Museum scientist Pat Holroyd and retired paleontologist Howard Hutchison have been exploring UCMP's vast collection of fossil turtles from Wyoming in hopes of tackling the little addressed question of how turtles and other aquatic reptiles respond to changing climates. These fossils have managed to tell the story of several ancient takeovers back in the Eocene, about 55 million years ago. The Eocene was when several abrupt global warming events took place - the first of which defines the start of the epoch, the Paleocene-Eocene Thermal Maximum (PETM) - and semi-tropical forests extended across the northern United States.
It turns out that with warming temperatures came a case of turtle wanderlust. While most groups of North American animals are thought to disperse via high latitude dispersal routes (like along the Bering Land Bridge or through Greenland) to the continent they call home, some reptiles, especially turtles and lizards, also opted to disperse from the south as new corridors opened up during the PETM. The eclectic mix of creatures in North America resulting from these long treks included pond turtles and tortoises from Asia and mud turtles and river turtles from Central America.
These foreign arrivals rapidly dominated their new environments, reminiscent of classic invasive species dynamics. And this doesn't only happen in the PETM. Another warming event later on in the Eocene has the same signature turnover, but with a new set of immigrants, including the ancient relatives of today's tortoises. But while the composition of North American turtles during these times shifted dramatically in favor of the migrants, there is no sign that there were any extinctions of the locals. They merely got shunted into relatively smaller abundances.
So it is critical to understand dispersal and dispersal routes in order to understand how the composition of a fauna changes in response to climate, stresses Pat. It'll be interesting to see how the turtles respond to the modern age of global warming.