814 Biogeography and Evolution

Northwestern, Cordillera Central, and Atlantic Lowlandareas:22,13, and 13%, respectively. The Southwestern areahas a lower but essentially equal value, 10%. The absenceof endemics from the two Meseta Central faunal areas sug-gests that their recognition as discrete units is suspect.

GENERAL PATTERNSThe analysis above indicates that three general patterns ofdistribution reflect different histories for groups in the fau-nal areas. It should not be surprising that the autochtho-nous Middle American Element and Central American Com-ponent are major contributors to most of these faunas.The robust representation of South American Element taxaemphasizes the rote of the isthmian region as an area offaidy recent, geologically speaking, mixing of tropicalMiddle American and South American biotas.

The three patterns are as follows:1. Faunal areas in which a combination of Middle Amer-

ican and South American elements make up 65% ormore of the herpetofauna (SW, A)

II. Faunal areas in which a combination of Middle Ameri-can Element and Central American Component taxamake up 65% or more of their composition, withMiddle American genera predominating (PS, AS, CC)

III. A faunal area where Middle American Element andCentral American component genera are nearly equalin contribution (NW)

The areas constituting the Meseta Central Occidental andMeseta Central Oriental are anomalous as the only puta-tive areas in which Central American genera (40:30%)substantially outnumber Middle American (30:28%) ones.They also resemble the NW area in having relatively lownumbers of South American Element representatives.

The Cordillera de Talamanca area is unique and ambigu-ous as to placement, since it could be referred to pattern 1(MA = 43% + SA = 28% = 71%) or pattern II ( MA =43% + CA = 23% = 66%).

The degree of species endemism within these areas shedsfurther light an the Situation. The high degree of endemismcombined with its unique combination of taxa strongly sug-gests a long-term isolation of the NW fauna from the oth-ers. The relatively high level of endemism in SW, A, and CTfaunal areas seems to indicate separate histories within asingle pattern (1).

Within pattern II, there appears to be no basis for sepa-rating out PS and AS as distinctive faunal areas, since theyare similar in generic composition and proportional repre-sentation of historical units. The Cordillera Central is notsupported as a separate faunal area but appears to be atone end of a gradient in historical unit contributions from

Figure 15.7. Faunal areas for the Costa Rican herpetofauna.

PS + AS -+ MOC - MORC - CC. If PS + AS + CC arecombined the level of species endemism is 15%, similar tothat for other recognizable faunal areas.

The Meseta Central Occidental (MOC) and Meseta Cen-tral Oriental (MOR) have no endemic species and mostclosely resemble NW in generic composition. As Scott (1969)pointed out long ago, they appear to form a geographic tran-sitional area between patterns III (MA = 34% + CA = 36%= 70%) and II, that is, the NW and upland areas. TheMeseta Central Occidental more closely resembles theNorthwest region in proportions of historical contributions( MA = 30% + CA = 40% = 70%) and the Meseta Cen-tral Oriental (MA = 28% + CA = 38% = 66%) the up-lands (PS = AS + CC): an balance both Meseta faunas arebest regarded as part of the Upland Fauna because of thegreater similarity in generic composition compared withthat of the Northwestern region.

In summary, the following discrete recognizable faunalareas (fig. 15.7) appear to have had separate histories andare the biogeographic areas whose history will be traced inchapter 16:Lowland

Pacific Northwest (NW)Southwest (SW)Atlantic (A)

Upland/HighlandMontane Slopes and Cordillera Central (SCC)

HighlandCordillera de Talamanca (CT)

16

PALEOGEOGRAPHIC BACKGROUNDMobile Plates, Blocks, and Island ArcsAmong the most exciting scientific discoveries of the sec-ond half of the nineteenth century was the realization thatthe earth's outer layer consists of a number of rigid, mobileplates riding an a deeper elastic, nearly liquid plastic layer.The plates underlying the oceans are about 65 km deep,and those making up the continents are as deep as 140 km( box 16.1). The uppermost portion of the plates forms theearth's rocklike crust. Oceanic crust is very dense, highlymagnetized, and relatively thin, about 5 km thick. Conti-nental crust is much lighter, less magnetized, and muchthicker than oceanic crust-about 35 to 40 km thick. Thetwo kinds of crust are also composed of different kinds ofrocks. In the course of geologic history continental and oce-anic crust have maintained their integrity, and interactionsbetween the various plates at their boundaries producedmany of the most prominent features of the earth's geogra-phy. In addition, the plates have not remained static in posi-tion through time; in the Permian what we now recognize ascontinents formed a single continent, Pangaea, whose con-stituent plates have separated and drifted apart over the in-tervening 225 million years.

The geography of Central America is the result of a com-plex geologic development over the past 75 million yearsinvolving the interactions of five of these mobile tectonicplates (fig. 16.2). The current structure of the Land portionsof the area consists of four primary crustal blocks:1. Mayan block: mostly continental crust with its southern

border at the Motagua fault system in Guatemala

815

Development of the Herpetofauna

C ENTRAL AMERICA as aland-positive region has a Jong and complex geologic andclimatological history spanning some 90 million years fromthe Tate Cretaceous to the present (fig. 16.1). To explain theevolution of the Costa Rican herpetofauna it is first neces-sary to describe something of this broader history in orderto establish the context. The following paragraphs highlightthe most important events in earth's history that shaped theregion and in turn are responsible for the patterns of distri-bution described in chapters 14 and 15.

2. Chortis block: mostly continental crust with its southernboundary at the Santa Elena fault in northern Costa Rica

3. Chorotega block: accretionary crust, with its southeast-ern border at the Gattin fault

4. Chocö block: accretionary crust with its boundary withthe South American plate at the Romerol fault

The Mayan, Chortis, and Chorotega blocks are bordered anthe Pacific margins by the Middle American trench, whereoceanic crust is being subducted under the lighter continen-tal and accretionary crust. The Chocö block is similarly bor-dered an the Pacific by the Colombia trench subduction zone( box 16.2). It should be noted that before about 25 millionyears ago (Ma) the Cocos and Nazca plates were part of theFarallon plate (Atwater 1989).

The following paragraphs and accompanying figures( figs. 16.3 to 16.6) present a summary of the major geologicevents affecting the origins of the extant herpetofauna, basedprimarily an a synthesis of data from Coates and Obando( 1996), Donnelly et al. (1990), Escalante (1990), Frisch,Meschede, and Sick (1992), Kerr and Iturralde (1999), Lu-cas (1986), Mann (1995), Marshall and Sempre (1993), Pin-dell and Barrett (1990), Rage (1978,1981,1986,1995), andSavage (1982).

The initial fragmentation of the supercontinent Pangaeainto a northern land mass, Laurasia, and a southern one,Gondwanaland, was essentially completed by the middleof the Jurassic epoch, about 160 Ma (Barron et al. 1981).

Box 16.1Principal Types of Crusts FormingUpper Layers of Earth's SurfaceOceanic: formed mainly at midoceanic ridges and

hot SpotsContinental: formed by mantle differentiation, mag-

matism, sedimentation, and metamorphism;mostly ancient Precambrian crystalline rock

Accretionary: formed from a mixture of rock typesprincipally at a subduction zones

mWLm

Figure 16.2. Tectonic features of Mesoamerica referred to in the text.Crustal blocks: Chocö Chor otega, Chorus, Maya. Faults (F): R =Romeral, SC = South Caribbean. Fracture zones (FZ): G = Gatün,M = Motagua, SE = Santa Elena. Tectonic plates (PL): CAR = Ca-ribbean, NA = North American, SA = South American. Trenches (T):C = Colombian, MA = Middle American; GI = Galäpagos Islands,HE = Hess Escarpment, EPR = East Pacific Rise.

By 140 Ma the southern continent began to fragment, andby 80 Ma South America had become fully separated fromAfrica by seafloor spreading along the Mid-Atlantic ridge.Thus South America was completely isolated from otherLandmasses (fig. 16.3). This series of events doubtless set thestage for the origin of the extant families of amphibians andreptiles and was responsible for their association, blurredsomewhat by later dispersals, with either Laurasia or SouthAmerica.

During this time North and South America were sepa-raten by a wide proto-Caribbean seaway created by seafloorspreading of the Atlantic system (Pindell and Barrett 1990).This gap, however, was ultimately replaced by an accre-tionary land bridge that developed from activity along thesouthwestern margin of the Caribbean plate. As the platemoved generally northeastward by the late Cretaceous, anancient Isthmian Link came to connect the two continents( fig. 16.4). This isthmus (the Proto-Antilles) was composedof rock that ultimately would fragment into the GreaterAntilles.

At this time the Maya block was in its present positionas the principal component of what would become easternMexico and northern nuclear Central America, and theChortis block lay well to the west. Far to the southwest werea series of volcanic islands that would later coalesce into theChorotega and Choc6 blocks, forming by subduction of theFarallon plate under the Caribbean plate.

By the end of the Paleocene the Land bridge between Northand South America had undergone fragmentation, and someparts were submerged as the Caribbean plate continued its

Development of the Herpetofauna 817

Box 16.2Important Tectonic Features ofthe Central American Region

Plate margins are the areas of greatest tectonic activ-ity, marked by extensive volcanism, numerous earth-quakes, and faults and mountain building.

Seafloor spreading: occurs where plates are diverg-ing; magma rises at a midoceanic ridge through arift in the crust and spreads equally and slowly atright angles an both sides of the rift to harden andform new oceanic crust; for example, the EastPacific Rise and the boundary between the Cocosand Nazca plates

Subduction: occurs where plates are converging anone another and the denser oceanic crest subductsunder the continental crust an the opposing plateto form a deep ocean trench; for example, theMiddle American and Colombian trenches

Transform faults: occur at right angles to midoceanicridges as plates grind past one another in fits andstarts along plate margins to reduce strain; for ex-ample, faults across the East Pacific Rise

Hot spots: occur where magma erupts through theoceanic crust to produce strings of volcanoes as aplate moves over the hot spot; for example, theGaläpagos Islands.

northeasterly journey (Marshall and Sempere 1993). Theseevents eliminated any terrestrial connection between thelandmasses and completely isolated the faunas of North andSouth America during most of the Cenozoic. By the middleEocene the Chortis block and the lower Central Americanisland arc were also moving eastward, and the former be-came sutured to the Maya block by the end of that epoch,about 38 Ma (fig. 16.5).

The Oligocene and early Miocene saw the Chorotega andChoc6 block volcanic islands moving into the now nar-rowed oceanic gap between the Chortis block and northernSouth America (fig. 16.6). These islands became more ex-tensive as subduction under the now bordering Caribbeanplate added additional volcanoes to the arc. This narrow-ing gap would be successively closed from the late Mioceneto the Tate Pliocene (fig. 16.7) by the increasing uplift ofthe area through subduction of the Cocos ridge under theChorotega block and the collision of the Choc6 block withSouth America (fig. 16.8).

Complete closure of the Isthmian Portal between the Pa-cific Ocean and the Caribbean Sea was effected in the middle

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Figure 16.4. Mesoamerican region in the Tate Cretaceous-early Paleocene. Plates (PL): CAR = Caribbean,FAR = Farallon, NA = North American, SA = South American. Location of Chortis and Maya blocks.CRP = magmatic arc forming part of present-day Costa Rica and Panama. Dotted line equals approximatehoundary hetween NAPL and SAPL.

Figure 16.5. Mesoamerican region in the Eocene. Abbreviations as in figure 16.4.

Figure 16.6. Mesoamerican region in the early Miocene. Abbreviations as in figure 16.4.

SAPL

820 Biogeography and Evolution

aFigure 16.7. Isthmian Link area in the middle (a) and late (b) Miocene.

Figure 16.8. lsthmian Link area in the Pliocene. Chorotega and Chocd blocks sutured at Gatün fault (GF);principal marine barriers indicated.

Pliocene, 3.4 to 3.1 Ma (Coates and Obando 1996). At thistime a continuous land corridor was established between theChortis block region and South America. However, otherevidence (Cronin and Dowsett 1996) indicates that the clo-sure was temporary and that some exchange between At-lantic and Pacific marine faunas occurred between 2.8 and2.5 Ma. This would explain the apparent two pulses of dis-persal of northern terrestrial forms into South America andSouth American forms into lower Central America in latePliocene and Pleistocene times as discussed below.

b

Uplands, Mountains, and Volcanoes

The Mesoamerican region is characterized by a complex se-ries of elevated areas. Many of these support endemic taxain their herpetofaunas. They are also often invoked as dis-persal routes for various faunal components. Thus the dis-tribution and timing of uplift and possible previous con-nections of these highlands with other areas are basic tounderstanding current faunal distribution patterns (Cserna1989; Ferrusqufa 1993; Ortega, Sedlock, and Speed 1994

Development of the Herpetofauna 821

Figure 16.9. Upland areas of tropical Mexico, east of the Isthmus of Tehuantepec: SMOG = Sierra MadreOccidental, SMOR = Sierra Madre Oriental, SMS = Sierra Madre del Sur, TV = Transverse Volcanic Range.

for Mexico; Weyl, 1980 for Central America) (figs.16.9 and16.10).

Principal Upland AreasSierra Madre Oriental (Mexico): paralleling the Gulf coast,

originally uplifted during the Laramide orogeny under in-fluence of the now extinct Farallon-North American sub-duction tone; minimal Cenozoic volcanism; most recentuplift Miocene-Pliocene

Sierra Madre del Sur (Mexico): originally formed in theearly Paleogene and probably related to subduction in theextinct Farallon-North American subduction tone; min-imal Cenozoic volcanism; most recent uplift in the Mio-cene-Pliocene

North Central American Sierras: a series of subparallelranges forming an arc open to the north extending fromChiapas, Mexico, across Guatemala and Honduras tonorthern Nicaragua; probably originally formed duringsuturing of the Chortis block to the Mayan block and

transfer of the former to the Caribbean plate; minimalCenozoic volcanism; Pliocene uplift.

Tertiary VolcanicsSierra Madre Occidental (Mexico): originally formed above

former Farallon-North American plate subduction zone;extensive Tertiary volcanism; Miocene uplift

Central American: lying eastward of the Quaternary vol-canic chain from Guatemala to southern Nicaragua; re-lated to movements of the Chortis block; extensive vol-canism from Miocene through Pliocene; Pliocene uplift

Costa Rica-Panama: mountains from central Costa Ricato eastern Panama originally formed as an island arc atCocos-Caribbean subduction tone; extensive volcanismthrough Miocene; Miocene uplift

Quaternary VolcanicsPacific Volcanic Chain: from Mexico-Guatemala border

along Pacific versant to central Costa Rica; formed by

822 Biogeography and Evolution

Figure 1 6.10. Upland areas of Central America: CPC = Costa Rica-Panama cordilleras, NCAS = NorthernCentral American sierras, PVC = Pacific volcanic chain. The Central American Quaternary volcanic chainlies generally to the east of the PVC and partially underlies it.

subduction at Middle American trench; many still activevolcanoes often sitting an top of earlier volcanic ranges;uplift continuing today

Transverse Volcanic Belt (Mexico): formed by subduction ofCocos plate under North American plate; many still ac-tive volcanoes; uplift continuing today

PALEOCLIMATES AND VEGETATIONIn addition to drifting landmasses and the uplift of moun-tain masses, the current distribution of the herpetofaunashas been shaped by changing climatic factors through theCenozoic. Estimates of past climates are based principallyan analyses of oxygen and carbon isotope composition, fos-sil faunal (especially marine) assemblages, fossil pollen pro-files, and fossil plant assemblages (Frakes, Francis, and Syk-tus 1992; Graham 1994, 1996, 1997, 1999a,b,c; Burnhamand Graham 1999). Terrestrial angiosperm foliar physiog-nomy reflects to a substantial extent climatic conditions, sofossil floras are invaluable indicators of climate, regardless

of taxonomic composition, and palynofioras often providethe only estimates of past climate as reflected by vegetation.

In this section 1 will briefly review the major climatic shiftsthat have affected the herpetofauna and describe in broadstrokes the changes in vegetation that occurred in our areaof study. Frakes, Francis, and Syktus (1992) recognized twoclimate modes, periods during which similar climates pre-vail, as characteristic of Phanerozoic times. Cool modes areperiods of global refrigeration, ranging from those in whichglaciation occurs to those where ice is formed at high lati-tudes only during the winter. Warm modes are periods whenclimates are globally warm with little or no polar ice. LateCretaceous and Paleocene times were among the very warm-est and most humid of times in earth's history (fig. 16.11).The polar regions were free of ice, with temperatures warmenough to allow forests and associated reptile and amphib-ian faunas to live there. Mean global temperatures were 6°Chigher than today. Sea levels were high in the Tate Cretaceous,about 200 m above current levels, and large areas of the con-tinents were covered by extensive seaways (Vail, Mitchum,

Eustatic Curve5

- m Climate

Figure 16.11. General pattern of climatic and sea level fluctuations,Eocene to present.

and Thompson 1977). By the Paleocene the seaways hadgradually receded. Vegetation over most of what is now theAmericas consisted of tropical and subtropical broadleafevergreen forests (rainforest). Temperate forests includingbroadleaf evergreen and deciduous formations grew sur-rounding the poles. The tropical assemblage and the northtemperate one have been recognized as the Neotropical Ter-tiary and Arcto-Tertiary geofloras, respectively (Axelrod1958; Axelrod and Ting 1960).

Development of the Herpetofauna 823

By the time the Proto-Antillean Isthmus began to breakup in the Tate Paleocene, tropical and subtropical evergreenforests ranged north to 60 to 65°, temperate broadleaf for-ests to 70° , and a previously minor component of the Arcto-Tertiary geofiora, conifer forest, began developing aroundthe pole (fig. 16.12a).

The onset of the overall cooling trend for the rest of theCenozoic began in the Eocene at about 50 to 55 Ma. Thisearly cool period saw displacement of the vegetational beltssouthward, and a number of temperate forests componentsranged even farther southward along the developing uplandsof western North America and eastern Mexico. At lower ele-vations a transitional zone appears to have been present be-tween subtropical evergreen and temperate evergreen forestsat about 50'N (fig. 16.12b). Although there were later warmmodes in the Cenozoic with some shifting north and southof climate-vegetation zones, tropical and subtropical vege-tation became displaced farther and farther south through-out the rest of the era (fig. 16.12c) (Wolfe 1985).

Superimposed an the cooling trend was a drying trendthat was associated with the mountain building of the west-ern North American cordilleras and the increasing conti-nentalization of climate. The drying trend is already notice-able in the Oligocene, since broadleaf deciduous forestsdominated eastern North America and subtropical decidu-ous vegetation covered most of what is now the southwest-ern United States and adjacent Mexico (fig. 16.13a). The lat-ter vegetation corresponds to the Madro-Tertiary geofloraof Axelrod (1958).

The early Miocene (fig. 16.13b) was a period of warmingand increasing aridity. This coupled with uplift of the SierraMadres in Mexico completed the isolation of tropical Meso-america and its highlands from biotas to the north by the de-velopment of semiarid to desert vegetation across Americawest of the Mississippi and south through central Mexico.The Tate Miocene and earliest Pliocene (6 to 4.8 Ma) formeda period of severe cyclic glaciations in which average annualtemperatures in Middle America were depressed as much as6°C below present ones. Continuing drying trends, however,prevented any substantial movement southward of temper-ate forests, so that they came to be compressed between thetaiga and scrubland-grassland belt (fig. 16.14).

After a brief interval of warming and a rise in sea level,the rest of Pliocene, from the initial closure of the Pana-manian seaway (3.1 Ma) throughout the Quaternary, sawrepeated cooling and warming episodes as polar glacierswaxed and waned. Interglacial periods in tropical Meso-america would have climate and vegetation patterns similarto those at present. Glacial periods saw temperature depres-sions of 5 to 6°C so that upland vegetation was displaceddownward and mixed with lowland communities. Contraryto earlier authors (Clapperton 1993; Haffer 1969; Webb1985), in lower Central America and the Amazon at least,

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aFigure 16.12. General distribution for forest vegetation in the early Paleocene to middle Eocene: CF =conifer forest, STF = subtropical rainforest, TDF = tropical deciduous forest, TF = tropical rainforest,Temp F = temperate forest, Temp RF = temperate rainforest.

a

b

b

c

Figure 16.13. General distribution of vegetation, Oligocene to Miocene. Abbreviations as in figure 16.12.Temp DF = temperate deciduous forest. Subtropical vegetation, not shown because of scale, forms transi-tion between tropical and temperate formations. MT = Madro-Tertiary woodland, chaparral, and scrubvegetation.

Figure 16.14. Neogene trends in vegetation produced by increasedcooling and drying and continentalization of climates. Abbreviationsas in figures 16.12 and 16.13.

Development of the Herpetofauna 825

interglacial intervals were not periods of increased dryness( Colinvaux 1993, 1996). 1 will return to this matter later inthe discussion of the biogeographic patterns for montaneamphibians and reptiles.

Cooling DISPERSALS, VICARIANCE, AND FAUNALASSEMBLAGE: THE BIG PICTUREThe history of the Tropical Mesoamerican herpetofauna in-cludes a series of concordant dispersal events interspersedwith periods of fragmentation of formerly continuous rangesfollowed by diversification within the now separate areas( vicariance). The earliest major vicariance to affect the pres-ent-day fauna was the gradual fragmentation of Pangaeainto northern (Laurasia) and southern (Gondwanaland)landmasses in the middle Jurassic (about 160 Ma). Duringthis period of separation all extant families of amphibiansand reptiles probably evolved as further fragmentation ledto the present pattern of continental units (table 16.1). Forthis discussion, that initial separation is called vicariance V o .

Sometime in the late Cretaceous the accretionary islandarc described above came to lie between North and SouthAmerica and through uplift formed an isthmian connectionbetween the two continents (the Proto-Antilles). This con-nection lasted 5 to 10 million years and mediated a major

Table 16.1. Historical Sources for Suprageneric Groups of the Mesoamerican HerpetofaunaGroup Laurasian Pangaean GondwananCaecillians CaeciliidaeSalamanders PlethodontidaeFrogs and Toads Rhinophrynidae Pipidae

Ranidae BufonidaeLeptodactylidaeHylidae.CentrolenidaeDendrobatidaeMicrohylidae

Lizards Phrynosomatidae CorytophanidaeEublepharidae IguanidaeGekkoninae (pt.) HoplocercidaeXantusiidae PolychrotidaeScincidae (pt.) SphaerodactylinaeDibamidae GymnophthalmidaeTeiidae Gekkoninae (pt.)Bipedidae Scincidae (pt.)Anguidae AmphisbaenidaeXenosauridaeHelodermatidae

Snakes Loxocemidae Typhlopidae AnomalepididaeUngaliophiidae Leptotyphlopidae BoidaeColubridae (pt.) TropidophiidaeElapidae Colubridae (pt.)Viperidae

Turtles KinosternidaeDermatemydidaeChelydridaeEmydidae

Crocodilians Crocodylidae (pt.) Crocodylidae (pt.)

826 Biogeography and Evolution

dispersal event (D 1 ) with apparent extensive faunal exchangebetween the two continents. All evidence points to an ancientcontinuity and essential similarity of a generalized tropi-cal herpetofauna that ranged over tropical North, Middle,and South America in the Tate Cretaceous-Paleocene times.Descendants of this assemblage are represented today bythe South and Middle American Elements defined in chap-ter 15. These elements are associated with the Neotropical-Tertiary geoflora defined above, which at this time rangedfar to the north (70°), and its derivatives throughout theCenozoic. To the north of this fauna ranged a temperate,Laurasian-derived unit associated with the Arcto-Tertiarygeoflora, now represented by Old Northern Element her-petofaunal taxa.

By the end of the Paleocene the Proto-Antillean Isthmusbecame fragmented as the Caribbean plate moved north-eastward to separate the continents and isolated northernand southern fragments of the generalized tropical herpeto-fauna in North and South America to constitute a secondmajor vicariance event (V 1 ). Differentiation in situ withinthe two fragments during the next 54 million years cre-ated the distinctive tropical elements of the two herpeto-faunas that became intermixed with the establishment ofthe new Isthmian Link in the Pliocene. Phylogenetic analy-sis of several groups of xenodontine snakes (Cadle 1985),the coral snakes of the genus Micrurus ( Slowinski 1991,1995), the Trogs of the genus Eleutherodactylus ( Lynch1986; Savage 1987), and the anole lizards (Guyer and Sav-age 1992) strongly support the significance of this vicari-ance (fig. 16.15).

In the Eocene, probably related to the Eocene cool pe-

South American

CentralAmericanClade

LAnt

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South AmedcanClade

Ant

SA

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SouthAmerican

CentralAmedcanCladeClade

L

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b

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Central American

South America

Antilles

Central AmericaDactyloa

Other Anoles

Norops

d

Figure 16.15. Phylogenetic relationships reflecting separation ofNorth and South America in the early Paleogene: (a) xenodontinesnakes; (b) venomous coral snakes (genus Micrurus): C = collarisgroup, F = fulvus group, L = lemniscatus group, M = mipartitus

group; (c) frogs of the genus Eleutherodactylus; ( d) anole lizards.Ant = Antilles, G = Greater, L = Lesser.

Europa!Western North Western North Mesoamerica!

America

America

SouthAmedca

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äz m

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Figure 16.16. Phylogenetic relationships: (a) bolitoglossine salaman-ders illustrating incorporation of one lineage (BOL = Bolitoglossaplus twelve genera) into the Central American biogeographic compo-nent (after Wake and Larson 1987); (b) Mesoamerican batugarineturtles of the genus Rhinoclemmys showing vicariance event V 3 .

riod and emerging uplands in western North America andMexico, a substantial number of Old Northern groups be-came integrated with tropical taxa in Mexico (dispersalevent

D2). Most prominent among these organisms wereplethodontid Salamanders and colubrine snakes, but severalfreshwater turtles and a variety of lizards are also repre-sented (table 15.9). As the former continuity between the re-gion and what is now the eastern and far western UnitedStates was affected by mountain building and the subsequentcooling and drying trends for the rest of the Cenozoic, thesecomponents became disjunct in Mexico (Axelrod 1975;Graham 1997). This disjunction (Rosen 1978) allowed dif-ferentiation of the Central American Component of the OldNorthern Element (fig. 16.16a), which from the Oligoceneonward evolved in association with the Middle AmericanElement.

Thus the initial organization of what was to become theMesoamerica herpetofauna involved a pair of vicarianceevents: complete geographic isolation from South Americaand fragmentation and isolation of the Central AmericanComponent from its northern congeners, by a combinationof physiographic and climatic factors. By the Oligocene,most of the genera or their ancestors, which now form theOld Northern and Middle American Elements (table 15.9),were present in the region.

A momentous physiographic development, the uplift ofthe main mountain axis of Mexico and Central America,created one major dispersal event (D 3 ) and two importantadditional vicariance events. This process seems to have hada north-to-south sequence, with the Sierra Madres of Mex-ico present as upland areas beginning in the Oligocene, andthe highlands of Nuclear Central America developing in theMiocene. The final sequence of uplift was in lower CentralAmerica leading to the closure of the Panamanian Portal

in the Pliocene. As the uplift proceeded, Middle AmericanElement and Central American Component taxa dispersedsouthward (D 3 ) over the emerging landmass. A primary vic-ariance effect of the uplift was to gradually fragment whatwas a rather homogeneous Mesoamerican herpetofaunainto several geographic assemblages, most notably in thelowlands, where the orogenic effects an the northeast tradewinds produced a partial rain shadow along the Pacific ver-sant from Sinaloa, Mexico, to Costa Rica. This process ledto the replacement of humid conditions and evergreen veg-etation by subhumid to semiarid climates and deciduousand thorn forest formations. As pointed out in chapter 14,many species and most genera of lowland groups in CentralAmerica are found an both the Pacific and Caribbean coastalstrips. Duellman (1966b, 1988) and I (Savage 1966a, 1982)also emphasized the relative homogeneity of the herpeto-fauna an each lowland versant. Nevertheless, a considerablenumber of sister species pairs reflect the impact of the moun-tain barrier. An example supporting the significance of thisvicariance event (V3 ) is provided by the turtle genus Rhino-clemmys ( fig. 16.16b) based an the study of Lahanas (1992).

As the mountains were uplifted, the distributions of cer-tain other groups, perhaps originally associated with the lowuplands of earlier times, became fragmented onto the threemajor highland areas today constituting the backbone ofMiddle America. This fragmentation has led in some casesto the development of endemic montane isolates from an-cestors with a formerly continuous north-to-south range.However, this explanation does not seem satisfactory in allcases, and 1 will return to the problem of montane specia-tion in a later section.

The final major factor in shaping the herpetofauna of Cen-tral America was the complete emergence of the Panaman-ian Isthmus in the Pliocene to directly connect North andSouth America. Reconnection led to the dispersal (D4 ) ofmany South American Element genera northward and per-mitted immigration into South America by Old Northernand many Middle American stocks. Concordant dispersalevents (D 4 ), have brought sixty-five living generic-level taxaof clearly South American origin across the Isthmus to con-tribute to the Central American herpetofauna (table 15.9).Most of these groups are restricted to the region from east-ern Panama to Costa Rica, so that the South American in-fluence is minimal over most of Mesoamerica. Similarly, thegreatest number of Old Northern and Middle American ge-neric level taxa and species are found in northwestern SouthAmerica, but many range southward to the Amazon basinor beyond.

The recent herpetofaunas of Central America, exceptthose in eastern Panama, are based an a fundamental coreof autochthonous Middle American groups whose history inthe region goes back at least to the early Tertiary. Coexist-ing with this unit throughout the region are a series of en-

1 0-

20 -

30 -

40

180 -

Ma

Development of the Herpetofauna 827

Figure 16.17. Area cladogram showing sequence of major vicariance(V) and dispersal (D) events responsible for present biogeographicpatterns for the terrestrial herpetofauna: At = Atlantic, GA = GreaterAntilles, IL = Isthmian Link, NA = North America, P = Pacific,PGA = Proto-Greater Antilles, TM = Tropical Mesoamerica.

demic derivative stocks of Old Northern relationships thathave been in the region from Eocene-Oligocene times on-ward. Only in Panama and Costa Rica do South AmericanElement taxa contribute significantly to the fauna and pre-dominate in eastern Panama.

To summarize, the events leading to the present pattern forMesoamerica are as follows (figs. 16.17 and 16.18):

Vicariance (V 0 ): breakup of PangaeaDispersal (D 1 ): from the south over the Proto-Antillean

IsthmusVicariance (V 1 ): breakup of the Proto-Antillean IsthmusDispersal (D 2 ): invasion and integration of Old Northern

fauna with the tropical herpetofaunaVicariance (V 2 ): climate changes introducing a semiarid to

arid barrier between temperate North America and tropi-cal Mesoamerica

828 Biogeography and Evolution

ON

GT

Paleocene

a

d

Dispersal (D 3 ): concordant population of the emerging Isth-mian Link by Mesoamerican and Central American fau-nal units from the north

Vicariance (V 3 ): uplift of Mesoamerican highlandsDispersal (D4 ): from the south after emergence of the Pana-

manian Isthmus; this event subsumes two pulses that oc-curred about 1 million years apart

The Costa Rican Herpetofauna andthe Closure of the Panamanian PortalBy the end of the Paleogene the land-positive portions of theMaya and Chortis blocks were populated by a variety ofMesoamerican Element and Central American Componentgenera, many shared with tropical Mexico. The continuingeast-northeastward movement of the Caribbean plate by thistime had brought the Costa Rica-Panama island arcs into

b

Figure 1 6.18. Evolution of the Tropical Mesoamerican herpetofauna showing distribution of historicalsource units over time: CAC = Central American Component, GT = Generalized Tropical Fauna, MA =Middle American Element, ON = Old Northern Element, SA = South American Element.

e

c

approximately their present locations. Uplift of a series ofgradually emerging islands characterized this period for theChorotega block, but the islands were widely separated fromSouth America by a deep and wide seaway (fig. 16.7). Theearliest of these islands are represented today by a series ofPacific versant peninsulas: Santa Elena, Nicoya, Herradura,Osa, Burica, Sonä, and Azuero.

At this same time, although the Chortis and Chorotegablocks were sutured together at their margins, the contigu-ous Nicaragua depression and the Tempisque, San Carlos,and northern Lim6n basins formed a significant marine bar-rier to dispersal onto the islands from the north. Coates andObando (1996) suggested that this barrier continued as anelongated marine connection between the Pacific Ocean andCaribbean Sea into the Pliocene (3 Ma). Other reconstruc-tions (Van Andel et al. 1971; Donnelly 1989; Iturralde andMacPhee 1999; Perfit and Williams 1989; Pindell and Bar-

rett 1990) indicate a land-positive connection between theChortis and Chorotega blocks in the Tate Miocene (10 Ma).It seems likely that the connection was completed at theearlier time, as indicated by the late Miocene (6 Ma) mam-mal fossils of strictly northern affinities from central Panama( Whitmore and Stewart 1965). However, eustatic fluctua-tions may have opened and closed this and other intero-ceanic connections at various times before final closure ofthe Isthmian Portal.

In the Middle Miocene (ca. 15 Ma) the Panama portionof the arc began to be compressed by the South Americanplate, at which time the Chocö block was formed and a se-ries of land-positive areas emerged in the diminishing gap be-tween South America and the Chorotega block. These areasformed islands, continuing the isthmian archipelago south-ward, and today are represented by the Serranias San Blas-Darien and Maje of western Panama, the Serranaa Sapo-Baudo of western Panama and northwestern Colombia, andthe Dabeiba region of the Sierra Occidental of Colombia.

In addition, beginning about 5 Ma, the Cocos ridge be-gan to subduct under the central portion of the Chorotegablock to uplift the Cordillera de Talamanca. Thus by theearly Pliocene a continuous land connection formed a dis-persal corridor between the southern portion of nuclearCentral America and what is now Costa Rica and westernPanama. These events provided a landscape of coastal low-lands with a rapidly rising upland region that ultimatelyformed a continuous montane barrier 2,000 m in altitudeacross the Isthmus. Paleobotanical and paleoclimatic data( Graham 1987a,b, 1999a,b,c; Savin 1977; Savin and Doug-l as 1985) indicate that mid-Miocene climates were much thesame as today at these latitudes (ca. 10° N), and the princi-pal vegetation present was humid evergreen forests. How-ever, Graham (1987a) noted that a substantial number ofgenera represented groups now found in midaltitude situa-tions. These facts suggest that the basic habitats used by themodern herpetofauna were in place, except for the dry for-est component, by the mid-Miocene (5 to 10 Ma). It is littlewonder then that Mesoamerican Element and Central Amer-ican Component taxa dominate all areas an the Chorotegaand Chocö blocks, since they had the earliest and exclusiveopportunity to invade the emerging Isthmus from the north.

The final chapters in this process involved complete clo-sure of the seaway by further uplift of the Chocö block. Thefirst such episode occurred in the Pliocene, when sea levelwas 100 m lower than today (Vail and Hardenbol 1979),about 3.4 to 3 Ma. This event led to the concordant disper-sal of South American Element taxa into lower CentralAmerica. These earliest invaders are probably representedby the few genera of this unit now ranging north to Guate-mala or beyond.

Later, warming temperatures and rising sea level appearto have again connected the Caribbean Sea and the Pacific

Development of the Herpetofauna 829

via the Atrato region for a time, with final closure dated atabout 2 Ma (Cronin and Dowsett 1996). This resulted in asecond pulse of dispersal that continues to this day. Although1 have included them within dispersal event D 4 ( fig.16.17),they may be thought of as D 4a and D 4h . It is these dispersalsthat are responsible for the substantial representation ofSouth American taxa in Costa Rican faunal areas other thanthe northwest region.

The temporary opening and final closure of the seawayi n the Pliocene also may explain a puzzling aspect of SouthAmerican biogeography. Many Mesoamerican and CentralAmerican unit genera have relatively few South Americanrepresentatives, and their distributions are restricted tonorthern South America. Others, however, have wider dis-tributions and considerable species richness an the continent(e.g., Bolitoglossa, Norops, and Bothrops). It appears thatthere were also two pulses of dispersal southward, mirror-ing those just described for South American genera dispers-ing northward across the Isthmus. Middle American generashowing wide distributions and considerable speciation inSouth America probably represent the first wave of migrantsacross the Isthmus-for example, the salamanders of thegenus Bolitoglossa ( Hanken and Wake 1985) and some col-ubrine snakes (Atractus and Sibynomorphus, Cadle 1985),and perhaps the pitvipers of the genus Bothrops. Those withmore restricted ranges and less diversity would have arrivedin the second, more recent pulse after the second and finalclosure of the seaway.

The two-step closure model would also explain the fre-quent occurrence of endemic species of Mesoamerican Ele-ment and Central American Component genera in the Chocöregion of western Colombia or northern South America(e.g., the lizards of the genus Basiliscus and turtles of thegenus Rhinoclemmys). The ancestors of such forms wouldhave migrated across the first Pliocene land connection andbecome isolated from Central American populations. Dur-ing separation, allopatric speciation occurred to producethe South American form(s). In some cases, after the finalPliocene closure, dispersal has brought the descendant spe-cies into sympatry. Examples include Basiliscus basiliscus ofCentral America and northern South America and B. galeri-tus of the Chocö and Rhinoclemmys annulata and R. nau-suta, with the latter species in the Chocö. In other cases theymay continue to be allopatric: Kinosternon angustipons ( At-lantic slope Costa Rica and western Panama) and K. dunni.The South American genus Phyllobates mirrors this situa-tion in reverse with allopatric species in Costa Rica and Pan-ama (Phyllobates lugubris and P vittatus) and their sisterspecies (P aurotaenia) in the Chocö. All are probably iso-lated fragments of an ancestor that crossed over the firstPliocene land bridge.

Several authors have argued that the presence and diver-sity of Middle American taxa in South America, here asso-

830 Biogeography and Evolution

ciated with the first Pliocene dispersal pulse, shows theymust have arrived there by island hopping along the mag-matic arc before the emergence of the Isthmian Link in thePliocene. Hanken and Wake (1985) proposed that salaman-ders of the genus Bolitoglossa made such a journey in themiddle Miocene about 18 Ma based an a molecular clockderived from genetic distances. Zamudio and Greene (1997)evoke a similar scenario for colubrine snakes and Lachesis.Using mtDNA data, they established a molecular clock thatestimated the time of divergence between Central Americanand South American Lachesis as 6 to 18 Ma, again beforecompletion of the Isthmian Link. Given the unreliability ofmolecular clock estimates (Scherer 1990; Avise et al. 1992;Avise 1994; Gaut 1998), the evidence for island hoppingdispersal raises the issue whether dispersal could have oc-curred before the Pliocene by successive, sporadic connec-tions of islands in the arc. The two-closure model of the por-tal also needs further verification to satisfactorily explainthese and other distribution patterns.

By the late Pliocene most components of the present her-petofauna of Costa Rica and western Panama were in place.Mesoamerican Element and Central American stocks hadarrived from the north during the Miocene to inhabit theemerging Isthmian Link as it gradually extended towardSouth America. Uplift of the Talamanca massif had providedan upland center for differentiation of these same lineages.Simultaneously, it split the lowland area into Pacific and At-lantic regions, where unique faunal assemblages would de-velop. Final closure of the Panamanian seaway had led tothe migration of South American Element taxa northwardto contribute eventually to herpetofaunal differentiation anboth Pacific and Atlantic versants and in the uplands as well.

The Problem of Tropical Montane HerpetofaunasThree principal upland areas occur in tropical Middle Amer-ica: the Mexican Sierra Madres, the highlands of nuclearCentral America, and the highlands of Costa Rica and west-ern Panama. The first two are separated from one anotherby the low-lying Isthmus of Tehuantepec and the last two bythe Nicaragua depression. No evidence suggests that theseregions were ever connected by montane corridors at anytime in their histories, yet closely related taxa and sometimesdisjunct populations of arguably the Same species occur antwo or more of these highlands.

Three principal hypotheses have been proposed to ac-count for these patterns:

At times of maximum Ice Age cooling, a temperature de-pression of about 6°C would allow dispersal through thelowlands from one upland area to another

The upland taxa are derivatives of widespread lowland taxathat independently invaded each upland area from below

The ancestral taxa were originally widespread lowland ones

that were carried upward and fragmented as each high-land was uplifted

Recent studies an fossil pollen profiles by Colinvaux( 1993, 1996) have clarified the issue at least as it relates toclimate and vegetation in the Quaternary of Lower CentralAmerica. His principal conclusions are:

During glacial periods temperatures were depressed by 6to 8°C from Guatemala through Panama, and glaciersformed at the highest elevations in the Cordillera deTalamanca

During these periods there was not a simple downward de-pression of the vegetational zones by 800 m, a conditionthat would have eliminated all components of lowlandvegetation

Rather, the distributions of upland genera were compresseddownward, where they became mixed with lowland taxa

The lowlands of the region were covered during glacial pe-riods by these humid forests, with no indication of Ice Ageincrease in aridity

During interglacial periods the upland taxa were sorted outby moving back up the cordilleras as temperatures re-turned to levels equivalent to those at present

There have been repeated cycles of these events in Quater-nary times

Although this pattern is documented for the Quaternary,an earlier period of severe glaciation cycles that occurred inlatest Miocene to earliest Pliocene times (6 to 4.8 Ma) prob-ably showed a similar sequence of events. One may assumethat, like the flora, herpetofaunal assemblages responded tolong-term temperature changes.

It therefore seems likely that the populating of the moun-tains of nuclear Central America by upland genera from theSierras of Mexico occurred during a late Miocene to earlyPliocene glacial period. Upland taxa became mixed withlowland ones at this time and were able to disperse acrossthe Isthmus of Tehuantepec. During a subsequent warmingperiod the upland genera were sorted out of the mixed faunaby moving into the newly available montane habitats. Anexample of the result of such a process is the distribution ofthe lizard genus Abronia ( fig. 16.19).

Similar subsequent cycles would lead to a sequential In-vasion from north to south of more southern uplandsthroughout the Pliocene. An example of this pattern is thedistribution of the snakes of the Rhadinaea godmani group( fig. 16.20).

The formation of the Quaternary volcanic chain in Cen-tral America would expedite this process during the severalcooling and warming cycles. A general model for montanespeciation in the region (fig. 16.21) requires an initial dis-persion event followed by cycles of temperature depression( glacial periods) and release (interglacial periods). The ini-tial period of such a sequence is probably responsible for the

Figure 16.19. Distribution of the lizard genus Abronia.

differentiation of sister species in isolated ranges such as theCordillera Central and Cordillera de Talamanca. An ex-ample of this process is the genus Nototriton in Costa Rica( fig. 16.22).

Evolution of the Lowland Dry Forest HerpetofaunaThe northwestern region of Costa Rica forms the southernterminus of a continuous corridor of thorn woodland andsemiarid, deciduous, and semideciduous forests and patchesof savanna that extends from Sinaloa, Mexico, southwardalong the Pacific coast. The Meseta Central Occidental rep-resents a transitional zone between the Lowland Dry For-est of Guanacaste and northern Puntarenas Provinces andmore humid formations. Climates and vegetation similar tothose found along the Pacific coastal plain also occur anthe outer portion of the Yucatän Peninsula and in the rainshadow valleys an the Atlantic versant of nuclear CentralAmerica (fig. 16.23).

The disjunct distribution of several amphibians and rep-

Development of the Herpetofauna 831

tiles in the separate portions of these habitats suggests For-mer continuity. An example is the distribution of the spiny-tailed iguanas of the Ctenosaura quinquecarinatus group( fig. 16.24).

Elements of these vegetational formations are of Neo-tropical-Tertiary geofloral types, but this complex probablyrepresents a transition between the increasingly drier andcooler core of the Madro-Teritiary flora and tropical ever-green formations (Axelrod 1958). By late Oligocene timesthe relation between these might be visualized as a broadbelt of tropical evergreen forest bordered an the north bytropical deciduous forest. The latter in turn was bordered bythe horseshoe-shaped enclave of developing Madro-Tertiaryvegetation in what is now northern Mexico and the south-western United States (fig. 16.14). This element would con-tinue to expand over western North America for the rest ofCenozoic to produce the familiar semiarid live oak-coniferwoodland, chaparral, arid subtropical scrub, desert grass-lands, and subdesert and desert formations.

In the Miocene the northern limits of tropical conditions

Figure 16.20. Distribution of the snakes of the Rhadinaea godmani group.

I nitial dispersal

T° release}

}

T° release

Another dispersal

} ~ }uplift

T° depression

sympatry

} }more uplift

sympatry

A D F E C B Y X

Figure 16.21. Model for montane speciation involving cycles of temperature depression and release begin-ning in the Pliocene; letters indicate species, cladograms indicate phylogenetic results.

Figure 16.22. Distribution of the salamanders of the genera Cryptotriton and Nototriton: triangles = Cryp-totriton; shading = Nototriton; cross = sympatry between the two genera.

were being restricted farther and farther south. At the sametime the uplift of the Mexican Sierra Madres Oriental andOccidental and the Mexican Plateau drove an upland wedgesouthward that by the mid-Pliocene fragmented the low-land region into Pacific and Atlantic sections connected onlyacross the Isthmus of Tehuantepec.

Herpetofaunal associates of the tropical deciduous for-est belt were isolated at this time in three areas: along thewestern margin of the Gulf of Mexico; the outer margin ofthe Yucatän Peninsula, but doubtless extending more to thesouth than now; and western coastal Mexico. An exampleof a clade now found disjunctly in all three areas are the can-tils Agkistrodon bilineatus and A. taylori ( fig. 16.25).

During the rest of Neogene times the rain shadow effectof the rising nuclear Central American mountains combinedwith the Pacific climatic regimen saw the expansion south-ward of the subhumid to semiarid formations to replace hu-mid ones for the most part in the western lowlands, produc-ing considerable endemism over time. The herpetofauna ofthis corridor is more strongly influenced by members of theCentral American Component than any other representedi n Costa Rica. This fauna probably reached Costa Rica rel-

Development of the Herpetofauna 833

atively recently, during the time of Quaternary volcanismand after the uplift of the Nicaragua depression just to thenorth.

The distribution of this fauna in the Atlantic slope rainshadow valleys also appears to represent late Neogene frag-mentation of once continuous ranges. Stuart (1954) long agoproposed a subhumid corridor involving dispersal across in-termediate pine-oak forest barriers from the Pacific lowlandsat the Isthmus of Tehuantepec through the Grijalva valley ofChiapas, Mexico, in the Atlantic drainage and into the semi-arid Rio Negro and Rio Motagua valleys of Atlantic slopeGuatemala (see also Wilson and McCranie 1998).

Another explanation seems more parsimonious. It is likelythat these four areas and the dry Sula and Aguan valleys ofHonduras were part of a more or less continuous corridorduring glacial maxima in the Pliocene. Subsequent upliftof the Sierra Madre of Chiapas, the transverse ranges ofGuatemala, and the Sierra de Oma and Sierra de Sulaco inHonduras appear to have fragmented this corridor into itspresent components and allowed differentiation of some en-demic taxa.

The other isolated dry valleys of more southern Honduras

83 4 Biogeography and Evolution

Figure 16.23. Distribution of subhumid to arid climatic conditions in tropical Mesoamerica supporting de-ciduous forests, thorn forests, thorn woodlands, and savanna. Areas of these types in Panama show littlefaunal similarity to those from Costa Rica northward.

were probably connected across the low early Pliocene con-tinental divide with the Pacific slope dry corridor. Again Pli-ocene orogeny, reinforced by Quaternary volcanism, wouldhave isolated the Atlantic sector from the Pacific sector. Al-though there clearly were considerable expansions of dryforest conditions into areas where evergreen forest now pre-dominates in upper Central America during the Pleistocene,it is unlikely that the various isolated valleys were relinkedas in their Pliocene continuities.

Webb (1977,1978) and Webb and Rancy (1996) proposedthat a continuous "savanna" corridor extended throughCentral America and across the emerging Isthmian Link andserved as a major pathway for movements of fauna, as indi-cated by mammalian fossils, between the Americas in thelate Pliocene to mid-Pleistocene. Their concept of "savanna"was broad and was applied to all subhumid to semiarid habi-tats now represented in Central America, including thosediscussed in the previous paragraphs. Colinvaux (1996) re-cently has demonstrated that such a corridor could developonly during glacial periods but found no support in the fos-sil plant record that one could have existed. All evidence

from herpetofaunal distributions indicates that the Pacificlowland subhumid-semiarid corridor never extended farthersouth than its present location near 10° N in Costa Rica.

Colinvaux (1993, 1996) has further discredited the no-tion, first proposed by Haffer (1969), that at times of glacialmaxima in the Quaternary lowland evergreen forests andtheir faunas were replaced in tropical America, except in afew refugia, by dry-adapted formations. According to Haf-fer and others (Whitmore and Prance 1987), present-daydry forests and related associations have contracted rangenthat will expand and reconnect during the next glacial ex-pansion. The Pleistocene alternation of rainforest (inter-glacial) and dry forest (glacial) expansion and contractionswas postulated as being responsible for numerous specia-tion events. As described above, the distribution of dry for-est habitats and their faunal associates are the result of long-term changes in the physiography and climate of CentralAmerica in the Neogene, not slight Pleistocene fluctuations.Most important in this regard was the uplift of the centralmountain backbone of the region that emphasized the rainshadow effect an the Pacific slope and fragmented Atlantic

Figure 16.24. Distribution of the lizards of the Ctenosaura quinquecarinatus group.

dry forest habitats from those an the Pacific and from oneanother. The strong evidence for these vicariant processes asprimary for dry forest development adds support to Colin-vaux's refutation of the refugium hypothesis.

WHY THERE ARE SO MANY SPECIESBETWEEN TWO CONTINENTS,BETWEEN TWO SEASThe present Isthmian Link is relatively recent geologically,having its initial connection to the southern end of the Chor-tis block between 6 and 10 Ma. Its final emergence as a con-tinuous landmass between North and South America wasnot permanently effected until the late Pliocene, as recentlyas 2 Ma. Most of the species in the herpetofauna of CostaRica and Panama or their ancestors could have migratedonto the link no sooner than 6 or 7 Ma from the north andno more than 3 Ma from the south. Subsequent rapid spe-ciation must have occurred to produce the astonishing di-versity characteristic of the region today (table 16.2).

It is not surprising that at the species level the greatestdiversity is represented by the representatives of the autoch-thonous Mesoamerican Element and the coevolving Cen-

Development of the Herpetofauna 835

tral American Component that developed in situ in tropi-cal Middle America throughout the Tertiary (table 16.3).Fully 70% of the species found in Costa Rica and elsewherean the Link are derivatives of these historical units. Theremay haue been some minimal overwater dispersal to the is-land precursors of the Isthmus in the early Miocene, but itis clear that the central core of the herpetofauna dispersedfrom nuclear Central America alter the island chain becameconnected via the Chortis block. Without question, theavailability of the many unoccupied ecological niches foramphibians and reptiles led to substantial differentiation.Within the Central American herpetofauna 156 species areendemic to Costa Rica and western Panama. Of these en-demics, 54% are Mesoamerican Element species and 29%are Central American Component representatives. Of the to-tal Costa Rican-western Panamanian herpetofauna, Meso-american Element forms endemics make up 20% and Cen-tral American Component 11 % of the species.

The second principal factor contributing to differentia-tion was the continuing uplift and volcanic activity an theemerging Isthmus (Gardner et al. 1987). These processesopened new habitats and fragmented old ones. Volcaniceruptions and Lava flows constantly modified topography

83 6 Biogeography and Evolution

Figure 16.25. Distribution of the cantils Agkistrodon bilineatus and A. taylori, light and darker shading,respectively.

Table 16.2. Comparison of Species Numbers and Relative Species

altitudinal zones produced environments in the lowlandsRichness (ISR) for Selected Geographit Areas

most closely resembling those of midelevation forests (500to 1,600 m) at present. This situation allowed cool-adaptedmembers of the herpetofauna to migrate between separatemountain masses and volcanoes, where during times ofwarmer conditions the corridor was broken and each al-lopatric montane population had the opportunity for differ-entiation. The several glacial cycles created multiple oppor-tunities for this process to be repeated, with potential forfurther speciation events (fig. 16.21).

A fourth factor in the assemblage of the diverse Costa Ri-can herpetofauna was the reconnection of Central Americato South America by the completed Isthmian Link. This re-sulted in the invasion of Central America by South Ameri-can Element taxa long isolated from any relatives to thenorth. In Costa Rica and western Panama, endemics derivedfrom this source make up 12% of the endemics and 4% ofthe total species in the herpetofauna. Some of these speciesdoubtless are descendants of ancestors that crossed the firstbridge that closed the seaway, and others probably arrivedafter the second closure proposed by Cronin and Dowsett( 1996). In either case they arrived after the Mesoamerican

Introduced and marine species not included.Index of Species Richness (ISR) = N!kma

X 100.

and produced a constantly changing mosaic of isolated ar-eas where speciation could occur. The orogenic influencecannot be overemphasized as a factor in the diversificationof the many upland endemics in the region.

The fluctuations of cooling and warming cycles in the lat-ter part of the Cenozoic, in association with the continueduplift of the cordilleras, provided additional impetus forspeciation. At times of glacial maximum the compression of

Table 16.3. Numbers (Upper Figure) of Species and Percentage (Lower Figure)of Total Costa Rican Herpetofauna by Historical Source Unit

Seven marine species and six introduced species not included.

Element and Central American Component units were wellin place and make up a significant but smaller percentage(26%) of the total species in the fauna.

The final act, bringing our story to its dose, was the ratherrecent movement of dry forest taxa into northwestern CostaRica and onto the Meseta Central. Late Quaternary vol-canism was the principal factor in reducing the effects of thenortheast trade winds an the Pacific slope. Increasingly xericconditions in the northwest, unmitigated by monsoonal flowfrom the Southern Hemisphere, created an opportunity fornorthern forms to penetrate the formerly humid lowlands.The recency of these events is demonstrated by the absenceof any species strictly endemic to this region of Costa Ricaand the dose resemblance of its herpetofauna to that of thesubhumid to semiarid lowlands from Guatemala south.

The population of the Meseta Central Occidental by dryforest species may imply a recent ongoing dispersal event.On the other hand, both mesetas may have been covered bydry forest earlier in history, and perhaps their continuing up-lift has merely carried dry forest taxa upward in the process.

Development of the Herpetofauna 837

The former hypothesis suggests that the Subset of dry forestamphibians and reptiles an the Meseta are good disperserswith more to follow. The second hypothesis is that they areholdovers and are gradually slipping off the Pacific side fromthe Meseta Central Occidental as uplift continues, a processthat will ultimately lead to their extinction an the MesetaCentral Oriental.

Documentation of herpetofaunal diversity for Costa Ricaremains incomplete. Recognition and description of speciesnew to science by several authors is ongoing, and some ofthese works will doubtless appear while this book is in press.Still other species will be discovered in the future in under-explored areas of the republic as access increases and habi-tat destruction accelerates. Vaya bien to all of you searchersan this rich coast. May you too emerge from the forest of in-terlocking opposites an the Pass of Desengafio while fol-lowing the trail of the golden frog to new and exciting dis-coveries. ;Pura vida!

GroupOld Northern

MesoamericanSouth

American TotalsCAC EAC WACCaecilians 3 1 4

0.7 0.25 1Salamanders 37 - 37

10 0 9Anurans 4 2 66 59 131

1 0.5 17 15 34Total amphibians 41 2 69 60 172

10 0.5 18 15 45Lizards 8 - 4 36 21 69

2 1 9 6 18Snakes 51 4 1 60 6 133

13 1 0.25 16 4 35Turtles 2 8

1.6 0.5 2Crocodilians 1 1 2

0.25 0.25 0.5Total reptiles 65 6 5 97 38 211

17 1.6 1.3 26 10 55Grant total 106 8 5 166 98 383

28 2.1 1.3 43 26 100

Geographit Unit N Area (km2 ) ISR

United States 547 7,828,080 0.0007Mexico 997 1,972,545 0.0005California 126 441,015 0.03Florida 122 151,670 0.08Cuba 158 114,524 0.13Guatemala 325 108,889 0.29Oaxaca 355 95,364 0.37Panama 399 75,474 0.53Costa Rica 383 50,900 0.75