LATE CENOZOIC EVOLUTION OF TROPICAL LOWLAND VEGETATION IN VERACRUZ, MEXICO.

Abstract

Veracruz is physiographically the most diverse state in Mexico, with altitudes ranging from sea level to the highest peak in Mexico (Pico de Orizaba, 5747 m). This diversity in environments is reflected in a complex mosaic of vegetation estimated at 8000-10,000 species. Studies by GomezPompa (1973) have identified the principal plant communities of Veracruz and provide information on their ecology, composition and distribution. Recently paleobotanical data has become available that, with better understanding of present-day vegetation, reveal some trends in the Cenozoic evolution of lowland communities in Veracruz. An assemblage of 110 identified fossil pollen and spores has been recovered from lignites of the Paraje Solo formation near Coatzacoalcos, Veracruz, Mexico (Table 1). The age of the sediments is upper Miocene and the microfossils thus represent remnants of a vegetation occupying lowlands and adjacent slopes of the Sierra Madre Oriental approximately 10-12 million years ago. With the data available on modern vegetation, these fossils may be arranged into paleocommunities and a comparison made between fossil and modern biotas. Some similarities are evident, together with distinct differences, especially with regard to the Tropical Rain Forest, arid lowland vegetation, and communities suspected (but not proven) to result from anthropogenic influences. A summary of upper Miocene and modern vegetation from southeastern Veracruz is presented in Figure 1. The present-day communities are listed along the left and bottom of the chart with three east-west transects shown along the eastern escarpment from about 1700 m elevation to the Gulf Coast (Gomez-Pompa, 1973). Fossil taxa are placed according to known natural occurrences and may appear in more than one paleocommunity. In a few cases pollen of two genera could not consistently be distinguished in the fossil condition (Eugenia/Myrcia, Combretum/Terminalia, Hampea/Hibiscus). An occasional wellpreserved, critically oriented grain has indicated the probable presence of each genus, and all are included in reconstruction of the paleocommunities although estimates of the relative abundance of these taxa are not possible. The Palmae and Gramineae are listed only for palm stands and savannas but pollen of palms and grasses (and Compositae) are found in all samples, and the modern plants occur scattered throughout most types of vegetation in Veracruz. Pollen of the Lauraceae does not fossilize or survive processing techniques. The criteria used to estimate presence or absence and relative importance of various paleocommunities in the Miocene vegetation are a) occurrence of pollen of the dominants, b) presence of genera characteristically associated with a particular vegetation type, as opposed to wide-ranging genera occurring in several types of communities, and c) abundance of each pollen type, with compensation for distance from the depositional basin, differential pollen production, and nature of the pollination mechanisms (to the extent these data are available).