Central American Land Bridge Research Articles

Porphyry-related high-sulfidation mineralization early in Central American Arc development: Cerro Quema deposit, Azuero Peninsula, Panama

The 70.74 to 70.66 Ma age range for three molybdenite samples accompanying pyrite- and enargite-bearing assemblages effectively constrains an earliest Maastrichtian age for the high-sulfidation Au-Cu mineralization at Cerro Quema, Panama. The epithermal system was contemporaneous with emplacement of a composite dacite dome complex in a geotectonic setting transitional from mafic, primitive intraoceanic (Azuero Protoarc) to more evolved island arc magmatism (Azuero Arc), during initial construction of the Central American land bridge at the trailing edge of the Caribbean Large Igneous Province (CLIP). The molybdenite ages confirm the rapid evolution of the earliest stages of the Central American Arc, from subduction initiation at 75–73 Ma to arc maturation at 71 Ma. A porphyry connection is apparent at Cerro Quema and characterized by highly contorted, banded, and planar quartz-veinlet stockworks and sheeted zones in pyrophyllite- and sericite-bearing patchy-textured rock. These are cut by ledges of quartz, alunite, and dickite, which implies overprinting of the advanced argillic lithocap onto the underlying porphyry environment. Hydrothermal telescoping resulted from synmineralization uplift congruent with an actively emerging volcanic arc, which the Re-Os molybdenite dates accurately constrain at 71 Ma, presumably as a far-field effect of collision between the leading edge of the CLIP with parts of North and South America.

Roles of land bridges in global biogeography and ecosystems

Roles of land bridges in global biogeography and ecosystems

Phylogeography of the Central American lancehead Bothrops asper (SERPENTES: VIPERIDAE).

The uplift and final connection of the Central American land bridge is considered the major event that allowed biotic exchange between vertebrate lineages of northern and southern origin in the New World. However, given the complex tectonics that shaped Middle America, there is still substantial controversy over details of this geographical reconnection, and its role in determining biogeographic patterns in the region. Here, we examine the phylogeography of Bothrops asper, a widely distributed pitviper in Middle America and northwestern South America, in an attempt to evaluate how the final Isthmian uplift and other biogeographical boundaries in the region influenced genealogical lineage divergence in this species. We examined sequence data from two mitochondrial genes (MT-CYB and MT-ND4) from 111 specimens of B. asper, representing 70 localities throughout the species’ distribution. We reconstructed phylogeographic patterns using maximum likelihood and Bayesian methods and estimated divergence time using the Bayesian relaxed clock method. Within the nominal species, an early split led to two divergent lineages of B. asper: one includes five phylogroups distributed in Caribbean Middle America and southwestern Ecuador, and the other comprises five other groups scattered in the Pacific slope of Isthmian Central America and northwestern South America. Our results provide evidence of a complex transition that involves at least two dispersal events into Middle America during the final closure of the Isthmus.

Guentherus altivela Osorio, the first ateleopodid fish reported from the eastern Pacific Ocean

Guentherus altivela , long known from the west coast of Africa, is described from 14 specimens collected off the Pacific coast of Costa Rica and Panama. The eastern Pacific form is remarkably smiilar to literature descriptions and comparative material of the eastern Atlantic form. The Pacific population has a smaller mean number of anal rays (74.0 vs. 78.1), and a slightly longer head, upper jaw and first free pelvic rays. The present discovery fills a major gap in the cosmopolitan distribution of the Ateleopodidae. It is suggested that G. altivela is a trans-Atlantic species which populated the eastern Pacific before the last emergence of the Central American land bridge; its eventual discovery in the western Atlantic could be expected.

Key drivers controlling stable isotope variations in daily precipitation of Costa Rica: Caribbean Sea versus Eastern Pacific Ocean moisture sources

Costa Rica is located on the Central American Isthmus, which receives moisture inputs directly from the Caribbean Sea and the Eastern Pacific Ocean. This location includes unique mountainous and lowland microclimates, but only limited knowledge exists about the impact of relief and regional atmospheric circulation patterns on precipitation origin, transport, and isotopic composition. Therefore, the main scope of this project is to identify the key drivers controlling stable isotope variations in daily-scale precipitation of Costa Rica. The monitoring sites comprise three strategic locations across Costa Rica: Heredia (Central Valley), Turrialba (Caribbean slope), and Caño Seco (South Pacific slope). Sporadic dry season rain is mostly related to isolated enriched events ranging from −5.8‰ to −0.9‰ δ18O. By mid-May, the Intertropical Convergence Zone reaches Costa Rica resulting in a notable depletion in isotope ratios (up to −18.5‰ δ18O). HYSPLIT air mass back trajectories indicate the strong influence on the origin and transport of precipitation of three main moisture transport mechanisms, the Caribbean Low Level Jet, the Colombian Low Level Jet, and localized convection events. Multiple linear regression models constructed based on Random Forests of surface meteorological information and atmospheric sounding profiles suggest that lifted condensation level and surface relative humidity are the main factors controlling isotopic variations. These findings diverge from the recognized ‘amount effect’ in monthly composite samples across the tropics. Understanding of stable isotope dynamics in tropical precipitation can be used to a) enhance groundwater modeling efforts in ungauged basins where scarcity of long-term monitoring data drastically limit current and future water resources management, b) improve the re-construction of paleoclimatic records in the Central American land bridge, c) calibrate and validate regional circulation models.

Continental crust generated in oceanic arcs

Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intraoceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10Myr ago - coincident with the onset of subduction of more oceanic crust that originally formed above the Galapagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone - a process probably more common in the Archaean - can produce juvenile continental crust.

Neotropical Macrobrachium (Caridea: Palaemonidae): on the biology, origin, and radiation of freshwater-invading shrimp

Comprising >240 extant species, Macrobrachium Bate, 1868 is the most speciose caridean genus in Palaemonidae. It is generally considered as a monophyletic clade that lives exclusively in limnic and brackish habitats. Thus, it may provide a suitable model for the reconstruction of evolutionary transitions of euryhaline shrimp from ancestral state of living in the sea towards invasions of freshwater environments. Reviewing patterns of the biology (adaptative physiological and reproductive traits) and modern biogeographic distribution of this clade, I propose here a scenario for its tentative origin and evolutionary invasion of limnic inland waters, especially in the Americas. Macrobrachium shows: 1) a world-wide tropical to subtropical distribution, with only few species occurring in temperate regions and none in cold waters at high latitudes; 2) a clear preference for low salinity conditions (based on strong osmoregulatory capacities); 3) larval export strategies in coastal species (with diadromous migrations and invariably an extended larval development in estuaries); and 4) almost exclusively an abbreviated and lecithotrophic mode of larval development in hololimnetic inland species. The extant patterns of geographic distribution of this genus are strikingly disjunct, with completely separate groups of species occurring in the Indo-Pacific region (where maximum diversity occurs), inWest Africa, and in the Americas. In a clade with common ancestry, this pattern can only be explained with a Tethyan origin and dispersal. Hence, I suggest an origin of ancestral Macrobrachium in theMesozoic. As a consequence of the breakup of Gondwana and fragmentation of the Tethys Sea, mainly during the Late Cretaceous and Palaeogene, Neotropical and West African clades became isolated from the remaining congeners and thus can be considered as Tethyan relicts. In the Neotropics, the closure of the Central American landbridge in the Late Pliocene was a significant vicariant event that caused diversification of coastal Macrobrachium in the Caribbean region, so that two separate groups of extant species live now in the Atlantic and Pacific drainage systems. The distribution of hololimnetic inland species can be explained by continental floodings occurring during the Miocene sea level highstand. Due to concomitant sub-Andean subsidence and impounded by the uplift of the Andes, the huge Pebas wetland system was formed at that time, covering the vast proto-Amazonas-Orinoco catchment area with sub-Andean lowlands reaching from Columbia to southern Bolivia. This allowed for incursions of marine-derived lineages of euryhaline shrimp and other coastal animals throughout northern and central South America, followed by adaptive radiations in freshwater. After the breaching of the continental divide (mainly the Purus Arch) in central Amazonia, probably in the Late Miocene, the modern eastward flow of the Amazon developed, draining to the Atlantic Ocean. Fluctuation but trending to a general decrease in global sea levels during the Pliocene and Pleistocene caused fragmentation of previously interconnected inland waters, promoting vicariant diversification of hololimnetic lineages. However, some hydrological connections between different catchment areas remained transitorily open, providing limited dispersal routes that also may have influenced speciation, e.g., through hybridisation. Thus, the modern distribution patterns of Neotropical Macrobrachium reflect interacting effects of tectonic events including Andean orogeny and the closure of the Panamaian landbridge, Miocene marine incursions, and possible cross-drainage dispersal through historical connections between river basins.

Sedimentología v paleoecología de un estuario del litoral Pacífico del Istmo Centroamericano Primordial (Mioceno Medio, Costa Rica)

The rise of the Central American landbridge is characterized by a Campanian to Eocene island arc stage, an Oligocene to Miocene swell stage, and a Pliocene to Recent mountain chain stage. During the Oligocene to Miocene swell stage the volcanic edifices and insular shelves of the foregoing stage gradually grew together, forming the primordial Central American isthmus. Thus, extensive tidal flat areas established which occupied, especially during the Middle Miocene, large areas of the actual territory of Costa Rica. The present study deals with a Middle Miocene section situated at Punta Judas on the Pacific coast of Costa Rica. This section provides a nearly complete record of the depositional environments that characterize the Pacific estuaries of the primordial Central American isthmus. The 1300 m of mainly litharenitic volcaniclastic sediments are extre-mely fossiliferous, especially in mollusks, echinoids, and trace fossils. Thus, sedimentological interpretations can be choroughly controlled and complemented by paleoecological data. 10 different facies types could be identified that reflect a great variety of depositional environments ranging from open estuary through intertidal shoals and mangrove swamps to mesohaline marginal shallows, and even slightly hypersaline lagoons. Facies character and pattern in the subtidal area was largely controlled by the temporariness and volume of the terrigenous input which was transferred across the deltaic depocenter and overtaken by the estuarine circulatory regime Ecological successions that developped in response to the intermittent sediment supnlv commonly are characterized by native background communities (susnenpion feeiers - mollusks) and opportunistic after-event communities (sediment feeders-cetossils) senau SEILACHER (1980). In the intertidal area lower flats are typified by crosslaminated sand-stones, whereas in mid and upper flats mangrove swamp deposits are predominant. This fades unit frequently interfingers with granule and pebble conglomerates that entered from minor lateral river mouths. The coarse river bed load. is redistributed both in meandering tidal channels and in straight beach-parallel channels. The latter show an unidirecional filling pattern, the size ranges being fashioned into large graded sigmoidal foresets that end up in welldevelopped toe sets; mud drapes prove that these features have to be considered as ti, dal bundles. Tempestites are fairly frequent within the mangrove swamp deposits, but occur also in nearly all other facies types. Following these criteria it mustbe concluded that the Punta Judas estuary was laterally bounded by a mountainous hinterland and headed by a stream system that supplied large amounts of fine- to medium-sized volcaniclastic sediments. Despite the low salinities in its uppermost sector, the estuary was strongly controlled by esotidal currents which obviously followed an asymmetrical circulatory pattern. It was an essentially sandy estuary and must have reached dimension, comparable to the actual Gulf of Nicoya. There is no evidence for the existence of an outer bar, of barrier islands, or ebb- and flood-tidal deltas. Hence, thp Middlo Miocene Punta Judas estuary has to be considered as an open embayment. The Punta Judas section documents at least 2 complete filling cycles (pro-gradations). The return from shoreline to open estuarine environments always came off very abruptly. This suggests rapid sealevel rises, followed by gra-dual lowerings. The absence of both angular unconformities and hardgrounds proves that subsidence was fairly constant. Most probably, the Punta Judas estuarine embayment was a direct graben-like tectonic feature. These data coincide with the tectonical and geographical framework of the actual Pacific estuaries of Central America. Differences exist, however, in that the Punta Judas estuary reached salinities as low as the -mesohaline range. This is not common to the actual ones which are essentially normal marine to high polyhaline. Thus, we propose an environmental model which falls midway bet-ween the strongly tectonically controlled Pacific centroamerican type and the epicontinental type which characterizes mid latitude Atlantic coastal plains.

The Central American land bridge as an engine of diversification in New World doves

Aim The closure of the Central American land-bridge connection between North and South America 3.5 million years ago was a major biogeographic event that allowed considerable interchange of the previously isolated faunas of these continents. However, the role that this connection may have had in diversification of North and South American faunas is less well understood. The goal of this study was to evaluate the potential role of the formation of this land connection in generating diversity, through repeated rare dispersal events followed by isolation. Location North and South America. Methods We evaluated the role of the Central American land-bridge connection in avian diversification using a molecular phylogeny based on four gene regions for mid-sized New World doves. Diversification events were dated using a Bayesian relaxed clock analysis and internal calibration points for endemic island taxa with known island ages. Results The reconstructed phylogenetic tree was well supported and recovered monophyly of the genera Leptotila and Zenaida, but the quail-doves (Geotrygon) were paraphyletic, falling into three separate lineages. The phylogeny indicated at least nine dispersal-driven divergence events between North and South America. There were also five dispersal events in the recent past that have not yet led to differentiation of taxa (polymorphic taxa). Main conclusions Most of these dispersal-driven diversification events occurred at the time of or after the formation of the Central American land bridge, indicating that this land connection played a role in facilitating divergence via dispersal of doves between continents.

Magmatic history and evolution of the Central American Land Bridge in Panama since Cretaceous times

Chemical compositions for 310 igneous rocks from the Cordillera de Panama and the Sona and Azuero peninsulas were supplemented by 40 Ar/ 39 Ar dating and Sr-, Nd-, Pb-, and O-isotope analysis to determine the magmatic evolution and oceanic plate interactions over the past 100 Ma in western Panama. An initial phase of intraplate magmatism, having geochemical characteristics of the Galapagos hotspot, formed the oceanic basement of the Caribbean large igneous province from 139 to 69 Ma. Younger accreted terranes with enriched trace element patterns (accreted ocean island basalt [OIB]) were amalgamated between 70 and 20 Ma. A second magmatic phase in the Azuero and Sona peninsulas has trace element patterns (Sona-Azuero arc) suggesting the initiation of subduction at 71–69 Ma. Arc magmatism continued in the Chagres basin region (Chagres-Bayano arc) from 68 to 40 Ma. A third phase formed discrete volcanic centers across the Cordillera de Panama (Cordilleran arc) from 19 to 5 Ma. The youngest phase consists of isolated volcanic centers of adakitic composition (Adakite suite) in the Cordillera de Panama that developed over the past 2 million years. Initiation of arc magmatism at 71 Ma coincides with the cessation of Galapagos plateau formation, suggesting a causal link. The transition from intraplate to arc magmatism occurred relatively quickly and introduced a new enriched mantle source. The arc magmatism involved progressive transition to more homogeneous intermediate mantle wedge compositions through mixing and homogenization of subarc magma sources through time and/or the replacement of the mantle wedge by a homogeneous, relatively undeleted asthenospheric mantle. Adakite volcanism started after a magmatic gap, enabled by the formation of a slab window.

Prehistory of Native Americans on the Central American Land Bridge: Colonization, Dispersal, and Divergence

The Central American land bridge has served as a passageway for animals and humans moving between North and South America. Nevertheless, after the first waves of human immigration at the end of the Pleistocene, contact between the native peoples who remained on this isthmus and other peoples living in continental areas where civilization ultimately developed, is characterized, according to the field record, by the transfer of crops, technologies, and goods, until ca.1400 BP when speakers of Mesoamerican languages occupied the northwestern edge (Gran Nicoya). The ancestors of modern-day speakers of Chibchan and Chocoan languages underwent social and cultural diversification mostly within the confines of the land bridge. Some Precolumbian residents altered vegetation immediately after first arrival at least 11,000 years ago, and began to add domesticated crops to their subsistence inventory between 9000 and 7000 BP. Maize and manioc (or cassava), domesticated outside the land bridge, were introduced in Preceramic times, early in the period between 7000 and 4500 BP, and gradually dominated regional agriculture as they became more productive, and as human populations increased and spread into virgin areas. Diversity in material culture is visible ca. 6000 BP, and becomes more apparent after the introduction of pottery ca. 4500 BP. By 2000 BP culture areas with distinctive artifact inventories are discernible. Between 2500 and 1300 BP hierarchies among regions, sites, social groups, and individuals point to the establishment of chiefdoms whose elite members came to demand large numbers of costume and sumptuary goods. A few special centers with stone sculptures and low-scale architecture served a social universe larger than the chiefdom, such as clusters of recently fissioned social groups with memories of a common heritage. Social interactions on the land bridge, endowed with productive bottomlands, highland valleys, and coastal habitats, appear always to have been strongest among neighboring groups.

A Novel Abundant Family of Retroposed Elements (DAS-SINEs) in the Nine-Banded Armadillo (Dasypus novemcinctus)

About half of the mammalian genome is composed of retroposons. Long interspersed elements (LINEs) and short interspersed elements (SINEs) are the most abundant repetitive elements and account for about 21% and 13% of the human genome, respectively. SINEs have been detected in all major mammalian lineages, except for the South American order Xenarthra, also termed Edentata (armadillos, anteaters, and sloths). Investigating this order, we discovered a novel high-copy-number family of tRNA derived SINEs in the nine-banded armadillo Dasypus novemcinctus, a species that successfully crossed the Central American land bridge to North America in the Pliocene. A specific computer algorithm was developed, and we detected and extracted 687 specific SINEs from databases. Termed DAS-SINEs, we further divided them into six distinct subfamilies. We extracted tRNA(Ala)-derived monomers, two types of dimers, and three subfamilies of chimeric fusion products of a tRNA(Ala) domain and an approximately 180-nt sequence of thus far unidentified origin. Comparisons of secondary structures of the DAS-SINEs' tRNA domains suggest selective pressure to maintain a tRNA-like D-arm structure in the respective founder RNAs, as shown by compensatory mutations. By analysis of subfamily-specific genetic variability, comparison of the proportion of direct repeats, and analysis of self-integrations as well as key events of dimerization and deletions or insertions, we were able to delineate the evolutionary history of the DAS-SINE subfamilies.

THE HISTORY OF A NEARCTIC COLONIZATION: MOLECULAR PHYLOGENETICS AND BIOGEOGRAPHY OF THE NEARCTIC TOADS (BUFO)

Previous hypotheses of phylogenetic relationships among Nearctic toads (Bufonidae) and their congeners suggest contradictory biogeographic histories. These hypotheses argue that the Nearctic Bufo are: (1) a polyphyletic assemblage resulting from multiple colonizations from Africa; (2) a paraphyletic assemblage resulting from a single colonization event from South America with subsequent dispersal into Eurasia; or (3) a monophyletic group derived from the Neotropics. We obtained approximately 2.5 kb of mitochondrial DNA sequence data for the 12S, 16S, and intervening valine tRNA gene from 82 individuals representing 56 species and used parametric bootstrapping to test hypotheses of the biogeographic history of the Nearctic Bufo. We find that the Nearctic species of Bufo are monophyletic and nested within a large clade of New World Bufo to the exclusion of Eurasian and African taxa. This suggests that Nearctic Bufo result from a single colonization from the Neotropics. More generally, we demonstrate the utility of parametric bootstrapping for testing alternative biogeographic hypotheses. Through parametric bootstrapping, we refute several previously published biogeographic hypotheses regarding Bufo. These previous studies may have been influenced by homoplasy in osteological characters. Given the Neotropical origin for Nearctic Bufo, we examine current distributional patterns to assess whether the Nearctic-Neotropical boundary is a broad transition zone or a narrow boundary. We also survey fossil and paleogeographic evidence to examine potential Tertiary and Cretaceous dispersal routes, including the Paleocene Isthmian Link, the Antillean and Aves Ridges, and the current Central American Land Bridge, that may have allowed colonization of the Nearctic.

THE HISTORY OF A NEARCTIC COLONIZATION: MOLECULAR PHYLOGENETICS AND BIOGEOGRAPHY OF THE NEARCTIC TOADS (BUFO)

Previous hypotheses of phylogenetic relationships among Nearctic toads (Bufonidae) and their congeners suggest contradictory biogeographic histories. These hypotheses argue that the Nearctic Bufo are: (1) a polyphyletic assemblage resulting from multiple colonizations from Africa; (2) a paraphyletic assemblage resulting from a single colonization event from South America with subsequent dispersal into Eurasia; or (3) a monophyletic group derived from the Neotropics. We obtained approximately 2.5 kb of mitochondrial DNA sequence data for the 12S, 16S, and intervening valine tRNA gene from 82 individuals representing 56 species and used parametric bootstrapping to test hypotheses of the biogeographic history of the Nearctic Bufo. We find that the Nearctic species of Bufo are monophyletic and nested within a large clade of New World Bufo to the exclusion of Eurasian and African taxa. This suggests that Nearctic Bufo result from a single colonization from the Neotropics. More generally, we demonstrate the utility of parametric bootstrapping for testing alternative biogeographic hypotheses. Through parametric bootstrapping, we refute several previously published biogeographic hypotheses regarding Bufo. These previous studies may have been influenced by homoplasy in osteological characters. Given the Neotropical origin for Nearctic Bufo, we examine current distributional patterns to assess whether the Nearctic-Neotropical boundary is a broad transition zone or a narrow boundary. We also survey fossil and paleogeographic evidence to examine potential Tertiary and Cretaceous dispersal routes, including the Paleocene Isthmian Link, the Antillean and Aves Ridges, and the current Central American Land Bridge, that may have allowed colonization of the Nearctic.

Heavy-mineral analysis and clinopyroxene geochemistry applied to provenance analysis of lithic sandstones from the Azuero–Soná Complex (NW Panama)

The volcaniclastic sandstones of the Middle Eocene to Lower Miocene Tonosı́ Formation were deposited within the forearc realm of an island arc system which today is part of the southern Central American landbridge. Both heavy-mineral analyses and geochemical analyses of clinopyroxenes clearly reveal differences in the composition between sediments of the Lower and Upper Tonosı́ Formation, indicating changes within the tectonic setting as well as island arc evolution during time of deposition. Based on characteristic heavy-mineral suites three lithogroups can be determined and assigned to the Lower and Upper Tonosı́ Formation, respectively. Provenance analyses indicate that the sandstones of the Tonosı́ Formation were generally derived from an island arc setting. The abundance of a mafic heavy-mineral suite relative to heavy-minerals derived from metamorphic and sialic intrusive rocks indicates a convergent plate boundary setting for provenance of the Lower Tonosı́ Formation. This result is supported by the chemistry of clinopyroxenes within the heavy-mineral suite, which indicates evolved basaltic magma types sourced the detrital grains. Within the Hb–Ol + Id + Cpx brown–Opx + Cpx green-diagram most samples of the Upper Tonosı́ Formation plot in the field for divergent plate boundaries. The geochemical analyses also show that clinopyroxenes of the Upper Tonosı́ Formation additionally exhibit affinities to within-plate alkaline magma types. As an integration of our results we present a provisional two-stage model for the tectono-sedimentary evolution of the western Panamanian forearc area during the deposition of the Tonosı́ Formation.

A plate-tectonic model for the Mesozoic and Early Cenozoic history of the Caribbean plate

We present a model in which the Caribbean plate is an intra-American feature formed along the Caribbean spreading center as opposed to the current model that considers the Caribbean plate as a far-travelled crustal segment that formed in the Pacific region. Paleomagnetic data, which cover an age range from Jurassic through Paleocene, indicate the ophiolite complexes in Costa Rica and Panama formed in an equatorial position. The ophiolites did not fundamentally change their position relative to South America since their origin. Basaltic rocks of the lower part of the ophiolites are of mid-ocean ridge type suggesting formation at a spreading-center. They are interpreted to have formed as part of the proto-Caribbean crust at a spreading axis in an intra-American position during Jurassic and Early Cretaceous times. The upper part of the ophiolites is mainly built up by island arc and intraplate basalts. The island arc basalts evolved at the Central American landbridge which started in the Middle Cretaceous. The intraplate basalts are related to the Caribbean plateau basalt which thickened and stiffened the Caribbean crust in Middle Cretaceous to probably Campanian times. Changes in relative plate motions along the Middle American convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault-slip data in southern Mexico and Costa Rica. Kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern Mexican crustal complexes and along the northern boundary of the Chortı́s block in the Motagua–Polóchic fault system. This reflects the eastward drift of the Chortı́s block since the beginning of the Cenozoic. At this time the Chortı́s block became part of the Caribbean plate which moved more than 1000 km towards the east relative to the North and South American plates during the Cenozoic.

The American Proteaceae

The American Proteaceae are outliers from the main centres of diversity of the family in Australia and South Africa. There are about 83 species in eight genera which all belong to the monophyletic subfamily Grevilleoideae. Three genera, Embothrium, Oreocallis and Lomatia, are placed in the tribe Embothrieae (sensu Johnson and Briggs), four Euplassa, Gevuina, Panopsis and Roupala in the Macadamieae and the single genus Orites in the Oriteae. There are five genera endemic to America and three also have species in Australia and New Guinea (Gevuina, Lomatia and Orites). The Proteaceae appear to have arrived in South America via two routes. The larger genera Euplassa, Panopsis and Roupala, which are all endemic to America and have a general distribution in northern South America and south-eastern Brazil, are derived from Gondwanaland before it separated from South America. The remaining genera are distributed either in temperate South America or in the high Andes and appear to have arrived more recently via the Australia–Antarctica–South American connection. Three of these genera have species in both regions. The centres of species diversity of Euplassa, Panopsis and Roupala fall outside hypothesised forest refugia, indicating that they are not true rainforest species but species of seasonal habitats like those achieved at higher altitudes where they are commonly found. Two genera,Panopsis and Roupala, have reached Central America after the central American land bridge was formed six million years ago. The exact relationship to genera on other continents is still unclear and there is a need for a cladistic biogeographic analysis of the group based on both morphological and molecular data.

Ecogeographical Differentiation in Nectandra (Lauraceae), and its Historical Implications

AbstractFollowing a taxonomic revision of Nectandra, a genus of about 114 species of neotropical trees, character gradients are compared against geographical distribution, habitat preferences, and reproductive phenology of individual species, with the aim of reconstructing the spatial and temporal diversification of the genus. It is shown that Nectandra, together with Ocotea, Persea, and other genera of the Lauraceae, originated from a northern hemisphere matrix that immigrated into South America, perhaps as late as after the closure of the Central American land bridge in the Pliocene. The dramatic diversification of the genus within South America was facilitated by its ornithochorous dispersal system, implying ample processes of allopatric speciation. At present, speciation seems to take place preferably by processes of ecotypic (mostly edaphic) specialization, by which widely distributed, ecologically polymorphic species give rise to swarms of ecologically narrowly specialized satellite species.

Phytogeography of Talamanca Montane Quercus Forests, Costa Rica

Phytogeographical patterns of the vascular generic flora of the montane Quercus forests in the Cordillera de Talamanca, Costa Rice, have been studied. With the exception of orchids and bromeliads, 114 families and 253 genera have been recorded, of which eighty are trees, seventy-seven shrubs, forty-four herbs, twenty-one climbers and thirty-one ferns. About 75% of all genera are tropical in distribution. The remaining 25% is made up of temperate (17%) and cosmopolitan (8%) genera. The neotropical element is best represented and contributes to almost half of the genera (46%), whereas the tropical afroamerican element is worst represented (3%). This overall spectrum does not differ much from the spectrum of woody genera alone. Only the percentage of cosmopolitan genera is significantly reduced among the woody genera since most cosmopolitan genera are herbs (14%) and ferns (29%). Tree genera are mainly neotropical, tropical malayo-american and pantropical in distribution, while shrub genera are principally neotropical (over 60%, mostly Andean-centered), pantropical and northern or southern temperate. Herb genera are basically neotropical, pantropical and wide-temperate, climber genera mainly neotropical and pantropical, while fern genera expose fundamentally cosmopolitan, pantropical and neotropical distributions respectively. The Talamanca woody spectrum has been compared to spectra available from Mexican and Colombian montane Quercus forests. Analysis reveals a greater phytogeographical affinity with the Northern Andes than with the Mexican Neovolcanic Belt. This is probably due to the Nicaraguan depression, which separates mountain ranges on the Central American landbridge and thus serves as a mayor barrier to dispersal of montane plant taxa in the Neotropics. Also the drier climatic conditions prevailing in Mexican montane forests may play an important role in floristic (dis)similarity trends.

Interoceanic differences in adaptation: effects of history and productivity

Previously published evidence indicated that the degree of habitat specialization and enemy-related adaptation among shallow-water tropical marine species was greatest in the Indo-WestPacific (IWP), intermediate in the Eastern Pacific (EP), and lowest in the Western Atlantic (WA) and Eastern Atlantic (EA). New data on the incidence of unsuccessful shell breakage (measured a s the frequency of shell repair) and on the importance of shell breakage as a cause of death for shallow-water gastropods complicate, but do not invalidate, this picture. Columbellid and planaxid gastropods conforn~ to the expected interoceanic pattern of shell repair in that the lowest frequencies of repair were found in the Atlantic; but the Cerithiidae and Thaididae showed no interoceanic differences in repalr, and the Conidae exhibited a contradictory pattern in which repair was most frequent in the WA and least frequent in the IWP. Great intraregional variation existed in the contribution that breakage made to overall gastropod mortality, as inferred from analyses of dead shells. There was a tendency for breakage to be relatively more important as a cause of death in the IMrP and EP than in the WA, but the differences were not statistically significant. A review of other recent literature generally supported the hypothesis that the IWP biotas have undergone more specialization with respect to hab~ ta t and eneinyrelated adaptation than have marine biotas in tropical America. Available historical evidence suggests that these d~fferences are attributable to contrasting geological histories. In the IWP, extensive speciation, especially among well-armored molluscs, took place in the absence of significant extinctions. In the WA, widespread extinctions selectively eliminated armored gastropod taxa, which were not replaced through diversification of Pliocene and Pleistocene survivors. The apparent decline of planktonic productivity in many parts of the WA following uplift of the Central American land bridge during the Pliocene may have contributed to the observed pattern of molluscan extinctions. In the EP, where productivitv has remained h ~ g h , extinction rates were lower, and some highly adapted species have invaded from the IWP. The paper closes with a discussion of future directions in research.