Formation of the Isthmus of Panama

Abstract

The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.

Keywords: Central America; Evolution; GABI; Isthmian closure; ecology; land-bridge.

Fig. 1. Uplift of the Isthmus of Panama and global sea levels over the last 20 My.

Rates of uplift are estimated from changes in the age and depth of deposition of sedimentary units across the Panama Arc (table S1) and are therefore relative to sea level. Eustatic sea-level estimates (light blue and dark red lines) from the study of Miller et al. (131). The dark blue line indicates values averaged within time bins of 250 thousand years (ky) for the 0 to 9 Ma record.

Fig. 2. The Isthmus of Panama and northwestern Colombia with locations of some crystalline rocks (igneous and metamorphic) with reported Paleocene-Eocene radiometric ages (squares and triangles).

Montes et al. (15) proposed that Eocene detrital zircons in middle Miocene sediments of the Magdalena Basin (blue dots) could only be from plutonic rocks of the Panama Arc (dark green outline). However, several other rock units (for example, Western, Central, and Eastern Cordilleras plus the Sierra Nevada de Santa Marta; red outlines and table S2) exhibit radiometric ages in the same interval and therefore must also be considered as potential sources. From previous studies (–26, 30, 31, 132).

Fig. 3. Timing of divergence between Caribbean (blue) and Tropical Eastern Pacific (red) environments and ecologies in coastal, shallow, and deep waters.

(A and B) Estimated mean annual range of temperature [MART; a proxy for strength of upwelling estimated by measuring zooid size variation in fossil bryozoan colonies (133)] (A) and the relative skeletal weight of corals and coralline algae (68, 134) (B) in replicated bulk samples from coastal shelf sediments on the Isthmus of Panama. (C and D) Rates of accumulation of carbonate (CMAR) in deep-sea sediments (C) and estimated surface water salinity (reflected in the oxygen isotope record) (D) in Caribbean and Pacific surface waters. (E) Neodymium (Nd) values from fish teeth and foraminifers in Pacific and Caribbean basins (35, 38).

Fig. 4. Bayesian estimates of median molecular divergence times of sister taxa on either side of the Isthmus of Panama.

Only species for which both nuclear DNA and mtDNA data were available and for which divergence ages were calibrated by the fossil record are included. Error bars represent 95% HPDs. Numbers refer to species pairs in table S3.

Fig. 5. Frequency of appearances of immigrant vertebrate taxa or their oldest known descendants in opposing continents as observed in well-dated fossiliferous sediments in South and North America per million years.

See fig. S3 for further details.