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The Origins and History of the Central American Herpetofauna

Jay M. Savage
Copeia
Vol. 1966, No. 4 (Dec. 23, 1966), pp. 719-766
DOI: 10.2307/1441404
Stable URL: http://www.jstor.org/stable/1441404
Page Count: 48
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The Origins and History of the Central American Herpetofauna
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Abstract

The modern herpetofauna of Central America is comprised of approximately 625 species that have developed in response to ecologic diversity and historical opportunities provided by the stimulus of Cenozoic changes in physiography and climate. Of the 159 genera of amphibians and reptiles known from the region, 12 have widespread tropical distributions, 62 have patterns of differentiation and distribution centered on South America, 67 are essentially tropical Middle American groups, and 18 have distributions centered in extratropical North America. Genera with South American distributions are poorly represented in Central America north of Panamá and make up only 14% of the fauna north of Costa Rica. Under these circumstances the tropical Middle American assemblage is regarded as a distinctive unit, the Mesoamerican herpetofauna, equivalent in rank to the Nearctic and Neotropical units. The distribution of this fauna determines the limits of the Mesoamerican region, which extends from central Panamá to the limits of tropical conditions in México. Six major herpetofaunal assemblages are recognized in Central America: Mesoamerican-Eastern Lowland, Western Lowland, Guatemalan Highland, and Talamancan; Neotropical-Panamanian and Chocoan. Each of these assemblages is restricted to a particular geographic area (or biogeographic subregion) characterized by a series of tropical bioclimates and vegetation. The most important influences making it possible to estimate time of arrival and history of Middle American groups that today have similar distribution patterns are: 1) isolation of Central America from South America during late Mesozoic; 2) connection of the two areas by an intercontinental land bridge during Paleocene; 3) separation of Nuclear Central America from South America by the Panamanian marine portal from Eocene to Pliocene; 4) reconnection of the two areas by the Isthmian Link from early Pliocene onward; 5) uplift of the highlands of northern and lower Central America from Miocene to the present; 6) development of xeric climate and vegetation along the Pacific Coast during Pliocene to Recent. Four basic distribution patterns that correlate with these key events of Cenozoic history indicate primary historical units that have contributed to the Central American herpetofauna. These fundamental historical units or elements and their basic distribution patterns are: 1) South American Element, groups principally South American in distribution, but in some cases now ranging northward into Central America; 2) Middle American Element, groups that are primarily tropical Middle American in distribution and have their closest allies in Middle or South America or are endemics; 3) Old Northern, groups that are essentially extratropical in North America and the Old World and have their closest allies outside tropical America; 4) Young Northern, taxa with distributions centered on xeric extratropical portions of México and the southwestern United States, that appear to have been derived originally from tropical American ancestors. Historical complexes are subdivisions of the elements composed of species with similar distributional histories. It appears that in early Cenozoic the Americas were dominated by two major faunal assemblages. In subtropical and tropical America to at least as far as 40° N lat. a generalized tropical herpetofauna existed. To the north occurred ancestral Old Northern groups. The inundation of the Isthmian Link from Nicaragua to Colombia in late Paleocene eliminated the continuity of the tropical fauna north and south of the broad marine portal. The South American Element underwent evolution in the South American island from Eocene until Pliocene when the Isthmian Link was re-established. Sixty-nine genera belonging to this element are found in Central America, but only 23 of them occur north of Costa Rica. Two principal distribution patterns are shown by representatives of this element that occur in Central America: a) species associated with mesic conditions in Pacific Colombia and Ecuador, members of the Western South American Complex; and b) species associated with the xeric habitats of Caribbean Colombia and Venezuela, members of the Northern South American Complex. The Middle American Element differentiated to the north of the Panamanian portal during the Eocene-Pliocene separation from South America. Seventy-two genera belong to this element in Central America. Two basic distribution patterns are shown by taxa included in the element, indicating separate histories during middle to late Cenozoic: a) species found in the xeric lowlands from western México to central Costa Rica, the Western Mesoamerican Complex; and b) species with distributions centered on the mesic lowlands of the Caribbean versant, the Eastern Mesoamerican Complex. In some cases tropical families that had not penetrated across the Isthmian region by Eocene evolved in isolation: to the north as part of the Middle American Element, the Rhinophrynidae, Kinosternidae, Xantusiidae, Helodermatidae, and Xenosauridae; to the south as part of the South American Element, the Pipidae, Dendrobatidae, Centrolenidae, Atelopodidae, Pelomedusidae, Chelidae, and Aniliidae. In other cases elimination of the link led to evolution of clusters of genera north and south of the water barrier as shown in the Microhylidae, Iguanidae, Teiidae, Boidae, and Crocodylidae. The Young Northern Element is composed of groups that appear to have evolved during middle to late Cenozoic in the extratropical arid regions of western North America. Only seven genera belonging to this element are found in Central America. The Old Northern Element is represented in Central America by 18 genera. Three patterns of distribution suggest centers of differentiation and common origins. Apparently representatives of this element first reached Middle America in Eocene through migration southward along the developing Rocky Mountains. These groups later became fully isolated from their congeners in the eastern and western areas of the United States by the development of temperate arid conditions across northern México. The isolated tropical stocks became the Central American Complex, whose subsequent history was in close association with the Middle American Element. One group of species within the complex appears to have invaded the lowlands of Central America early in its history, while another group seems to have been associated with the margins of the Mexican Plateau. Several Old Northern taxa with affinities to the southeastern United States appear to be recent immigrants into the Caribbean lowlands. From Eocene onward, Central America has been dominated by the Middle American Element. The crucial period in the development of the Central American herpetofauna into modern aspect was the Pliocene. At the beginning of this period Middle American Element and Central American Complex groups were widespread over the region, with local centers of differentiation in the emerging highlands of north and lower Central America. Further uplift from Pliocene to Recent has emphasized the highlands as minor evolutionary centers and as barriers to lowland dispersal. Reconnection of Central America and South America through the Isthmian Link also permitted northward movement of South American Element groups south of Nicaragua. By middle to late Pliocene four additional historical components appear to have contributed to the fauna: 1) Middle American taxa associated with xeric climatic conditions in west México moved south along Pacific Central America as xeric habitats developed; 2) Young Northern Element groups apparently invaded the xeric Pacific lowlands and the Guatemalan highlands from the north; 3) upland Mexican groups of Old Northern relations also seem to have moved into the highlands of Chiapas and Guatemala; 4) South American xeric-adapted groups invaded Pacific coastal Panamá. From Pliocene onward these components expanded into Central America along a series of physiographic and climatically controlled dispersal routes while many groups of the basic Middle American and Central American units expanded their range southward into South America. The principal dispersal routes utilized by the faunal units are: a) an Atlantic lowland route from México to eastern Panamá that crosses over to the Pacific versant and continues to Ecuador, followed by expanding Middle and Central American groups, particularly southward, and by South American groups northward; b) a series of Pacific lowland routes that permitted Western Mesoamerican Complex and Young Northern Element forms to move southward to Costa Rica and some South American Element groups to disperse from northern South America to western Panamá. A moist area in the Golfo Dulce region effectively blocks the penetration of many South American forms into the xeric corridor of Pacific Central America and conversely limits the distribution of Middle American groups to the corridor; c) a highland route across the Isthmus of Tehuantepec that permitted exchange in both directions between the Chiapas-Guatemala-Honduras highlands and the mountains of southern México; followed southward by Mexican groups of Old and Young Northern affinities; d) a route across the Talamancan highlands of Costa Rica and Panamá; used by endemic groups and a few invaders of Old and Young Northern relations from the northern highlands. In every part of Central America, except in eastern Panamá and on the highest peaks, the fauna expresses in its distinctness from both the Nearctic and Neotropical assemblages its long and independent history, and fully validates the concept of the Mesoamerican unit as a separate major herpetofauna.

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