Utilization of the isthmian land bridge during the Cenozoic—paleobotanical evidence for timing, and the selective influence of altitudes and climate

Abstract

A comparison of plant microfossil assemblages from southern Central America (Costa Rica, Panama) and northern South America reveals distinct differences that extend into late Cenozoic time. Eight kinds of fossil palynomorphs (2.6%) are common in middle(?) to Late Eocene microfossil floras from central Panama and northern South America, increasing to only 21 taxa (8.9%) by Gatum time (present assignment-Middle Pliocene, ca. 4 Ma). This argues for continuous emergent land connection suitable for interchange of plants with limited seed dispersal capacity in the later part of the current 5-1.8 Ma range estimates. Glacio-eustatic changes in sea level of 150 m during the Quaternary, compared to maximum current land mass elevations of 34 m across Costa Rica/Nicaragua and 84 m across eastern Panama, further suggest that direct overland crossings were intermittent even during the Pleistocene. Paleophysiographic reconstructions based on paleobotanical data show that all taxa in the early Miocene floras could be accommodated within an altitudinal range from sea level to about 1200–1400 m, increasing to about 1700 m by Gatun time. Paleoclimatic data based on fossil palynofloras from the relatively low-lying islands and peninsulas constituting Central America for most of the Cenozoic indicate tropical conditions and vegetation generally similar to that of the modern lowlands until about the Middle Pliocene. At that time an increase in grass pollen (from virtually absent to a maximum of 7.5%), initial appearance of elements of a tropical dry forest, first evidence of a possible differentiation into a wetter Atlantic and drier Pacific side, and continued tectonic activity leading to greater topographic diversity mark the beginning of the modernization of southern Central American vegetation. Utilization of the isthmian land bridge by plants from North or South America, until post-middle Pliocene times, was limited primarily to those of tropical climates, living in lowland to premontane habitats, and with effective means of dispersal across at least moderate marine barriers. Other means of exchange, rather than direct migration across continuous or near-continuous land surfaces, could have included flotsam or abrupt long-distance dispersal throughout Cenozoic time.