An early Miocene (Aquitanian) mangrove fossil forest buried by a volcanic lahar at Barro Colorado Island, Panama

Abstract

We describe remains on an Early Miocene mangrove forest, based on a fossil wood assemblage, discovered on Barro Colorado Island, Panama. Sedimentological and stratigraphic analysis suggests that the fossil trees grew in marginal marine to coastal fluvial settings and were buried under a volcanic lahar flow in a single event. Radiometric analyses of a tuff associated with the fossils gives a date of ∼22.79 Ma indicating an assignment to the Aquitanian stage of Early Miocene. At this time, central Panama was part of a long and narrow peninsula with intense volcanic activity that connected it with North America and was separated from South America by the Central American Seaway. A total of 121 fossil wood specimens were located. Wood anatomy indicates that most of the identifiable specimens belonged to the same morphotype, which has anatomical traits similar to Sonneratia (Lythraceae), a mangrove tree that is native to Southeast Asia. We named this morphotype as a new fossil species: Sonneratioxylon barrocoloradoensis Pérez-Lara., sp. nov. Biomechanical estimates indicate that S. barrocoloradoensis had a mean tree height of 25 m with some specimens reaching 40 m, in contrast to modern Sonneratia and other extant mangrove forests, which generally have lower mean heights. The dominance of S. barrocoloradoensis, its similarity to Sonneratia, and the depositional setting suggest that the fossil wood assemblage on Barro Colorado Island comprised a mangrove forest growing along the coast of the volcanic chain of central Panama.