range of environmental changes will be considered in these studies including: (1) cyclic

and long-term, gradual climatic changes, (2) sea level changes, (3) vicariant geographic isolation

of large populations, (4) founder events isolating small populations, ( 5 ) relict populations and

others. The first set of studies will focus on shallow water marine taxa, and they will hopefully

expand to deep water marine and fresh water forms in the future.

This first study, appropriately published in the Proceedings of the Ninth International Ostracod

Symposium, whose theme was evolutionary biology, examines two major categories of geographical

isolation. First, isolation during the Cenozoic of small populations .of shallow water species on

remote Pacific atolls and islands. In theory, such isolation could lead to peripatric speciation

(sensu Mayr, 1982), which is equivalent to type Ib speciation of Bush (1975). Conversely, classic

dumbbell isolation of large populations by the development the Isthmus of Panama about 3 mya

might theoretically lead to allopatric speciation, type la of Bush (1975). This important distinction between types of allopatry was emphasised by Mayr (1982) because of his hypothesized

inverse relationship between population size and rate of divergence.



Geographical Isolation in Marine Ostracoda 875



GEOGRAPHICAL

ISOLATION

ON OCEANIC

ISLANDS

AND ATOLLS

Benthonic marine ostracods have no planktotrophic larval stage and therefore dispersal of

shallow water species across deep water is almost certainly passive. This situation is similar to that

of a terrestrial organism passively dispersed to small islands, but differs from that in some marine

gastropods having larval dispersal and gene flow among dispersed populations (Scheltema, 1971).

Study of the relationship between Atlantic/Caribbean and Pacific marine ostracods reveals interesting examples of taxa that disperse over long distances and show very little intraspecific variability

in carapace morphology among widely separated populations. However, taxonomic studies of

faunas from different regions have frequently not recognised the strong affinities of isolated populations. This may reflect an unfamiliarity with the literature from other regions or unstated assumptions by taxonomists about the endemic nature of shallow water ostracods. The evidence

presented below shows the conspecific nature of many small, widely dispersed populations of a

tropical species.



Herrnanites transoceanica Teeter, 1975

Teeter (1975) described the extant species Hermanites transoceanica from the Belize carbonate

platform near eastern Central America, and noted its wide distribution in the Pacific off New

Guinea and Hawaii. Holden (1976) identified specimens of the same species from the Pleistocene

of Midway Island as Jugosocythereis luctea (Brady, 1866), and Hartmann (1981) described the new

extant species Quadracythere insulardeaensis from eastern Australia based on specimens that are

also conspecific with Teeter’s species. Bonaduce et al., (1980) described Quudrucythere auricolatu



TABLE

1-LOCALITIESOF Hermanites transoceanica TEETER,1975t



1. Eastern Australia (Hartmann. 1981)

2. Rangiroa Atoll (Tuamoto Archipelago) (Hartmann, 1984)

3. Kwajalein Atoll (Marshall Islands), this paper

4. Okinawa Island (Ryukyu Islands), this paper

5. Enewetak Atoll (Marshall Islands), this paper

6. Rongelap Atoll (Marshall Islands), this paper

7. Ponape, this paper ,

8. Bikini Atoll (Marshall Islands), this paper

9. Guam (Mariana Islands), this paper

10. Guadalcanal (Solomon Islands), this paper

11. Shortland Island (Solomon Islands), (Titterton, 1985)

12. New Guinea, (Teeter, 1975)

13. Belize (Central America), (Teeter, 1975)

14. Bahamas, this paper

15. Virgin Islands (St. Thomas), this paper

16. Vera Cruz (Mexico), (Krutak, 1982)

17. Midway Island (Hawaiian Islands), (Holden, 1976) Pleistocene

18. Taiwan (Hu, 1981) Pliocene-early Pleistocene

19. Enewetak Atoll, Marshall Islands, this paper, Miocene-Pleistocene

20. #Pinaki and Niau, (Tuamoto Islands), this paper

21. #Pago Pago, (Samoa Islands), this paper

22. #Gulf of Aqaba, Red Sea, (Bonaduce et a!., 1980)

t : Numbers refer to map in Text-fig. 1. ## : Not shown in Text-@. 1. All samples are modern except 17,

18 and 19. Locality data-can be obtained from the author.



876 T.M. CRONIN



TEXT-FIG.1-Localities yielding Hermunites trunscoceunicuTeeter, 1975. Fossil samples are indicated by open circles, modem samples by solid circles (See Table 1 for list of localities). Since this paper was first written, this

species has also been identified in samples from Pinaki and Niau, Tuamoto Islands, South Pacific, and from

Pago Pago, American Samoa, from Truk, Majuro and Arno in Micronesia, and in the paper by Bonaduce et

ul., (1980) on ostracodes from the Gulf of Aqaba, Red Sea.



from the Gulf of Aqaba in the Red Sea for a species that is conspecific with H. transoceanicu.

Similarly, Hu (198 1) described the species Rudimella microreticulata from the Maanshan Mudstone

of southern Taiwan, and although citing Teeter (1975), Hu did not identify these Asian forms as

conspecific with H. trunsoceanica, although they are clearly so.

To study H. trunsoceunica in detail, I assembled fossil and modern collections from shallow

carbonate environments in the western equatorial Pacific and Caribbean and, where possible,

examined material from published studies and theses. Selected localities yielding H. trunsoceunica

are shown in Text-fig. 1 and are listed in Table 1. Text-fig. 2 plots carapace length versus height

for measured specimens and plots length and height of holotypes using measurements given by

Teeter (1975) and for Australian material from Hartmann (1981). As suggested by Teeter (1975),

H. transoceanica includes widely dispersed populations from a single, morphologically well-defined

species. The length/height plot shows the range of carapace sizes for fossil specimens (MiocenePleistocene) from the Marshall Islands is about the same as that from recent populations from the

same region. Measurements taken from Holden (1976) for fossils from Midway, and from Hartmann (1984) for modern material from Polynesia show specimens slightly larger, but not outside the

expected range for a species. Sexual dimorphism is weak in this species and the few specimens believed to be males are indicated. Carapace ornamentation shown in Plate 1 reveals a striking degree

of morphological stability over time (at least 6 million years) and geographical area. The fossil record

of this species shows that it lived in the Marshall Islands from the late Miocene until modern times,

on Taiwan during the late Pliocene or early Pleistocene, and on Midway during the Pleistocene.



Geographical Isolation in Marine Ostracoda 811



Hermanites transOceania



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MARSHALL ISLANDS fossil

MARSHALL ISLANDS

POLYNESIA

CARIBBEAN. BAHAMAS

BELIZE. CENTRAL AMERICA

SOLOMON ISLANDS, AUSTRALIA

HAWAIIAN ISLANDS fosd

MARIANA ISLANDS



AUSTRALIA



610



630



LENGTH I MICRONS)

TEXT-FIG.

2-Plot of carapace length versus height of Hermanites transoceanica Teeter, 1975. Specimenspresumed

to be male are indicated with the male symbol.Measurements of the holotype of H. transoceanica from Belize

were taken from Teeter (1975). Hartmann (1981) gives ranges of 500-530 microns and 280-290 microns as

measurements for length and height for Quadracyfhere insufardeaensis from Australia, and the midpoint of

these values was plotted.



The presence of this species in the Carribbean suggests it probably lived in this region before

about 3.0 mya when the formation of the Isthmus of Panama separated Atlantic and Pacific

populations. Specimens from Belize, the Virgin Islands and the Bahamas (Plate 1) cannot be distinguished from Pacific forms. Its presence in the Red Sea, however, leaves the remote possibility of

westward migration across the Atlantic during the last few million years.

Populations of H. trunsoceunicu were compared with two sympatric species, H . purvilobu (Hu,

1981), believecfto be a separate lineage, and H . mooneyi n. sp., the only known descendant species

evolving from H. trunsoceunica. Text-figure 3 plots length versus height for these three speciesshowing the similar dimensions of H. trunsoceunicu and H. mooneyi. However, Plate1 shows H. mooneyi

differing significantly from H. trunsoceunica in the development of surface ridges and reticulation.

The results for H. trunsoceunicu show that at least 10 times during the Neogene, and probably

dozens more, populations became established off remote islands or in isolated atolls, but maintained

morphological stability for several hundred thousand to several million years. Since the Miocene,

only one species has evolved from ancestral H . trunsoceunicu. Although short-lived species may

be as yet undiscovered, the evidence from well-studied fossil faunas from Midway (Holden, 1976),

Enewetak (Cronin, unpublished), Taiwan (Hu, 1984 and references), and Okinawa (Nohara and

Tabuki, 1985) shows no evidence for any. If new species had evolved during the Pliocene or

Pleistocene, one would expect to find living representatives in the many well-known tropical

Pacific faunas.



Geographical Isolation in Marine Ostracoda 819



PACIFIC- CARIBBEAN

HERMANITES



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ti iranscceanrca

ti mooneyi

ti Parvrloba



610



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650



IM~CRONS)



Tw-FIG. 3-Plot



of carapace length and height for three species of Hermanites. The plots of H. transoceanica

are the same as in TEXT-FIG.

2. See text for discussion.



It is appropriate to consider mechanisms to account for the dispersal of H. trunsoceunicu.

This species could not have actively migrated across deep water barriers as it does not have a

planktotrophic larval stage. Dispersal was almost certainly passive and there are several possible

mechanisms which are summarized by Teeter (1 973). The encysted (double-walled) eggs of fresh

water ostracods can withstand dessication and therefore, have probably been dispersed by

migratory birds, either on their feet, trapped in feathers or in the intestinal tract (Sandberg, 1964).

PLATE1-Figs.



1-8. Hermanites transoceanica Teeter, 1975. 1. Lateral view, left valve, female carapace (USNM

401834, Holocene, Guam). x 99. 2. Lateral view, left valve, female (USNM 401835, Holocene, St. Thomas,

Virgin Islands). x 99. 3. Lateral view, right valve, female (USNM 401836, Holocene, Rongelap Atoll, Marshall

Islands). x99. 4. Lateral view, left valve, female carapace (USNM 401837, Holocene, Acklins Island,

Bahamas). x99. 5. Lateral view, left valve, female carapace (USNM 401838, Holocene, Enewetak Atoll,

Marshall Islands). x99. 6. Lateral view, left valve, female (USNM 401839, Holocene, Bikini Atoll, Marshall

Islands). x99. 7. Lateral view, left valve, female (USNM 401840, late Pliocene, Enewetak Atoll, Marshall

Islands). x99. 8. Lateral view, left valve, female carapace (USNM 401841, middle to late Miocene,

Enewetak Atoll, Marshall Islands). X 99.

Fig. 9. Hermanites parviloba (Hu,1981). Lateral view, left valve, female (USNM 401842, early Pleistocene,

Enewetak Atoll, Marshall Islands). x 99.

Figs. 10-12-Hermunites mooneyi Cronin n. sp. 10. Lateral view, left valve,female (Holotype, USNM 401843,

Pliocene, Enewetak Atoll, Marshall Islands). x 99. 11. Lateral view, right valve, female carapace (Paratype,

USNM 401844, Pliocene, Enewetak Atoll, Marshall Islands). x 99. 12. Lateral view, left valve, ?male

(Paratype, USNM 401845, late Pleistocene, Enewetak Atoll, Marshall Islands). X 99.



880



T.M. CRONIN



Teeter (1973) considers avian transport unlikely for marine species because (1) eggs of marine

species are single-walled and are not known to withstand dessication; (2) many species live in the

subtidal zone and have a very small probability of contact with birds; (3) those tropical and

subtropical ostracod species that are geographically widespread are latitudinally restricted over

broad oceanic areas that are not related to north/south continental pathways of migratory birds.

Teeter (1973) similarly dismisses wind and fish as agents for widespread dispersal because of the

vulnerability of the single-walled egg to destruction.

The possibility that transoceanic shipping is the cause of widespread distributions of some

species has been discussed (Teeter, 1973), but an increasing number of species now have fossil

records back to the Miocene and Pliocene showing they were widely dispersed millions of years ago.

Teeter considers drifting on aquatic plants the most probable means of dispersal and notes the

occurrence of living ostracods on the marine alga Turbinaria, which lives near tropical coasts, and

on Sargassum near the Florida Keys. Sohn (1954) also reported benthonic marine ostracods living

on Sargassum that had drifted near Cape Cod, Massachusetts. Surface water currents therefore

could easily carry living ostracods on algae to new habitats on atolls and tropical islands. Another

passive mechanism is transport on drifting pumice rafts (Newton and Bottjer, 1985), but there are

no data on ostracods living on floating pumice. At present it is unclear how frequent these dispersal

events were and whether they were frequent enough to postulate continued, albeit occasional,

genetic interchange among seemingly isolated populations. In my opinioil, the probabilities are low

that significent genetic interchange occurred between Pacific atoll populations over the millions of

years of the species history.

In summary, if the taxonomic literature from the last decade were taken at face value, populations of H . transoceanica would be recorded as five species in four separate genera, Radimella,

Jugosocythereis, Hermanites, and Quadracythere. Actually, intraspecific stasis, both in the sense of

species integrity over geographic range (Van Valen, 1982) and in terms of temporal stability (Eldredege and Gould, 1972) have characterised this single species despite many opportunities thought

to be conducive to lineage splitting.



VICARIANT

ISOLATION

BY THE ISTHMUS

OF PANAMA

During the middle Pliocene 4 to 3 mya, the Isthmus of Panama formed as a barrier preventing

interchange between Pacific and Atlantic/Caribbean marine faunas (Keigwin, 1978; Quinn and

Cronin, 1984). This event represents a classic dumbbell allopatric split of large populations of

tropical species. Indeed, Vrba (1985) has cited it as a potential natural experiment for testing the

punctuated equilibrium model and Valentine and Jablonski (1983). pointed out that identifying

gradual patterns of change resulting from vicariant events such as the Isthmus would strengthen

evidence for punctuational patterns found elsewhere.

Ostracods from the Caribbean and western Atlantic are better known than are eastern Pacific

faunas, but strong affinites between populations from the two sides have long been known. In a preliminary report, Swain et al. (1964) identified Orionina vaughani (an “Atlantic” species) from the

Gulf of California, but later Swain (1967) renamed it 0. pseudovaughani for Pacific populations.

In a remarkable coincidence, Pokorny (1970) and Hazel (1977) independently identified a unique

species of Caudites having asymmetrical ornamentation of left and right valves and both called it

asymmetricus (Hazel’s name was nomen nudem). Hazel’s material was from the Pleistocene of

South Carolina, Pokorny’s was Holocene from the Galapagos. Subsequently, Hazel (1983) formally

described the Atlantic form as C. paraasymmetricus citing slight differences in the location of the

caudal process and position of the carinae. In ostracod faunas from the Clipperton Islands (Allison



Geographical Isolation in Marine Ostracoda 881



and Holden, 1971) and the Galapagos (Bate et al., 1981) similarities with the Caribbean have also

been noted for species of Paracytheridea, Neocaudites, Cytherelloidea, and Caudites. Carreiio (1985)

described Miocene-Pliocene ostracods from Maria Madre Island, Mexico and also noted strong

affinites in species of Loxocorniculum, Cativella, Puriana, and Triebelina.

In the majority of cases, while acknowledging morphological affinities between Pacific and

Atlantic populations, most authors either described new species based on relatively minor differences or postulated possible post-Isthmus faunal interchange to explain the similarities. I call

this the “pacifica-atlantica” tendency and suspect it reflects a strong bias by many that geographical

isolation should lead to allopatric speciation. There seems to be a reluctance to call populations

from opposite sides conspecific. Yet on morphological grounds alone, splitter and lumper alike

would not hesitate to call two populations conspecific if they were not separated by a land barrier

and thousands of miles.

Resolving questions of species identification and rates of morphological divergences are extremely important. Consequently, to test whether many Pacific and Atlantic/Caribbean populations

are really conspecific, I collected or obtained from colleagues Neogene material from numerous

formations in southern California and Baja, Mexico to compare pre- and post-Isthmus populations of genera found on both sides of the Isthmus. In this way, the monophyletic relationships of

descendant populations for each lineage can been demonstrated by establishing that a single contiguous (presumed “panmictic”) population existed before the Isthmus formed. New Pacific material includes the Imperial Formation (Quinn and Cronin, 1984), a unit long known for containing

mollusc and other macroinvertebrates with “Caribbean” affinities, the Santa Barbara and Pic0

Formations (Cronin et al., 1983), and in Baja, the Loreto, Tirabuzon (Carreiio, 1981), Salada,

Ysidro, Boleo, and San Ignacio Formations. Material from 10 Caribbean formations and at least

20 more formations from the southeastern U.S. was examined. The following paragraphs briefly

summarize the results for key species whose populations were split by the Isthmus, but which in

most cases shown negligible morphological change during the last 3 mya.

Cativella navis Coryell and Fields, 1937: This species is known from late Miocene to Holocene

deposits in numerous localities in the Caribbean, Central America, Atlantic Coastal Plain and

Baja, Mexico (Bold, 1974). In addition to many published occurrences, Cativella navis was found

in the Tirabuzon and Loreto Formations and modern sediments off La Paz (Baja, Mexico), the

Gatun Formation (Panama), the subsurface Pliocene of south Florida, the “Ecphora” zone of the

Jackson Bluff Formation (Florida), the Bowden Formation (Jamaica) and the Raysor Formation

(South Carolina). It is characterised by its small size three longitudinal ridges, the ventral one

sometimes having perforations. The medial ridge sometimes consists of a row of tubercles and can

be slightly curved or straight (Pl. 2, fig. 8). This species is easily distinguished from C. pulleyi

Teeter, 1975 and C. semitranslucens Couch, 1949 by its size, shape and ornamentation.

From examination of intra- and inter-population variability in Atlantic, Caribbean and Pacific

forms, I consider the following forms conspecific with C. navis: C . dispar Hartmann, 1959; C . semitranslucens and C. unitaria of Carreiio (1985); C. sp. A of Valentine (1976); C . dispar and Costa?

variabilocostata seminuda of Swain (1967, P1. 3, figs. 2a,b, 14). Plate 2 illustrates 5 specimens that

show the similarity of widely distributed populations. Cativella navis therefore represents a cohesive

group of populations showing very slight morphological variation among them. Before the Ithsmus

of Panama formed, and during its formation, early-middle Pliocene populations lived in Baja,

Mexico (Loreto, Tirabuzon Formations) and the Tres Marias Islands on the Pacific side, in Central

America (Gatun Formation), and in the Atlantic/Caribbean/Gulf of Mexico (Bowden, Conception,

Aquequexquite Formations, Pinecrest Member of the Tamiami Formation, and “Ecphora” zone of

the Jackson Bluff Formation). Today, modern populations are still known from the Pacific in the

Gulf of California, off the west coast of Baja, and off El Salvador, and in the Caribbean (see



882



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Chapter 61. Geographical isolation in marine species: Evolution and speciation in Ostracoda, I

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