(I've corrected the rather obvious mistake in the Science Centric heading above.)
Gradually, however, as an increasing amount of the planet's water became locked up in polar ice, the sea level dropped and more land was exposed. Vast regions of the earth became drier and warmer, including the region that would become western Pennsylvania. The coal swamps and lakes dried up, and many of the coal-forming plants became extinct. It was at this time that amphibian populations in what would become the Pittsburgh region shifted from mainly aquatic to mainly terrestrial, paralleling the change in climate from tropical to semi-arid. Vertebrates that had already begun adapting to terrestrial life - including amphibians closely related to Fedexia striegeli - became far more abundant, widespread, and diverse than their relatives who were still dependent upon cooler, moist environments.
Full article here