DISCUSSION

The Pleistocene fossil flora collected along the Río Puerto Viejo contains fragmentary leaves, cuticle, wood, fruits, and seeds, adequately preserved to allow detailed morphological and anatomical observations. This fossil flora sits within a gray layer of clay-rich volcanic material, which, in turn, is overlain by sedimentary deposits and modern soil horizon at the top of the riverbank (Horn et al. 2003). The depositional soil near Puerto Viejo is classified as old alluvial terraces, Pleistocene River terraces (Sollins et al. 1994). The fossiliferous gray layer was initially described as a fluvially reworked volcanic deposit (Alvarado 1990), but subsequent lab analyses of feldspar grains and detritus in this gray sediment indicated a primary airfall deposit (Horn et al. 2003). Except for a single leaflet, all the fossil leaves examined from this layer are fragmentary, and the sizes are under 8 cm long. The size range for extant species that are related to the fossil leaves include Pouteria, up to 45 cm long (Pennington 1990), Parinari up to 17 cm long (Prance 1972), Piper (multi-nerved species, Bornstein 1989) up to 20 cm long, and Costa Rica species of Ocotea/Nectandra up to 55 cm long (Burger and van der Werff 1990). The lack of large leaves may be due in part to the collection methods, which involved cutting out blocks of material that would fit in plastic bags ca. 20 cm x 20 cm in size. The condition of the leaves in the flora may also reflect the fact that larger leaves tend to become fragmented and destroyed in turbulent conditions (Wolfe 1971), and they do not travel very far via air currents (Roth and Dilcher 1978). In comparison, small leaves may be an overrepresentation of streamside plants, or differential aerial transport of more distant species (MacGinitie 1953; Wolfe 1971; Spicer 1981). The fact that almost all the fossil leaves are small and fragmentary suggests mechanical fragmentation of streamside plants in a fluvial setting. Further field examination needs to be done for a proper taphonomic analysis of the leaves, such as the direction of the midvein to explore if the leaves were oriented in the same direction and comparing the deposit with modern leaf deposits in similar environments along the Rio Puerto Viejo.

The Puerto Viejo Pleistocene sediments contain macrofossils of leaves, fruits, and seeds of 43 morphotypes, 11 of which have been determined to the Tribe, Family, Genus, or Species level (see Horn et al. 2003 for identifications of fossil wood, pollen, and spores). Of the undetermined morphotypes, most have an abundance of less than 0.4 %. Of the 11 identified taxa, five are fragmentary leaf/cuticle types, and six are whole fruit/seed types. Leaves that dominate the paleoflora are Parinari (50.3%) and Pouteria (31.3%). Other leaves are Laurophyllum (1.6%), Piperites (0.4%), and Ingeae (0.4%). Reproductive units that dominate the flora are Parinari (32.1%), followed by Oxandra (10.7%), Sacoglottis and Byrsonima (9.8% respectively), Pouteria (5.3%), and Legumincarpon (2.7%) (Table 1). Fossil Parinari represents half of the leaves and nearly one-third of the fruits recovered, with an extant distribution in the lowland tropics. Although Parinari is lacking in the modern flora of La Selva, it is present at nearby higher elevations of 500 m (Horn et al. 2003). Another extant lowland tropical species is Pouteria, with a high percentage of fossil Pouteria leaves in the fossil flora. Pouteria, which grows at La Selva today, increases in abundance from 300–600 m (Pringle et al. 1984; Lieberman et al. 1996). Sacoglottis and Byrsonima can be found living at La Selva and up to 500–1000 m (Burger and Zamora 1991). The close relatives of Laurophyllum are found in La Selva (Nectandra) and in higher elevations of 2300–2600 m (Ocotea) (Wilbur 1994; Lieberman et al. 1996). Oxandra seems to be lacking in La Selva and higher elevations near La Selva. Pentaclethra macroloba dominates the extant primary lowland forest of La Selva (35–150 m), is not dominant at 300 m (Pringle et al. 1984) and is lacking in the fossil flora. The preceding distributions suggest that the fossil flora has some connection to a slightly higher elevation tropical forest of present-day Braulio Carrillo National Park (Hartshorn and Hammel 1994; Lieberman et al. 1996), just south of La Selva. The Quaternary period included 18–20 glacial-interglacial cycles (Johnson 1982; Davis 1983). In the mountains of Costa Rica, cooling during Quaternary glacial intervals resulted in altitudinal shifts downward in species ranges and vegetation belts (Martin 1964; Hooghiemstra et al. 1992; Islebe et al. 1995; Islebe and Hooghiemstra 1997). Extrapolating from modern ranges of the preceding species and surface temperature lapse rates (Horn et al. 2003), the macrofossil flora of La Selva suggests paleotemperatures 2.5–3.1°C cooler than at present, which may coincide with fluctuations in the Quaternary climate. The fossil flora is similar to the modern Costa Rican tropical forest, which has a biogeographic connection with tropical forest of Northern South America (Gentry 1990). This biogeographic connection shows up in the fossil record of Central and Northern South America (Burnham and Graham 1999). Genera and families in common with previous reports of the fossil flora of Costa Rica and Northern South America include Parinari, Sacoglottis, Piperaceae, and Annonaceae (Engelhardt 1895; Berry 1921a, 1921b, 1922, 1923, 1936; Wijninga and Kuhry 1990; Wijninga 1996).