Studies in Neotropical Paleobotany. V. The Lower Miocene Communities of Panama-The Culebra Formation

Abstract

The lower Miocene Culebra Formation of central Panama consists of lignites and lignitic shales in a predominantly estuarian sandstone sequence. Forty-one palynomorphs have been identified from the lignites, and the following ten are most abundant: Monolete fern spore type 2 (20%), Manicaria-type palm pollen (10%), Cyathea (8%), Cryosophia-type palm pollen (8%), Selaginella (7%), Synechanthus-type palm pollen (6%), Rhizophora (6%), monolete fern spore type 1 (5%), Hampea/Hibiscus (4%), and Lycopodium (2%). Ferns constituted 25% of the fossil flora, palms 24%, and lowland vegetation types isolated 71%. The most prominent paleocommunities were the tropical moist forest represented by 30 genera that can occur in this vegetation type (including an only moderately developed mangrove swamp), premontane wet forest (25 genera), tropical wet forest (22 genera), and possibly some form of the premontane moist forest (12 genera). Communities of higher elevations and dry to arid habitats (including savannahs) were poorly represented to absent. Paleoenvironments were similar to those of the present, including annual rainfall estimated at about 275-325 cm (107-128 inches). The rainfall probably decreased slightly with elevation, similar to the pattern shown in a transect from sea level to about 1,172 m from present-day Atlantic Costa Rica (353.6-188.9 cm). Annual temperature probably ranged between about 21.degree. C and 32.degree. C, as on adjacent Barro Colorado Island today. These estimates are consistent with the general paleotemperature curve derived from 18O studies of marine invertebrates, and with emerging paleobotanical data from other Central America Tertiary floras. The geographic affinities of the Culebra flora are distinctly Central and North American, with all of the 41 taxa identified represented in the modern flora of Panama. Pollen of the Gramineae and shrubs of open, drier habitats continue to be rare or absent in Tertiary formations from southern Central America.