Tuesday, February 8, 2011

Bermuda rock lizard takes an long journey

Rock lizard on rock.
 Imagine you're a lizard living under a rock on the coast-land of South Carolina (officially a state in, oh, 50, 000 years.) You're small--about 3 inches from snout to tail.

You scurry around hunting crickets and crustaceans, bask in the morning sun, and don't expect to leave your coastal abode for your entire 20-year life. But, low and behold, the sky darkens, the wind kicks up in furious, chaotic sweeps. A full blown hurricane picks you up, whirls you around, and drops you back down on the island of Bermuda, about 1,000 km from home rock.

This is what scientists think may have happened to Plestiodon longirostris, the Bermuda Rock Lizard, in the late Pleistocene era. Since volcanoes formed the Bermuda islands 2 million years ago, there was no land bridge or way to walk from their home habitat to their new vacation paradise. And, though humans may have lived in North American as early as 50,000 years ago, it's unlikely they would have had the technology or motivation to sail to Bermuda, lizard in tow. The lizard must have colonized Bermuda somehow.

 "Although we can only speculate how these colonizing individuals dispersed over water, we note that both hurricanes and ocean currents are known to transport living lizards and debris to and from islands, and that the powerful Gulf Stream ocean current runs along eastern North America to the mid-Atlantic Ocean," according the the paper, published in PLoS ONE last week.

Zee island, mon.
Other terrestrial species may have piggy-backed to Burmuda on wind or ocean currents: the Burmuda turtle and a few bird species lived on the island during the Middle Pleistocence (781--126 thousand years ago) according to the fossil record. Other modern species have since found the island as well.

"Another reptile (indeed, the only other potentially native reptile), the diamondback terrapin (Malaclemys terrapin), is a likely very recent immigrant that descended from populations of the same species that currently inhabit the eastern United States," says the recent article.

But, the Bermuda Rock lizard and the recently extinct Bermuda turtle are special: within thousands of years of their maritime odyssey, the mainland species died out. Bermuda has become it's only residence, a phenomenon scientists call paleoendism.

The Bermuda Rock lizard is an ancient species--recent date shows it diverged from it's (clade) about 16million years ago, well before modern Plestiodons existed and well well before the Bermuda island was even formed (2 million years ago.) No fossils of this species have been found in North America. No ancestors of the species exist anywhere in the world today.  

Zee research, mon.
"We are therefore left with the remarkable conclusion that a two million-year-old island contains the sole survivor of an ancient lineage that predates the existence of Bermuda by well over 10 million years."

Eh. Could be overstating it, but you get the point. The Bermuda Rock lizard's lineage was maintained ONLY through their residence on the island. Hence, the author's compulsion to call their article, "Bermuda as an Evolutionary Life Raft..."

So, what was it about Bermuda that enabled the Bermuda Rock Lizard and the Bermuda turtle to survive? It certainly wasn't a steady habitat. Sea levels in Pleistocene Bermuda were extremely variable, limiting land space and killing off many bird species in the process. Lack of predators could have made life easy for both. (Compounded by the Plestiodons unusual survival technique of thrashing their tale until a predator bites it off. No biggy--they just grow it back.)  The answer is--who knows how they survived. We weren't there to witness it.

There are now estimated to be fewer than 500 Plestiodon longirostris on the island of Bermuda, their only home. In coming years, the lizard species may well mirror the fate of the extinct Bermuda turtle.

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