Zircons are crystals that form when magma cools. Newly-formed zircons contain some uranium, but no lead. Therefore, any lead found in an old zircon must have originated in the radioactive decay of uranium. Thus, zircons are a favorite tool for dating in geology. (as well as dinner and a movie ;-)).
Zircons found in streams in Colombia and Panama have dated the closing of the gap and formation of the "land bridge" at 13-15 million years.
The biggest mystery is why animals, especially mammals like this racoon ancestor,
did not make the mass movement across the land bridge until 3-5 million years ago (as seen in the fossil record). Perhaps the "land bridge" was more like a series of volcanos with shallow waters in between so it took ten million years for a solid, continuous land bridge to form.
This research was performed by Camilo Montes et al. This article also contains a link to terror birds (if you dare). [And a new terror bird species has just been discovered as reported on April 13, 2015.]
Any ideas as to what was going on in the 10 million years between the possible closing of the North and South American continents and the mass mammal migration (say that 5 times fast!)?